Symposium EQ10—Advances in Metasurfaces, Metamaterials and Plasmonics—Materials Design, Manufacturing, Applications and Industrial Aspects

Plasmonic nanoparticles (e.g., silver and gold nanoparticles) exhibit unique optical characteristics. When these plasmonic nanoparticles are placed close enough, the plasmonic coupling effect occurs, which makes the scattering spectrum shift to a longer wavelength. In addition, the spectral shift of scattering light differs according to the arrangements, inter-particle distances, and materials of the plasmonic nanoparticles. Due to their unique optical characteristics, they are utilized in various applications, from sensors to imaging methods. For example, these plasmonic nanoparticles can be used as a ruler by utilizing their distance-dependent scattering spectra.
Histone is a protein at chromatin in a nuclear of cell. The histone has a tail and various histone modifications are at the tail. Its numerous combinations of modifications are called histone code. This histone code is determined by reader and eraser enzymes. The determination of enzymes is affected by various causes including senescence of a cell or diseases and it affects the topology of the chromatin. For example, Heterochromatin Protein 1 (HP1), a reader protein, identifies and combines with the H3K9me3 histone modifications. H3, K9, and me3 represent histone 3, 9th lysine, and trimethylation, respectively. HP1 concentrates together, making chromatin compact and it is called heterochromatin. The interaction between HP1 and another protein, lamin A, makes the heterochromatin to be located near the nuclear membrane. However, as a cell becomes senescence, H3K9me3 tends to concentrate inside the nuclear and create sites where the histones are closely packed. It is called senescence-associated heterochromatin foci (SAHF). To date, various histone modifications are found and it is revealed that some of them are closely related to diseases like cancer. Thus, understanding the relationship between histone modifications and topological change of chromatin during a cell’s senescence becomes significant because of the relation between the distribution of histone modifications and the diseases.
The conventional method for imaging the distribution of the histone modifications, using organic fluorescence dye for example, has encountered limits including poor resolution and blurred images. Furthermore, the distribution of the histone modifications cannot be distinguished when organic dyes are at a close distance. Therefore, plasmonic nanoparticles are implemented in this study by utilizing their unique optical characteristics. Plasmonic nanoparticles are used as a probe for imaging the distribution of the histone modifications for the first time to overcome the drawbacks of the conventional methods.
In this work, the scattering spectra are calculated with various arrangements and inter-particle distances of plasmonic nanoparticles. Some arrangements showed unique scattering spectra at certain inter-particle distances. These spectra were distinctive since they exhibited additional dominant scattering peaks (i.e., double peaks) while other spectra have only a single peak. Furthermore, the unique spectra are more often found when silver and gold nanoparticles are placed together. Dominant peaks are found even at the shorter wavelength owing to the silver nanoparticles. The combinatory peaks of scattering spectra make each scattering spectra distinguishable. Not only the distinctive scattering spectra but also a broader redshift of scattering spectra exhibited when silver and gold nanoparticles are placed together. Based on the simulation results, the distributions of the histone modifications in a cell were predicted. The distributions predicted were ranging from 2D to 3D arrangements with inter-particle distances of from 1 nm to 7 nm. We believe that this novel visualization method would be useful for monitoring the progress of diseases and also further understanding of the optical characteristics of plasmonic nanoparticles would lead us to advanced optical applications.

Optical fibers have been studied extensively due to their remarkable features such as flexible handling, long-distance light transportation, and strong light confinement. These features enable a variety of applications such as optical trapping, optical communications, sensing, and medical imaging. Particularly, the fiber-based endoscope allows in vivo imaging of biological samples. During the fiber imaging, the scattered light from the object is first collected by the lens, such as ball lens or gradient index lens (GRIN), then guided into the fiber core. However, these two bulky lenses suffer from chromatic aberration that hinders obtaining clear images over wavelengths of interest.
Metasurface, a planar version of metamaterials, has shown a great capability to modulate its optical properties [1] such as amplitude, phase delay, polarization, chirality, and nonlinear coefficient. Especially, dispersion engineering using metasurface, modulation of phase delay over wavelengths of interest, enables diffraction-limited achromatic focusing and imaging, i.e. achromatic metalens [2, 3]. Interfacing fiber with the achromatic metalens could be the ultimate solution to deal with the aberration that exists in fiber-based imaging. Several efforts have been made to fabricate metasurface interfaced with fiber by using electron-beam lithography (EBL) [4], focused ion-beam milling (FIB) [5], and chemical etching techniques [6]. However, these fabrication methods limit the type of materials and geometry of meta-atom, thus prohibiting the design of broadband and large achromatic metalens on top of the fiber. Instead, laser nanoprinting based on two-photon polymerization may provide a platform for optically functionalizing a fiber facet with three-dimensional (3D) arbitrary microstructures [7].
In this presentation, I will introduce an entirely new concept of an achromatic metafiber that focuses light coming out from the fiber facet over wavelengths of interest. The achromatic metafiber consists of achromatic metalens microprinted on a telecommunication single-mode fiber (SMF-28). The 3D meta-atoms of which height is a geometric degree of freedom unlike conventional meta-atoms can provide large variation of group delay, capable of realizing high time-bandwidth product. As a demonstration, direct scanning confocal imaging with our achromatic metafiber is facilitated over entire telecommunication wavelengths. We believe that our compact achromatic metafiber envisages many photonics applications such as hyperspectral imaging in endoscopy, in vivo deep-tissue imaging, and wavelength-multiplexed fiber communications.
References
[1] Jung, C. et al. Metasurface-Driven Optically Variable Devices. Chem. Rev. (2021) doi:10.1021/acs.chemrev.1c00294.
[2] Khorasaninejad, M. et al. Metalenses at visible wavelengths: Diffraction-limited focusing and subwavelength resolution imaging. Science (80-. ). 352, (2016).
[3] Chen, W. T. et al. A broadband achromatic metalens for focusing and imaging in the visible. Nat. Nanotechnol. 13, 220–226 (2018).
[4] Wang, N., Zeisberger, M., Hübner, U. & Schmidt, M. A. Boosting Light Collection Efficiency of Optical Fibers Using Metallic Nanostructures. ACS Photonics 6, 691–698 (2019).
[5] Yuan, G. H., Rogers, E. T. & Zheludev, N. I. Achromatic super-oscillatory lenses with sub-wavelength focusing. Light Sci. Appl. 6, e17036–e17036 (2017).
[6] Kuchmizhak, A., Gurbatov, S., Nepomniaschii, A., Vitrik, O. & Kulchin, Y. High-quality fiber microaxicons fabricated by a modified chemical etching method for laser focusing and generation of Bessel-like beams. Appl. Opt. 53, 937 (2014).
[7] Plidschun, M. et al. Ultrahigh numerical aperture meta-fibre for flexible optical trapping. Light Sci. Appl. 10, 57 (2021).

Phase gradient metasurfaces are revolutionizing optical system design, providing a way to shrink the thickness of any component down to the nanoscale, from lenses and prisms to polarizers. Tailoring diffracted light using non-uniform arrays of nanoscale antennas, the performance of these metasurfaces often matches if not surpasses their refracting counterparts. For example, meta-lenses with comparable resolution to high-end infinity corrected objectives 1000 times their size and bright, artifact-free metasurface holograms have both been realized. With most metasurface elements being resonant, it has been suggested that they are also ideal for dynamic light wave manipulation, potentially unlocking exciting application areas, including LiDAR and AR/VR. However, the very weak refractive index modulation found in most materials leads current metasurface designs to require impractically large electrical tuning voltages. Exciting demonstrations have been given using special materials, such as liquid crystal or epsilon-near-zero materials, but these come with limitations, including slow switching speeds, poor resolution, and often extremely strong absorption.
Here, we reveal a route to dynamic metasurface wave shaping free from these limitations by amplifying weak index modulation. Specifically, we theoretically and experimentally introduce a new high quality (Q) factor metasurface platform, capable of generating arbitrary reflected phase profiles over an extremely narrow bandwidth in the infrared. These metasurfaces are composed of silicon nanoantennas sitting above a metallic ground-plane. The nanoantennas are patterned to support high-Q dipolar guided mode resonances (DGMRs). Due to the lack of absorption and the formation of an image dipole in the ground plane, the reflected phase from each element varies from 0-2π across the resonance, while maintaining unity reflectance. Importantly, the DGMRs or localized to individual nanoantennas. So, by independently tuning the resonant frequency of each element, the reflected phase of light at the center wavelength can be defined as a function of position across the array. As the element spacing can be subwavelength, very precise phase profiles can be realized, resulting in a high numerical aperture and minimizing diffractive artifacts. By simulating the reflection from an electrically biased Si-LNO metasurface, we show that beamsteering can be achieved with just a few volts applied to each element, which is a direct result of the high Q of the DGMRs.
As experimental proof of the dynamic high-Q meta-reflect-array concept, we have realized fixed beam steering metasurfaces with a structurally tuned steering direction. While a dynamically tunable device requires resonant shifts to be realized via a bias dependent index change, the same physics applies to a system with resonance shifts resulting from slight structural variations of the antennas. Our nanoantenna design involves columns of silicon block pairs sitting on a sapphire substrate, with the slight difference in length between neighboring blocks determining the Q. After e-beam patterning, the chip is coated with PMMA and silver to produce the reflect-array. Phase tuning of each nanoantenna is achieved by altering the width of the smaller block. We vary this parameter across the array to generate linear phase gradients near the operating wavelength of 1485nm. Measuring the change in resonant deflection angle for devices with different numbers of elements per supercell, we confirm that arbitrary phase slopes can be realized. The relatively high-Q of the modes, Q≈500, allows the very small structural variation, just a 14nm (or 6%) difference between largest and smallest blocks, to produce strong diffraction. This small structural change is a proxy for needing only a very weak bias in a dynamic system. The Q here was also limited mostly by e-beam resolution, so we expect even better performance with a dynamic system.

Chiral metal nanoparticles are reported to have interesting chiroptical properties, which attracts researchers in the bio-sensing and nano-optics fields. The gold 432-hellicoids, the particles of interest in this work, were synthesized with chiral peptides and showed Circular Dichroism (CD) [1]. The distinguished three-dimensional chiral shape of the nanoparticle is thought to attribute the optical chirality.
However, the origin of the chiroptical property of the 432-hellicoids has not been fully understood yet. To understand the physical ground of it, analyzing the possible induced plasmon modes in single-particle-level through simulations and measurements is the most first step.
Electron Energy Loss Spectroscopy (EELS) in Transmission Electron Microscope (TEM) is a powerful characterization tool for this work. Scanning Transmission Electron Microscopy Electron Energy Loss Spectroscopy (STEM-EELS) has superior spatial resolution compared to any other characterization tools and good energy resolution. It enables sub-particle-level plasmon mapping, and the three-dimensional imaging is also possible with those maps at several different sample tilt-angles.
In this work, Cs-corrected and Monochromated TEM (Themis Z, Thermofisher) was used to map the surface plasmons of a single 432-hellicoid nanoparticle. The Spectrum-Images (SI) with the energy range in 0 eV to 3 eV and 5nm-sized pixel were obtained for each sample tilt-angle. The samples were tilted along the axis of the holder (alpha-tilt) from -60 degree to +60 degree in the increment of 10 degree. The 13 SIs were post-processed to get information on plasmon energy and the spatial distribution, then used to reconstruct three-dimensional mapping of plasmon modes. The widely known MNPBEM, the plasmon simulation tool that numerically solves Maxwell equation, was used to interpret the experimental data [2].
This work enables deep understanding of the plasmons in a single chiral nanoparticle. It also can be a step forward to great advancement in establishing design strategy its potential future application like chirality-dependent bio-sensing.
[1] Lee, Hye-Eun, et al. "Amino-acid-and peptide-directed synthesis of chiral plasmonic gold nanoparticles." Nature 556.7701 (2018): 360-365.
[2] Hohenester, Ulrich. "Simulating electron energy loss spectroscopy with the MNPBEM toolbox." Computer Physics Communications 185.3 (2014): 1177-1187.

Symmetry dominates the structure and function of the universe. Symmetry dictates the properties of many nanomaterials and nanostructures with low, but still defined, symmetries often display markedly different properties compared to their higher symmetry counterparts. An excellent example of symmetry-dependent properties can be found in the optical properties of Au cuboctahedra of O_h symmetry which display a single, dipolar resonance and Au nanorods with D_nh symmetry which display two dipolar plasmon modes corresponding to the long and short axes of the rods. While numerous routes have proven experimentally promising at systematically reducing NP symmetry, use of surface protecting groups such as silica or collapsed polymer shells have shown promise at restricting growth on NP seeds. Herein, poly(styrene-b-polyacrylic acid) (PSPAA) is used to asymmetrically passivate cubic Au seeds through competition with CTAB ligands. The asymmetric passivation via collapsed PSPAA caused only select vertices and faces of the Au cubes to be available for deposition of new material (i.e., Au, Au-Ag alloy, and Au-Pd alloy) during their overgrowth. The resulting NPs form nanobowl-like structures with some degree of branching, being reduced symmetry guided by the asymmetric seed passivation. Through experiment and simulation, the eminent optoelectronic properties of this class of nanomaterials are probed, with applications such as Surface Enhanced Raman Scattering (SERS) being employed to elucidate the structures’ viability as nanosensing platforms.

Plasmonic nanoparticles have been used extensively for nonlinear optical frequency mixing in recent years due to their broadband field enhancement and the large ultrafast nonlinearities they offer. Through careful design of the particle resonances and shapes, a bespoke nonlinear optical response can be achieved, such as a second-order response in a non-centrosymmetric arrangement of Au nanoparticles. The control of the nonlinear output can be maximised when arranging the nanoparticles in a metasurface for harnessing the power of designer ‘flat optics’ with capabilities such as frequency- and polarisation-dependent routing of photons. However, while many proof-of-principle studies have been published on classical coherent frequency mixing of multiple beams, the practical aspects of spontaneous photon pair generation rates in metasurfaces and a comparison with conventional photon pair sources is rarely addressed.
Here, we apply the Stimulated Emission Tomography (SET) technique to frequency mixing in plasmonic nanoantennae to quantify their expected performance as novel wavelength-agile correlated photon pair sources. Introduced by Liscidini and Sipe less than ten years ago, SET conveniently enables characterising photon pair generation by studying the corresponding stimulated process, thus bridging the gap between nonlinear optics and quantum optics. We first introduce our sample and setup and explain how the numbers of stimulating and generated photons are determined in our experiment. Next, the classical frequency mixing efficiencies and the estimated spontaneous photon pair generation rates are presented. Below the damage threshold of the particles, we find a generation efficiency for spontaneous parametric down conversion of about 1 photon pair per second per nanoparticle. This analysis is followed by a characterisation of Joint Spectral Density (JSD), which allows us to quantify the spectral entanglement of the biphoton output. Here, the SET technique provides increased spectral resolution, limited only by the spectral width of the stimulating beam, compared to an SPDC experiment with spectrally resolved coincidence measurements. Our sample does not yet offer the ability to directly detect spectrally correlated photons, however, with our experimental capabilities we would be able to easily characterise a variety of candidate nonlinear metasurfaces. The talk will close with perspectives on applications of wavelength-agile pair sources for quantum imaging in previously inaccessible wavelength ranges.

Conductive polymers have been having a fundamental role in the development of organic electronics and show great potential for tuneable photonic systems. The fact that organic materials are soft and flexible and are often compatible with high throughput fabrication methods gives them great technological potential in developing flexible, wearable, and low-cost devices. Besides the extensive use for emissive devices in optoelectronics, there has been a large interest for electrochromic displays and smart windows, where their tuneable absorption properties are used. However, instead of having just a few electrochromic states available, a novel compelling application is in reflective displays in colour. While those displays have the advantage of a high eye comfort and visibility against the sun, coupled with very low power consumption, the bottleneck is the low brightness, having no other light sources than ambient light. This can be addressed by having an electroactive tuneable structural colour that can cover the entire visible spectrum, with no need for a traditional subpixel division. Plasmonics, optical nanocavities, and photonic crystals have been proposed as electroactive structural colors in the visible, but only a few were shown to be able to cover the entire visible spectrum and achieve a high reflectance. Most of the organic-based devices used redox electroactive polymeric or gel-like structures with redox sites, but conductive polymers have only been recently considered.
Electrochromism has been the most exploited optical property of conductive polymers, but their redox tunability makes them a viable choice for tuneable optics. That is, because along with the absorption also the permittivity and the thickness or volume can be electrochemically changed. We have demonstrated that conductive polymers based on poly(3,4-ethylenedioxythiophene) (PEDOT) exhibit a negative permittivity in certain spectral regions and can be used as plasmonic materials [1]. Moreover, we also showed that PEDOT can be structured by photolithography in combination with vapour phase polymerization, making possible to directly obtain images by structural coloration, with a certain degree of tunability [2]. We focus here on our recent efforts in studying and exploiting conductive polymers for electroactive photonic devices. We show that some novel polymers, such as thienothiophenes and suitably engineered polythiophenes, exhibit a relatively low absorption in the visible for all the redox states, which make them good candidates for electroactive tuneable reflective structural colors. To test their optical performances as actuating materials, we successfully integrated those polymers in optical nanocavities inside an electrochemical cell [3]. We achieved complete coverage of the visible spectrum, maintaining a high reflectance for all the colors while using a low voltage range. This is made possible by a synergistic combination of the variation of the optical properties and thickness change of the polymeric spacer. We offer here an overview and spectroelectrochemical characterization of the properties of conductive polymers in respect to the change of their redox state and the use in organic photonics. Besides electroactive structural coloration, those materials might find applications in other types of optical metasurfaces, with potential also in the infrared region, and could also be used in the field of electrochemical actuators, due to their thickness changes.
[1] S. Chen et al., Nature Nanotechnology 2020, 15, 35-40
[2] S. Chen et al., Advanced Materials 2021, 33 2102451
[3] S. Rossi et al., Advanced Materials 2021, 2105004

Changing climate conditions are driving fundamental shifts to our marine and freshwater ecosystems. Phytoplankton are microscopic organisms responsible for half of global photosynthetic carbon fixation, but certain phytoplankton taxa can produce powerful, low molecular weight biotoxins that contaminate drinking water sources, harm wildlife, are detrimental to human health, and pose economic threat to coastal communities. Understanding how environmental drivers impact plankton nutrient cycling and toxin production is key to advancing climate resilience but remains an outstanding challenge. Dominant methods of studying these marine toxins and their genes are based on liquid chromatography with tandem mass spectrometry (LC/MS-MS) and polymerase chain reaction (PCR), but these techniques are costly, require sophisticated infrastructure, and lack remote, autonomous, real-time detection capabilities central to understanding coupled hydrographic and climate dynamics.
In this talk, I will describe the development of a nanophotonic sensor for real-time aquatic biotoxin detection using surface-functionalized high quality factor (high-Q) silicon metasurfaces. I will discuss the design of our sensor, which is formed of sub-wavelength silicon nanobars, of dimensions 500 nm x 600 nm x 160 nm, periodically modulated to enable free space coupling to guided mode resonances. We demonstrate that these guided mode resonances generate a strong local field enhancement through high quality factors exceeding 1,000 both in simulation and in fabricated devices. We show that increased field penetration into the surrounding medium and long resonant lifetimes together enable sensitive and quantitative readout of the local dielectric environment, where small perturbations generate strong spectral shifts to the resonant mode. We demonstrate that by using free space coupling into the guided mode resonances, the optical responses of individual resonators can be spatially resolved and simultaneously read out as a change in resonance wavelength, or as a change to the scattering intensity on a 2D CCD array using a home-built infrared optical microscope. We apply this platform to the detection of two aquatic biotoxins: domoic acid, a neurotoxin produced by the marine diatoms, Pseudo-nitzschia spp., responsible for human and wildlife mortalities, and microcystin, a liver toxin produced by cyanobacteria that poses a major threat to drinking and agricultural water supplies. We show that by employing an antibody binding assay with tailored surface functionalization, our high-Q metasurfaces can exhibit strong shifts in resonance wavelength of ~5 nm, offering a straightforward approach to detection with high antibody binding specificity. Finally, I will discuss the integration of our high-Q metasurfaces with the Environmental Sample Processor, an autonomous robotic water sampler developed at the Monterey Bay Aquarium Research Institute (MBARI), that offers a pathway for bringing the optical bench to the sea through in situ sample acquisition, processing, and analysis.

Optical transdimensional meta-structures, including single- and multi-layer metasurfaces, comprise a class of optical metamaterials with reduced dimensionality and allow for the miniaturization of conventional refractive optics into planar structures. Metasurfaces and transdimensional meta-structures can exhibit exceptional abilities for controlling light, the anomalously large photonic density of states, and, similar to their bulk counterpart, can provide imaging with subwavelength resolution. Such transdimensional design is expected to facilitate new physics and enhanced functionality for nanostructures that are distinctly different from those observed with their three-dimensional analogs. The work focuses on investigations of novel nanoresonator and nanoantenna designs using an alternative material platform such as layered materials instead of conventional solutions with silicon and moderate-index dielectrics.

We analyze lattice resonances in the arrays of nanoscatterers made of high-index materials, including those with high losses. We show that the formation condition of lattice resonances, such as the number of particles in the array of finite size and lattice periodicity, depends on the resonance quality factor. Narrower resonances require a larger number of scatterers in the lattice. Furthermore, the condition depends on whether it is electric or magnetic resonance and of what order. In addition, lossy nanoscatterers result in the formation of broader lattice resonances and require a smaller number of particles in the array.

Enhancing fluorescence of quantum emitters while shaping radiation properties at the source bears great potential for establishing bright sources with controllable radiation patterns. This is of interest in different fields of research and applications ranging from quantum networks to active metasurfaces. The last decades provided great evidence that dielectric as well as metallic cavities can accelerate radiative transitions of emitters by orders of magnitude based on the Purcell effect [1]. In the case of dielectric cavities, emission enhancement is due to modal coupling to high quality cavities, which however are fundamentally limited in bandwidth and mode size. In contrast, plasmonic metal structures are not diffraction limited, which allows for the creation of cavities with extremely small mode volumes and electrical control for direct emitter manipulation at the source. On the emitter side, erbium is of high interest due to its paramount technological significance defining the optical communications C-band.
Here, we demonstrate strong enhancement of Er³⁺- ion fluorescence by means of efficient coupling to and de-focusing from a hybrid plasmonic waveguide platform [2,3]. Our results unambiguously demonstrate acceleration of quantum emission over several orders of magnitude across the entire C-band based on a single hybrid plasmonic waveguide. Additionally, efficient coupling to a highly-confined waveguide mode allows to resolve Stark-split transitions of Er³⁺-ions at room temperature which commonly requires low temperature environments due to phonon broadening. In this context, plasmonic platforms provide the means to address and manipulate emission centres electrically and optically on the nanoscale.
[1] E. M. Purcell, H. C. Torrey, and R. V. Pound, Phys. Rev. 69, 37 (1946).
[2] N. A. Güsken, M. P. Nielsen, N. B. Nguyen, S. A. Maier, and R. F. Oulton, Opt. Express 26, 30634 (2018).
[3] R. F. Oulton, V. J. Sorger, D. A. Genov, D. F. P. Pile, and X. Zhang, Nat. Photonics 2, 496 (2008).

In this talk, we present and discuss our recent advances on metasurfaces based on highly nonlocal features, stemming from long-range resonant interactions and lattice phenomena. Different from conventional metasurface approaches, engineered nonlocality offers tailored spectral control, both temporally and spatially, ideal for signal processing applications, and combined with enhanced light-matter interactions. We achieve these features by combining quasi-bound states in the continuum with geometric phase variations in engineered metasurfaces, tailoring at will the supported eigenwaves. The resulting metasurfaces support ultrasharp responses selective to the impinging wave properties, effectively realizing ultrathin transparent films that highly reflect light only when illuminated by selected polarization, frequency and wavefront spatial distribution of choice. The demonstrated wavefront selectivity of nonlocal metasurfaces opens exciting opportunities for augmented reality, secure communications, thermal emission management, optical modulators and enhanced light-matter interactions for nonlinear and quantum optics.

Although many geometrically complex structures have been accomplished in colloids by imposing intrinsic chirality of the nanostructure and/or exerting local chiral bias during chemical process and self-assembly, chiral 3D plasmonic superstructures on the surface/interface using solution process hitherto remains virtually unexplored, which is essential for practical applications of their intriguing electromagnetic/optical properties. In addition to the challenge in aligning the main anisotropic axis of the chiral nanoparticles (NPs) with the normal vector of the substrate surface, aggregation of the particles with high-density deposition causes undesirable inter-particle coupling effects; A significant loss in chiroptical efficiency of the NPs, the lower g-factor, have been reported in many studies upon depositing them on the substrate. In this work, we established a new path to pattern chiral metasurfaces using circularly polarized light (CPL). Direct chirality transfer from photons to nanostructures enabled the fabrication of CPL-induced nanotwisters on the surface area with light exposure. The rotational axis of twisted assemblies is well aligned with the normal vector of the substrate surface, leading to high g-factors. This new technique offers fast (minutes) printing metasurfaces on many substrates including plastic, glass, and flexible polymer substrate without a prior seeding process. The spectral range for chiroptical activities of the printed structures can be modulated by altering the wavelength of light source. The photon-to-matter chirality transfer shown in this technique has the potential to be applied to other light-sensitive materials outside of plasmonic, such as semiconductor and dielectric materials. This simple and versatile fabrication technique can be used for extensive applications, including but not limited to photonic, optoelectronic, and electromechanical devices as well as enantioselective catalytic and sensing systems.

Dynamic tuning is a critical step towards advanced functionality and improved bandwidth of metamaterials. In the visible spectrum, the realization of full spectral color tuning is inhibited by materials in which large absorption accompanies index changes, particularly at blue wavelengths. Here, we demonstrate a reversible, continuously tunable platform for active photonic devices based on anatase TiO₂, a material popular for its high index and low absorption coefficient in the visible regime. For the first time, we quantify the change in complex refractive index due to electrochemical intercalation of cations in TiO₂ to design dynamic metasurfaces for manipulation of visible light. In hybrid metasurfaces incorporating TiO₂ thin films, shifts in reflectance spectra of over 100 nm are achieved via electrochemical lithiation at low voltages. The dynamic range, speed, and cyclability of such devices indicates that the TiO₂/LTO system is competitive with established actuators like WO₃, with the additional advantage of reduced absorption at high frequencies. Further, we provide a path to large-area, optically-addressed, all-dielectric metasurfaces using electrochemical reductive doping of hydrogen in TiO₂ nanoparticles at sub-volt biases.

Metal plasmonic nanorods both interact strongly with light and can be aligned by an external electric field, presenting a new paradigm for displays and high-speed optical devices. While the utility of these materials has been clearly forecasted, the limits of their performance have formerly remained untested. We now demonstrate highly dense nanorod phases and map nanorod alignment at concentrations across several orders of magnitude. First, we refine organic ligand and solvent chemistry to achieve stable Au nanorod suspensions. We fabricate custom plasmonic pixel cells with precise optical path lengths and directly measure the pixels’ exceptional optical densities in a lab-designed apparatus incorporating laser illumination and high-sensitivity detectors. We apply alternating current electric fields to the pixels, rapidly inducing orientational order among the nanorods, resulting in a large contrast in transmitted optical power. Finally, we reveal nanorod behavior through in situ optical microscopy during alignment. By formulating, understanding, and controlling highly concentrated and optically dense plasmonic nanorod phases, we begin to exploit their properties for dynamic optoelectronic applications.

We report the design and fabrication of a low-loss all-dielectric active metasurface suitable for beam steering in a reflection geometry with an efficiency of ~20%, using electro-optically tunable InP multiple quantum wells as an active material. The InP-based metasurface exploits a guided-mode resonance (GMR) to achieve narrow linewidths operating in the telecommunication C-band (λ=1.55μm ) and exhibits a phase shift of 200° upon refraction index modulation of the multiple quantum wells. We have used a full-wave simulation-based optimization approach, using beam directivity as the objective function, to design an InP-based GMR active metasurface that enables beam steering with higher directivity and sidemode suppression than is achievable in conventional iterative designs. Using this approach, we show that InP-based GMR metasurfaces can steer the beam up to polar angles of 32° while maintaining a reflectance of >20% and a sidemode suppression ratio of > 7 dB.
Our metasurface design is based on a localized GMR (L-GMR) structure, which confines the GMR mode laterally to a ridge waveguide. Typically, the ridge waveguide mode is localized in two dimensions but light cannot be coupled from free space. The GMR mode, on the other hand, can be excited from the free space but is localized only in one dimension. The proposed L-GMR structure is composed of a ridge waveguide with a shallow coupling grating patterned on top of it. Due to the shallow grating, the incident light efficiently couples to the waveguide mode. The L-GMR exhibits the favorable properties of the GMR mode – high Q-factor and a 2π phase shift around the optical resonance. The highly localized mode of the L-GMR opens the possibility to use an L-GMR mode as a ‘pixel’ of a tunable metasurface. In this case, modulating the refractive indices of the L-GMR will result in localized changes in the phase and amplitude of reflected light. Thus, the L-GMR mode enables a cavity-less dielectric material to achieve active control of the wavefront at a subwavelength scale.
Finally, we discuss the fabrication of InP active metasurfaces, whose electro-optically tunable elements consist of InP waveguides with smooth sidewalls. We will also report reflectance and beam steering measurements for InP metasurfaces when driven by quasi-static electro-optical phase modulation of the MQW layer.
To conclude, we will provide an outlook on the potentially wide range of applications for all-dielectric active metasurfaces. Due to the ability to dynamically control the wavefront of the reflected or transmitted light with high optical efficiency, all-dielectric active metasurfaces can form the basis of future chip-scale light detection and imaging as well as ranging (LiDAR) systems.

Dielectric and semiconductor nanomaterials with a high refractive index are important for realizing low-loss optical antennas and meta-surfaces. Inspite of this, only a few materials are commonly used and it is therefore reasonable to ask if better materials exists. In this work, we employ high-throughput computational screening and Mie theory to evaluate the performance of more than 2000 binary materials and find several interesting candidates. We specifically identify boron-phosphide as a very interesting candidate material in the visible and UV. We synthesize nanoparticles of this material using laser reshaping and evaluate its performance using both far-field and near-field measurements, clearly demonstrating that these particles support Mie resonances. Our work is a clear example of how a combination of computational screening and material synthesis can be used to guide the adoption of new materials in nano-optics.

Plasmonic hot electrons are high energy charges form as a natural outcome of the nonradiative plasmons decay in metallic nanostructures. Applications of hot carriers can be generally categorized in two subsets defined by the spatio-temporal characteristics of hot carriers. In the first set of applications, the confinement of hot carriers within the host plasmonic metal plays the key role. This regime of operation often relies on the electron and lattice heating, stemmed from the electron and electron-phonon thermalizations. Photothermal effects and the conventional Kerr-type optical nonlinearity fall into this category. The second set of applications, which has received a greater attention, relies on the interfacial transport of hot carriers over a Schottky junction that is formed at the interface of plasmonic metals and an adjacent dielectric material. Photovoltaics, photocatalysis, sub-picosecond nonlinear optics, and optoelectronics are among applications that immensely benefit from the transport of hot carriers. In these applications, understanding the transient dynamics of hot carriers from the generation to the transport and relaxation is the key to unlock the ultimate potentials this branch of plasmonics holds. Here, we study the dynamics of hot-carrier transport in terahertz (THz) regime by monitoring the time variation of the radiated THz fields following the interfacial transport of hot carriers. THz time domain spectroscopies enable us to exclusively monitor the transport dynamics regardless of other competing transient effects that occur with similar timescales and probabilities. The role of device symmetry, propagation vector of hot carriers, and electrical biasing will be discussed.

A liquid crystal (LC) consists of nanometer-sized birefringent molecules with a high aspect ratio. In the nematic phase, the polarization of the LC can be easily tuned by applying an electrical bias and this is widely used in modern display technology. In recent years, a solid format of liquid crystal, namely, the liquid crystal elastomers (LCE) has captured notable attention in the field of liquid crystals due to its unique elastomeric properties. In a microscopic view, a LCE is a rubber-like material, in which nematic phase liquid crystal molecules are embedded and cross-linked in a prescribed deformable network. The network is usually made up by a polymer that has a two level energy system. When the electrons are excited (optically, electrically or thermally) from the ground state to the excited state, the network can be made to stretch. Attached to the deforming network, the embedded liquid crystal molecules also rotate, leading to not only dimensional changes, but also a change in the state of polarization for the light that is transmitted through the LCE film.

Previous work has studied the mechanical tunability at macro- or meso- scales, where reversible deformation of LCE films can be observed through optical and thermal excitation. In our work, we take full advantage of such tunability (dimensional and polarization change) of the LCE material and use the material to tune the optical resonances of nanophotonic devices. In one concrete application, we sandwich a 5 nm thick LCE film between 100 nm diameter gold nanoparticles and a gold substrate to form a Metal-Insulator-Metal (MIM) surface gap plasmonic devices. Through UV light excitation, the thickness of the LCE gaps shows 5%-10% change which leads to an easily observable (~20 nm) resonance wavelength shift in the visible range. We envision that the LCE, as a material platform, could be applied to a broad range of metasurface and photonic devices to achieve reversible optical tuning.

Plasmonic nanoparticle lattices are periodic arrays of metal nanoparticles that combine advantages of both plasmonic and photonic systems. The localized surface plasmons of the metal particles can hybridize with photonic modes defined by the array spacing to result in collective excitations known as surface lattice resonances. This talk will describe advances in the fabrication of nanoparticle lattices based on the selective growth of functional nanoscale coatings as well as their integration with responsive materials. We will discuss emerging applications of this platform, including reconfigurable flat lenses, photo-electrocatalysis for hydrogen evolution reactions, and photothermal heating elements in programmable, self-regulatory materials systems.

Wavefront shaping and control is essential for advancements in optical technologies including LiDAR, AR/VR, and LiFi systems. Metasurfaces offer a promising route for these technologies, as they allow for precise control of the amplitude, phase, and polarization of light in a sub-wavelength-thick footprint. However, most current metasurface designs are static, relying on fixed geometric patterning and lacking tunability once fabricated. Here, we design high quality factor (high-Q) metasurfaces that enable electro-optically tunable phase and amplitude control of the constituent nanoantennas. Our metasurface consists of a uniform array of subwavelength nanobars of silicon atop lithium niobate. By introducing subtle geometric perturbations into each constituent nanobar, we excite guided mode resonances with normally-incident light. Using full-field simulations, we show how these guided mode resonances excite strongly enhanced fields localized in the lithium niobate, with mode quality factors exceeding 1000 in both theoretical and experimental demonstrations. Applying a voltage across individual nanoantennas shifts the spectral position of the high-Q resonance, modifying the antenna phase and amplitude. We computationally design a fully-reconfigurable, solid-state beam steerer, diffracting light to varying angles between 18-31° with up to 86% efficiency. Near-continuous beam-steering can be achieved by modifying the bias across each antenna and utilizing the near 2π phase modulation range with an applied voltage not exceeding ±25 V to reconfigure the phase gradient. We also show how this platform can enable dynamic modulation of other optical transfer functions, including beam-splitting and lensing. We can additionally use this platform to realize an amplitude-tunable metasurface platform, operating at low powers and high switching speeds. To reduce the nanoantenna length required to generate high-Q resonances, we pattern the ends of the notched bar with photonic crystal mirrors, further confining the guided mode to realize tunable pixelated arrays. This allows us to achieve quality factors of over 1000 in metasurface arrays with lateral dimensions less than 15 μm. Our high-Q electro-optic metasurfaces provide a foundation for light-weight, fully reconfigurable, solid-state classical and quantum information processing.

Confining the free-space wavelength to extremely sub-diffractional dimensions is a fundamental nanophotonics objective. Recently, this has been explored in hyperbolic media: strongly anisotropic materials where the in- and out-of-plane permittivity tensor components are opposite in sign, such that the isofrequency contour becomes an unbounded hyperboloid. While these properties occur naturally in some materials, here we explore hyperbolic metamaterials (HMMs), where optical anisotropy is engineered in superlattices of alternating dielectric and metallic layers, expanding design flexibility and spectral tunability. In this case, we fabricate HMMs consisting of high- and low-doped cadmium oxide (CdO) for applications in the infrared (IR). High-power impulse magnetron sputtering (HiPIMS) allows rapid growth of CdO thin film structures with consistently high crystal quality which enables high mobilities (up to 500 cm²/V-s), and thus low optical losses. CdO exhibits a Drude-like dielectric function in the IR which is tunable by doping to control carrier concentration. Numerous donor dopants including Y³⁺ and In³⁺ enable the CdO epsilon-near-zero crossing point to be tuned across the mid- to near-IR (λ ~ 2 – 6 μm), opening a wide spectral range where the real part of the permittivity is positive (“dielectric”) for a low-doped film and negative (“metallic”) for a high-doped film. Thus, optical anisotropy can be engineered by layering CdO films with alternating doping levels. We further show that homoepitaxial CdO film stacks exhibit strong carrier confinement between layers, creating sharp optical “interfaces,” while minimizing scattering and strain-induced losses which are introduced otherwise when layering structurally/chemically heterogeneous dielectric and metallic materials.

To design CdO HMMs, we employ effective medium theory (EMT) which, due to the high carrier confinement of these structures, is successful for modelling the optical response. These calculations allow us to map out spectral regions which support Type I (ε_z(ω) < 0, ε_x,y(ω) > 0) and Type II (ε_x,y(ω) < 0, ε_z(ω) > 0) hyperbolic modes as well as effective dielectric (ε_x,y(ω), ε_z(ω) > 0) and metallic (ε_x,y(ω), ε_z(ω) < 0) behavior. Structures were fabricated with individual layer carrier densities ranging from 3×10¹⁹ cm^-3 to 3×10²⁰ cm^-3, layer thicknesses between 50 to 140 nm, and total thicknesses between 1 to 1.5 μm. Attenuated total reflectance (ATR) measurements confirm the presence of both Type I and Type II hyperbolic modes, and match up well with transfer matrix method (TMM) simulations. In this presentation, we demonstrate fine spectral tunability of CdO HMMs across the mid-IR by controlling carrier density, total thickness, and filling fraction between high- and low-doped layers.

Analogous to the bianisotropic phenomena in electromagnetics, Willis coupling has been widely employed for acoustic metamaterials as the linear momentum, which is coupled only to velocity by mass density at the microscale, is also coupled to strain at the macroscale. For piezoelectric materials responding to electric fields, the theory for electro-momentum coupling is recently discovered which shows that the macroscopic momentum of these materials can be additionally coupled with non-mechanical stimuli. Understanding the theoretical bounds of the electro-momentum coupling coefficient can provide helpful information on the performance space of piezoelectric metamaterials for various applications such as acoustic sensing. However, the theoretical bounds have not been reported in previous literature. In this work, we aim to derive tight bounds on the electro-momentum coupling imposed by energy conservation. This allows us to design metamaterials actively capable of manipulating waves by stimulus modulation while ideally taking advantage of the full extent of coupling. Moreover, machine learning algorithms will be used to complement the computational approach to design microstructures of piezoelectric metamaterials which can reach this theoretical maximum coupling value for tunable properties.

Monolayer 2D transition metal dichalcogenides (TMDCs) have recently emerged as promising materials for a new generation of optoelectronic devices due to their direct bandgaps and high light emitting efficiency. Further efficiency improvement and control of light emission from vertical heterostructures based on TMDCs have driven the development of various device architectures with photonic cavities with additional efforts to achieve lasing modes. Plasmonic gap structures present a particularly intriguing system due to tightly confined fields. Large field enhancements can be easily reached by positioning plasmonic nanoparticles on a metal film with a thin spacer layer or 2D materials. Such plasmonic nanocavities have been demonstrated to drastically enhance light emitting efficiency from TMDCs. In this work, we study the effect of plasmonic nanocavities on electrically induced light emission in vertical van der Waals heterostructures based on WS2 monolayers. The devices are fabricated from large-scale CVD materials allowing batch production of hundreds of devices in a single fabrication process. Every layer, including nanostructures, is aligned-transferred to avoid heat induced damage of WS2 layers during direct metal deposition. While being a transparent top electrode, graphene is employed to facilitate current injection in the active material WS₂. Using hBN tunnelling barriers between graphene/gold contacts and WS₂ layers, the current leakage is minimised for efficient electron-to-photon conversion. In presence of plasmonic cavities, the devices demonstrate modified light emission spectra indicating the potential coupling with gap plasmon modes.

Metasurface design methods mainly rely on full electromagnetic simulations. Although they provide high accuracy, the time and computational power requirements of these simulations become a limiting factor with increased structural complexity. In recent years, deep learning (DL) based simulation and design approaches are emerged as an alternative solution to the computational cost problem. However, there are several challenges to be overcome to achieve feasible and widely applicable DL based models. Addressing high degrees of design freedom while ensuring manufacturable designs one of the main challenges. Fabrication limitations are either addressed in low dimensional design space or not taken into consideration by most DL based approaches. Additionally, most of the existing DL based metasurface models are valid for limited operation conditions that are defined by the parameter space of the training data. This brings the insufficient generalizability of models as another fundamental problem.

Here, addressing these problems, we introduce a deep learning enabled framework to design free-form metasurface unit cells, where the target design space is subject to the constraints of top-down nanofabrication techniques. First, a shape generation method is developed to create a diverse library of fabrication-friendly meta-atoms. Then, applying our recently proposed wavelength normalization approach [1,2] to this dataset, a combined architecture of variational autoencoder and fully connected layers is trained as a forward (from structure to response) model. In addition to an unseen validation set extracted from our library, the forward model is also tested on cross-polarization, material dispersion, and spectral ranges outside our dataset, which the model wasn’t explicitly trained for. All in all, our forward model is proven to be a successful surrogate to electromagnetic simulations that provide high accuracy predictions for a wide span of design and problem settings, for ultra-low computational cost. In the final stage, we realized inverse design of free-form metasurfaces while ensuring manufacturability conditions.

[1] Tanriover, I.; Hadibrata, W.; Aydin, K. Physics-Based Approach for a Neural Networks Enabled Design of All-Dielectric Metasurfaces. ACS Photonics 2020, 7 (8), 1957–1964.
[2] Tanriover, I.; Hadibrata, W.; Scheuer, J.; Aydin, K. Neural Networks Enabled Forward and Inverse Design of Reconfigurable Metasurfaces. Opt. Express 2021, 29 (17), 27219–27227.

At present deep space exploration is limited by long transit time and excessive mission costs. Conventional spacecraft make use of chemical or electric engines, which are fundamentally limited. Light sailing is an alternative propulsion which offers a conceptually different approach to space travel. In this talk I will overview our work on solar and laser driven propulsion for space exploration.

First, I will survey our work on laser driven propulsion. I will show that with relatively moderate laser beams (~1 MW and ~1m aperture), which are available today, novel regimes for spacecraft maneuvering and orbiting on interplanetary and Earth orbital missions emerge. I will discuss nanophotonic designs that optimize the tradeoff between mass, reflectance and a need for a radiative cooling. I will then discuss implications of utilizing smaller and more affordable systems for near-term space exploration and outline a roadmap for future interstellar missions.

Next, I will highlight an alternative approach for deep space missions with the use of extreme solar sailing. I will demonstrate that lightweight solar sails are potentially capable of reaching >30 AU/year velocities (i.e., more than 8 times faster than the Voyager – fastest space probe ever built), and can pave the way to fast and scalable exploration of the solar system and beyond. I will then survey material challenges associated with such solar sailing and highlight our ongoing experimental and theoretical effort.

Genetic analysis methods are instrumental in a variety of settings, spanning human, animal, and ecosystem health. Current nucleic acid detection methods such as polymerase chain reaction (PCR), next generation sequencing (NGS), and gene microarrays rely on “traditional” optics such as fluorescence or absorbance for signal transduction, necessitating sample amplification and additional optical tagging processes. Nanophotonic sensors promise to provide a label-free platform to sensitively detect gene fragments for rapid, compact, and point-of-care genetic screening. However, photonic devices have generally faced two challenges: 1) limitations in sensitivity due to broad spectral features from low quality factor resonances and 2) challenges in scalability and multiplexing, due to poor control over free space diffraction.
Here, we experimentally demonstrate a new platform for multiplexed nucleic acid detection based on high quality factor (high-Q) metasurfaces. First, we show how free space radiation can be coupled into guided mode resonances through structural perturbations along chains of silicon nanoparticles to produce high-Q resonances with Q factors exceeding 1,000 in aqueous buffer. Due to the localization of the resonant mode along a 1-D chain of silicon nanostructures, we demonstrate the ability to individually tune and measure responses from high-Q “pixel” sensing elements at densities of ~1,000,000 sensors per cm². We then functionalize our metasurfaces with self-assembled monolayers (SAM) of DNA probes. Through both calculations and experimental measurement, we observe resonant wavelength shifts of 0.5-1.0 nm through each processing step as multiple layers of molecules are grafted to the resonator surfaces to form the DNA SAM and target nucleic acid strands are flowed over our sensors. In order to produce dense, multiplexed genetic panels, we utilize an acoustic bioprinter to pattern our metasurface sensing elements individually with distinct DNA probe sequences at a droplet resolution of ~30 um. Specific target detection of four different respiratory viruses, including coronavirus and influenza, are observed simultaneously through changes in the transmitted light intensities and wavelengths of 100 individual resonators in parallel. Measurable signal changes are detected within 10 minutes of target nucleic acid introduction. Finally, we discuss how this platform can be utilized for label-free and massively multiplexed gene assays, enabling sensitive, rapid, and cost effective genetic analyses.

Two entities are defined as chiral if they are mirror images of each other, but not superimposable upon translation and rotational transformations. From the biochemical standpoint, molecular chirality determines the function and activity of proteins and enzymes; small conformational changes in proteins have been connected to neurodegenerative diseases. In drug discovery, pharmacological activity is heavily impacted by the stereochemistry of chiral pharmaceuticals.
Circular dichroism (CD) spectroscopy, which measures the difference in the transmitted intensities of left and right circular polarized light (CPL) passing through a sample, is conventionally used for characterizing the chiroptical properties of biomolecules and their resulting structure. However, the size mismatch between biomolecules and the wavelength of light leads to a small differential absorption cross-section for CPL, hence yielding a weak signal and limiting the structural resolution of this technique.
Intending to circumvent the deficiencies of CD spectroscopy, a diverse number of recent studies have demonstrated important chiroptical effects by using metallic plasmonic nanomaterials: Lithographically fabricated or chemically synthesized structures with intrinsic geometric chirality exhibited strong differential transmission or scattering of R- and L-CPL. Furthermore, the near-field interaction between the localized surface plasmon (LSP) of an achiral nanostructure and surrounding chiral molecules can induce a CD signal at the wavelength of the LSP resonance. Most of this research, however, relied on gold nanostructures with resonances in the visible to infrared, whereas biomolecules exhibit optical activity in the ultraviolet. Consequently, the interaction between molecular and plasmonic chirality under resonant conditions persists as an uncharted area with potential for greater enhancements and improved chiral sensing.
The optical properties of Al allow it to be used as a plasmonic material for spectral response ranging from the NIR into the deep UV. Its reactivity is a challenge, but which researchers can mitigate or overcome by employing faster deposition rates.
Here we fabricate Al gammadion arrays which display strong chiroptical activity ranging from the near- (from 250 to ~300 nm) into the far-UV (λ < 250 nm) regions of the spectrum, important for protein tertiary and secondary structural characterization, respectively. The polarization response of the gammadion arrays is determined by extracting its full Mueller matrix with spectroscopic ellipsometry in the range of 210–400 nm. We calculated the circular diattenuation (CD) of the arrays using an analytical inversion of the Mueller matrix, which is a rigorous approach for determining the optical activity of bianisotropic materials (containing both circular and linear anisotropies).
To study the resonant interaction of molecular and plasmonic chirality, we deposit thin (< 40 nm) films of amino acids via sublimation on the nanostructures. The large chiroptical signal of gammadion arrays containing handedness dissymmetry overshadows any information on plasmonic-molecular chiral interaction. To circumvent this issue, we employ racemic arrays which display some remanent CD, due to inherent fabrication heterogeneities. This residual signal, however, allows us to observe the aforementioned interaction.
The presence of an aminoacid film of a given handedness (L- or D-) perturbs the UV-chiroptical response of the array differently than the corresponding enantiomorph or a racemic film. Furthermore, this effect is dependent on the region that the biomolecule display CD. In this work, we will discuss how resonant chiral interactions open a new route to more direct, fingerprint-like, sensing of chiral analytes than what is currently employed.

Three-dimensional elements, with refractive index distribution structured at subwavelength scale, provide an expansive optical design space that can be harnessed for demonstrating multifunctional free-space optical devices. We present three dimensional dielectric elements, designed to be placed on top of the pixels of image sensors that provide multiple functionalities like sorting and focusing of light based on its color, polarization and incidence angle. The devices are designed via iterative gradient-based optimization to account for multiple target functions while ensuring compatibility with existing nanofabrication processes. This approach combines arbitrary functions into a single compact element, even where there is no known equivalent in bulk optics, enabling novel integrated photonic applications. We analyze how the device behaves for input parameters that it was not designed for and investigate how the arrangement of the pixels in the image plane affects the device performance. We present preliminary experimental data in the mid infrared.

We demonstrate switching of frequency harmonics using an electro-optically tunable metasurface in which a spatial phase gradient is superimposed with a high-frequency, temporally varying phase front. Active metasurfaces comprising of arrays of programmable nanoantennas have recently garnered widespread interest owing to their power for versatile wavefront shaping. Until now, researchers have primarily focused on controlling the spatial features of the scattered light (i.e., amplitude, phase, and polarization) as a function of applied bias. However, these functions are realized using a quasi-static signal where the externally applied stimulus is not varied in time. The introduction of a temporally varying waveform enables the creation of higher-order frequency harmonics that provide control over the spectral content of the scattered light.

We implement a time-modulated antenna array by integrating a plasmonic, indium tin oxide (ITO) based active metasurface into a radiofrequency network. Using electro-optic effects, we report a modulation in the phase and amplitude of the reflected light at an operating wavelength of 1550 nm. The active metasurface consists of two interdigitated electrodes that address alternating metasurface elements. By applying a common temporally varying waveform onto both electrodes, we first measure higher-order harmonics that are separated by up to 5 MHz from the central frequency. The conversion efficiency of the desired frequency harmonic is given by the nonideal phase and amplitude response of the active metasurface, i.e., a phase shift smaller than 2π and a covarying, non-unity amplitude. To maximize the conversion efficiency and therefore the signal-to-noise ratio of the frequency spectrum, we implement an optimization algorithm that alters the applied waveform in real-time. With the use of an additional nonresonant phase shifter, we then introduce a time delay of half a period into the waveform applied onto one of the electrodes. This generates a spatial phase shift of 180° between neighboring unit cells, thus enabling the creation of a space-time modulated switchable diffraction grating. As a result, frequency harmonics are steered into desired directions, as determined by the designed grating period. Finally, we demonstrate a modified metasurface architecture with 32 independently addressable electrodes. We explore ways in which this design can facilitate the realization of continuous steering of frequency harmonics as well as multi-channel optical beam shaping on a single chip.

Photonic crystals have been used for various applications with their distinctive optical properties. They feature synergetic optical properties which combine the controllability of selective wavelength in transmittance and reflectance, light confinement, and light scattering effect inside the periodic structure.¹ The spectacular colors of natural photonic crystals are produced by complex periodic structures, which have proven to be challenging to synthesize artificially. A variety of tools for the fabrication of artificial photonic crystals have been demonstrated, for instance, template-assisted fabrication for inverse opals, silicon double inversion, and laser lithography. However, the inherent constraints of material limitation, complexity, and time-consuming fabrication have hindered the development of high-performance photonic crystals for practical applications.

In this research, we utilized a transfer-printing technique, derived from immersion transfer-printing (iTP)², to fabricate 3D photonic crystals solely consisting of quantum dots (QDs). Following template-guided formation of QD films using spin-casting, simple contacting of polydimethylsiloxane (PDMS) mold selectively removed QDs on undesired areas. Hence, the transferred 2D QD patterns can provide distinct periodicity with a low defect density for the desired functionality of photonic crystals. In order to fabricate the 3D structures with preserving the high crystal quality by capping materials between each layer, the individual 2D layers of QDs were stacked with freely controlled angle variations. The stacked QD patterns for 3D photonic crystals can exhibit useful optical characteristics, in mainly two aspects, combining enhanced light absorption and extraction efficiency. Both contribute to the overall photoluminescence (PL) enhancement. Periodic empty space in photonic crystals can confine traveling light and longer interaction between the incident light and QDs induces increased light absorption. As light scattering on periodic nanostructure lowers total internal reflection, the QD photonic crystals can achieve much enhanced light extraction efficiency as well. We successively fabricated 3D QD-based photonic crystals with a thickness of up to a few hundred nanometers, and it showed a PL enhancement compared to conventional QD films. We characterized the absorptance and extraction efficiency of photonic crystals by means of experiment and calculation, validating the origin of PL enhancement in the structure. Our developed fabrication tool for the bottom-up nanoparticle-based photonic crystals can potentially be exploited for metamaterials³ with unconventional functions or properties such as negative index, delay of carrier lifetime, optical waveguides, and so on.

Reference
1. Eileen Armstrong and Colm O'Dwyer, Journal of Materials Chemistry C 3, 6109 (2015).
2. Tae Won Nam et al., Nature communications 11, 3040 (2020).
3. Nikolay I. Zheludev Science 328, 5978 582-583 (2010).

Visible light driven catalysis has typically been performed with nanomaterials such as gold and silver to take advantage of their high carrier concentration, which produces a Localized Surface Plasmon Response (LSPR) in the interaction with light. Due to their low melting point and expense however, alternatives such as TiN and ZrN are increasingly investigated and gaining interest for the fields of photocatalysis, photochemistry, biophotonics, sensing and waveguiding, among others.
In this work, we present the novel non-thermal plasma synthesis technique to make plasmonic ZrN with particle size of roughly 10 nm.¹ The plasmonic peak is measured in solution to be at around 620 nm in the visible spectrum, making ZrN an attractive and efficient alternative for light driven catalysts. The ZRD and TEM measurements confirm a cubic rock salt structure of ZrN nanoparticles. Catalysis experiments are performed with the goal of reducing heavy metals from water such as platinum and chromium (VI) Chromium (VI) is a known carcinogen², while its reduced species chromium (III) is a nutrient for some plants and is required to metabolize fats and sugars in humans.³ ZrN has shown the ability to reduce chromium (VI) to chromium (III) much faster than TiO2 using only water as an oxidizing agent without requiring the addition of methanol, another known carcinogen, to speed up the reaction time. We find that ZrN is much more efficient in reducing Cr (VI) to Cr (III) compared to the more commonly used semiconductor TiO2, when a broad source light, containing both UV and Visible light is used. For comparison, AU nanoparticles were unable to reduce a measurable amount of chromium (VI) under the same light conditions. Additionally, we also see that ZrN is able to reduce chromium (VI) in the dark by heating the solution in a bath of water while TiO2 and Au were not able to. Other catalysts experiments reducing chromium (VI) were performed in a solution illuminated with the same 500 W HG-XE arc lamp light source (Newport 67005) and a monochromator to spectrally resolve the light to show quantum yield measurements of Cr (VI) reduction using only visible light, convincingly demonstrating that the photocatalysis process is due to the plasmonic response of ZrN. Interestingly, we find that ZrN was able to reduce more Cr (VI) than TiO2 with light at 350 nm as well as in the visible part of the spectrum. Experiments reducing Chloroplatinic acid (H2PtCl6) comparing TiN⁴ and ZrN using the addition of methanol to enhance the reaction were both able to reduce platinum (IV) to platinum, ZrN is much more efficient.
(1) Exarhos, S.; Alvarez-Barragan, A.; Aytan, E.; Balandin, A. A.; Mangolini, L. Plasmonic Core-Shell Zirconium Nitride-Silicon Oxynitride Nanoparticles. ACS Energy Lett. 2018, 3 (10), 2349–2356. https://doi.org/10.1021/acsenergylett.8b01478.
(2) Gibb, H. J.; Lees, P. S. J.; Pinsky, P. F.; Rooney, B. C. Lung Cancer among Workers in Chromium Chemical Production. Am. J. Ind. Med. 2000, 38 (2), 115–126. https://doi.org/10.1002/1097-0274(200008)38:2<115::AID-AJIM1>3.0.CO;2-Y.
(3) Anderson, R. A. Essentiality of Chromium in Humans. Sci. Total Environ. 1989, 86 (1–2), 75–81. https://doi.org/10.1016/0048-9697(89)90196-4.
(4) Barragan, A. A.; Hanukovich, S.; Bozhilov, K.; Yamijala, S. S. R. K. C.; Wong, B. M.; Christopher, P.; Mangolini, L. Photochemistry of Plasmonic Titanium Nitride Nanocrystals. J. Phys. Chem. C 2019, 123 (35), 21796–21804. https://doi.org/10.1021/acs.jpcc.9b06257.

It is very challenging to diagnose Alzheimer’s disease (AD) at the early pathological stage. Imaging of β-amyloid (Aβ) peptides using positron-emission tomography (PET) or quantification of Aβ in cerebrospinal fluid (CSF) has been proposed for AD diagnosis, however, these methods are costly and very invasive. Detection of blood-based Aβ peptides such as Aβ40 and Aβ42 is considered to be a promising diagnostic for AD, which is of a great challenge due to their low abundance in the blood, requiring a sensitive sensing method. Here, we demonstrate a SERS nanoprobes-based immunoassay for the sensitive detection of Aβ peptides in the human blood. Silver nanogap shells (AgNGSs) bearing Raman label compounds were prepared as surface-enhanced Raman scattering (SERS) nanoprobes, producing strong and stable SERS signals without overlapping. The nanogap size of the AgNGSs was effectively controlled by the reaction kinetics mediated by various Raman label chemicals, leading to the significant enhancement of the SERS signals up to 1.7x10⁷. The AgNGS nanoprobes, conjugated with an antibody specific to Aβ40 or Aβ42, could selectively detect the AD biomarkers Aβ40 and Aβ42 from the human serum through their SERS signals. The AgNGS nanoprobes-based immunoassay could detect the amyloid peptides at concentration as low as 0.25 pg mL⁻¹.The AgNGS nanoprobes-based immunoassay can be extended for the sensitive and selective detection of various protein biomarkers.

Mesoscale polyhedron-like structures, hereon referred to as mesopyramids, are formed by annealing an ~30 nm thick Au film supported on Si(100) in vacuo to approximately 750 ^oC. The base of the mesopyramids is typically on the order of tens of microns and the height 3-5 mm. Their surface topography consists of channels and plateaus that are 100-200 nm across and tens of microns in length, which is ideal for the launching and propagation of surface plasmon polaritons (SPP). It is proposed that SPP scattering can be exploited for electro-optic coupling. The SPP activity of the mesopyramids is examined using confocal microscopy, where the mesopyramids are coated with ZnO and GaN to create electro-optic devices. Electrical current is passed through the semiconductor layer while simultaneously acquiring visible light reflectivity spectra from the mesopyramids. The reflectivity spectrum experiences a blue shift proportional to the applied voltage/current. The blue shift is attributed to polarization of the semiconductor layer, i.e. a change the materials dielectric response function.

Colloidal crystals (CCs), composed of monodisperse polymer latex particles, find extensive use in various sensing applications. Their photonic properties in combination with an adjustable response to external triggers make them a versatile class of materials. However, in some cases, the information provided by one single type of CC is insufficient. We have found that iridescent thin films with a unidirectional gradient of a physical property can circumvent this problem. Localized optical characterization of such a gradient nanostructure allows for a much more diagnostically conclusive readout compared to homogeneous films. While most literature on gradient CCs focuses on a change in interparticle distance of non-close-packed particle assemblies, we have recently shown the fabrication of a different type of gradient material. Infusion-withdrawal coating enables the fabrication of ordered thin films consisting of two isometric particle types with different thermal properties arranged in a compositional gradient (https://doi.org/10.1002/adma.202101948). The characterization and calibration of these structures proceeded via the incorporation of trace amounts of fluorescent nanoparticles.
Enhancing the fluorescence intensity and thereby the accuracy of this evaluation is difficult, as higher concentrations of tracer particles would disrupt the periodic mesostructure. Here, we improve upon this by utilizing post-synthesis fluorescent labeling of the latex particles themselves. A reversible swelling/deswelling procedure allows incorporation of a hydrophobic dye of choice while maintaining the monodispersity and capability of co-crystallization with the pristine building blocks. Not only does this allow a drastic enhancement of the signal intensity, but it also makes the incorporation of more than one different, tailor-made particle type feasible. The photonic properties, i.e., optical stop-band, are fully retained thereby. We prepare particles labeled with Nile red (red-fluorescent) as well as others with Coumarin 1 (blue-fluorescent). Self-assembled particle mixtures and fluorescence microspectroscopy thereof show that the intensity ratio of the two non-overlapping emission spectra provides a highly accurate readout of the local composition, independent of the sample thickness. This, in turn, allows us to examine more complex gradient profiles in addition to the simple linear case. Regarding infusion-withdrawal coating, the shape of this profile can theoretically be shown to depend directly on the relative pumping rates of the two syringes. Non-linear gradients can therefore be prepared in a controlled manner without sophisticated pumping programs. The improved calibration using the post-synthesis labeled particles provides sufficient accuracy to characterize these samples and correlate the experimental results with theoretical calculations. The simplicity of this coating method and the nevertheless complex gradient nanostructures open a wider range of applications for this emerging type of colloidal assembly.

Broadband perfect absorbers have been researched intensively for decades for their near-perfect absorption optical property which can be applied to various applications. However, achieving large-scale and heat-tolerant absorbers has been remained challenging work because of costly and time-consuming lithography methods and thermolability of materials, respectively. Herein, we demonstrate a thermally robust titanium nitride broadband absorber which has a polarization-independent absorption of 95.4% under normal incidence angle in average and high absorption at incident angles up to θ = 40^o at visible and NIR wavelength (400 < λ < 900 nm). The near-perfect absorption of our device is still maintained even after heating at the temperatures ＞600 ^oC. Such a large-scale, heat-tolerant, and broadband near-perfect absorber will provide further useful applications in solar thermophotovoltaics, stealth, and absorption controlling in high-temperature conditions.

The discovery of the Willis coupling, described as the non-local coupling of momentum and strain, exhibited by architected metamaterials has led to the theorization of further non-local coupling phenomenon, one of which is electro-momentum coupling. Electro-momentum coupling has the potential to realize electric field sensing or elastic wave generation through the bulk metamaterial structure. Electro-momentum coupling entails the coupling of the momentum of the metamaterial to its electrical properties. This coupling is facilitated through the integration of piezoelectric materials into the metamaterial, allowing for the phenomenological bridging of strains to electrical potential. This metamaterial structure must essentially combine the piezoelectric effect along with Willis coupling in order to realize the electro-momentum coupling. This type of coupling at the macroscale is unprecedented and empirically unobserved as of now. In this research, we aim to experimentally confirm the theoretical observations of recent works proposing the functionality of unidimensional stacked metamaterial structures.
Primarily, this work involves the precise fabrication of metamaterials using an advanced custom designed multimaterial 3D-printing method capable of printing piezoelectric materials. This method allows for the precise tailoring of the material distribution in the metamaterial structure as required by theoretical calculations. The metamaterial consists of barium titanate (BaTiO₃), lead zirconate titanate (PVT4), and polyvinylidene fluoride (PVDF) all of which are tightly integrated into the metamaterial structure in periodic unit cells. Current experimental efforts involve the testing of a unidimensional sample and empirically observing electro-momentum coupling. These experiments involve the application of a dynamic load onto the metamaterial and measuring its transient displacement field while simultaneously measuring the unidirectional electric field distribution. Through this work, we aim to provide the first empirical observations of this coupling phenomenon.

The electro-optic Pockels and Kerr effect make it possible to tune the refractive index of a material with an applied electric field. Remarkably, these effects are reversible and show a very short reaction time. If it were possible to use these phenomena to extract light from the surface of a planar waveguide, this would provide laser displays, projectors, and scanners of unsurpassed performance.
So far, tunable coupling between waveguide modes and free-space modes often relies on mechanical components or surface acoustic waves, limited in terms of reaction time. By placing a thin film resonator in the center of an electrooptic waveguide, we show that a continuum of waveguide modes can be tuned between zero and maximized coupling efficiency (80%) into free-space modes. We term this first phenomenon bound continuum coupled to the continuum (BCC), describing a continuum of waveguide modes with access to the continuum of free-space modes, showing zero coupling efficiency. Realized by the Pockels effect, this phenomenon is a promising way towards mechanics-free optics. Additionally, the proposed geometry allows for the absence of self-interference so that the electrooptic waveguide does not function as bandwidth-limiting second resonator. In addition, the symmetry enables switching in the entire visible spectrum.
We demonstrate a local switching behavior and contrast ratios for red (C=1000) and green (C=500) light of our polymeric thin film resonator inside a symmetric lithium tantalate waveguide. Furthermore, we estimate the performance of laser displays and scanners based on the demonstrated concept.

Several techniques have been developed for directing the self-assembly of nanoparticles into ordered two- and three-dimensional supercrystals. Interparticle separations, lattice type, particle size, shape, and composition, as well as microcrystal habit may be individually manipulated to produce a macroscopic solid. In recent years, the computational modelling of these 3D periodic structures has proven challenging due to the multiscale nature of the physical properties. On one hand, the nanoscale nature of the building blocks requires very fine spatial discretization of the computation domain to describe the near-field nature of the localized surface plasmons. On the other hand the microscale nature of the superlattice requires a very large simulation domain. To tackle this problem, two approaches are generally taken (i) an effective medium theory approach which neglects the nanoscale effects to focus on the overall optical properties of the supercrystal, and (ii) the use of a unit cell with periodic boundary conditions which neglect the overall habit of the supercrystal to focus on nanoscale behaviors. This second approach is used for the calculation of the photonic band structure of these periodic structures. However, it fails to describe the photonic properties rising from finite-size effects such as Fabry-Pérot resonances, whispering gallery modes, and decrease of the photonic mode life-time. Here, we developed a computational approach, based on the finite-difference time-domain (FDTD) electrodynamic method to accurately calculate the photonic band structures from finite, microscale 3D supercrystals of cubic, spherical, and rhombic dodecahedral habits. We discuss how they differ from the band structures computed for infinite structure and compare them to traditional optical quantities such as transmission and reflection.

We theoretically and experimentally investigate the optical and thermal performance of gate-tunable conducting oxide metasurfaces [1], which are illuminated with laser irradiation at high power densities. Our gate-tunable metasurfaces use indium tin oxide (ITO) as active light-modulating materials, which undergo an epsilon-near-zero (ENZ) transition under applied electrical bias. We show that even under an applied bias, when over 70% of the incoming light is absorbed in a 1-nm–thick charge accumulation layer in the ITO layer, the localized optical absorption results in a negligible local temperature rise for these active metasurfaces under high power density irradiation (>1 kW/cm²). Despite the localized absorption, the calculated temperature variation within the metasurface is <0.3 ^OC upon steady state illumination, indicating that active metasurfaces have potential for high power density operation.
Next, we experimentally probe the gate-tunable performance of our metasurfaces upon high-power illumination. We measured the metasurface reflectance as a function of applied voltage under focused laser irradiation with a laser spot size of 6-8 μm at an operating wavelength of 1565 nm. At three different high power irradiance values of 1.8 kW/cm², 467.5 kW/cm², and 567 kW/cm², the electrically tunable optical response of the metasurface is similar to that for low power irradiance. We develop a theoretical model, which projects that at an irradiance of 567 kW/cm², absorbance of 85%, and 8 μm laser spot diameter, the temperature increase of the metasurface is ~102 ^OC. To estimate how this increased temperature affects optical properties of the ITO-based active metasurface, we measured the temperature-dependent optical constants of each constituent layer (Au, Al₂O₃, ITO) of an ITO-based active metasurface at temperatures up to 150 ^OC. We observe that the optical constants of an ITO film encapsulated by an 8 nm-thick Al₂O₃ film are unchanged at temperatures up to 150 ^OC.
Finally, we provide an outlook and discussion regarding potential applications of high power density conducting oxide metasurfaces. Our analysis shows that conducting oxide metasurfaces can tolerate the power densities needed in higher power applications, ranging from free space optical communications, to light detection and ranging (LiDAR), as well as laser-based additive manufacturing.
[1] Y.-W. Huang, H. W. H. Lee, R. Sokhoyan, R. A. Pala, Thyagarajan, K.; Han, S.; Tsai, D. P.; Atwater, H. A., Gate-Tunable Conducting Oxide Metasurfaces. Nano Lett. 2016, 16, 5319-5325.

Excited-state reactions are an essential phenomenon in many biological, chemical, and physical processes. However, current methods to study these excited-state reactions often involve large, expensive and elaborate instrumentation. Here, we investigate the reaction kinetics of a few well-known classical fluorescent-based processes through a novel approach called, plasmonic current. Plasmonic current operates when a fluorophore near-to a metallic nanoparticle island film can non-radiatively transfer its energy to the surface of the metal, inducing electron hopping across the metal and subsequently generating current. The magnitude of the current is influenced by the type of metal, spacing of the metal nanoparticles, spectral properties of the fluorophore and the distance of the fluorophore from the metal. Studying reaction kinetics through plasmonic current can potentially simplify the process and open doors to analyzing various other reactions that would otherwise involve complex instrumentation and moreover experimental geometries. Some applications of this technology include solar energy conversion, biological sensing, and chemical sensing, to name but just a few.

We combine flexible flat membrane modeling with spin-stabilization and nanophotonic design motives based on anisotropic optical scattering to report passive structural and dynamical stabilization of laser-propelled lightsails. Near-term interstellar exploration of exoplanets as envisioned by the Breakthrough Starshot initiative relies on propulsion of ultrathin membrane-like probes, or lightsails, to relativistic speeds via a directed laser source [1]. Successful deployment will depend on the ability to preserve the overall structural integrity of the lightsail while ensuring stable beam-riding behavior [2]. To date, studies on passive stabilization of laser-driven, photonically engineered lightsails assumed rigid-body dynamics [3-10]. In reality, lightsails with extreme area-to-mass ratios will be flexible and thus capable of developing shape deformations that could not only result in structural instability, but also compromise beam-riding ability.

We shed light on the flexible-body dynamics of flat silicon nitride membranes by using finite-element-based time-domain simulations in the linear elastic regime. Silicon nitride is a promising material candidate due to its low absorption in the near-infrared propulsion band and its wafer-scalable fabrication. Spin-stabilization is utilized to prevent shape distortion and collapse of the lightsail without needing to introduce additional mass or material. To passively stabilize such spinning flat membranes, we present a nanophotonic design based on asymmetrically diffracting optical metagratings. With the design search accounting for gyroscopic effects, we highlight how varying the spinning frequency and width of the Gaussian beam can disrupt or enable stable dynamics. Specifically, for a laser beam with a peak intensity of 1 GW/m² at 1064 nm and a Gaussian beam profile of w = 0.3 m, our simulations show how a meter-sized flexible flat membrane with a mass of 0.87 g, spinning at 120 Hz and patterned with our metagrating design will self-stabilize for a minimum duration of 5 seconds even if being initially displaced by x = y = 0.05 m and tilted by 2° (pitch and roll). Assuming constant, but realistic material properties, we also confirm that the sails develop strain and exhibit peak temperatures that are sufficiently below silicon nitride’s maximum tensile strain limit and crystallization temperature, respectively.

Our results pave the way towards more realistic simulations of lightsail deployment and propulsion from the microscopic to the macroscopic scale. Moreover, our open-source simulation tool can be readily expanded to include other types of optical interaction or experimental temperature-dependent material properties, while allowing to develop strategies for damping of the lightsail’s lateral oscillations and payload integration.

[1] Harry A. Atwater et al., Nature Materials (2018)
[2] Ognjen Ilic and Harry A. Atwater, Nature Photonics (2019)
[3] Elena Popova et al., Mathematical Methods in the Applied Sciences (2017)
[4] Zachary Manchester and Abraham Loeb, The Astrophysical Journal Letters (2017)
[5] Prateek R. Srivastava et al., Optics Letters (2019)
[6] Joel Siegel et al., ACS Photonics (2019)
[7] Ying-Ju L. Chu et al., Physical Review Letters (2019)
[8] Mohammad M. Salary and Hossein Mosallaei, Laser & Photonics Reviews (2020)
[9] Niels Gieseler et al., Optics Express (2021)
[10] Avinash Kumar et al., Physical Review Applied (2021)

Dynamic tuning of metasurfaces raises many potential applications. Such tuning can be achieved by changing either the material or geometric properties of the metasurface building blocks. In particular, two-dimensional transition metal dichalcogenides (2D TMDCs) are of interest as a novel material platform to integrate with metasurfaces. At atomic-layer limit, the possibility to excite excitons offers a strong “Materials Resonance” that can efficiently couple to the geometric resonances of dielectric and plasmonic nanostructures. The strength of excitonic resonance and its spectral location can be effectively controlled by methods such as electrical gating, material strain and dielectric screening. Such modulation can be harnessed to induce large changes in the optical material properties. However, the availability of high-quality, large-area 2D TMDCs is still limited by current synthesis and exfoliation methods and can be impacted by large-area patterning techniques.

Here we present optical tunable antennas and metasurfaces combing large-area monolayer/hetero-bilayer TMDCs and plasmonic structures. We show how intralayer/interlayer excitons can couple with plasmonic resonators and provide emission that is polarized in-/out-of-plane respectively and is controlled by solid-state electrical gating. With an Au-assisted large-area exfoliation method, we achieve scalable optoelectronic devices capable of amplitude, phase and propagation control. By applying 0.5V/nm bias across a gate oxide and electrically doping monolayer WS₂, we can reversibly and fully quench the excitonic resonance at 615nm. We further functionalize the plasmonic-assisted 2D emitters at room temperature, which not only enhances the light-matter interaction, but also actively redirects the photoluminescence from a patterned monolayer WS₂. We also demonstrate that interlayer excitons existing in device-scale hetero-bilayer TMDCs can be directly imaged and manipulated by judicious design of plasmonic antennas. Combining both in- and out-of-plane control in scalable TMDCs will open a promising avenue for tunable nanophotonic devices and metasurfaces.

Metasurface concepts have enabled the creation of a wide range of passive, flat optical components for optical wavefront shaping, polarization-conversion, and redirection of light. These elements have by now reached very high performance levels. However, for many optoelectronic devices (e.g. solar cells, sensors, and displays) it is not sufficient to have metasurfaces that serve as mode converters between different types of free-space waves. We also need metasurfaces that effectively interface free-space photons with optical elements that emit or absorb light waves. In this presentation, I will describe how initial work on the design of metasurface electrodes in organic solar cells and sensors has led to the development of advanced OLED displays.

Metasurfaces made up of tunable material can reconfigure the optical properties, hereby, contributing new opportunities for functional. It can perform diverse optical responses depending on external stimuli, which can be employed in visually discernable displays and anti-counterfeiting markets. Here, we demonstrated the dual-mode anti-counterfeit metasurfaces via single-step hydrogel nano-imprinting. The hydrogel metasurface has encrypted the information from the near and far-fields separately. The initial state can decode the information 1 the format of holography imaging in a far-field under the RCP rotated laser beam. On the other hand, information 2 is encrypted in near-field that can detect by optical microscopy. When the metasurface was exposed to a high humid condition such as exhaled breath, the hydrogel meta-atom can swell then aggregate with each other. Through the process can be revealed hidden information (2) in the near-field. In terms of mass-production and functionality, the dual-mode anti-counterfeiting metasurface can be expected to be actively applied to the future anti-counterfeiting industry.

Plasmonic metamaterials are engineered metallic structures designed to manipulate electromagnetic waves. Typically, such materials are made of periodically arranged elements, so-called meta-atoms, on a lattice significantly smaller than the wavelength of interest. This structuring locally mimics the quasi-homogeneous light-matter interaction of ordinary materials, but gives access to optical properties far beyond the scope of natural materials, including negative refraction, near-zero refractive index, and strong chiro-optical effects. On the other hand, fully connected metallic network metamaterials (NMs) mimic the electro-magnetic response of bulk metals. In contrast to natural metals where the plasma frequencies typically fall in the ultraviolet range, that of NMs can be tuned to an arbitrary smaller wavelength.

Double-net metamaterials (DNMs) feature two intertwining but spatially separated metallic network domains. These DNMs exhibit intriguing energy propagation mechanisms which are not yet completely understood. In contrast to the microscopic field overlap coupling mechanism between two plasmonic meta-atoms, two intertwining metallic networks can couple to each other through charge oscillations. This leads to a new type of longitudinal bulk mode that resembles an electron acoustic wave in a hydrodynamic double plasma (HDP). Simulation data confirms that these coupled charge oscillations exist within DNMs and that they propagate energy. Interestingly, in contrast to single networks that give rise to an effective plasma frequency, below which no propagating modes can be found, the new wave exists at arbitrarily small frequencies.

Here, we present our current understanding of this new mode that is based on pure charge oscillations with vanishing homogenized electro-magnetic fields and therefore largely decoupled from vacuum radiation. We discuss different network geometries, symmetry limitations and potential applications involving bound states in the continuum based on the new electron-acoustic wave.

Plasmonics have been recognized as a promising platform that may premise versatile applications. During the last decade, we have performed a comprehensive approach for the design and synthesis of multifunctional hybrid nanomaterials to seek their potential applications as key elements in nanotechnology. Researchers have extended the potential of plasmonics to unconventional domains, among which plasmon-enhanced electrocatalysis has shown promising evidence. Here we investigate the promise of plasmonic materials with unique light-interacting properties (localized surface plasmon resonance) for emerging application in energy storage. Next, we propose to integrate plasmonic effects into representative piezoelectric materials toward plasmo-phototronics. Generation of extra level of carriers is expected to relieve the operation voltage via near field effect and the emissive light can be amplified by plasmonic coupling.

The intensive local light-matter interaction in the vicinity of chiral plasmonic metasurfaces facilitates their robust application for chiral medium sensing.¹ However, the target-specific design of the ultrasensitive sensor is hard to achieve due to the severe complexities in inverse engineering of the plasmonic metasurfaces. Even several researches experimentally demonstrated the significance of correlation between the sensing capability and the features of plasmonic metasurfaces, architecting the on-demand nanostructure for specific purposes is still challenging due to the absence of well-organized but simple methodologies.² In this regard, artificial intelligence may offer the programmable basis of chiral plasmonic metasurfaces to overcome the limitations, based on its extraordinary capacity to deal with intricate and non-intuitive problems.³ On this, we successfully retrieved the chiroptical properties of the representative optically active plasmonic model (Born-Kuhn) and their optical responses to the adjacent medium based on a deep learning-based study. Herein, the plasmonic Born-Kuhn model, vertically displaced coupled nanocuboids, was found to be suitable for the systematic study thanks to its availability of analytic interpretation and capability of straightforward modulation in dimension.⁴ At first, a bidirectional neural network predicting the chiroptical response of a single plasmonic Born-Kuhn model, a fundamental descriptor of chirality, was constructed by employing multi-task and tandem deep neural networks. Then, by combining the multi-task deep neural networks with a genetic algorithm, we explored the spectral change that occurred by the intervention of various refractive indices and chirality parameters and figured out specific configuration of the on-demand ultrasensitive sensor for the target medium. Once the dataset for training was prepared using the finite-difference time-domain (FDTD) method under batch simulations, the neural network was found to be exponentially resource-saving than repeated numerical computations. Our study provides insight into deep learning-based cost-efficient strategy architecting plasmonic nanostructure for a specific purpose and serves a general methodology in designing nanophotonic metasurfaces.
References
[1] M. Kadodwala, et al., Nature nanotechnology, 2010, 5.11: 783-787.
[2] B. Kanté, et al., Nature Physics, 2020, 16.4: 462-468.
[3] H. Suchowski, et al., Light: Science & Applications, 2018, 7.1: 1-8.
[4] H. Giessen, et al., Nano letters, 2013, 13.12, 6238-6243.

Light can be influenced by permittivity changes in optical resonators, enabling optical sensors, modulators and switches. The performance of these devices is given by the relative change of intensity per given permittivity change and is proportional to their quality factor Q and sensitivity S. In classical systems an increase of Q is accompanied by a decrease of S and vice versa. In contrast to that, bound states in the continuum (BICs) show superior performance as they make it possible to increase the radiative Q factor independently from S. This, however, usually requires devices of large lateral size while state-of-the-art top-down fabrication processes are only compatible with small scales. It is assumed, that reducing the necessary device size by the reduction of the BIC propagation length leads to a decrease in performance.
Here, we demonstrate a hybrid waveguide exhibiting hybrid photonic-plasmonic BICs which were fabricated using a cost-efficient scalable fabrication method. The use of plasmonics enables a strongly reduced device size while at the same time maintaining high performance: we theoretically show that in an integrated diffraction geometry the performance of hybrid BICs exceeds that of dielectric BICs by about one order of magnitude.
The ability to produce hybrid waveguides of high symmetry on large scales opens up new opportunities for BICs in general and enables printable freestanding BIC waveguides, their combination with silicon substrates, and 3D stacking for the realization of both complex and cost-efficient high FOM* sensors and modulators.

Significant modifications in the kinetics of electrochemical reactions are observed in cyclic voltammetry experiments in the conditions of a plasmonic environment and light illumination. In nanostructured gold, the effect of light on anodic and cathodic currents is much pronounced than that in the gold flat system. The electron transfer rate shows a three-fold increase under photoexcitation. Applying these findings to the electrochemical deposition of polyaniline, we experimentally show the possibility to control film growth with light.

Highly doped transparent conducting oxide (TCO) thin films with near-zero permittivity can support epsilon-near-zero (ENZ) modes ^1,2, which leads to perfect light absorption and a large electric field intensity enhancement within the film. Extreme nonlinear light-matter interactions in ENZ thin film have led to enhanced high harmonic generation ^3,4 and giant enhancement of the optical Kerr effect (OKE) ^5,6 with ultrafast (sub-picosecond) recovery time. Surprisingly, the self-refraction counterpart of high harmonic generation, i.e., the higher-order Kerr effect (HOKE), in ENZ materials has remained unnoticed until now. Also, all previous works on the giant enhancement of OKE report the saturation of the refractive index change induced by optical nonlinearity with increasing E-field intensity of the incident wave, showing the ultimate limit of self-phase modulation. Although saturation of the imaginary part of the complex refractive index can be understood in terms of saturable absorption-type characteristics of the ENZ thin films, no proper explanation can be found in the existing literature for the observed saturation effect for the real part of the refractive index. Moreover, in the case of extreme nonlinear light-matter interactions, similar to what is observed in gaseous media ⁷, in an ENZ film, HOKE, i.e., the contributions from fifth, seventh, and higher odd-order nonlinearities to self-phase modulation may become prominent. However, no such observation was made to date or perhaps overlooked entirely.
Here, we report the experimental observation of strong HOKE in aluminum-doped zinc oxide (AZO) film grown by atomic layer deposition technique with a thickness of 76 nm and ENZ wavelength of 1590 nm. The Z-scan experiments were performed with femtosecond laser pulses of various wavelengths (1100-1600 nm) and angles of incidence (0-70^o). We observed a substantial contribution from the fifth-order nonlinear process to nonlinear refraction near the ENZ wavelength and at oblique incidence, which resulted in the transition from self-focusing to self-defocusing type OKE. The physical mechanisms leading to this observation are twofold- i) excitation of ENZ modes and the associated field intensity enhancement, and ii) increase of the effective nonlinear light-matter interaction length at an oblique incidence that leads to the enhancement of higher-order nonlinear refraction caused by cascaded third-order processes ⁸. We introduce novel nonlinear metrics to quantify the enhancement of the higher-order nonlinear refraction coefficients.
Finally, our observation of the interplay between the third-order and the fifth-order contributions to the self-phase modulation shows a limited performance of TCO films in all-optical modulators. This observation sheds new light on the optical nonlinearity of TCO films which got overlooked in the past. Many interesting alternative applications can be found based on such a novel observation, where one needs a fast switching from self-focusing to self-defocusing effects. An example is a femtosecond scale switching from an all-optical modulator to an all-optical isolator operation. Additionally, HOKE observed in TCO films could be beneficial for quantum information science. One would be interested in generating >2 photons entangled states via multi-wave (>4) mixing processes that need a significant contribution from HOKE.
References:
1. Campione, S., et al., Optics express (2016) 24 (16), 18782
2. Vassant, S., et al., Optics express (2012) 20 (21), 23971
3. Yang, Y., et al., Nature Physics (2019) 15 (10), 1022
4. Tian, W., et al., ACS omega (2020) 5 (5), 2458
5. Alam, M. Z., et al., Science (2016) 352 (6287), 795
6. Reshef, O., et al., Optics letters (2017) 42 (16), 3225
7. Bache, M., et al., Optics letters (2012) 37 (22), 4612
8. Saltiel, S., et al., Optics letters (1997) 22 (3), 148

Indium tin oxide (ITO) exhibits strong field enhancement and nonlinear optical response around its epsilon-near-zero (ENZ) wavelength, in which the real part of the permittivity switches from a positive to negative value. The ENZ wavelength of a particular film can be configured by adjusting free carrier concentrations through deposition parameters. We present here the detailed process parameters of a high temperature reactive sputter process which can produce ITO films with an ENZ wavelength from 1.1µm to well beyond 3µm. We then perform z-scan and second harmonic generation (SHG) measurements to determine the nonlinear response, and correlate this to the optical, electrical and structural properties of the films. Finally, we design and fabricate a vertical metamaterial which is a multilayer ITO-SiO₂ device similar to a previously demonstrated NIR perfect absorber. We show that this structure greatly enhances SHG over a single ITO thin film.

Indium Tin Oxide (ITO) has a metallic response in the near- mid- infrared but becomes dielectric in the visible [1]. When exposed to laser pulses, the distribution of conduction band electron induces changes in the real and imaginary parts of the permittivity [2]. When an electromagnetic wave couples with the electrons in the conduction band of metallic nanostructures, thanks to the nonlinear plasmonic response we can provide optical modulation that can be exploited and used to create integrated optical components [3].
Here, we present a photophysical study on ITO grating, realized using the femtosecond micromachining technology, and comparing its properties with the one of an ITO thin film. The geometries, dimensions, and pitch of the various gratings analyzed are obtained using direct ablation in a controlled atmosphere of a homogeneous thin layer of ITO deposited on a glass substrate. This solution guarantees us precision, repeatability, and extreme manufacturing flexibility.
Ultrafast pump-probe spectroscopy has proven to be a powerful technique to study out-of-equilibrium phenomena, being applicable over a broad range of photon energies from THz to x rays. In pump-probe, a medium is first excited with a short pump pulse and the photoinduced dynamics are probed by a time-delayed broad-band probe pulse.
We characterize both the plasmon and inter-band temporal dynamics. To excite the plasmon of ITO we tuned our Optical Parametric Amplifier at the resonance in the NIR (1550nm) and to probe the modulation of the transmitted spectra from the grating we generate a broad-band probe in the visible range by focusing our fundamental beam into a thin sapphire plate.
We observe a modulation of the visible spectra of the probe induced by the excitation of the material plasmon in the NIR. Exciting the plasmon induces a modulation of the permittivity of ITO, this change in permittivity changes the refractive index that changes the transmission efficiency of the grating.
When using ultrashort laser pulses we can have the generation of several Coherent Artifacts (CAs) of considerable intensity that can completely or partially distort the first hundreds of femtoseconds of relaxation dynamics, causing loss of information about early electronic processes under investigation. In particular, in our measurement there is a strong Cross Phase Modulation artifact (XPM)[4], limiting our capabilities of extracting information of the first 200fs.
Here, we introduce a novel AI-driven method to remove XPM artifacts from ultrafast pump-probe dynamics. We propose XPMnet, a Neural Network (NN) model able to operate directly on raw pump-probe data and efficiently retrieve the embedded electronic dynamics of physical interest in material systems. We designed and developed XPMnet as a supervised ML model. We structured the model architecture as a Convolutional Neural Network trained on a labeled dataset, which consisted of simulated pairs of XPM-affected pump-probe instances and related electronic time dynamics, the latter carrying the physically relevant information about the sample under investigation.
[1] Mryasov, O. N., and A. J. Freeman. "Electronic band structure of indium tin oxide and criteria for transparent conducting behavior." Physical Review B 64.23 (2001): 233111.
[2] Roxworthy, Brian J., et al. "Understanding and controlling plasmon-induced convection." Nature communications 5 (2014): 3173.
[3] Guo, P., et al (2016). Ultrafast switching of tunable infrared plasmons in indium tin oxide nanorod arrays with large absolute amplitude. Nature Photonics, 10(4), 267.
[4] M. Lorenc, M. Ziolek, R. Naskrecki, J. Karolczak, J. Kubicki, and A. Maciejewski, “Artifacts in femtosecond transient absorption spectroscopy,” Appl. Phys. B 74, 19–27 (2002).
[5] F. Emmert-Streib, Z. Yang, H. Feng, S. Tripathi, and M. Dehmer, “An introductory review of deep learning for prediction models with big data,” Front. Artif.Intell. 3, 4 (2020).

Time-gradient materials and metasurfaces have opened up new applications for manipulating light actively, and have unlocked new optical phenomena. In particular, ultrafast optical driving of epsilon-near-zero (ENZ) materials has attracted great interest as it allows for fast refractive index changes, arising from large nonlinearity, sub-picosecond timescale response, in turn enabling all-optical dynamic frequency-conversion and beam-steering applications.
Here we report the observation of nonreciprocal beam steering and frequency shifting on hundreds of femtosecond time-scales. Ultrafast pump-probe measurements were applied to indium-tin-oxide (ITO) bulk material and metasurfaces, with ENZ wavelength in the near infrared range. A wavelength and time-dependent study demonstrates maximum frequency of 18nm and refraction angle shifting of 0.7 degrees when permittivity approaches zero as predicted, which also allows for THz modulation speeds. The results indicate that ENZ materials and metasurfaces are promising for future applications of frequency conversion and dynamic optical steering.

Ultrathin solar cells have the potential to reduce cell cost and achieve higher efficiencies, which have been the main driving forces for the photovoltaic industry. Recently, emerging market for high-specific-power solar cells has added importance of making solar cells thinner, as it can offer desired values such as flexibility and weight-reduction. However, implementation of the ultrathin solar cells is hampered by the substantially lower efficiency than that of bulk cells. This is mainly due to the incomplete absorption as the thickness of the cells is much smaller than the absorption depth of the materials. Therefore, for ultrathin cells, not only require effective antireflection coatings but light-trapping layers are also needed to realize high-efficiency. While macroscopic random surface texturing is widely used for both antireflection and light-trapping purposes for bulk cells, it is impossible to employ such textures on ultrathin cells where the thickness is smaller than the texture size.
In this work, we show how a single nanostructured layer can be judiciously designed to offer both broadband antireflection and light-trapping functions for ultrathin Si solar cells. We use high-index nanostructures as our building blocks which can support Mie resonances to provide strong light absorption and scattering in a controllable fashion. Broadband antireflection is achieved by placing the Mie resonators in a dense array. At the Mie resonances, strong reflection from the high-index Si cell can be countered by optimizing the destructive interfere with the strong scattering from the Mie resonators. At the longer wavelengths, the array can further serve as a homogeneous medium that supports Fabry-Perot resonances to cancel out the reflection from the substrate. However, such dense arrays often cannot couple light in the substrate when the interparticle distance is smaller than the effective periodicity. To achieve light-trapping functions simultaneously, we utilize differently sized Mie resonators within a repeating unit cell so that the dense array can effectively couple light into diffracted modes in the cell. This redirection of light into the plane of the cell increases the optical path length and therefore light absorption in the thin active layer. We experimentally demonstrate the nanostructured light-trapping antireflection coatings on a 2.8-µm-thick c-Si solar cell, realizing power conversion efficiency of up to 11.2%. The enhanced optical absorption by the proposed design is translated into higher short-circuit current and power conversion efficiency, which are improved by 48% and 51% respectively, compared to a cell without any photon management schemes.

Strong coupling of e.g. excitons and cavities was shown to affect energy eigenstates of interacting constituents, cause Rabi splitting of the dispersion curves, and enable scores of intriguing physical and chemical phenomena. In particular, the effect of strong coupling on the Fabri-Perot cavity’s work function has been reported in Ref. [1].
In this study, we report on the Kelvin probe workfunction measurements of the Fabri-Perot cavity’s components, such as metallic films and dye-doped polymers. (i) The work functions of thick Al, Ag and Au films were as expected. (ii) The work functions of thin Au films (≤50 nm) were smaller than those of thick films. (iii) Dye doped polymer (R6G: PMMA) deposited on top of thick Au film reduced the work function significantly. (iv) At the same time, dye-doped polymer deposited below Au films resulted in a significant increase in the work function. (v) Surprisingly, the work functions of Au-based Fabry-Perot cavities were not much different from those of thick Au films.

Light-matter interactions occurring in weak or strong coupling regimes can enable scores of physical and chemical phenomena ranging from Förster energy transfer to rates of chemical reactions. Thus, it has been shown that the rate of photodegradation of semiconducting polymer P3HT can be inhibited by metal-dielectric environments, including MgF₂-coated metallic films and lamellar structures of metal and dielectric layers [1]. An even stronger effect was observed at the strong coupling of P3HT with resonant Fabri-Perot cavities [2].
In this work, we study the effects of metal-dielectric environments on another phenomenon of technological importance – photopolymerization of the [2,2′-bi-1H-indene]-1,1′-dione-3,3′-diyldiheptanoatecarboxylate monomer. Experimentally, thin films of the monomer were deposited on a variety of metal-dielectric substrates and exposed to radiation of the Xe lamp (converting the monomer to a polymer). The amount of the remaining monomer was monitored by measuring the strength of the 480 nm absorption band that was present in the monomer and did not exist in the polymer. We did not observe any reduction of the photopolymerization rates by metallic or metal-dielectric substrates. On the contrary, the photopolymerization kinetics become faster, some by a little and some by far. Thus, silver substrate accelerated the rates of polymerization by nearly fourfold. The effects produced by Ag/MgF₂ substrates or gold substrates were more modest. More experimental details and the data analysis will be presented at the conference.
[1] V. N. Peters, T. U. Tumkur, G. Zhu & M. A. Noginov, “Control of a chemical reaction (photodegradation of the p3ht polymer) with nonlocal dielectric environments”, Scientific Reports 5, 14620 (2015).
[2] V. N. Peters, Md Omar Faruk, J. Asane, R. Alexander, D. A. Peters, S. Prayakarao, S. Rout, M. A. Noginov, “Effect of Strong Coupling on Photodegradation of the Semiconducting Polymer P3HT”, Optica 6, 318-325 (2019).

Active photonic metasurfaces control the amplitude and phase of light over a surface and allow for dynamic and total light field manipulation. Such metasurfaces could be broadly applied to many fields such as LiDAR, AR/VR and medical sensing. Over recent years, there has been an expanded effort to improve dynamic nanophotonic devices by employing a variety of physical phenomena that modulate the interaction of light with sub-wavelength components. Mechanical modulation techniques offer high promise due to their ability to alter the optical permittivity over the surface to a greater extent than any other modulation technique. However, the mechanical actuation of dense arrays of nanoscale structures has proven to be exceedingly difficult. Here, we demonstrate mechanical modulation of the gaps between plasmonic nanoparticles and a metallic surface. This is achieved through the deformation of a compliant gap filler. Plasmonic gaps confine light to nanometer sized regions, allowing for small movements to significantly modulate the optical scattering properties of structures. Further, by using acoustic waves, mechanical forces can be sculpted across a surface at a spatial frequency like that of optics and modulated with GHz level bandwidths. These aspects together provide the ability to dynamically alter large optical phase gradients across a surface at high speed and create a new class of light modulation devices.

In this study, long-range excitation energy transfer (EET) between molecular aggregates in the presence of plasmonic silver nanorods is explored. Transition matrix elements of biological chromophore complexes are shown to be greatly enhanced when located near the surface of silver nanoparticles, notably demonstrating distance-independent electronic coupling up to several tens of micrometers. We show that these enhancements can lead to energy transfer rates increased by up to a billion-fold across the entirety of the visible spectrum and into the near-infrared. These results illustrate that it is possible to devise plasmonic nanostructures such that the electronic properties of multi-chromophoric systems can be drastically perturbed by the spatio-inhomogeneous electromagnetic fields generated by them, allowing for EET well beyond the Förster limit.

As carbon emissions increase and global warming becomes more serious, the concept of zero energy building (ZEB) has emerged, which covers the heating and cooling loads and other energy consumption of buildings with renewable energy such as photovoltaic. Achieving ZEB requires either increasing the building's own energy production or reducing energy loss. In particular, since energy loss is transferred in the form of heat, conduction/convection losses can be controlled by using highly insulated materials or phase change materials, but radiative energy loss which is the form of the electromagnetic wave is difficult to control. Thermal energy radiated from the building to the outside environment and solar energy incident to the building increase the heating or cooling load of the building, resulting in higher energy consumption. Each energy is a form of the electromagnetic wave in the long-wavelength infrared (LWIR) and near-infrared (NIR) bands, and in the case of windows and exterior walls, additional control of the visible waveband is essential for light transmission or aesthetics of the building exterior. Herein, a hierarchical Polydimethylsiloxane (PDMS)-based metamaterial is proposed to simultaneously control three wavebands, which are visible, NIR, and LWIR. Solar irradiation and atmospheric emission spectra were considered to tailor optical properties for controlling incident and outgoing energy. Moreover, the material was designed by using COMSOL Multiphysics software and fabricated without a photolithography process. By measuring the transmittance of visible waveband, the reflectance of NIR waveband, and the emissivity of LWIR waveband according to the control characteristics required for each energy type, the waveband controllability of the proposed metamaterial was confirmed.

The infrared (IR) sensor applied to the modern IRST system detects radiation signature emitted from the target, and the IR signal in the 5-8 μm band cannot reach the sensor due to the atmospheric transmission window effect. Therefore, it is important to reduce the IR signal in the “detection band” to prevent from being attacked. In particular, the long-wavelength infrared (LWIR) signal in 8-12 μm is dominantly generated on the aircraft’s fuselage surface, which is relatively cold compared to plume. In order to reduce the IR signal generated from fuselage, it is necessary to design a surface that reduces the signal in the detectable LWIR band and selectively emits the signal to the undetected band. For the purpose of selective absorption/emission in the IR band, dielectric multilayered materials or metal-dielectric-metal (MDM) layered metamaterials are being actively studied. In case of MDM metamaterials, many researchers mainly used lithography or nanoimprinting fabrication methods. However, these processes are expensive, time consuming, and difficult to process over a large area. For this reason, we suggested a cost and time efficient and large-area fabrication method for IR selective emitter of hole-structured MDM layer using PS microsphere. We designed hole-structured periodic metamaterials using the hexagonal packing of microspheres. For monolayer hexagonal packing of PS microsphere, we used sodium dodecyl sulfate (SDS) solution. The hole-structured MDM metamaterial mainly changes the absorptivity/emissivity characteristics depending on the hole diameter, hole pitch, and dielectric material’s thickness. To analyze the optical properties of the metamaterial according to the microsphere’s diameter, we fixed the other parameters, which are dielectric thickness and hole diameter, and then, we conducted numerical simulation using COMSOL Multiphysics®. In addition, we measured the reflection and emission of fabricated specimen using FT-IR and compared with the numerical simulation results. As the diameter of the PS microsphere increased, the pitch between the particles in the hexagonal packed monolayer increased, and accordingly, the hole pitch of the metamaterial also increased. Since the resonance frequency of the MDM structure can change with different hole pitch, we finally confirmed that our metamaterials can controlled selective emission characteristics into the undetected band with different microsphere diameters.

Light displays distinct interactions with materials, depending on its physical properties such as the wavelength, directionality, and polarization. This enables a variety of optical sensing and imaging applications that detect and distinguish incident light waves from an optical scene based on these properties. These are typically performed using a conventional photodetector with a set of bulky external elements to filter light waves with properties of interest, which makes dense integration a significant challenge.
Alternatively, optical resonances in nanostructures can be harnessed not only to control the flow of light, but also to achieve light detection that is sensitive to certain properties of light. Especially, it has been demonstrated that plasmonic nanostructures can be used to detect circularly polarized light with a specific handedness. However, as they rely on inefficient hot electron emission, it is desirable to explore whether such photodetectors can be created using high electrical-performance semiconductors such as silicon.
Here, we illustrate how a thin silicon film can be nanopatterned to display high differential absorption for left- and right-handed circularly polarized light. This is enabled by polarization-controlled excitation of guided-mode resonances through periodically arranged dislocations into a silicon nanowire array. We also present the experimental demonstration of photodetectors using our dislocated metasurfaces. The proposed structures could potentially be used in a wide range of optical sensing and imaging applications.

The integration of plasmonic materials with chiral structures is currently revolutionizing a variety of nanoscience fields including nanophotonics. Previously, our group developed a new class of chiral nanostructures by combining the crystallographic control of nanoparticles and enantioselective chemistry between organic and inorganic materials. By exposing high-Miller-index surfaces to colloidal gold nanoparticles and inducing enantioselective interactions with cysteine and glutathione, 432 symmetric gold nanoparticles with significant chiral morphology were synthesized. Further studies revealed that the formation of chiral morphology was determined by the crystallographic growth pathways and cooperative chirality expression. In this context, we utilized dipeptides for the first time to encode chirality on the nanostructures. Based on our previous work, we expanded the sequence of chiral peptides and enabled precise tuning of chiral morphologies and chiroptical responses of gold nanoparticles. Importantly, we demonstrated that the formation of chiral morphology can be systematically understood based on crystallographic analysis.
Herein, we closely investigated the geometric structure of nanocrystals in colloidal synthesis using chiral shape modifiers from a crystallographic perspective. We explored the effects of dipeptides on chiral morphology, providing an in-depth understanding of the interplay between particle growth and chirality formation. We are convinced that the efforts to crystallographically analyze the chiral morphology of nanocrystals allows us to ultimately understand the three-dimensional structure of chiral nanomaterials and the resulting chiroptic response. We believe that the analysis of the effect of the peptide sequences on chiral shape development suggests fundamental principles for the design of the morphologies and properties of nanoparticles for versatile applications as well as for peptide incorporation during nanoparticle growth.

Nanometer-scale metal particles exhibit chemical and physical properties which differ significantly from macroscopic metal phases, including strong interaction with visible light due to their localized surface plasmon resonances. In particular, silver nanoparticles (Ag NPs) have had extensive attention due to the low-cost and abundance of elemental Ag. The strong and tuneable interaction of visible light with Ag NPs have found many applications.
In many of these applications, it is essential to reliably and precisely control the size and density of Ag NPs on conducting transparent supports, such as indium tin oxide (ITO). Electrochemical deposition is an attractive growth method. However, electrochemical deposition of Ag on ITO naturally leads to scattered, low density irregular nanoparticle growth owing to the low surface energy and high nucleation barrier of ITO. While it is well known that surface properties of the substrate have a key role in nucleation dynamics, most studies involving ITO substrates do not pay complete attention to the physical properties of the surface, and often the only parameter specified is the sheet resistance.
In this talk, we elucidate the influence of crystallographic orientation of the ITO surface on silver nanoparticle growth. We provide a careful and quantitative study of Ag nanoparticle nucleation and growth on two batches of ITO substrates. Even though all ITO substrates come from the same supplier and have the same nominal specifications, XRD scans clearly differentiate between two types of films based on the ratio between the (222) and (400) related peaks.
We find that the preferential presence of lower index surfaces (e.g. (100)) leads to few orders of magnitude lower particle density compared to the preferential presence of higher index surfaces (e.g. (111)). Furthermore, the density on the lower index surface is strongly dependent on the nucleation pulse potential, while the higher index surface is barely affected. A geometrical analysis of multiple individual particles shows that ITO orientation also affects growth dynamics, where preferential presence of (100) surfaces leads to more isotropic nanoparticle growth.
Controlling metal particle density and size is crucial in many applications. Beyond careful control of electrochemical parameters, here we show that neglecting the macroscopic surface characteristics of ITO (and any other polycrystalline substrate) can lead to major ambiguity in nucleation and growth dynamics. We show here that XRD characterisation prior to deposition is a simple additional measure to tailor metal nanoparticle electrochemical growth on ITO.

The sub-diffractional confinement of light can be achieved through the user-designed sub-wavelength polaritonic structures, which find applications in nonlinear optics, enhanced molecular sensing, light generation, and communications. Localized surface phonon polariton (LSPhP) modes provide ideal options to achieve extreme sub-diffractional photon confinement within the mid-infrared spectral region. LSPhPs are induced through light coupling with oscillating ionic charges (optic phonons) within nanostructures fabricated from polar crystals. These excitations possess narrow spectral linewidths with the resonances able to be tuned through structure design. However, while certain objectives can be fulfilled within periodic structures featuring a single repeated (1x1) LSPhP element in the unit cell, there are limitations resulting from such simple designs. For example, while one can periodically space polar nanostructures at distances on the order of the free-space wavelength of light to induce diffractive coupling between localized and propagating modes, this necessarily reduces the filling fraction and thereby lacks the stronger field enhancements. Moreover, a limited range of LSPhPs modes are supported for simple unit cell designs. This thus raises the question: can we further modify the LSPhP unit cell to manipulate new polariton properties? By increasing the complexity of the unit cell for subwavelength polaritonic structures, new polaritonic resonant properties can potentially be engineered. Similar to atoms in different arrangements forming molecules, interacting nanostructures in metallic complexes can give rise to collective modes. Similarly, to design new LSPhP resonant properties, more repeatable elements can be added into the unit cell. Such designs are especially interesting for sensing applications as altering the composition and subarray structure could allow for additional polaritonic resonances, enhanced spectral tuning, and improved definition of the local near-fields. However, the LSPhP system with increasing unit cell complexity remain largely unexplored.
In this work, we explore subarray designs of subwavelength nanopillars etched into 4H-SiC substrates, which can support low-loss, LSPhP modes with collective behaviors. In the subarray design, the unit cell (1x1, 2x2, 3x3, 4x4) is scaled to smaller size based on the number of elements in the subarray. By introducing more complex unit cells, excitation of collective LSPhP modes resulting from near-field interactions between neighboring elements are observed. Referring to the terminology for the electromagnetic analog of molecular orbital theory, we report on the observation of collective symmetric and anti-symmetric modes supported within LSPhP subarrays. Interestingly, we observe a quasi-mode-matching of the collective symmetric mode. In addition, the anti-symmetric mode is found to be primarily dictated by the near-field interactions within the unit cell. In contrast, the symmetric mode is observed as an extended excitation with long range interactions between adjacent unit cells. Moreover, these previously unexplored collective LSPhP modes are found to be robust to defects introduced into the unit cell, opening up possibilities for symmetry-protected states in collective modes. As such, our subarrays represent a novel modal design platform that offers the low-loss, subdiffractional confinement of light associated with LSPhPs, but expanding the precise spectral tuning of the electromagnetic resonances, near-field distributions and potential to be hybridized with other resonances resulting from the subarray architecture.

Dielectric Metasurfaces, composed of deep-subwavelength nanostructures, can sculpture the phase, amplitude, and polarization of light at the nanoscale. Their multifunctional response at the single-pixel level, combined with their compact form factor and integrability with other CMOS compatible devices make them key components of miniaturized photonic platforms, leading to numerous applications in spatial domain wavefront manipulation of light[1]. Recently, temporal-domain shaping of scalar waveforms of a linearly polarized ultrafast pulse has been demonstrated using dielectric metasurfaces[2]. Here, we show, both theoretically and experimentally, that a single-layer transmission-mode dielectric metasurface can be leveraged to simultaneously and independently tailor the temporal vectorial and spatial properties of an ultra-broad bandwidth, near-infrared femtosecond pulse, achieving arbitrary control of complex electric-field transients at the femtosecond timescale.
The spatiotemporal profile of a p-polarized femtosecond pulse of ≈10 fs duration (full-width at tenth-maximum bandwidth of ≈ 80 THz, centered at 800 nm) is tailored by manipulating the constituting discrete frequency lines. A 4-f Fourier-transform pulse shaper spatially disperses and focuses the frequency lines at the Fourier plane, where a metasurface is positioned to implement a complex masking function sampled by 200 super-pixels. Each super-pixel contains a two-dimensional (2D) array of rectangular silicon nanopillars orientated at 45° with respect to the horizontal direction, providing parallel phase modulation along the two birefringent axes. In this way, each super-pixel can independently impart a 2D spatial phase function as well as an overall phase-shift to the two orthogonal polarization components of frequency lines that reside within the super-pixel. The shaped frequency lines are then subsequently recombined into a spatiotemporally engineered femtosecond pulse.
This approach, leveraging ultra-high spectral resolution of a Fourier-transform pulse shaper and multifunctional responses at the nanoscale provided by the dielectric metasurface, offers ready design flexibility that can enable the synthesis of a vast variety of complex ultrafast spatiotemporal beams. To demonstrate the versatility of this approach, exotic femtosecond pulses, exhibiting a rich set of time-varying instantaneous polarization states and structured wavefronts within a single pulse, are designed and experimentally demonstrated. Comprehensive analyses, including numerical simulations and analytical modeling, are also performed to explain their spatiotemporal evolution.
In conclusion, we have demonstrated tailoring of both the temporal and spatial degrees of freedom of an ultra-broad bandwidth, near-infrared femtosecond pulse using a single-layer transmission-mode dielectric metasurface. Such an approach further promotes the already intriguing applications of metasurfaces, revealing new possibilities in the field of ultrafast science and technology.
[1] N. Yu and F. Capasso, Nature Materials 13, 139 (2014).
[2] S. Divitt, W. Zhu, C. Zhang, H. J. Lezec, and A. Agrawal, Science 364, 890 (2019).

Proteins are harbingers of human health. Measuring the proteome provides a snapshot of the functional output of the genome, a crucial insight given that as many as 100 different proteins can be generated from a single gene. Sensitive proteomic measurements can better enable early disease diagnosis, health monitoring, and significant opportunities for personalized medicine. However, proteomic profiling is inherently challenging due to the wide dynamic range of possible protein concentrations, as well as the lack of signal amplification techniques, like PCR, that exist for nucleic-acid based tests. Current methods for protein detection and monitoring include enzyme-linked immunosorbent assays (ELISA), lateral flow assays, and protein microarrays. These techniques face tradeoffs on speed, sensitivity, and multiplexing capabilities due to their reliance on “traditional” optics such as fluorescence or absorbance.

Here, we develop a nanophotonic assay that can quantitatively and rapidly detect protein binding. Unlike existing assays that rely on secondary tagging, we utilize high quality factor (“high-Q”) nanophotonic Si metasurfaces that possess extremely sharp scattering linewidths, comparable to a laser (<1 nm); these high-Q surfaces generate resonant scattering intensities that are proportional to the bound protein load. Our chips consist of sub-micron Si nanobars functionalized with a protein probe – the SARS-CoV-2 receptor-binding-domain (RBD) protein for our initial work. Importantly, we optimize monolayers of a zwitterionic, polyethylene glycol (PEG)-lyated matrix to minimize nonspecific adsorption. Additionally, we modify the spike protein with a polyhistidine-tag, orienting the antibody recognition site and allowing for increased primary antibody binding. We illuminate the metasurface in the near-infrared (1000-1500 nm) and measure the dynamic optical transmission upon introducing SARS-CoV-2 antibodies, including IgG and IgM. Upon antibody binding, we observe a clear 2 nm resonant-shift in transmission within 10 minutes. We show specific and sensitive detection of these antibodies, with linear responsivity spanning the micromolar to near-single-molecule range. Finally, we demonstrate that multiplexed antibody testing is also possible on a single chip, using a home-built acoustic bioprinting functionalization platform with ~10 um molecular droplet resolution.

Rapid identification of mycobacterium tuberculosis (MTB) infection and antibiotic susceptibility testing (AST) is essential for effective patient treatment, prevention of community spread, and combating antimicrobial resistance. To date, culture-based techniques remain the gold standard for the identification of TB strains and determination of their antibiotic susceptibility. Unfortunately, such identification and AST can take several days to multiple weeks in well-equipped labs, due to the slow growth rate of MTB. One promising alternative to culture-based cellular fingerprinting is Raman spectroscopy, an optical characterization technique which probes vibrational modes of a sample. It relies upon the inelastic scattering of photons incident on a sample and measures their wavelength shift after interacting with a sample. Raman scattering offers three key advantages over alternative cellular fingerprinting approaches. First, it is label-free and generalizable to all kinds of cells; the intrinsic vibrations of the cells serve as native labels. Second, Raman can be fast, with sub-second acquisition times, and accurately measured with minimal sample preparation. Lastly, because peaks in a Raman spectrum are unique to the vibrational modes of particular biomolecules, Raman cell signatures can be used to assess and monitor the composition, structure, and vitality of that cell and uniquely differentiate it from another cell. Despite these advantages, obtaining and utilizing Raman spectra from cells presents a challenge due to (1) Raman’s inherently weak scattering efficiency and low signal-to-noise ratio and (2) its complexity in spectral interpretation.

To address these two issues, we present a combined nanophotonic-machine learning platform that uses resonant nanophotonic surfaces, known as high-quality factor (high-Q) dielectric metasurfaces, to provide critical Raman signal enhancement. Using FDTD simulations, we demonstrate a metasurface of subwavelength silicon nanoblocks that are biperiodic in width to generate a high Q resonance. These nanoblocks have a length of 425 nm, a biperiodic width of 105 and 95 nm, and a height of 200 nm. By tuning the nanoblocks’ dimensions, we place a high-Q resonance at the Raman excitation wavelength that will be used to enhance the local electric field amplitude as well as a broad magnetic dipole Mie resonance at the MTB’s Stokes-shifted Raman region to provide strong, broadband Raman signal enhancement . Collectively, these two resonances provide a maximum Raman signal enhancement of over 105 over a wide spectral range of 1500 cm-1.

We fabricate these metasurfaces using electron beam lithography and experimentally verify this enhancement by placing dried bacterial samples on the metasurfaces and measuring their Raman spectra. We use four unique strains of antibiotic-resistant Bacillus Calmette-Guérin (BCG), an attenuated form of Mycobacterium Bovis, as a model for Mycobacterium tuberculosis, as well as a control BCG strain. We classify the antibiotic resistant BCG strains according to their Raman spectra using logistic regression. Specifically, we use a 80/20 train-test split, in which 80% of the data is used to train the model and the remaining 20% is held out and tested on the model in order to evaluate the accuracy of the model. Through this machine learning model, we are able to identify and differentiate the 5 BCG strains with ~98% accuracy. Through this combined nanophotonic-machine learning platform, we have demonstrated a rapid and accurate platform for bacterial identification and classification using Raman spectroscopy.

COVID-19 pandemic, caused by respiratory syndrome-coronavirus-2 (SARS-CoV-2), is an ongoing global challenge. Although the reverse transcription polymerase chain reaction (RT-PCR) test is effective in detecting SARS-CoV-2 virus as a gold standard clinical diagnosis, it is insufficient as a preemptive test to prevent the rapid transmission of SARS-CoV-2. To further improve the diagnostic rapidity, direct and sensitive approaches for label-free and/or amplification-free detection of SARS-CoV-2 have been presented. Representatively, electrochemical sensors based primarily on carbon/graphene electrodes have been successfully demonstrated to provide diagnostic results on clinical samples in minutes using immunological responses without an additional pre-sampling process. Even so, these electro-signaling strategies still require expertise and specialized equipment with limited scalability of the sensing area, and are less intuitive for ordinary users. In practical applications for virus detection, the most intuitive method is to easily identify optical or color changes with the human eye, as is the case with typical diagnostic techniques such as fluorescence immunoassays and enzyme-linked immunosorbent assays (ELISA). Although several optical-based diagnostic techniques for rapid detection of SARS-CoV-2 without labeling or amplification have been reported including colorimetric assays or surface plasmon methods using gold nanostructures or particles, they are still challenging in sensitivity and observability compared to methods involving amplification and labeling processes. Over the past few decades, advances in optics to make it possible to control the speed of light propagating through optically dispersive media presents many opportunities for wise use of light. Slow light promotes stronger light-matter interactions, which also have applications in biosensing by providing additional control over the spectral bandwidth of these interactions. Previously reported slow light-based biosensing platform mainly utilize nanovolume-scale structural slow light in the form of nanospheres or nanocrescents as surface-enhanced Raman scattering substrates. Meanwhile, as a simple structure for effectively controlling light, Gires-Tournois resonators using non-linear effective phase changes have been recently studied as flat metasurfaces including a dynamic phase-change absorber, achromatic surface cloaking and lensing, and colored perovskite solar cells. This strong designable modulator, which can effectively control light with various media combinations, has not yet been optimized for biosensing applications including low-index virus detections through light control such as slow light that induces strong light-matter interactions. Here, we introduce the Gires-Tournois immunoassay platform (GTIP) for amplification/label-free rapid detection of SASR-CoV-2. In slow light conditions, GTIP exhibits an intuitive colorimetric response to viral particles of SARS-CoV-2. Viral distribution and concentrations are provided through chromaticity analysis from virus cluster images obtained by microscopic scanning. For practical applications, GTIP, attached to an everyday item such as a face mask, also detects sprayed droplet transmission in infectious environments such as person-to-person conversations and coughing. Moreover, for extended applications with versatility, GTIP provides optimal structures for analytes of different refractive indices and sizes with the design parameters of the tri-layer configuration.

THz radiation’s unique characteristics have drawn significant attention in the medical research community. Of particular interest in biosensing is the fact that a lot of biomolecules have vibrational and rotational energy levels that lie in the THz region. Thus, molecules like proteins and DNA display a characteristic frequency response in that part of the electromagnetic spectrum which can be used for label-free, non-destructive detection of biomolecules [1]. The main obstacle in current solutions is the attenuation of the signal through water absorption which reduces the sensitivity and specificity. Additionally, the bulkiness and cost of existing THz instrumentation also hinder commercialization of diagnostic platforms, thus limiting their use to research laboratories.

A potential avenue for tackling these problems is through the use of THz dielectric metasurfaces. By incorporating the benefits of THz radiation and the unique abilities of metasurfaces, they have the potential to cater to the demands of biosensing.

Currently, THz sensors rely on testing dry samples which is unrepresentative compared to the in-vivo environments. To overcome this, it is necessary to examine small volume samples with reduced hydration. This can be a challenge, however, due to the large-scale difference between sample and wavelength. A potential solution to this is to confine the THz radiation using metasurfaces and microfluidic channels [2]. Most of the research has been focused on metallic metasurfaces, but more recently dielectric-based metasurfaces have been demonstrated which exhibit the same capabilities of manipulating light as metallic ones, without the inherent absorption losses related to them.

When it comes to the design of biosensors, dielectric metasurfaces can be made to exhibit a strong electric field enhancement and energy localization. This confinement enhances light-matter interaction. When combined with microfluidics which will allow testing of small volume samples with precise liquid control, the metasurface can be made highly sensitive to small changes in the analyte concentrations.

A noteworthy approach that has been gaining a lot of research interest, regarding the further enhancement of the localization of energy, and thus the sensitivity of sensors, is based on the quantum mechanical phenomenon of bound states in the continuum (BIC) [3]. By introducing asymmetry in the unit-cell structure of dielectric metasurfaces in the form of supercavity modes, it is possible to realize extremely high Q-factors as well as a significant enhancement in the electric field confinement, thus providing a new avenue for achieving ultrasensitive biosensors.

In that context, here we present numerical simulations of a dielectric THz metasurface biosensor based on BICs. The periodic unit-cell structure consists of an array of silicon cuboids, positioned on top of a quartz substrate. By modifying the geometrical parameters of the structure, we were able to leverage the physics of BICs to tune both the resonance and the Q-factor of the metasurface as well as obtain a strong field enhancement. We also demonstrate sensitivity to local permittivity by simulating the resonance shift in the presence of an analyte model.



References:
[1] http://dx.doi.org/10.1016/j.tibtech.2016.04.008
[2] https://doi.org/10.1007/s10762-021-00792-9
[3] https://onlinelibrary.wiley.com/doi/10.1002/adma.201901921

Hyperbolic Metamaterials (HMMs) are structured optical nanomaterials that exhibit, due to their intrinsic anisotropy, optical topological transitions (OTTs)[1], leading to exceptional optical behaviour such as an unbounded density of optical states or to super collimation. This has been demonstrated in applications like improved efficiency in solar energy, which require high-temperature stability.
In recent years, there has been a growing interest in the stability of multilayered HMMs under high-temperature conditions. However, most of these studies have focused on refractory metal-dielectric pairs with applications in the near-IR, while the stability of the metal-semiconductor HMMs showing OTTs in the visible has gathered little attention. In our work, we examine the evolution of Ag-Si layered HMMs as function of thermal treatments. Silver was chosen over other commonly used metals in plasmonics (eg. Au or Al) because thermodynamic considerations point out at higher thermal stability.
The samples were produced by magnetron sputtering and annealed in ultra-high vacuum at temperatures ranging from 200 to 800C. The optical characterization involved Reflectometry measurements with polarized and unpolarized light and was assisted by transfer matrix calculations and finite element modelling. The structural and chemical characterization was carried out by FIB-SEM, XRD and RBS.
We demonstrate that thermal annealing of the HMM induces a multilayer instability. In a first regime, the expected HMM behaviour is observed, with broadband low reflectivity in agreement with our calculations. This optical regime persists even at 300C and although structural degradation can already be measured at that temperature. This initial degradation can be understood as enhanced roughness, and the measured lower reflectance is in agreement with previous numerical studies [3]. At higher temperatures, a second regime involving an order-disorder transition is found. Silver agglomerates inducing phase separation, and optically the system becomes an effective reflector. Finally, in the third regime, the Ag-to-Si ratio starts changing due to silver evaporation. We examine the different regimes of the multilayer instability and the different phenomena that may influence it.
Furthermore, we show that changing the stacking order in unit cell of the HMM directly impacts the onset and pathway of the instability, as well as the transition to the third regime. This behaviour has been found to hold for systems that differ in the bilayer thickness, composition and number of stacked layers, opening the door to enhance the stability of HMMs for applications in harsh environments.
[1] Krishnamoorthy et al. Science 336, 205–209 (2012).
[2] Baranov et al. Nat. Mater. 18, 920–930 (2019).
[3] Andryieuski et al. Opt. Express, OE 22, 14975–14980 (2014).

Diffractive optics, light guide gratings and metalenses are of significant interest for emerging applications including virtual and augmented reality (AR/VR) devices, precision imaging and compact, aberration free optical systems. While great strides have been made in the design and feasibility demonstrations for these devices, significant challenges remain for their high-throughput, cost effective manufacturing. We describe a rapid, reliable and scaleable additive manufacturing process for “printing” these components using a variation of nanoimprint lithography (NIL) and crystalline metal oxide nanoparticle-based inks.
Metalenses are ultrathin planar lenses comprising periodic high aspect ratio and high refractive index nanofeatures that efficiently diffract light. Despite the compact metalens form factor with heights less than a micron, their performance can match and even exceed that of bulkier refractive components. In addition to making lenses more compact and lightweight, metalenses possess more degrees of freedom than traditional optical elements, and can be designed to achieve a desired amplitude, phase, or polarization distributions by tuning size, shape, pitch and material properties. High refractive index (RI) transparent metal oxides such as titania are the materials of choice for practical metalenses as they are optically and mechanically stable and their high refractive index enables the use of nanofeatures with achievable aspect ratios. These materials properties are also sought after for diffractive optics and waveguide gratings, particularly for use in VR headsets, AR glasses, head-up displays, and other optical applications.
Current approaches to all-inorganic metalens fabrication are subtractive in nature and comprise a lengthy sequence of time and materials intensive steps, including many cycles of atomic layer deposition (ALD), electron beam lithography, and reactive ion etching that in all require 10s of hours and must be repeated for each wafer. Here, we report a one-step additive manufacturing process to fabricate metalenses, diffractive optics, and waveguide gratings for visible wavelengths within minutes. Nanostructures with aspect ratios larger than eight and critical dimensions smaller than 60 nm were produced using NIL and a titanium dioxide nanocrystal-based imprint material, resulting in inorganic structures exhibiting a refractive index of n=1.9. RI can be increased to 2.1, when desired, by post processing. The NIL approach requires a silicon master for the desired optical elements, which is fabricated once via subtractive processing. 100s to 1000s of polymer replications of the silicon master can then be created by casting and curing crosslinked siloxane films on the master. Each of those siloxane replicas can then be used as stamps for the additive imprint process for the fabrication of many optical wafers containing arrays of lenses. In this way, the cost of the master fabricated by subtractive processing is amortized over 10,000s to 100,000s of optical wafers instead of being used for the fabrication of each wafer.
As a demonstration, we fabricated metalenses with numerical apertures of 0.2 and focusing efficiencies over 50%. Manufacturability of 400 um diameter lens arrays over large areas was assessed by performing 15 manual imprints in 30 minutes (2 minutes of process time per imprint) with a single stamp. We have used such stamps for greater than 30 imprints while preserving lens quality, paving the way for high-throughput low-cost manufacturing. Metalenses with a diameters of 4 mm, diffractive gratings and lightguides were similarly fabricated.

We have developed a time-domain simulation tool to study the structural viability of ultralight membrane-like laser-driven lightsail space probes, which have been proposed as a technologically viable means of achieving relativistic propulsion for interstellar exploration. [1,2] While recent efforts have discussed the optical and material property requirements for lightsail membranes [3-5], identifying various shapes and nanophotonic designs for beam-riding stability [6-8], considerably less is known about the structural viability of the proposed sailcraft, which must be self-supporting membranes and also capable of carrying a scientific payload. We have identified several routes to achieving structurally stable sailcraft based on the mechanical and optical properties of real materials including Si and SiNx, taking into account the challenge of propelling a lightsail with an inhomogeneous mass distribution resulting from carrying a payload -- without introducing collapse, rupture, overheating, or destabilization of the lightsail membrane. Our results pertain both to shaped specular lightsails as well as flat membranes with self-stabilizing nanophotonic designs.

By modelling lightsails as flexible meshes rather than rigid bodies, we can investigate topics such as shape deformations, tensile strength limits, nonuniform mass distributions, temperature gradients, and internal reflections of light within the sail envelope. For example, we have studied specular curved lightsail shapes such as cones, paraboloids, and hemispheres of up to 10 m2 aperture area, and found that while many can provide beam-riding stability when spin-stabilized on the order of 100 Hz, many such shapes cause reflected light to become highly focused onto other portions of the sail, causing local temperature excursions exceeding 300K above the average sail temperature. Other designs are too weak to allow adequate spin-stabilization or attachment of a discrete payload. We have simulated several payload integration schemes including tethered (trailing) payloads, lumped centrally attached payloads, and distributed payloads, identifying viable approaches for carrying payloads up to 1 g in mass. Such nonuniform mass distributions require careful attention to the lightsail membrane and connecting structures to prevent tensile failure of the membrane at the points of attachment. We will also present our ongoing efforts to develop nanophotonic optical and thermal shielding for sailcraft payloads, which is required to prevent damage to conductors and semiconductor materials in the intense propulsion beam. Our presentation will include a discussion of various design regimes to ensure sailcraft stability and survivability, including simulated trajectories and animations of shape deformations.

<!--[if supportFields]><spanstyle='font-family:"Times New Roman",serif'><span style='mso-element:field-begin'></span><spanstyle='mso-spacerun:yes'> </span>ADDIN EN.REFLIST <span style='mso-element:field-separator'></span></span><![endif]-->1. Lubin, P. JBIS 2016, 69, 40-72.
2. Breakthrough Starshot Initiative. https://breakthroughinitiatives.org/initiative/3
3. Atwater, H. A.; Davoyan, A. R.; Ilic, O.; Jariwala, D.; Sherrott, M. C.; Went, C. M.; Whitney, W. S.; Wong, J. Nat Mater 2018, 17, (10), 861-867.
4. Brewer, J.; Campbell, M. F.; Kumar, P.; Kulkarni, S.; Jariwala, D.; Bargatin, I.; Raman, A. P. arXiv preprint 2106.03558 2021.
5. Ilic, O.; Went, C. M.; Atwater, H. A. Nano Letters 2018, 18, (9), 5583-5589.
6. Ilic, O.; Atwater, H. A. Nature Photonics 2019, 13, (4), 289-295.
7. Swartzlander, G. A. J. Opt. Soc. Am. B 2017, 34, (6), C25-C30.
8. Salary, M. M.; Mosallaei, H. Laser & Photonics Reviews 2020, 14, (8), 1900311.

Even in the 21^st century, bacterial infections are treated empirically. Antibiotics are prescribed preemptively, because a proper diagnosis relies on culture growth which can delay patient treatment, increasing morbidity. Yet the unnecessary administration of powerful, broad-spectrum antibiotics leads to the proliferation of antibiotic resistance, which currently causes over 35k deaths annually in the US alone. In order to reduce the time required for antibiotic susceptibility tests (AST) needed for informed diagnosis, we use spectral data from plasmonic architectures for metabolomic fingerprinting in order to determine phenotypic susceptibility or resistance to antibiotics. Bacterial responses to stress are critical to their survival in dynamic and challenging environments. Stresses can be general, such as from nutrient deprivation, or specific, such as from targeted antibiotic activity. Regardless of source, bacterial stress response results in rapid and profound shifts in metabolite concentrations within cells to maintain homeostasis. This platform complements existing genomic tests that only provide feedback on known antimicrobial resistance mechanisms.
Yet metabolomics approaches introduce an enormous parameter space, e.g., the E. coli metabolome contains over 2600 different metabolites. We overcome this challenge with plasmonic nanogaps with tunable chemistry providing selective interactions with classes of metabolites to rapidly collect large surface enhanced Raman scattering (SERS) data sets on the scale of minutes for analysis with machine learning (ML) algorithms. Just as one can smell the difference between coffee and chocolate amongst multiple odors, SERS+ML rapidly measures and classifies spectral features of bacterial metabolite signatures in response to antibiotics, which are correlated with antibiotic lethality mechanisms. In order to overcome the longstanding challenge to produce large, high quality and reproducible data sets that are needed for ML analysis, electrohydrodynamic flow is used as a driving force to drive chemical reactions between gold nanospheres in a microfluidic cell. The nanogap distance is controllable on the length scale of angstroms, which is critical as the nanogap distance determines performance in SERS devices. It is low-cost, scalable, and reproducible chemical assembly method able to detect individual molecules over mm² areas. Calibration data evaluated in an implemented convolutional neural network (CNN) regression model trained on SERS data of Rhodamine 800 resulted in limits of detection (LOD) and quantification (LOQ) of 10 fM (~10^-5 ng/mL) with prediction accuracy (r² value) of 0.96 over a dynamic range of 6 orders of magnitude.
Analysis of SERS data using a generative model, the variational autoencoder (VAE), is able to identify metabolic fingerprints associated with antibiotic efficacy in the in the cell lysate data. The high interpretability of the VAE generated spectra allows us to identify useful vibrational information which guides additional targeted data collection. Culture-free and easily acquired datasets of bacterial metabolites augment training data to improve predictive models. Greater than 99% accuracy is achieved with unsupervised Bayesian Gaussian Mixture analysis using just a single spectrum from each class representing different antibiotic exposure conditions when using this data informed transfer learning. Our results show differentiation of bacterial populations of ESKAPE pathogens based on antibiotic susceptibility in 10 min when using SERS + ML. This enormously reduces the amount of time needed to validate phenotypic AST with conventional cell growth assays and outlines a promising approach towards rapid phenotypic AST.

High quality factor (“high Q”) cavities have revolutionized information processing, communications, sensing, and nonlinear optics by increasing photon storage times and significantly enhancing light-matter interactions. However, when the size of dielectric cavities is reduced to the nanoscale, resonant modes start to resemble point sources, scattering an incident wave in many different directions. While this scattering has been leveraged to create remarkable metasurfaces that precisely control the phase, amplitude, and polarization of light in an ultrathin footprint, metasurfaces generally exhibit high radiative loss rates and thus low Q-factors. Here, we present a general strategy for crafting high quality factor resonances in a phase gradient metasurface, and apply these results to achieve 1) multiplexed molecular detection of nucleic acids and proteins 2) efficient electro-optic modulation and 3) nanoscale nonlinear and nonreciprocal effects at low power. To create a high-Q metasurface, we introduce subtle structural perturbations to individual resonators to weakly couple free-space light into otherwise bound modes. We experimentally demonstrate control over the quality factor and resonant wavelengths in this scheme, achieving record metasurface Q’s greater than 14,000. We highlight this scheme’s general applicability by designing and fabricating high-Q metasurfaces that act as beamsteerers to different angles, beam splitters, and lenses. Next, we show how high-Q metasurfaces can enable multiplexed nucleic acid and proteomic detection. Our high-Q metasurfaces are functionalized with aptamers to target specific RNA gene sequences and proteins, then illuminated with a laser diode; the scattered intensity provides a quantitative measure of the bound molecular concentration. As a proof-of-concept, we focus on Influenza A, B, RSV, and SARS-CoV-2 genes and proteins; through “multi-color” printing of the probe nucleic acid, we show multiplexed detection of millions of sensors within 15 minutes. Finally, we show how high-Q metasurfaces can be integrated with electro-optic and nonlinear materials for low-power metasurface modulation. Here, an array of uniform lithium niobate-on-Si antennas is individually addressed with an electrical bias, leading to a full 2 pi phase variation. We show how near-continuous beam-steering can be achieved by modifying the bias across each antenna, en-route to fully reconfigurable, solid-state information processing spanning LiDAR, LiFi, AR/VR, and quantum communications.

Looking up to the sky, there are the most powerful renewable heat source and cold sink: The sun and the deep space. In the past decade, both solar heating and radiative cooling have shown immense promise in reducing the demand on fossil fuel-based heating and cooling. For many applications involving varying thermal environments, such as building HVAC and personal thermal management, it is essential to have both heating and cooling functionalities in a dynamic and tunable way. However, such tuning involves ultrawideband (from UV to mid-IR) photonic engineering, and the required properties are the opposite for solar and thermal radiation frequency regimes. Moreover, it is preferable that the tuning is electrical and does not have moving part.
In this talk, I will introduce our recent progress of adaptive solar heating and radiative cooling (A-SHARC) device. We developed two key components to accomplish the A-SHARC: (i) an ultra-wideband transparent conductor and (ii) a reversible electrochemical reaction for tuning the device solar absorptivity and mid-IR emissivity. We showed that monolayer graphene and gold microgrid composite can serve as the ultra-wideband transparent conductor with high transmittance (T_UV-Vis = 85.63%, T_near-IR =87.85%, and T_mid-IR = 84.87%) and low sheet resistance (R_s = 22.4 ohm/sq). Enabled by the transparent conductor, we perform metal electrodeposition to vary the emissivity between 0.12 and 0.94 (Δε = 0.82). By manipulating the interfacial chemistry, we can control the metal morphology to become near-percolating nanoparticle clusters that work as a plasmonic selective absorber for solar heating at low emissivity. When cooling is needed, reversing the bias can dissolve the metal nanoparticles back to visibly transparent state, exposing the underlying mirror for solar reflectivity and electrolyte for high thermal emissivity. The optimal solar absorptivity (α) and thermal emissivity (ε) of solar heating and radiative cooling mode are (α, ε) = (0.60, 0.20), and (0.33, 0.94), respectively. The dual-band solar and mid-IR electrochromic device can bring vast opportunities for applications in heat management, camouflage, display, and building energy efficiency.
Reference: ACS Energy Lett. 2021, 6, 3906−3915. doi.org/10.1021/acsenergylett.1c01486

Equipping heterogeneous catalysts with catalytic functions of natural enzymes can increase the efficiency and selectivity of the catalysts and potentially also enable the enantioselective catalysis of nonbiological reactions. In contrast to the progress made with the pioneering examples of homogeneous and single-atom catalysts afforded by coordination and ligand field tuning, the progress in the rational engineering of metallic and inorganic electrocatalysts for electrochemical reactions has been limited. Representative examples include molecular imprinting on metallic surfaces applied to asymmetric electrosynthesis of optically active alcohols from aromatic ketones and the introduction of organic modifiers in electrocatalysts to provide additional binding interactions for intermediates in the carbon dioxide reduction reaction. On the basis of the current understanding of the importance of local geometry and atomic arrangements of electrocatalysts, we investigate the correlation between metallic surfaces defined by high-Miller-index planes and electrocatalytic selectivity and explore the potential of using kink-controlled nanoparticles as a platform to design catalysts with enzyme-like functions. We aim to control the kink/step density and the three-dimensional (3D) coordination at active kink sites.
Enantioselective oxidation of organic molecules by heterogeneous electrocatalysts is challenging because of the difficulty to control the asymmetric structures of the active sites on the electrodes. Here, we show that the distribution of chiral kink atoms on high-index facets can be precisely manipulated even on single gold nanoparticles; and this enabled stereoselective oxidation of hydroxyl groups on various sugar molecules. As a result, the lower limit of detection for β-D-glucose was 1 nM, a lower value than for any of the synthetic catalysts reported to date, and the device sensitivity was comparable to that of the current sensing device containing glucose oxidase. This study provides a synthetic platform to translate the biological principles of designing geometrically structured active sites to electrochemical devices by controlling nanoparticle kink sites, specifically on concave high-index facets.

Over the centuries, a wide range of optical components has been realized to control intrinsic properties of light waves, including frequency, linear momentum, and polarization. More recently, we see the emergence of flat optical elements termed metasurfaces that can reshape optical fields with the help of dense array of metallic and semiconductor nanostructures. Such structures support strong optical resonances and this has opened up a number of functionalities beyond the capability of their bulky counterparts. However, many fundamental challenges still remain. As one concrete example, it is a formidable task to manipulate optical wavefront at any desired wavelength and incident angle from overlapping beams. If solved, it will enable a new class of optical devices for emerging technologies such as augmented and virtual reality (AR/VR), light detection and ranging (LiDAR), optical computation and information processing, and ultrafast nonlinear optics. Conventional optics (e.g., a lens or a grating) and metasurfaces constructed from low quality factor (Q) plasmonic- and Mie-nanostructures cannot achieve such functions. They perform similar, highly-correlated optical behaviors across broad range of the wavelengths and incident angles.
Precise spectral and angular control has been the domain of photonic crystals, guided-mode resonator (GMR), and bound states in the continuum devices that host nonlocal optical resonances. Their properties have enabled narrow-band optical filters, high-sensitivity sensors, low-threshold lasers, and elements that only shape beams at certain wavelengths and incident angles. However, their nonlocal optical responses at different wavelengths are thus far not fully independent and most of them operate in a static manner. As such, it is highly desirable to realize novel optical systems that can offer fully independent optical functions for different wavelength and incident angle and can facilitate such functions in a dynamic manner. Here, we demonstrate a new class of nonlocal, high-Q metasurfaces that enables 1) fully decoupled optical functions and 2) dynamic wavefront control only at a desired wavelength and incident angle. Our approach is based on a unique design of GMR structures that incorporates atomically-thin, patterned materials as the surface-relief gratings on the top of dielectric slabs. Atomically-thin gratings make the GMR structures exhibit distinct optical transfer functions only for a narrow range of wavelengths and angles and let these functions extremely sensitive to the optical properties of the grating materials.
As the first example, we showcase a highly transparent, rainbow-free diffractive optical element by placing a 3-nm-thick Si diffraction grating on the top of a Si₃N₄ slab waveguide. Spectrally-selective optical absorption properties of the Si facilitate efficient (>10%) resonant diffractions in a near-infrared wavelength (850 nm) as well as highly suppressed rainbow diffraction in the visible wavelengths which is one of the most severe issues in contemporary AR/VR technologies. We also demonstrate an eye tracking prototype that allows precise eye gaze tracking (1° accuracy) as well as significantly suppressed rainbow diffractions below 0.1%. Next, we demonstrate ultracompact, free-space optical modulators based on monolayer transition metal dichalcogenide (TMDC) as a switching medium. TMDC monolayers host strong exciton resonances that enable highly tunable dielectric constant by various external stimuli such as electric bias, local strain, and chemical/optical doping. We fabricate GMR structures with monolayer WS₂ gratings that accomplish high-contrast modulation (from 10% to 50%, and vice versa) of both transmissivity and reflectivity on resonance. The dispersion properties of the fabricated structure manifest the exciton features of the WS₂ monolayer and initial modulation experiments exhibit clear modulation of the optical signal.

Bacterial bloodstream infections (BSIs) account for over 40% of hospital deaths and are one of the most expensive diseases in US hospitals. Unfortunately, while clinical detection requires a concentration of 10^7-10^9 colony forming units (CFU) of pathogens per mL of blood, patients with a BSI typically have only 1-10 CFU/mL. Consequently, diagnosis is slow as hospitals rely on bacterial culturing steps for detection and identification. Raman spectroscopy has shown great promise for sensitive, specific, and rapid bacterial identification. However, one challenge impeding clinical translation of Raman is reproducibility due to challenges in sample handling.

Here, we use acoustic droplet ejection (ADE) to rapidly split a sample of blood spiked with bacteria and infused with gold (Au) nanorods into microdroplets, each containing red blood cells (RBCs), pathogens, and mixtures —to allow for culture-free, sensitive and specific bacterial identification using surface-enhanced Raman scattering (SERS). ADE is a printing method whereby a radio frequency burst signal excitation at a transducer’s resonant frequency generates a focused acoustic beam used to eject a droplet from the free surface of fluid, through radiation pressure at the liquid surface. This is an advantageous printing method for biological samples in that it is nozzle-free, and thus avoids sample contamination, nozzle-clogging, and generation of shear forces on cellular samples. We utilize our custom-built 150 MHz focused transducer to eject ~10 um microdroplets, optimized for cellular diameters. These droplets are ejected downwards onto a gold coated glass slide, fixtured on a motorized xy stage, to generate droplet arrays for analysis. Ejection stability is monitored via a stroboscopic camera setup. In parallel, we synthesize Au nanorods using CTAB and NaOL surfactants with an aspect ratio chosen for NR resonance at 850 nm, optimal for the interrogation of our bacterial samples.

Experiments are conducted with clinically relevant solutions. Bacterial samples are grown to log phase in Lysogeny broth media and washed. These samples are then spiked into a solution of 10% EDTA, washed and purified murine RBCs, and Au nanorods. 200 uL of solution is placed in the ejector and droplets are printed into a grid on a gold coated slide. Single droplet SERS measurements are then each droplet using a 785nm incident laser with 10x magnification. Scanning Electron Microscope (SEM) images of the sample are subsequently collected to confirm the presence of bacteria, RBCs, and nanorods within the droplets. By controlling the concentrations of Au nanorods and by making our substrate hydrophobic, we generate droplets containing bacterial cells with an even coating of nanorods.

With this setup, we obtain identifiable bacterial SERS spectra from our cellular prints with enhancements of ~1000X compared with control samples printed from bacteria-only samples. Furthermore, we isolate the bacterial SERS spectra from a mixed droplet containing both bacteria and red blood cells. Finally, we print these samples onto an agar plate and grow cultures from our droplets, verifying cell viability after printing. In coupling our acoustic printer with a spectroscope, we show cellular SERS measurements generated through ADE of clinically relevant samples, paving the way for rapid, culture-free detection of bacterial bloodstream infections.

Nonreciprocal devices (i.e. optical isolators and circulators) allow light to pass in one-direction only, which is a critical feature for the stable operation of photonic systems. Unfortunately, conventional lorentz isolators remain large (>100um) and are a limiting step in the development of densely-integrated photonic networks. Here, we fabricate and demonstrate high-Q metasurfaces for sub-micron nonreciprocal Raman amplification en route to optical isolation.
In Stimulated Raman Scattering, photons at the pump frequency (omega_p) stimulate emission at the Stokes frequency (omega_s), which results in the amplification of a signal that is injected at the Stokes frequency. We facilitate amplification by fabricating a metasurface with high-Q resonances at both the pump and signal frequencies, resulting in a doubly-resonant high-Q metasurface that more efficiently achieves Raman amplification. Our metasurface is composed of Si disks 540 nm in diameter and 230 nm in height, which support electric (TE) and magnetic (TM) Mie modes. This highly symmetric metasurface geometry also supports antisymmetric counterparts to the ordinary, symmetric TE and TM modes, but these modes are not accessible from free-space radiation, leaving these modes in the dark (i.e. bound in the continuum of free-space radiation). By reducing the symmetry of the metasurface lattice through the introduction of biperiodic diameters (diameter_diskA = 500nm, diameter_diskB = 580nm), the anti-symmetric TE and TM modes become accessible to free-space radiation over a narrow spectral linewidth. As a result, the linewidth of the resonance (and hence its quality-factor) can be controlled by the degree of asymmetry, where smaller asymmetry results in larger quality-factors. Experimentally, we have achieved quality factors exceeding 14,000, which enables us to observe nonlinear mechanisms like Stimulated Raman Scattering at lower power thresholds and in this subwavelength formfactor.
When we illuminate the metasurface using two lasers detuned by the Raman-shift in silicon, we observe Raman amplification in the silicon that is enhanced when illuminating at the metasurface resonance frequency(s). When we consider this process in a circularly-polarized basis, wherein the pump is circularly-polarized and the gain medium (i.e. the silicon metasurface) supports such circulating modes, spin-selection rules arise at the Stokes frequency that break Lorentz reciprocity. This nonlinear process enables nonreciprocal gain for a signal at the Stokes frequency and results in one-way, nonreciprocal Raman lasing in a subwavelength (230nm thick) device layer.

Herein, we report on a novel methodology and characterization of a finely tunable hyperbolic metamaterial (HMM) composed of self-assembled van der Waals crystals of h-BN and graphite. Bulk metamaterials were obtained by sintering well-stacked h-BN/graphite precipitate by spark plasma sintering. We established a facile and large-scale synthetic method of designing and realizing bulk metamaterials. The fabrication process consists of two steps: (1) self-assembly via controlled surface modification of the exfoliated nano building blocks and (2) densification under high pressure and current of nanohybrid powders. The final samples have a thickness of up to centimeters, readily further scalable. Intentionally controlled exfoliation and restacking enable us to obtain the h-BN/graphite HMM, in which building blocks layer thickness are varied systematically. The new hyperbolic metamaterials are composed of the alternating nanolayers of h-BN and graphite/graphene. They display a significant modulation in the position and intensity for both type-I and type-II hyperbolic resonances. Most importantly, their hyperbolic property can be tuned by controlling the thickness of building blocks and their relative concentration in bulk materials. Small amount of rhombohedral BN (r-BN) phase was also found to modulate the hyperbolic dispersions in the bulk phases, in which the r-BN polymorph is introduced by rhombohedral graphite (r-G) phases in graphite layers. This is the first example of bulk hyperbolic metamaterials, of which properties are tuned by controlling the chemical composition and crystal structure. Their permittivity obtained by Kramer-Kronig relation exhibits the capability in negative refraction of incident light and is delicately altered by introduction of graphite layer and r-BN phase. The resulting bulk material also exhibits unique reflectance spectra as a result of distinct interactions with unpolarized and polarized transverse magnetic and electronic beams. The outcome of our work can be a new platform for tailor-made synthesis of various bulk metamaterials without complicated nanofabrication techniques. It also gives an insight to a creation of new hyperbolic materials operating in a desirable range of frequencies.

Surface-enhanced infrared absorption (SEIRA) is a powerful technique for optically detecting and sensing substances of interest with high sensitivity and specificity. To improve SEIRA sensing performance, various types of resonant nanophotonic structures have been employed to enhance light-matter interactions. Such nanophotonic structures are usually designed to channel the energy of the incident light into deep-subwavelength volumes, often referred to as hot spots, to achieve exceedingly high field enhancement. However, the drawback of this commonly used strategy is that the delivery of target analytes into the hot spots becomes increasingly difficult and inefficient as the hot spots are designed to be ever smaller, especially when the hot spot dimensions are comparable to typical sizes of molecules. This drawback associated with deep-subwavelength hot spots has become a key factor that limits the performance of various types of SEIRA sensors in practical application settings. An effective approach to addressing this issue is to deliver the target analytes before forming hot spot structures. For example, metallic nanoparticles coated with analyte thin films can form super-crystals with nanometric gaps which function as the hot spots [1, 2]. Another demonstration of SEIRA sensors based on graphene acoustic plasmon resonators realized effective delivery of analytes into nanometric gaps by first spin-coating the thin analyte film on gold nanoribbons and subsequently transferring graphene onto the analyte film to form the complete SEIRA sensor structure [3]. Although these SEIRA sensor designs and device preparation methods effectively addressed the analyte delivery issue, they involve relatively complex procedures and hence may not be suitable for point-of-care applications.

Here, we demonstrate a high-performance SEIRA sensor based on nano-patch antennas which employ liquid metal (i.e., liquid gallium) as the ground plane. Target analytes are first introduced onto the surface of lithography-defined gold nano-strips on the sensor chip, and then the sensor chip is simply brought into contact with a droplet of liquid gallium at room temperature to form the complete nano-patch antenna structures, so that the target analytes are sandwiched between the liquid gallium and the gold nano-strips and experience highly enhanced electromagnetic field. Our liquid-gallium-based SEIRA sensors demonstrate state-of-the-art sensing performance for nanometric analyte thin films, such as monolayer 1-octadecanethiol (ODT) and thin PMMA films. The operation procedure is simple and suitable for point-of-care applications. Our experimental results also suggest that liquid gallium does not damage or affect the properties of the target analytes. Moreover, the liquid gallium can be conveniently and completely removed from the sensor chip surface, so that the sensors can be reused.

Reference
[1] K. Bian, H. Schunk, D. Ye, A. Hwang, T. S. Luk, R. Li, Z. Wang, H. Fan, Nature Communications 2018, 9, 2365.
[2] N. S. Mueller, E. Pfitzner, Y. Okamura, G. Gordeev, P. Kusch, H. Lange, J. Heberle, F. Schulz, S. Reich, ACS Nano 2021, 15, 5523.
[3] I. H. Lee, D. Yoo, P. Avouris, T. Low, S. H. Oh, Nature Nanotechnology 2019, 14, 313.

Chiral plasmonic nanostructure provides a new route to achieve ultrasensitive characterization of molecular chirality as it can enhance chiral light-matter interactions in the vicinity (local hot-spot) of nanostructures [1,2]. However, these approaches tend to be error-prone due to the spatially varying handedness of local hot-spots and stochasticity of the target molecular orientations, vibrations, and local concentrations [3]. In this regards, applying uniform and strong chiral field offers viable route to overcome these limitations. Previously, we presented a novel gold helicoid nanoparticle with 432 symmetry which can preserve their handedness in 3 dimensions by peptide-assisted method [4,5]. Based on the unique property of 432 helicoid III nanoparticles (helicoids), here, we demonstrate a collective circular dichroism from periodically assembled helicoids (2D helicoid crystal) for highly sensitive and robust enantioselective sensing. The collective resonance [6] of helicoids in crystal results in collective spinning of helicoid dipoles upon circularly polarized light (CPL) illumination and this collective behavior leads uniform boost up of chiral fields on the 2D helicoid surface. The 2D helicoid crystal exhibited highly sensitive response to the molecular chirality change at sub millimolar (mM) scale which was substantiated by distinct polarization resolved color distribution of 2D helicoid crystals in different enantiomeric solutions. Integration of 2D helicoid crystal in microfluidic systems enabled trace sample analysis and practical application of 2D helicoid crystal for the monitoring of chirality alteration in nucleotides (microRNA-21) and proteins (SNARE complexes). We believe that our collective circular dichroism based strategy offers a truly new paradigm and rational design rules of chiral plasmonic structures for robust enantioselective sensing.
References
[1] Tang, Y. et al., Phys. Rev. Lett., 104, 163901 (2010).
[2] Ho, C. S. et al., ACS Photonics, 4, 197 (2017).
[3] Hentschel, M. et al., Sci. Adv., 3, e1602735 (2017).
[4] H.-E. Lee et al., Nature, 556, 360 (2018).
[5] S.W. Im et al., Adv. Mater., 32, 1905758 (2020).

Enhancements in light-matter interactions at the nanoscale have enabled new spectroscopic and nonlinear applications as well as the miniaturization of existing optical devices. Guiding and confining light to sub-wavelength dimensions has become a key focus in nanophotonics, which enables these applications. One strategy to achieve such nanoscale control is to harness hybrid excitations between light and coherent charge oscillations in materials, known as polaritons. While plasmon polaritons can be engineered across a broad spectral range, the short charge carrier scattering times in metals (≈ fs) result in high losses that limit their applicability. In contrast, hyperbolic phonon polaritons (HPhPs) enable strong confinements, low losses, and intrinsic beam steering capabilities determined by the refractive index anisotropy. The discovery of HPhPs in hexagonal boron nitride (hBN) has generated significant interest due to their remarkable optical properties and long lifetimes (≈ ps). Previous studies have predicted the phonon lifetimes in hBN can be increased by monoisotopic enrichment (¹⁰B), however, the measured values at room temperature were significantly lower than the theoretical predictions.

In this study, two scanning-probe techniques: photothermal induced resonance (PTIR) and scattering-type scanning near-field optical microscopy (s-SNOM) are used to map infrared (6.4 µm – 7.4 µm) HPhPs in large (up to 120 μm × 250 μm) near-monoisotopic (> 99 % ¹⁰B) hBN flakes. To avoid interference from polaritons launched from different asperities (edges, folds, surface defects etc.), wide scans are necessary (≈ 40 µm). Translating the real space images in Fourier space (0.05 µm^-1 resolution), we report precise measurements of HPhP lifetimes (up to ≈ 4.2 ps) and propagation lengths (up to ≈ 25 µm). With respect to naturally abundant hBN, we demonstrate an 8-fold improved, record-high (for hBN) propagating figure of merit (i.e., with both high confinement and long lifetime) in ≈ 99 % ¹⁰B hBN.
Approaching the theoretically predicted values, we show that wide near-field scans are critical to enable accurate estimates of the polaritons lifetimes and propagation lengths. Equally, the incidence angle of light, with respect to both the sample plane and the flake edge, needs to be considered to extract correctly the dispersion relation from near-field polaritons maps. Overall, the measurements and data analyses employed here elucidate details pertaining to polaritons propagation in isotopically enriched hBN and pave the way for developing high-performance HPhP based devices.

We have developed a high throughput spectrometric technique addressing single biological entity (bacteria) resolution. This novel technique will be used to develop a novel imaging technique based on mechanical frequency shift of a nanomechanical resonator to generate a mechanical image of single particles and bacteria. The physical principle behind this technique is the modulation of the light absorption by the particle, which is translated into a thermo-mechanical effect on the nanomechanical resonator. This idea was recently demonstrated by using plasmonic gold particles of 100 nm in diameter [1] and to mechanically image viruses and bacteria cells (in press) of 700 nm in diameter. We will show not only the optomechanical coupling that emerges in the cavity formed by a plasmonic nanoparticle onto a free-standing silicon nitride membrane, but also the thermomechanical coupling by using dielectric particles and biological entities. The optical absorption depends on the scatterer material; therefore, it is possible to unambiguously discern in between different dielectric particles and bacteria cells of the same size by simply analyzing the mechanical frequency shift while shinning with a laser [2]. The optical absorption can also be tuned by playing around with the optical resonances of a photonic crystal, which allows to actively tune the mechanical resonance frequency.
We experimentally demonstrate the effect of the localized surface plasmon resonance (LSPR) of a single gold nanoparticle (AuNP) of 100nm in diameter on the mechanical resonance frequency of a free-standing silicon nitride membrane by means of optomechanical transduction. The key effect to explain the coupling in these systems is the extinction cross-section enhancement due to the excitation of the LSPR at selected wavelengths. The extinction is the combination of the absorption and the scattering of the particle; therefore, it becomes a hot-spot on the membrane consequently tuning the mechanical resonance frequency through thermomechanical effects. Despite the low-quality factor exhibited by the broad optical resonances, these systems represent an attractive alternative when compared with other optical cavities in literature because they are easily excited by free-space light beams. This issue is of crucial importance because the free-space optomechanical coupling largely decays when the size of the mechanical system is below the wavelength[3,4], which, is needed to achieve high mechanical frequency regime. In order to validate this coupling, we have developed an interferometric readout system with an integrated tunable laser source, which allows us to perform the first experimental demonstration of nanomechanical spectroscopy of deposited AuNPs, dielectric nanoparticles and biological entities onto the membrane, allowing the differentiation in between single bacteria cells and particles by the frequency shift and polarization sensitivity.
[1] Ramos, D., et al, “Nanomechanical Plasmon Spectroscopy of Single Gold Nanoparticles”, Nano Lett., 18, 7165-7170 (2018).
[2] Ferreiro-Vila, E., et al, “Micro-Kelvin Resolution at Room Temperature Using Nanomechanical Thermometry”, ACS Omega 6 (36), 23052-23058 (2021).
[3] Ramos, D., et al, “Silicon nanowires: where mechanics and optics meet at the nanoscale”, Sci. Rep., 3, 3445, (2013).
[4] Ramos, D., et al, “Optomechanics with Silicon Nanowires by Harnessing Confined Electromagnetic Modes”, Nano Lett., 12, 932-937 (2012).

The elemental sulfur, a by-product from the petroleum refining process, has a great potential for low-cost polymeric mid-wavelength infrared (MWIR, 3-5 μm) optics owing to its high transmittance in the IR spectrum regime. For applying polymeric sulfur to a high-performance optical polarizer, the optical simulation must be preceded for investigating geometries dependent optical properties including the transmission of transverse magnetic field (T_TM) and extinction ratio. However, in the nano-fabrication point of view, the realization of simulated design remains a challenge for polymeric sulfur via thermal nanoimprinting due to the high viscosity of polymeric sulfur in conjunction with the large surface tension of nanostructures originating from their high surface area-to-volume ratio. Herein, we demonstrate high fidelity 1D nano-gratings patterned and spacer thickness controlled polymeric sulfur-based MWIR polarizer (SM-polarizer) as a result of the conjunction with simulation and investigation of processing conditions. Before considering the processing conditions, a theoretical analysis is conducted to investigate optimal spacer thickness, width, height, pitches, and gold layer thickness for high T_TM and extinction ratio. For realizing the simulated geometries of SM-polarizer, we investigate thermal nanoimprinting lithography (thermal NIL) conditions including pressure, temperature, and time for high fidelity nano-feature of SM-polarizer. Then, the spacer thickness, where the Fabry-Perot resonance occurs, is tailored via a spin-coating process by adjusting concentrations of polymeric sulfur solution. Finally, the SM-polarizer demonstrates high T_TM with a high extinction ratio, measured by using the FT-IR method and confirmed by simulations.

Metallic nanoparticles exhibit a plasmonic resonance : they strongly absorb or scatter light around the resonance wavelength. One of their applications is the generation of structural colors [1].
This work aims at a better understanding of the optical properties of disordered assemblies of metallic nanoparticles embedded in a thin film stack, using both experimental and theoretical approaches. Experimentally, monolayers of silver nanoparticles are grown by magnetron sputtering deposition with in-situ monitoring of the nanoparticles growth [2] and their absorption spectra are measured by spectrophotometry. Theoretically, we use analytical models [3] and numerical methods [4,5] to investigate how optical properties are affected by the electromagnetic interaction between the particles and their environment (neighboring particles and interfaces).

References:
[1] A. Kristensen, J.K.W. Yang, S.I. Bozhevolnyi, S. Link, P. Nordlander, N.J. Halas, N.A. Mortensen, Plasmonic colour generation, Nature Reviews Materials, 2 (2016).
[2] Q. Hérault, Mécanismes de croissance des couches minces d’argent par pulvérisation cathodique magnétron, phd thesis (2019)
[3] A. Reyes-Coronado, G. Morales-Luna, O. Vazquez-Estrada, A. Garcia-Valenzuela, R. G. Barrera, Analytical modeling of optical reflectivity of random plasmonic nano-monolayers, Optics Express, 26, 12660 (2018)
[4] M. Bertrand, A. Devilez, J. P. Hugonin, P. Lalanne, K. Vynck, Global polarizability matrix method for efficient modeling of light scattering by dense ensembles of non-spherical particles in stratified media, J Opt Soc Am A, 37, 70 (2020)
[5] R. Lazzari, I. Simonsen, GranFilm: a software for calculating thin-layer dielectric properties and Fresnel coefficients, Thin Solid Films, 419, 124 (2002)

Fogging of surfaces poses a real safety issue for a plethora of applications, such as windows or eyewear. Several approaches have been investigated how this issue can be handled in a passive, energy-neutral way. Previous research has focused on altering the wettability by micro-/nanotexturing surfaces. Another alternative pathway is to exploit the sunlight in order to passively heat the surface. However, solar-based solutions are either broadband absorbing or thick, rendering the surface either intransparent or difficult to integrate into existing systems. Here we show a thin percolating Au-TiO2 metasurface which absorbs nearly 30% of the sunlight over just 11 nm. Based on the percolation effect, the surface remains its transparency in the visible, yet it stronlgy and broadly absorbs in the near-infrared.
Due to its ultrathinness, the metasurface can be readily applied on any—including compliant—substrate and can easily be integrated into existing systems. Its photothermal response allows superior fog prevention and removal. The real-world applicabilty is demonstrated under non-ideal outdoor conditions, showing that the metasurface is ready for industrial implementation.

Optical spectroscopy has been a key characterization technique in a variety of settings from scientific study to industrial and healthcare applications¹. Traditional spectrometers use a dispersive element such as a diffraction grating or prism to achieve an angle-dependent light dispersion, accompanied with focusing optics that focus the incoming light on the detector. This combination of free space elements results in good performance at the expense of bulky size¹. However, in order to proliferate its use and envision regular consumers to achieve in-situ material characterization down to molecular level detection, it is imperative to realize high-performance ultracompact dispersive devices or spectrometers that can be integrated into smartphones². In addition to the traditional performance parameters of benchtop spectrometers such as high resolution, high throughput, and large spectral range, it is vital that these handheld spectrometers also have a wide acceptance angle. Not only is this important for increasing the throughput of the devices, but a large angular tolerance makes the spectrometer more practical by allowing it to tolerate more misalignment between spectrometer and the spectroscopy target . This is crucial for handheld applications so that untrained users can hold the spectrometer in their hands and obtain accurate measurements without observing the same precise alignment practiced in a fixed laboratory setting.
In this work, we have drawn inspiration from the spectrally tunable interlaced nanophotonic facet lenses found on the compound eyes of long-legged flies (Dolichopodidae)³ and realized a wide angle, high resolution light-dispersing structure for ultra-compact photo-spectrometry. Based on the sub-wavelength nanostructures on curved multilayer for wide angle spectral filtering by long-legged flies³, we developed a flat multilayer system with scattering nanostructures to induce similar wide-angle dispersive properties. This bio-inspired dispersive element (BioDE) comprises different sets of 1-dimensional random nanostructures positioned on top of a distributed Bragg reflector (DBR) interference filter. This architecture ensures a large, constant output angle dependent dispersion (via multilayer periodic layers) that is independent of the input angle (via random nanostructures), allowing it to achieve a huge angular tolerance of 30° with significantly larger optical throughput over 10-fold compared to most compact spectrometers. We further engineered the BioDE to disperse light only in the near-infrared (NIR) and transmit visible light without dispersion permitting imaging in the visible. These two features, when combined with an imaging lens and a CMOS sensor, allow the BioDE simultaneously to take a photo in the visible spectrum and perform spectroscopy in the near-infrared (NIR) region in a compact platform. Finally, the BioDE was integrated onto a preexisting CMOS image sensor of a smartphone camera to demonstrate the hybrid functionality of imaging and spectroscopy.
References
[1] Savage, N. Spectrometers. Nat. Photonics 3, 601 (2009)
[2] McGonigle, A. J. S. et al. Smartphone spectrometers. Sensors 18, 223 (2018)
[3] Stavenga, D. G. et al. Photoreceptor spectral tuning by colorful, multilayered facet lenses in long-legged fly eyes (Dolichopodidae). J. Comp. Physiol. A 203, 23–33 (2017)

Classification of features in a scene typically requires conversion of the incoming photonic field into the electronic domain. Recently, an alternative approach has emerged whereby passive structured materials can perform classification tasks by directly using free-space propagation and diffraction of light. In this manuscript, we present a fundamental study of such systems and establish the basic features that govern their performance. We show that system architecture, material structure, and input light field are intertwined and need to be co-designed to maximize classification accuracy. We show that a single layer metasurface can achieve classification accuracy better than conventional electronic linear classifiers, with an order of magnitude fewer diffractive features than previously reported. Furthermore, we find that once the system is optimized, the number of diffractive features is the main determinant of classification performance. The slow asymptotic scaling with the number of apertures suggests a reason why such systems may benefit from multiple layer designs. Finally, we show how the results extend to incoherent light fields, and propose a solution to overcome the inherent linearity in that case.

With Au being one of the most desirable plasmonic metals, it becomes a daunting synthetic challenge to impose a spiral geometry upon a growing nanostructure because the asymmetries of this morphology are in stark contradiction to the underlying symmetries of its fcc crystal structure. Without stringent chemical controls its emergence is no more probable than a five-sided snowflake. Here, we demonstrate a seed-mediated light-driven synthesis in which three sequentially applied symmetry-breaking events drive the nanostructure along a growth pathway yielding helical gold spirals. In doing so, we demonstrate a first-of-its-kind substrate-based synthesis while advancing the techniques, strategies, and mechanistic knowhow needed to direct isotropic materials along growth trajectories yielding tailored chiral geometries.

Dynamic tunablity adds unique functionality to optoelectronic devices. However, achieving dynamic tunability while maintiaining other performance parameters, is a scientifc challenge. In this talk, I will present dynamically tunable structural color dispalys, infrared emission control and infrared detection for practical applications.

Nanostructured materials can exhibit novel properties beyond what is possible in traditional bulk materials due to the increased influence of surfaces, geometry, and interfaces on the fundamental length and time scales intrinsic to energy carriers such as phonons, electrons, and magnons. In particular, phononic crystals—periodic structures with a specific symmetry embedded in an elastic medium—represent a promising route for engineering properties essential for a variety of applications [1]. By tailoring the properties through the nanoscale geometry, this approach has already shown promise for advancing thermoelectrics, interfacial transport materials, thermal diodes, and ultralight high-strength materials. However, experimentally accessing or theoretically predicting the behaviors and properties of complex nanostructured materials with dimensions on ~10s of nanometer scale is challenging, prohibiting the incorporation of promising discoveries into new technologies.
In this work, we nondestructively characterize the mechanical and structural properties of silicon metalattices—artificial 3D solids which are periodic on the sub-100nm length scale—using dynamic extreme ultraviolet (EUV) scatterometry. To fabricate these samples, silica nanosphere templates are first assembled into a colloidal crystal with periodicities on the sub-50 nm scale. The interstitial space between the nanospheres is then infiltrated with silicon, and finally the silica template can be removed, leaving an intricate silicon network of more capacious regions connected by thin necks. Due to the small feature sizes in these systems, we harness tabletop sources of ultrafast, coherent EUV beams provided via high harmonic generation [2] in a pump-probe scatterometry technique. We launch nanoscale surface acoustic waves (SAW) from laser-excited periodic transducer arrays and precisely monitor their dispersion using our dynamic scatterometry tool. By comparing measured frequencies to different finite element models of the metalattice structure, we can simultaneously extract the structural and mechanical properties of these complex, nanostructured materials.
In previous work, we utilized our EUV scatterometry technique to characterize the mechanical and structural properties of silica-filled silicon metalattice films with sphere diameters of 14 and 30 nm [3]. By tuning the wavelength of the launched SAW and measuring the frequency, we extracted the Young’s modulus and thickness of the film, finding the results in agreement with nanoindentation and cross-section electron microscope images. Here, we nondestructively measure the mechanical and structural properties of a low-porosity silicon metalattice film—where the empty metalattice has been re-infiltrated with silicon resulting in a porosity and interconnectivity which cannot be predicted—for the first time. By using unit cell and full structure finite element simulations, we simultaneously extract the Young’s modulus, Poisson’s ratio, thickness, and porosity—the latter being vital to predicting the metalattice properties. We compare our porosity and structural results to those observed in scanning transmission electron tomography reconstructions. Thus, we provide combined and correlative measurements of complex, nanostructured samples with feature sizes on 10-nm scale and below, which are critical for the progress of new nano- and quantum technologies.
[1] Nature 503, 209 (2013). [2] Science 280, 1412 (1998). [3] Nano Lett. 20, 3306 (2020).

In the past decade, optical metamaterials have demonstrated the capability to efficiently manipulate the phase, polarization, and emission of light with the usage of nanostructures – a fundamental shift from conventional refraction and propagation optics. The recent use of epsilon near zero (ENZ) materials has enhanced the capabilities and possibilities of metastructures, leading to unique optical properties such as perfect light absorption, non-reciprocal magneto-optical effects, and large optical nonlinearity. By constructing multilayer ENZ thin films and metal-oxide-semiconductor ENZ heterostructure, broadband and tunable ENZ properties and perfect absorption can be achieved [1]. However, a critical challenge in developing broadband and tunable ENZ metastructures is the complex design of ENZ multilayer stack or heterostructure involving different physical parameters such as ENZ wavelength, thicknesses, optical losses, carrier concentrations, material options, and excitation mediums and angles. Finding the optimal parameters of ENZ heterostructures is a complex issue because changing one parameter adversely affects another.

In this work, we demonstrate the first use of a residual generative neural network [2] to optimize broadband and tunable perfect absorption properties of ultrathin ENZ materials. The code for the neural network is written in PyTorch, a machine learning tool for python, and runs using Google’s high-performance graphics processing units (GPUs). The neural network determines the thickness of the thin film ENZ layer as well as the ENZ material parameters, such as carrier density. From here, a physics simulator returns the absorption, reflectance, and transmittance spectra of the ENZ thin film stack and is subsequently evaluated by a loss function – determining whether the network’s outputs were optimal or not. As the neural network continually trains, the global optimal thickness and materials are outputted by the network. Our initial results using the experimentally obtained ITO material properties demonstrate that the neural network can generate three-stack layers with a maximum absorption above 99.99% and a broad absorption (>80%) bandwidth hundreds of nm wide. These results in machine learning designed ENZ materials and heterostructures will provide broadband and tunable perfect absorption in ultrathin thickness (<10 nm), favorable for minimizing the dimensions in applications such as the absorbing layer in CMOS image sensors, smart-window layers, perfect absorbing layers for spacecrafts, and solar cell generation nanolayers, thus this will make a significant impact on the performance of imaging/display, solar/thermo-photovoltaics, and optical communication technologies.

References
¹ A. Anopchenko, et al.. Field-Effect Tunable and Broadband Epsilon-Near-Zero Perfect Absorbers with Deep Subwavelength Thickness, ACS Photonics 5, 2631 (2018).
²J. Jiang, J. Fan. Multiobjective and categorical global optimization of photonic structures based on ResNet Generative Neural Networks. Nanophotonics, 10, 361 (2021).

To enhance the Raman scattering efficiency of light by molecules, various enhancement techniques relying on either stimulated or surface enhanced Raman scattering (SERS) have been developed. Since the first SERS signal observed from pyridine adsorbed on a roughened silver electrode in 1973, various types of nanostructures for SERS have been designed, for instance, rough metallic surfaces, nanoparticles, and metallic tips. Remarkable enhancement of Raman scattering efficiency has been achieved by using those SERS techniques, but they are either limited by the poor control of the scattered light, narrow bandwidth of the resonance frequency, or restricted area of field enhancement.
In this talk, we present a unique plasmonic waveguide approach to Raman scattering enhancement with precise control of both the incident light intensity and the scattered Raman light. Raman scattering is enhanced in a plasmonic waveguide via two mechanisms. Firstly, the local intensity of the pump light is increase by efficient nano-focussing into a plasmonic slot waveguide with approximately 30% coupling efficiency. Secondly, within the optically confined slot waveguide environment, molecules experience increased local vacuum fluctuations. Since the waveguide presents both nano-focussing and confinement over a broad spectral range, it provides a broadband Raman enhancement.
In particular, we report directional broadband Raman scattering of light by 4-Aminothiophenol (4-ATP) molecules which are chemically bound as a single molecular layer onto well designed plasmonic slot waveguide. The waveguides are decorated with optical antennas that allow light to couple in and out of the slot waveguide with 30% efficiency, evaluated experimentally. Since we observe strong Raman signals from these waveguide, we have been able to spatially resolve the coupling in of the excitation laser and coupling out of Raman scattering. This enables us to precisely investigate how the scattered Raman photons of molecules couple into the waveguide, propagate and couple out via the antennas. In this way, we have experimentally determined the spontaneous Raman β factor in a plasmonic waveguide for the first time. We have demonstrated that 99% of the Raman photons are coupled into the waveguide and coupled out through the antenna pairs of the waveguide.
The high Raman β-factor is due to the largely enhanced spontaneous Raman scattering rate into the waveguide mode. The enhancement mechanism can be understood analogously to fluorescence emission enhanced by the Purcell effect, which is due to increased vacuum fluctuations and increased density of states. While Raman scattering in highly localised metallic hotspots offer high enhancement factors for a few molecules, here, a plasmonic waveguide offers predictable broadband enhancement for many molecules with a greatly improved interaction volume compared to other SERS approaches.

Two bodies at different temperatures experience a radiative heat transfer mediated by the electromagnetic field even when they are separated by vacuum. While this energy transfer is limited by Stefan-Boltzmann's law at large separation distances, in the second half of the 20th century the development of fluctuational electrodynamics has theoretically shown that it can be enhanced in a spectacular fashion in the near field [1,2] (distances in the micron range and below around ambient temperature), in particular in the case of polar materials supporting resonant surface modes of the electromagnetic field such as surface phonon polaritons [2]. Since then, these theoretical predictions have been confirmed by a large number of experiments, involving different geometries and temperature ranges, and stimulating in turn further theoretical investigations (see [4] and references therein).
These developments have paved the way to the exploration of the role played by near-field radiative heat transfer in a variety of applications, including heat-assisted data recording, infrared spectroscopy, energy conversion techniques and thermotronics, i.e. the design of thermal equivalent of circuit elements.
In this context a significant attention has been devoted to the rectification of radiative heat flux in the near field [5], consisting in a configuration implying a strong asymmetry of heat flux when exchanging the temperatures of the two bodies. This behavior has been explored by exploiting phase-change materials [6-14], superconductors [15,16] or many-body effects [17].
In our work we consider a system made of two parallel films, made of silica and vanadium dioxide, a metal-insulator phase-change material with a transition around ambient temperature. We show that, by covering both films with a graphene sheet and by acting on the chemical potential of both graphene sheets, the efficiency of such a device can be strongly modulated and more specifically enhanced by up to 14% at a separation distance of 100 nm. More specifically, our system is expected to reach an efficiency of 85%, among the best recently proposed rectification systems in the same distance and temperature ranges. By studying the dependence on the separation distance and the graphene chemical potential along with the flux spectral features, we highlight the crucial role played by graphene plasmons in the increased coupling between the two films. These results pave the way to the active manipulation of heat flux at the nanoscale.
References
[1] S. M. Rytov, Y. A. Kravtsov, and V. I. Tatarskii, Principles of Statistical Radiophysics (Springer, New York, 1989).
[2] D. Polder and M. van Hove, Phys. Rev. B 4, 3303 (1971).
[3] K. Joulain, J.-P. Mulet, F. Marquier, R. Carminati, and J.-J. Greffet, Surf. Sci. Rep. 57, 59 (2005).
[4] S.-A. Biehs, R. Messina, P. S. Venkataram, A. W. Rodriguez, J. C. Cuevas, and P. Ben-Abdallah, Rev. Mod. Phys. 93, 025009 (2021).
[5] C. R. Otey, W.T. Lau, and S. Fan, Phys. Rev. Lett. 104, 154301 (2010).
[6] P. van Zwol, K. Joulain, P. Ben-Abdallah, and J. Chevrier, Phys. Rev. B 84, 161413(R) (2011).
[7] P. Ben-Abdallah, S.-A. Biehs, Appl. Phys. Lett. 103, 191907 (2013).
[8] W. Gu, G.-H. Tang, W.-Q. Tao, Int. J. Heat Mass Transf. 82, 429 (2015).
[9] Y. Yang, S. Basu, and L. P. Wang, J. Quant. Spectrosc. Radiat. Transfer 158, 69 (2015).
[10] A. Ghanekar et al., Opt. Express 26, A209 (2018).
[11] F. Chen, X. Liu, Y. Tian, and Y. Zheng, Adv. Eng. Mater. 23, 2000825 (2021).
[12] K. Ito, K. Nishikawa, A. Miura, H. Toshiyoshi, and H. Iizuka, Nano Lett. 17, 4347 (2017).
[13] A. Fiorino et al., ACS Nano 12, 5774 (2018).
[14] I.Y. Forero-Sandoval et al., Phys. Rev. Applied 14 034023 (2020).
[15] J. Ordonez-Miranda et al., J. Appl. Phys. 122, 093105 (2017).
[16] E. Moncada-Villa and J. C. Cuevas, Phys. Rev. Appl. 15, 024036 (2021).
[17] I. Latella and P. Ben-Abdallah, Phys. Rev. B 104, 045410 (2021).

Active metasurfaces have been proposed as one attractive means of achieving high-resolution spatiotemporal control of optical wavefronts, having applications such as LIDAR and dynamic holography. Currently, dynamic metasurfaces are faced with two central problems. First, tuning the resonances of meta-atoms does not only affect the phase of the optical wavefront, it also leads to sharp changes in the magnitude of the reflected/transmitted light. Second, most tunable materials exhibit only a modest change in the optical index, and they cannot sufficiently tune the resonances of the meta-atoms over the full, desired 2π range. Consequently, complete tunable coverage of the full 0-2π phase range while maintaining a constant and significant light amplitude has so far remained elusive.

In this presentation, we unveil a novel electrically -tunable metasurface design strategy that operates near the avoided crossing of two resonances, one a spectrally narrow, over-coupled resonance and the other with a high resonance frequency tunability. This strategy displays an unprecedented upper limit of 4π range of dynamic phase modulation with no significant variations in optical amplitude, by enhancing the phase tunability through utilizing two coupled resonances. A proof-of-concept metasurface is illustrated using quasi-bound states in the continuum (qBICs) and graphene plasmon resonances, with results showing a 3π phase modulation capacity with a uniform reflection amplitude of ~0.65.

Two-dimensional transition metal dichalcogenides (2D TMDCs) are a promising materials platform to integrate with nanophotonic structures due to their strong and unique optical responses in the visible spectral range. In particular, their strong exciton resonances are stable even at room temperature and allow facile coupling with resonant semiconductor and plasmonic nanostructures. Furthermore, the optical properties of 2D TMDCs are very susceptible to external stimuli because they are atomically thin. Their exciton resonance location (and hence the optical permittivity) can be controlled by methods such as electrical gating, dielectric screening, and strain. This paves the way towards the realization of practical and tunable nanophotonic devices.
In this work, we investigate the coupling between plasmonic nanostructures and monolayer tungsten disulfide (WS₂) using strain and electrical gating approaches. By straining the monolayer WS₂, we are able to shift the exciton resonance location by more than half of its linewidth (FWHM). Hence, we can detune the spectral location of the exciton resonance from that of the plasmon resonance. Through electrical gating, we control the strength of the exciton resonance. In particular, we have achieved >90% suppression of the exciton resonance. This allows us to actively control the coupling between the two resonances. By comparing experimental results with full field simulations and coupled mode theory, we study the spectral evolution and nature of the coupling in the plasmonic-WS₂ structure. Since we can control the scattering amplitude and/or phase of this coupled system, they can serve as building blocks for tunable nanophotonic devices and metasurfaces. Further understanding of the different coupling regimes will also guide the design of optical devices with different functionalities such as photodetectors and modulators.

Vanadium dioxide (VO₂) has been widely exploited for reconfigurable photonics because of the large changes in its optical refractive index across its insulator-metal phase transition (IMT) at ~70 °C. The IMT temperature of VO₂ is determined by the stability of electron hybridization, which is sensitive to the strain environment, providing various modulation routes such as strain engineering via substrates, electrical gating, chemical doping, and defect engineering via ion implantation. Previously, we demonstrated that the IMT can be shifted close to the room temperature via introduction of structural defects into the VO₂ film by a high-energy ion implanter. Here, we advance this technique by engineering the defect density of VO₂ using a commercial focused ion beam (FIB) system, enabling us to locally tune the IMT temperature via a single-step mask-free lithography process.

First, we experimentally characterized the dependency of IMT temperature on the FIB fluence. A ~50-nm VO₂ film was deposited by sputtering on a sapphire substrate. Then, twelve 200-by-200-µm regions were irradiated using focused 30-keV Ga ions at room temperature with different ion fluences ranging from 0 to 20×10¹³ cm^-2. Note that we fabricated 200-by-200-µm regions to enable far-field optical characterization. In principle, much smaller resolution in irradiation can be achieved using a commercial FIB system, which we confirmed via microscopic Raman spectroscopic mapping. The modulated IMT temperature and complex refractive indices of all irradiated regions were extracted using a combination of temperature-dependent Fourier-transform infrared (FTIR) reflectance measurements and spectroscopic ellipsometry.
We found that with FIB fluences < ~7×10¹³ cm^-2, the IMT temperature can be continuously lowered by ~15 °C with no substantial changes in the refractive-index contrast between the insulating and metallic phases. For the higher fluences, though the IMT temperature can be further reduced, it is accompanied by a significantly broadened transition width and smaller refractive-index contrast. The low-fluence (i.e., < 7×10¹³ cm^-2) modulation is generally favorable for reconfigurable photonics because it can simply tune the IMT temperature (i.e., the triggering threshold of a VO₂-based device) without trading off the optical contrast between the device states.
As a proof of concept, we designed a dual-band transmission filter. Our filter consists of periodic arrays of gold aperture antennas on top of a 50-nm VO₂ film on a sapphire substrate. The apertures were designed into two sub-groups with different aperture lengths of 0.9 µm and 1.15 µm, thus supporting two resonant transmission modes centered at 3.6 µm and 4.8 µm, respectively. For the pristine VO₂ film (i.e., with no FIB engineering), this filter features two transmission bands when the VO₂ is in the insulating phase, and both transmission bands are simultaneously suppressed when the VO₂ is in the metallic phase. Using FIB irradiation, VO₂ areas adjacent to different sub-groups of gold apertures can be engineered to have distinct IMTs. For example, if the VO₂ areas around the shorter apertures are irradiated with ion fluence of 6.7×10¹³ cm^-2, those areas will be transformed to the metallic phase at ~65 °C while the other VO₂ areas stay in the insulation phase, resulting in suppression of the transmission band at 3.6 µm while leave the other band unaffected—a multi-state optical filter. We fabricated the designed dual-band transmission filter via an e-beam lithography process, and the experimental transmittance spectra are in good agreement with our simulations. FIB-assisted defect engineering on these fabricated filters is in process, and we expect to show experimental characterizations of these optical filters with individually tunable transmission bands in our presentation.

Recent years have witnessed a significantly growing need to develop free-space optical modulators for future optical communications, computer interconnections, and phased-array optical elements. Unfortunately, conventional semiconductors typically show very weak optical modulation effects due to its negligible exciton resonances and very thin accumulation/depletion layers. In sharp contrast, transition metal dichalcogenide (TMDC) monolayers support very strong exciton resonances thanks to its weak dielectric screening, and can be efficiently modulated via electrical gating due to its sub-nanometer thickness. Therefore, TMDC monolayers show great potential to build high-efficiency, ultra-compact free-space optical modulators. However, the sub-nanometer thickness of TMDC monolayers also prevent efficient light-exciton interactions. For this reason, only a ~ 0.5% reflection modulation can be observed for suspended monolayer WS₂ at room temperature.

Here, we demonstrate a monolayer semiconductor free-space optical modulator that affords efficient light modulation by placing a piece of monolayer WS₂ on top of a judiciously engineered silver metasurface gating pad spaced by a 15nm-thick Al₂O₃ gating oxide. The metasurface pad supports surface plasmon polaritons with a periodic perturbation to open the coupling channel with the free-space light. We experimentally show that this platform enables 10% reflection modulation and the modulation on/off ratio reaches 3dB with merely a 3V gating voltage. Consequently, it improves the reflection modulation effect by one and half orders of magnitude compared with the suspended monolayer WS₂. An AC modulation experiment is conducted as well, which verifies the good reversibility of the fabricated optical modulator. We further extend the concept from reflection modulation to phase modulation by designing an asymmetric metasurface grating as the gating pad, where the first order diffraction efficiency of the reflected beam is electrically modulated. We emphasize that the entire fabrication process is cleanroom standard and scalable. The fabrication starts from a millimeter-scale monolayer WS₂ flake exfoliated from bulk crystal, and finally 150 optical modulators are successfully patterned on the single chip.

The observed, significantly enhanced modulation results from designing an optimized photonic environment surrounding the monolayer WS₂. On the one hand, the radiative decay rate of excitons is tremendously enhanced by the Purcell effect through the coupling between the excitons and the surface plasmon resonance supported by the metasurface pad. On the other hand, the total radiative and non-radiative decay rate of the system can be finely tuned by gently modifying the perturbation of the surface plasmon polaritons. Therefore, the critical coupling can be achieved at one designed point, and thus the modulation on/off ratio is maximized. A temporal coupled mode theory is developed to predict the reflection spectrum of the coupled system, and it is in excellent consistence with the full-field simulations. We also build an optical rate-equation model to quantitatively link the modulation to the electrical gating voltage by revealing that the total decay rate of the excitons is doping dependent. Our full-field simulations suggest that a 40% reflection modulation as well as a 10dB on/off ratio can be realized for an optimized device.

These results demonstrate an unprecedented free-space optical modulator platform by merging a highly tunable excitonic monolayer semiconductor in an optimized photonic environment to boost the light-exciton interaction and offer the critical coupling. They can lead to many practical applications, ranging from free-space optical communications to Lidar systems for autonomous driving. The successful integration of nanophotonics with monolayer semiconductor optoelectronic devices further paves the way towards multi-functional, ultra-compact two-dimensional meta-devices.

Flat optics, represented by metamaterials and metasurfaces, foresees a promising route to ultra-compact optical devices. Conventional designs of such meta-structures start with a certain structure as the prototype, followed by extensive parametric sweeps to accommodate the requirements of phase and amplitude of the emerging light. Regardless of how computation-consuming the process is, a predefined structure can hardly realize the independent control over polarization, frequency, and spatial channels, which hinders the potential of metasurfaces to be multifunctional. Besides, achieving complicated and multiple functions calls for designing meta-systems with multiple cascading layers of metasurfaces, which introduces exponential complexity. We have developed a series of deep-learning enabled generative frameworks for the inverse design of plasmonic structures in response to on-demand optical properties, with extended case studies and experimental demonstrations. Moreover, we further present a hybrid deep learning framework for designing multilayer meta-systems with multifunctional capabilities.
Metasurfaces composed of meta-molecules with spatially variant building blocks, such as gradient metasurfaces, are drawing substantial attention due to their unconventional controllability of the amplitude, phase, and frequency of light. However, the intricate mechanisms and the large degrees of freedom of the multi-element systems impede an effective strategy for the design and optimization of meta-molecules. We propose a hybrid artificial intelligence-based framework consolidating compositional pattern-producing networks and cooperative coevolution to resolve the inverse design of meta-molecules in metasurfaces. We leverage the proposed framework to design metallic meta-molecules for arbitrary manipulation of the polarization and wavefront of light. Moreover, the efficacy and reliability of the design strategy are confirmed through experimental validations. This framework reveals a promising candidate approach to expedite the design of large-scale metasurfaces in a labor-saving, systematic manner.
We further introduce a deep learning framework for the discovery and design of a multilayer multifunctional meta-system, which is too complicated to be accomplished through conventional design processes. As solid illustrations, we report three examples designed by our framework: a polarization-multiplexed dual-functional beam generator, a second order differentiator for all-optical computing, and a space-polarization-wavelength multiplexed hologram. These functions are barely achievable by single-layer metasurfaces, and the devices are hardly approachable by conventional design means. The designed devices are constituted of arbitrary patterns, which indicates the extremely high degrees of freedom of the structures involved.
The machine learning methods developed here is applicable to the design of other photonic components and systems, including photonic crystals, chip-scale silicon devices, and quantum-optical devices. This design methodology is also significant to other disciplines of natural sciences, such as the design of nano materials, searching for new topological insulators, planning of chemical syntheses, prediction of protein structures, and many more. Consolidating the power of artificial intelligence into scientific research foresees the emergence of novel devices beyond human design capacities, discovers underlying physics from nebulous or counter-intuitive observations, and pushes forward the limits of knowledge in optics, applied physics, and materials science.

Finely tuned optical thin films and metasurfaces allow for precise control of optical phenomena at the light matter interface. Epsilon Near Zero (ENZ) materials with a particular frequency where their electrical permittivity approaches zero can support ENZ modes which leads to large electric field intensity enhancement within the film and perfect light absorption. The field confinement and perfect absorption can be electrically tuned by biasing the ENZ material in metal/insulator/semiconductor heterostructure via the formation of accumulation/depletion layer of the ENZ materials [1,2]. However, the tunability of the ENZ properties is generally limited in most of the ENZ films thicker than 10 nm because of the small Debye length of the active accumulation/depletion region.

In this work, we utilize two-dimensional ITO ENZ material as active material to develop highly tunable perfect absorber in the NIR regime. Previously studied ENZ thin films containing ITO layer thicknesses in the dozens to hundreds of nanometers range result in weak field confinement and electrical tunability. By utilizing a liquid metal printing technique [3], we are able to fabricate monolayers and bilayers of ITO with atomical layer thicknesses of 1.5 and 3 nm respectively, confirmed by atomic force microscopy. We utilize a reflection and transmission setup to characterize the NIR optical properties of bilayer ITO which as of now, has not been reported by any other group. The optical properties of such mono- and bilayers can be tuned throughout the entire ITO layer rather than confining the change in optical properties to just the ITO interface as shown in previous studies [1,2]. Our numerical simulations reveal strong absorption in the telecommunication wavelength due to the ENZ mode excitation in a heterostructure made of ITO bilayer, thin hafnium oxide spacer, and gold electrode. In addition, post-fabrication tuning of absorption wavelength > 100 nm is predicted in the heterostructure with an applied electrical bias of ± 5 V. This study is an important step in developing ultrathin tunable ENZ devices for linear, nonlinear, and quantum phenomena.

References

[1] A. Anopchenko, L. Tao, C. Arndt, H. W. Lee, “Field-effect tunable and broadband Epsilon-near-zero perfect absorbers with deep subwavelength thickness,” ACS Photonics 5, 2631 (2018).
[2] Y.W. Huang et al., “Gate-Tunable Conducting Oxide Metasurfaces,” Nano Letters 16 (9), 5319-5325 (2016).
[3] R. S. Datta et al., “Flexible two-dimensional indium tin oxide fabricated using a liquid metal printing technique,” Nature Electronics 3, 51–58 (2020).

Among magneto optical (MO) metasurfaces, magnetoplasmonic systems based on nano-ferromagnets (FMs) have attracted significant attention thanks to the interplay between the magnetism and the Localized Plasmon Resonances (LPRs) of the metallic nanoparticles. The combination of these two functionalities enhances the magneto-optical response and adds tunability to the plasmonic system.
In the present work we investigate the possibility of using atomic layer deposition (ALD) as a means for creating the ferromagnetic component of magnetoplasmonic metasurfaces. We aim to expand the spectrum of possibilities using the exceptional processing advantages and precision of ALD. Among the advantages are the vast range of materials, the Ångstrom scale precision during deposition, and the compatibility with different substrate types. Besides, ALD layers are covalently bound to the substrate, adding the dimension of enhanced stability and a higher degree of control and tunability of the final properties of the metasurface.
In the case under study, the material of choice is nickel, one of the prototypical materials for magnetoplasmonic applications. Nickel thin films and structures were obtained after ALD of nickel oxide, using Bis(cyclopentadienyl) nickel (Ni(Cp)₂) and ozone, and subsequent reduction to nickel by annealing the substrate in a H₂/N₂ atmosphere.
Nickel nanoarrays were fabricated by initially coating a quartz substrate and subsequent patterning. After the ALD deposition, a negative resist was spin coated, patterned by e-beam lithography (eBL), and finally used as a mask to transfer the pattern (nanodisks) from the resist to the underlying thin film by anisotropic etching. After removing the resist, the nickel oxide nanodisks were converted to metallic nickel by reduction in H₂/N₂.
To characterize the magneto-optical response and the LPRs of the ALD material, nanoarrays with a fixed square lattice and a periodicity of 550 nm were fabricated with different nanodisks diameters ranging from 100 nm to 350 nm. The characterization with a magneto-optical Kerr effect (MOKE) microscope and the measurement of the optical extinction spectra of the plasmonic nanostructures confirmed the suitability of the material for technological applications.
Alternative nanoarrays fabrication routes were also investigated. A bottom-up pathway, starting with the deposition of a positive resist and followed by ALD of the ferromagnet, was developed. This etch-free procedure can be easily integrated into multilayer device fabrication and is compatible with different kinds of substrates among which two-dimensional materials.
To showcase the potential of the bottom-up fabrication, a two-dimensional flake of MoS₂ was patterned by e-beam litography and decorated with ultra-thin nickel magnetic domains using ALD.
TEM analysis of the fabricated samples demonstrated that ALD can be used for the production for novel types of nickel based metasurfaces including two-dimensional nanocomposites.

Conventional optical fibers, which guides light based on total internal reflection between a high-index core and low index cladding, have been studied extensively and used in a broad range of applications, including telecommunication, remote sensing, imaging, spectroscopy, etc. This type of fiber, though robust, still does not transmit information at the maximum theoretically possible speed, due to the high index of the medium that light is propagating through. Epsilon-near-zero (ENZ) materials, which have its real part of the permittivity reduced to zero at a certain wavelength, have been studied for its unique optical properties and multiple potential applications in photonics [1]. Recent studies have shown that ENZ material can be utilized in optical fiber designs to achieve novel properties such as enhanced optical confinement and optical sensing [2].
In this work, we demonstrate a capillary optical fiber design, with a hollow air core where light propagates through, and cladding made of indium tin oxide (ITO) ENZ material. Near the tunable ENZ wavelength, chosen to be at telecommunication wavelength of 1550 nm in this work, the material has vanishingly small permittivity, leading to an index of refraction less than that of air. Thus, this structure creates a similar index profile as a conventional step-index fiber, with a high index core, and a lower index cladding, which allows for total internal reflection. Compared to a traditional optical fiber, a hollow core fiber design offers drastic improvement in speed of transmission, since light propagates faster in air than in glass. Our full-wave electromagnetic simulations show that the addition of a layer of ITO ENZ coating (with experimentally obtained dispersion) can reduce the loss of the waveguide by ~70% compared to the structure without ITO. Furthermore, the loss of the material itself has a large contribution to the overall loss of the hollow core fiber, thus further engineering of ENZ materials with lower imaginary part of the permittivity can further enhance the air-guiding performance of the fiber. In addition, we apply similar structure to existing hollow core fiber designs, such as hollow core photonic bandgap crystal fiber [3], as well as anti-resonant fiber [4]. The resulting spectra show similar behavior to those of the simple capillary fiber, and the performance can also be further improved as the loss of the ENZ material is reduced. This study opens up a new pathway for novel hollow core fiber designs, and may find potential applications in in-fiber imaging, sensing, etc.
References
1. Liberal, I.; Engheta, N. Nature Photonics, 11(3), 149 (2017).
2. Minn, K.; Anopchenko, A. Scientific Reports 8, (2018).
3. Russell, P. Science 299(5605), 358-362 (2003).
4. Wei, C.; Weiblen, R.; Menyuk, C.; Hu, J. Adv. Opt. Photon. 9, 504-561 (2017).

Using large-scale, real-time, quantum dynamics calculations, we present a detailed analysis of electronic excitation transfer (EET) mechanisms in a multiparticle plasmonic nanoantenna system. Our simulations provide a quantum-mechanical description (at an electronic/atomistic level of detail) for characterizing and analyzing these systems, without recourse to classical approximations. We also demonstrate highly long-range electronic couplings in these complex systems and find that the range of these couplings is more than twice the conventional cutoff limit considered by Förster resonance energy transfer (FRET)-based approaches. Furthermore, we attribute these unusually long-ranged electronic couplings to the coherent oscillations of conduction electrons in plasmonic nanoparticles. This long-range nature of plasmonic interactions has important ramifications for EET; in particular, we show that the commonly used “nearest-neighbor” FRET model is inadequate for accurately characterizing EET even in simple plasmonic antenna systems. These findings provide a real-time, quantum-mechanical perspective for understanding EET mechanisms and provide guidance in enhancing plasmonic properties in artificial light-harvesting systems.

Journal of Materials Chemistry C, 6, 5857-5864 (2018)
Journal of Chemical Theory and Computation, 13, 3442-3454 (2017)

Nitrogen vacancy (NV) centers in diamond have emerged as a leading platform for solid-state quantum sensors with coherent times exceeding one second even at room temperature. The ability to optically measure quantities such as electric field, magnetic field, and temperature makes the NV system appealing for a wide range of applications ranging from biomedical devices to geology and navigation devices. A particular area of focus has been on quantum microscopy for wide-field imaging applications, which take advantage of the atomic length scale of the diamond color center. The central challenge remains in maximizing optical signal from the near-surface color centers and optimizing sensitivity per area. We report a quantum sensing metasurface consisting of periodic arrays of plasmonic nanostructures that support plasmonic surface lattice resonances. The metasurface is optimized to readily couple with external radiation and concentrate the optical field within a micron-scale NV layer mediating efficient spin-photon interactions. This plasmonic quantum sensing metasurface (PQSM) achieves shot-noise-limited sensing with a standard camera, eliminating the need of single-photon detectors, and boosts sensitivity via several ways: probing the infrared singlet transition with absorption rather than fluorescence for efficient collection, using homodyne measurements to achieve unity contrast, and scaling to large imaging surfaces. By combining these aspects, our theoretical calculations predict sub-nT Hz^-1/2 sensitivity per 1-μm² sensing area [1]. In this presentation, our recent progress on an experimental demonstration of resonant quantum sensing metasurfaces will be discussed. The projected performance makes the studied PQSM appealing for the most demanding applications such as imaging through scattering tissues and spatially resolved chemical NMR detection.

Reference:
[1] L. Kim, H. Choi, M. E. Trusheim, and D. R. Englund, “Diamond Spin Microscopy on a Plasmonic Quantum Metasurface,” To appear in ACS Photonics (2021).

Tunable optical devices powered by metasurfaces provide a new path for functional planar optics. In particular, lenses with tunable focal lengths could play a key role in various fields with applications in imaging, displays, and augmented and virtual reality devices. Here, we present a method for designing electrically controllable varifocal metalenses at visible wavelengths by incorporating a metasurface designed to focus light at two different focal lengths with liquid crystals to actively manipulate the focal length through the application of an external bias. At a single wavelength in the visible spectrum, using an optimized low-loss a:Si-H we successfully demonstrate a bifocal metalens with a focusing efficiency of ~45% at both focal points, as well as showing proof of electrically tunable imaging. Furthermore, we present a method for designing an RGB achromatic bifocal metalens using a computational algorithm and experimentally realize it through a scalable nanoimprinting technique using a TiO2 nanoparticle embedded resin.

Ultrafast modulation of plasmonic systems heavily rely on the nonlinear optical properties of materials. The third-order Kerr-type nonlinearity, a change in the real and imaginary component of the refractive index that is dependent on the intensity of an excitation signal, especially plays a key role in this scheme. For plasmonic systems in particular, the Kerr-type nonlinearity is highly dependent on the dynamics of hot carriers, defined as high energy electrons and holes generated through the nonradiative decay of plasmon resonances. During the decay of plasmons, high energy carriers with a non-equilibrium energy distribution are formed, which quickly thermalizes through electron-electron scattering. Electron-phonon scattering, followed by the previous sequence, then brings excited electrons to an equilibrium condition with the metal lattice. From a microscopic perspective, the Kerr nonlinearity of an optically excited plasmonic metal indeed follows the characteristic timescales attributed to the formation and relaxation of hot carriers described previously. Understanding these characteristic timescales is thus pivotal for efficient design of active plasmonic platforms.

In this work, we present a comprehensive picture that reveals the interplay of hot-carrier transient dynamics and the linear/nonlinear optical response of a 1D plasmonic crystal. The sensitivity of lattice resonance modes to the in-plane momentum of impinging light allows the spectral tuning of the resonance wavelength. Therefore, exploring the interdependence of hot-carrier relaxation dynamics and active linear and nonlinear resonance properties of the devised plasmonic crystal can be explored over a wide spectral range. Furthermore, spectral tuning allows the observation of interacting resonance modes. This characteristic offers the opportunity to investigate how the interaction between resonance modes modifies the mutual dependence of hot-carrier relaxation dynamics and the optical resonance properties. As prospective applications, we demonstrate the ultrafast control of resonance band separation and coherent control of light attributes through hot-carrier dynamics. We believe our results offer new design principles for designing all-optical modulation based active plasmonic devices.

The subwavelength confinement of optical energy within the nanogap formed between adjacent plasmonic nanostructures leads to a multitude of fascinating properties that act as the foundation for new nanotechnologies. Here, we demonstrate a nanofabrication process for the assembly of substrate-based gold trimers with sub-5 nm air-filled vertical nanogaps. The stepwise process proceeds along a pathway that sees (i) the high-temperature vapor-phase assembly of standalone gold nanostructures, (ii) the formation of a conformal sacrificial oxide layer around each nanostructure using atomic layer deposition (ALD) where the role of the oxide is to precisely define the nanogap width, (iii) the glancing angle deposition (GLAD) of a gold-antimony bilayer that, when heated, assembles into a linear Au-oxide-Au-oxide-Au trimer as the sacrificial antimony layer is lost to sublimation, and (iv) an etching process that selectively removes the oxide to form air-filled vertical nanogaps. The trimers show a high degree of alignment that leads to a polarization dependent extinction spectrum. The work advances the possibility of using low-cost, high-throughput, and scalable vapor-phase techniques to form plasmonic nanostructures separated by nanogaps and, in doing so, provides the building blocks needed to enable on-chip plasmonic devices.

Dynamic control of wavefront shaping is essential for optical technologies spanning communication, computation, and sensing. While metasurfaces promise high-fidelity light control in a highly reduced footprint, most current designs are static and rely on fixed geometric patterning that once fabricated cannot flexibly tune the device response.
Here we present high quality (high-Q) factor phase gradient metasurfaces as a route to efficient nonlinear modulation of wavefront shaping devices including beamsteerers and lenses. Incorporating high-Q resonances within the metasurface design enables strong electromagnetic field confinement within individual nanoantennas, without compromising the metasurface transfer function. Further, high-Q metasurface resonances make the constituent nanoantennas more sensitive to subtle changes in refractive index, which improves the modulation of tunable devices via optical nonlinearities without sacrificing device efficiency.
We construct our metasurface lens from a series of nanoscale silicon bars. Including subtle, periodic perturbations to individual nanobars allows guided modes in the silicon to couple to free space radiation. In both theoretical and experimental investigations we have demonstrated Q factors well above 10^3 can be achieved by these guided mode resonances. This further corresponds to near field intensity enhancements in the affected nanobars on the order of 10^4x or more. Using full-field simulations, we show how applying a nonlinear perturbation locally modulates the refractive index and shifts the spectral position of the high-Q resonance within individual nanoantennas. Here we construct a cylindrical metalens with integrated high-Q structuring and investigate the optical Kerr effect as a mechanism for modulating the focal characteristics as a function of power. In computational studies, we achieved shifts in the focal position from 4 um to 6.5 um, as the power was increased from only 0.1 mW/um^2 to 0.5 mW/um^2. Additionally, a 4-fold decrease in the normalized focal intensity with increasing incident intensity is observed at the focal spot. Importantly, this power-limiting metasurface can be readily extended for multi-wavelength operation by integrating additional notches into the adjacent Si bars with varying bar widths. Furthermore, utilizing other nonlinearities, such as electro-optic effects, promises further advancements to this modulation scheme. By biasing across an individual nanoantenna, the spectral shifts of each local resonance also allows us to control the phase and amplitude at a set frequency. This effect can demonstrate a full 2pi phase variation to construct phase gradient transfer functions defined by the applied field, rather than varying the size, spacing, or geometry of the constituent nanoantennas. Our presentation will discuss not only the design but also fabrication and characterization en route to a versatile metasurface platform that can reconfigurably achieve a variety of transfer functions.

Short and coherent electron beams play an increasingly important role as probes of materials dynamics at atomic length (~0.1 nm) and time (~100 fs) scales and are being explored as patterning tools in lithography applications. Here, we demonstrate nanostructured plasmonic photocathodes that enhance and exploit multiphoton photoemission to generate femtosecond electron beams with engineered spatiotemporal characteristics. We present studies of two cathode geometries: nanogroove arrays for high current beams and plasmonic lenses for single or multiple nanoscale beams originating from a flat surface. Photocathodes were fabricated using e-beam lithography and then characterized in a newly commissioned 20 kV DC photogun testbed at Lawrence Berkeley National Laboratory. We first present photoemission characteristics of nanogroove arrays, including measurement of the 4-photon photocurrent yield and asymmetric emittance. We then illustrate characteristics of gold plasmonic lens emitters excited by near-infrared light using electromagnetic simulations, which predict sub-100 nm emitted electron beam size and sub-10 fs emitted pulse durations. Prototypes fabricated using e-beam lithography have sub-nm surface roughness and smooth plasmonic resonance modes characterized by cathodoluminescence spectromicroscopy. Finally, we present preliminary photoemission measurements from plasmonic lens cathodes. These results demonstrate a concept for designing nanoscale and ultrashort electron beams while paving the way for broader investigation of plasmon-enhanced photoemitters for scientific and industrial applications.

Polarization of light is crucial for a wide variety of classical and quantum optics-based applications such as chemical and biological imaging, birefringence sensing and polarization encoding of qubits and implementation of optical quantum computing. Here¹, we demonstrate electronically reconfigurable, diverse broadband polarization conversion at telecom wavelengths with atomically thin van der Waals heterostructures, using trilayer black phosphorus (TLBP) integrated in a Fabry-Pérot cavity. The quasi-1D exciton-dominated over-unity birefringence coupled with the inherent crystal symmetry-dictated anisotropy in TLBP enables a broadband operation window of the entire telecom E, S and C bands (1360-1580 nm), for a critically coupled resonant optical cavity. The strong light-matter interaction of TLBP with the optical cavity mode further enhances the intrinsic optical anisotropy in the complex reflectivity plane and allows generation of polarization states over the entire Poincaré sphere via spectral and input azimuth tuning. In addition, the extreme susceptibility of the exciton binding energy and its Bohr-radius to electrical doping provides near-complete suppression of its dipole oscillator strength, enabling active control of the polarization state across nearly half the Poincaré sphere. Both linear to circular (tunable quarter-waveplate) and linear cross-polarization (tunable half-waveplate) conversion schemes, among numerous other trajectories, were achieved (for example at ~1444 nm) via electrical gating (up to ~8x10¹²/cm²) demonstrating superior control over the reflected ellipticity and azimuthal angles. Specifically, for the aforementioned two cases, the ellipticity was tuned from 0^o to 44^o and the azimuthal was tuned from 24^o to 90^o, outperforming most existing polarization modulators. A two-dimensional control map can thus be employed to program arbitrary polarization on the Poincaré sphere by an appropriate combination of the device orientation and applied voltage, for the same device. Furthermore, we highlight strategies to realize ultra-high efficiency devices with lossless cavities and detuned (to the TLBP exciton) working wavelengths. Our finding of atomically thin devices for high dynamic range polarization modulation highlights the potential of BP as a promising element to miniaturizing polarization-sensitive electro-optic modulators, metasurfaces and spatial light modulators.
1. “Broadband electro-optic polarization conversion with atomically thin black phosphorus” – Biswas et al., Science 374, 448–453 (2021).

Deterministic Inverse Design of Lithography-Free, Tamm Plasmon Thermal Emitters with Multi-Resonant Control

Mingze He^1,6*, J. Ryan Nolen^2,6, Josh Nordlander³, Angela Cleri³, Nathaniel S. Mcllwaine³, Yucheng Tang⁴, Guanyu Lu¹, Thomas G. Folland^{1, 5}, Bennett A. Landman⁴, Jon-Paul Maria³, Joshua D. Caldwell^{1, 4*}
1. Department of Mechanical Engineering, Vanderbilt University, Nashville, Tennessee, USA
2. Interdisciplinary Materials Science Program, Vanderbilt University, Nashville, Tennessee, USA
3. Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania, USA
4. Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, Tennessee, USA
5. Department of Physics and Astronomy, The University of Iowa, Iowa City, Iowa, USA
6. These authors contributed equally

* Email: mingze.he@vanderbilt.edu; josh.caldwell@vanderbilt.edu

The development of cheap and effective light sources in the infrared is highly desired for numerous applications, such as non-dispersive infrared (NDIR) gas sensing. Thus, wavelength-selective thermal emitters (WS-EMs) are of particular interest due to the lack of cost-effective light sources in the mid- to long-wave infrared. Yet, most proposed WS-EMs employ patterned nanostructures, thereby requiring high-cost, low-throughput lithographic methods, and are therefore inappropriate for many applications. An alternative solution is Tamm plasmon polariton (TPP) heterostructures composed of a distributed Bragg reflector (DBR) on a conductor, typically a noble metal, resulting in an absorptive resonance with a high quality (Q)-factor at near-normal incident angles. As only thin-film deposition is required for fabrication, TPP-EMs can be grown at wafer-scale with relatively low cost and minimal fabrication steps, offering a promising candidate for WS-EMs.
Despite the broad potential of TPP-EMs, designs of such structures is challenging, as most applications require simultaneous control over both emission frequencies and corresponding Q-factors, and suppression of emission at other frequencies. Although aperiodic TPP-EMs can potentially fulfill those requirements, the large parameter space required makes the design extremely challenging. Furthermore, in experimental reports only noble metals have been used, which due to the fixed plasma frequency that falls in the visible range, severely restricts the spectral control that can be achieved, while also being incompatible with CMOS processing.
Here we present an inverse-design algorithm to efficiently optimize TPP-EMs comprised of an aperiodic DBR grown on an n-type, In-doped cadmium oxide (CdO) film, offering individual control of multi-peak WS-EMs [1]. We experimentally validate our design approach, and realize single-, dual- and triple-band TPP-EMs over a broad spectral range, with all structures exhibiting excellent agreement between experiments and simulations. Furthermore, we illustrate the design capabilities of CdO-based TPP-EMs by demonstrating the ability to match the resonance frequencies, lineshapes, and amplitudes of arbitrarily shaped spectra, including the ability to define Q-factors at any given frequency (e.g., 27 - 10,117 at 2360 cm^-1). Finally, we stress that such functionality is not possible within noble-metal-based TPP-EMs, but instead is enabled by the broadly tunable plasma frequency of CdO. The combination of our efficient inverse-design algorithm with these material advancements facilitates the realization of cost-effective, wafer-scale, CMOS-compatible, and lithography-free TPP-EMs for numerous applications, including multi-gas NDIR and free-space communications.
[1] He, M., Nolen, J.R., et al. Deterministic inverse design of Tamm plasmon thermal emitters with multi-resonant control. Nat. Mater. (2021). https://doi.org/10.1038/s41563-021-01094-0

Photonic metasurfaces—two-dimensional arrangements of resonant nanoparticles with designer optical responses—have emerged as a game-changer in the field of flat optics [1,2]. However, most metasurfaces exhibit fixed properties after fabrication, and adding tunability expands vastly the scope of the metasurface use scenarios. Here, we review our recent results in metasurfaces tunable at various time scales, from seconds to femtoseconds; see Fig. 1. We start by using liquid crystals as switching agents for metalenses with electrically actuated focal spots at low (< 10 V) voltages and high focal spot tunability [3]. The second part of the talk demonstrates pico- and femtosecond-scale tuning in semiconductor metasurfaces that provide an ultrafast platform for all-optical information processing [4]. Finally, we outline the potential use of tunable metasurfaces in polarization synthesizers, nonlinear and quantum optics, and frequency conversion on demand [5,6].
References
[1] Kildishev, A. V.; Boltasseva, A.; Shalaev, V. M. Planar photonics with metasurfaces. Science 2013, 339, 1232009.
[2] Yu, N.; Capasso, F. Flat optics with designer metasurfaces. Nat. Mater. 2014, 13, 139−150.
[3] Bosch, M.; Shcherbakov, M. R.; Won, K.; Lee, H.-S.; Kim, Y.; Shvets, G. Electrically Actuated Varifocal Lens Based on Liquid-Crystal- Embedded Dielectric Metasurfaces, Nano Lett. 2021, 21, 3849−3856.
[4] Shcherbakov, M. R. et al. Ultrafast all-optical tuning of direct-gap semiconductor metasurfaces, Nat. Commun. 2017, 8, 17.
[5] Karl, N. et al. Frequency Conversion in a Time-Variant Dielectric Metasurface, Nano Lett. 2020, 20, 10, 7052–7058.
[6] Bosch, M.; Shcherbakov, M. R.; Fan, Z.; Shvets, G. Polarization states synthesizer based on a thermo-optic dielectric metasurface, J. Appl. Phys. 2019, 126, 073102.

Thresholdless lasing is a long-sought goal for many applications. Yet, the inherent working principle behind lasers hinders its realization due to the necessity to reach population inversion. A new approach to overcome this limitation is based on exciton-polaritons. These hybrid light-matter excitations can undergo a phase transition to a coherent state, known as Bose-Einstein condensate, removing the requirement for population inversion.

The main limitation in achieving thresholdless exciton-polariton lasing is hindered by non-radiative and radiative losses that are inevitably present in microcavities. In this work we show the achievement of state-of-the-art polariton lasing threshold in a easy to fabricate GaAs/AlGaAs waveguide heterostructure. We focus on reducing the threshold by improving the fabrication technique to limiting exciton non-radiative channels, while at the same time we remove radiative losses using symmetry-protected bound states in the continuum (BIC). Furthermore, we will show how the topological properties of the condensate are transferred to the emitted light, giving rise to beam vortices in the far field.

In conclusion, this work unveils the possibilities introduced by dressed photons coupled to topologically protected states to realize low power and low cost coherent light sources.

Metasurface science is further expanding field of optics and photonics by providing alternative device platform of ultra-compact and multifunctional flat optical devices. Along with the advances in design principles for metasurfaces, the metasurface can realize multi-functionality within a single optical element, in which multiple degrees of freedoms of light can be modulated simultaneously at will.
In this talk, I will introduce metasurface-driven advanced imaging techniques. First, I will show an electrically tunable varifocal metalens that operates at visible wavelengths [1]. By combining a spin-multiplexed metalens with a liquid crystal (LC) cell, we are able to successfully demonstrate active switching between focal planes on the scale of milliseconds. A single meta-atom contains two degrees of freedom of phase information (i.e. geometric and propagation phase). Thanks to its dual phase modulation, a spin-dependent bifocal function is realized. The bifocal metalens is integrated with LC modulator where a circular polarization state of passing through metasurface is determined according to the applied electric bias. Within a millisecond, a focal point can be switched and also it enables electrically-tunable zoom change. Secondly, I will propose a point cloud generating metasurface for advanced 3D depth imaging or LiDAR application [2]. The proposed flash-type LiDAR is able to spread 10K dots over 2π full space with over 80% diffraction efficiency. The unit meta-atom is rigorously designed via convolution theorem at the reciprocal space to gain equal intensity point cloud. With an aid of stereo matching method, we simply capture distorted 3D point clouds and reconstruct 3D object (or depth information of the object). Such beam scanning-free LiDAR method has a variety of advantages like compactness, robustness and faster data processing over scanning-type LiDAR systems.
We believe such metasurface-driven advanced imaging techniques will further accelerate real-life applications such as ultracompact focus-tunable camera systems, indoor LiDAR sensor for face/object/motion recognition [3], advanced augmented/virtual/mixed reality displays [4].
[1] I. Kim* et al., Advanced Science 8, 2102646 (2021)
[2] G. Kim et al., Nature Nanotechnology (submitted)
[3] I. Kim et al., Nature Nanotechnology 16, 508 (2021)
[4] I. Kim et al., Advanced Materials 32, 2070378 (2020)

Plasmonic nanomaterials are expected to be a key technology that will merge electronics, electromechanical devices, and photonics components on the same device. Here we present a novel route for manufacturing plasmonic metamaterials on the surface of crystalline and amorphous SiO₂, ubiquitous materials found in electronics and optics/photonics applications.
Plasmonic nanotemplates were manufactured on crystalline (alpha-quartz) and amorphous (fused silica) SiO₂ substrates using a combination of high-energy ion beam irradiation and chemical etching. Ion irradiations were conducted with 20 MeV Ni⁶⁺ ions and 40 MeV I⁷⁺ ions at different fluences to produce ion track structures. Subsequent dry chemical etching of the ion tracks with HF vapor was carried out, resulting in nanowells of various sizes and morphologies. Nanowell depth, anisotropy, and aspect ratio can be controlled by altering the etching time and etchant concentration. By depositing a colloidal suspension of gold nanoparticles (AuNPs) into the nanowells, a large reinforcement of the plasmon-phonon coupling was observed thanks to the presence of plasmonic hot spots in the interparticle spaces. SEM and AFM microscopy reveal that AuNPs preferentially migrate to and agglomerate in the nanowells, leaving the specimen surface devoid of particles. Linear growth in the Raman scattering intensity as a function of AuNP concentration up was found at low AuNP concentrations. At higher AuNP concentrations, the Raman intensity decreased exponentially following the Beer-Lambert Law. Up to a 200-fold enhancement was observed in the first micron of the material. Furthermore, ellipsometry measurements reveal changes in the effective dielectric function of the surface layer. Our work presents a new route for functionalizing the surface of SiO₂ and producing thin metamaterial layers, the properties of which can be tuned through the choice of ion energy, fluence, beam angle, etching conditions, and nanoparticle deposition.

Metasurfaces have provided a versatile platform for wavefront shaping of light at the deep lateral subwavelength scale with the complementary metal–oxide–semiconductor (CMOS) compatible processes. The variation of the size and shape of the static metasurfaces enables us to assign any desired profile of optical characteristics including the amplitude, phase, and polarization.
Recent studies also have shown that such tuning can be achieved in time-varying fashion, being referred to as reconfigurable/tunable metasurfaces. There has been considerable research that has successfully demonstrated the modulation of the amplitude, phase, and polarization. However, these studies exhibit limited efficiency and purity. For example, although we could adjust the phase profile and accomplish the beam steering for a certain direction, the ratio of the intensities contained in the incident to steered beams is too small to be used in real products, e.g., the efficiency less than 1%. In addition, there is substantial energy leaked into undesired direction, which deteriorates the purity, e.g., the side mode suppression ratio less than 0 dB. Therefore, it is strongly necessary for us not only to conduct research for the general phenomenon, but also to seek the answer to the question of efficiency and purity.
In this presentation, we briefly introduce the history of the tunable metasurfaces and address the origin of the limited efficiency and purity. Then, we suggest our approach to boost up the performance of the proposed tunable metasurface that facilitates the efficiency beyond 50% and the side mode suppression ratio over 10 dB.

Physical Unclonable Functions (PUFs) represent a security primitive that has been introduced two decades ago [1] to obliterate the risks associated with the digital storage of authentication keys. Thanks to their intrinsic structural random disorder and uncontrollable manufacturing variations, a PUF is unique and “unclonable” and can be interrogated following a challenge-response pairs (CRPs) scheme which makes them ideal candidates for non-digital storage of authentication keys. Among the different types of PUFs, optical strong PUFs have been demonstrated as strong photonic materials whose tamper resistance overcomes the attack resilience of their electronic counterpart. However, despite their great potential, the number of Strong Optical PUFs demonstrated so far is still very limited due the material constraints, their response stability and scattering strength [2].
In this scenario, we propose an optically driven reconfigurable strong optical PUFs. By harnessing the light responsive nature of the scattering properties of a dye-doped polymer dispersed liquid crystals (PDLC),we determine the performance of the PUFs in term of stability, and key complexity. The proposed strategy allows to irreversibly reconfigure the scattering potential of the PUF in case of malicious attack. Reconfigurable complex photonic systems thus offer a cheap, reliable and highly entropic media from which cryptographic keys can be extracted on demand for secure authentication protocols based on optical physical functions.

References
[1] Pappu, R., Recht, B., Taylor, J. and Gershenfeld, N., Physical one-way functions. Science, 297(5589), 2026 (2002).
[2] Gao, Y., Al-Sarawi, S.F. and Abbott, D.,. Physical unclonable functions. Nature Electronics, 3(2), 81 (2020).

The research leading to these results has received funding from AFOSR (grant n° FA9550-21-1-0039) and Ente cassa di Risparmio di Firenze (2017/0881).

The development and research on metamaterials opens the doors for futuristic technologies using their custom-designed properties to interact with and manipulate the flow of light. AR glasses, beam-steering antennas and enhanced solar powered systems seem to be in the palm of our hand. The effectiveness of such plasmonic parts depends vastly on the size of the interactive material and research is driving into the direction of fabricating larger connected and homogeneous surfaces of metamaterials, which allows exploitation for custom-designed thermal emission, and photo- and electro-luminescence.
A promising approach for the nano-fabrication of these metamaterials is the well-established block co-polymer self-assembly, where the differently polarized ends of the polymer cause the polymer-solvent system to align in either spheres, lamellas or complex three-dimensional “gyroid” formations. We use various fabrication protocols and polymers to reach the requirements in thickness, surface area and the level of order necessary for bespoke collective emission properties. By using a new solvent evaporation annealing technique involving a high boiling-point additive, birefringent optical properties indicate large-scale reproducible homogeneous network areas beyond previously reported results. The polymer in use is PI-PS-PEO (polyisoprene-b-polystyrene-b-poly(ethylene oxide)), which results in a 3D gyroid network morphology with a unit cell size of approximately 50 nm. Replicating these structures into different metals results in high quality polarizing optical metamaterials with large single crystal domains on the order of 0.5 mm², suitable to be used for polarized fluorescence and sensing applications.
Concurrently, heated plasmonic materials can produce a strongly directional and therefore coherent thermal emission signal. This far-field thermal coherence, which seems to go against conventional wisdom, has been demonstrated in 2002 using phonon polaritons in a SiC grating with a period of about 6.25 μm, which is more than 100x larger than the period of the self-assembled structures.
Here we show different approaches to fabricate and characterize chiral micrometer-scale ordered metamaterials made of platinum, whose thermal emission yields circular polarized light in the near-IR regime.

What we can see with our own eyes or observe using standard optical imaging systems is limited to a small fraction of the information that the detected light actually carries. Two-dimensional (2D), flat images, such as a photo, only reveal the intensity and color of the light coming to us from an optical scene. However, light contains a wealth of information on the three-dimensional (3D) position, angle of incidence, spectral context, coherence, amplitude, phase, polarization, optical angular momenta, among others. In fact, if light interacts with media, nature will give us structured light [1] spatially varying in the named properties depending on the interaction. Different techniques nowadays allow for the analysis of some of these properties, e.g. digital holographic phase contrast microscopy, but they require complex experimental systems with bulky non-versatile optical components and/or data post-processing. Especially, 3D non-paraxial structured light could not be made available so far, thus, crucial information about the nature of interacting media stays hidden. The difficulty, in this case, lays in capturing not only the nanoscale complexity but also the 3D polarization nature of light, i.e., significant contributions of longitudinal in addition to transverse electric field components. The ability to access these properties could have a transformative impact on the development of imaging systems for today’s world, as in robotics, surveillance, augmented/virtual reality, (bio)medicine, or optical fields as nanophotonics.
To extract the full electric field information of non-paraxial structured light to advance today’s imaging technologies, recent advances in nanophotonics can provide promising innovative tools and methods to perform this task. Following this path, we create a molecular nanosensor which is able to reveal the typically invisible non-paraxial field properties of light by a single camera shot [2]. Our sensor is formed by a functional monolayer of self-assembled fluorescent sulforhodamine B silane produced on a silica glass cover slip. The arranged dipoles in the monolayer interact with a spatially varying non-paraxial electric field, giving a characteristic response which is sensitive to the amplitude, phase, and 3D polarization of light.
To demonstrate the capability of our approach, we introduce non-paraxial field customization by tightly focusing paraxial structured laser light fields [2,3]. We tailor these paraxial input fields by an amplitude, phase, and polarization modulation system [4] with a phase-only spatial light modulator as key component. This customization approach allows providing exemplary non-paraxial fields for our investigation on demand. We numerically as well as experimentally demonstrate the resulting, characteristic response of our monolayer sensor to these fields, successfully visualizing the non-paraxial field properties.
In an inverse approach, taking advantage of on-demand light fields, we can apply customized non-paraxial fields for selective dipole excitation. By adapting the input field properties, we are able to adjust the dominating direction of electric field oscillation and match it to the orientation of the selected dipole. Thus, our approach holds the potential for the in-depth analysis of, for instance, plasmonic nanostructures, 2D materials, or single quantum emitters.
[1] H. Rubinsztein-Dunlop et al. J. Opt. 19 (2017), 013001; E. Otte, Structured Singular Light Fields, Springer (2021).
[2] E. Otte et al., Nat. Commun. 10.4308 (2019).
[3] E. Otte et al., Opt. Express 25 (2017), 20194.
[4] C. Alpmann et al., Sci. Rep. 7.8076 (2017).

Space-Time metamaterials are a family of engineered structures in which some of the materials parameters can change in time instead of (or in addition to) changing in space. Such four-dimensional metastructures offer exciting possibilities for light-matter interaction. In this talk, I will present some of our most recent results in this topic, and will discuss some of their salient features with potential applications in temporal imaging, temporal beaming, nonreciprocity, and temporal deflections. Some of the fundamental aspects of wave interaction with such media, such as causality, and temporal and spatial dispersions for temporal and spatial boundary conditions will be highlighted. Physical insights into our findings will also be presented.

In recent years, intense research efforts have been undertaken to understand the roles of symmetry and topology in non-Hermitian physics, with photonics acting as a convenient testbed. Manipulating topology in photonics is enticing due to the promise of devices that are robust against defects. Such devices would have behaviors encoded in their topology rather than their geometry. Already, topological photonics in Hermitian systems has yielded remarkable demonstrations of unidirectional edge states and topological photonic insulators. Combining topology and non-Hermitian physics holds further promise for the observation of rich physical phenomena.
The synergy between topology and non-Hermiticity in photonics holds immense potential for next-generation optical devices that are robust against defects. However, most demonstrations of non-Hermitian and topological photonics have been limited to super-wavelength scales due to increased radiative losses at the deep-subwavelength scale. By carefully designing radiative losses at the nanoscale, we demonstrate a non-Hermitian plasmonic-dielectric metasurface in the visible with non-trivial topology. The metasurface exhibits higher-order (>2) parity-time symmetry. The device exhibits an exceptional concentric ring in its momentum space, indicating a non-Hermitian Z₃ topological invariant of V = -1. Additionally, the metasurface can be tuned through a PT phase transition using only the in-plane angle. Fabricated devices are characterized using Fourier-space imaging for single-shot bandstructure measurements. Our results demonstrate the potential of combining topology and non-Hermiticity for nanophotonic devices with a highly tailored directional response and topological protection.

Efficient excitation of a triplet (T₁) state of a molecule has far-reaching effects on photochemical reaction and energy conversion systems. Since optical transition from a ground singlet (S₀) to a T₁ state is spin-forbidden, it is generated via intersystem crossing (ISC) from an excited singlet (S₁) state. Although the ISC is enhanced by utilizing a heavy atom effect, energy loss during ICS, i.e., S₁→T₁ relaxation, is inevitable in the process. Here, we propose a general approach to directly excite a T₁ state from a ground S₀ state via magnetic dipole transition, which is boosted by enhanced magnetic field induced by a dielectric metasurface. The enhanced magnetic field promotes the magnetic dipole transition that allows the transition between different multiplicities (i.e., S₀→T₁). As a dielectric metasurface, we propose a hexagonal array of silicon (Si) nanodisks; the nanodisk array induces strongly enhanced magnetic field on the surface due to the coupled toroidal dipole (TD) resonance.[1] Ru(II) complexes (RuPc(py)₂) are placed on the dielectric metasurface as a benchmark to monitor the S₀→T₁ transition enhancement by the optical resonance of the metasurface. We prove this concept by measuring the phosphorescence (T₁→S₀ transition) spectra under different excitation wavelengths corresponding to the S₀→T₁ energy and demonstrate a large enhancement of the phosphorescence of the molecule by tuning the optical resonance of metasurfaces to the S₀→T₁ transition wavelength.[2]
A Si nanodisk array is fabricated by nanosphere lithography[1] and on top of it, a RuPc(py)₂ doped PMMA layer (30 nm in thickness) is deposited. The metasurface exhibits TD resonance at 805 nm. The PLE spectra detected at phosphorescence emission of the RuPc(py)₂ at 860 nm are recorded for metasurfaces and a reference sample. Compared to the featureless PLE spectrum of the reference sample, RuPc(py)₂ molecules on the metasurface show a distinct PLE peak around 805 nm; the phosphorescence intensity is 35-fold enhanced. The peak corresponds to the TD resonance wavelength of the metasurface. Because 805 nm is very far from the peak of the S₀→S₁ transition (630 nm) of RuPc(py)₂ molecules, the efficient excitation around 805 nm is most likely via the S₀T₁ magnetic dipole transition enhanced by the intense magnetic field under TD resonance. Excitation to the T₁ state by 805 nm light means that a photon energy of more than 400 meV is saved compared to the process involving the ISC. In the presentation, we will further discuss the validity of the model by quantitatively comparing the experimental results with electromagnetic numerical simulations.
[1] H. Hasebe, H. Sugimoto, T. Hinamoto, M. Fujii, Adv. Opt. Mater. 2020, 8, 2001148.
[2] H. Sugimoto, H. Hasebe, T. Furuyama, M. Fujii, Small 2021, 2104458.

Metamaterials, artificial structures that enable various electromagnetic (EM) responses not observable in natural materials, are paid attention owing to their arbitrary responsibility comes from their design. Metamaterial electromagnetic absorbers(1) are widely explored as alternatives of conventional ferromagnetic materials for high frequency (>1 GHz). However, resonant absorption mechanism affords only narrow absorption bandwidth and metamaterial absorbers need to be complex structure with multiple resonances in order to gain broadband absorptive band.
In this work, we report 3D conical helix metamaterials with broadband absorption in GHz region, inspired by similar design for optical region(2). Less than 30% fractional absorption band was demonstrated in numerical simulation and experiment. Geometric parameters of conical helix to obtain both high absorption intensity and broad bandwidth are clarified.

(1) Landy N. I., Sajuyigbe S., Mock J. J., Smith D. R. and Padilla W. Phys. Rev. Lett. 2008, 100, 207402.
(2) Mudachathi R. and Tanaka T. Opt. Exp. 2019, 27, 26369.

Metalenses—flat lenses made with optical metasurfaces—promise to enable thinner, cheaper, and better imaging systems. Achieving a sufficient field of view (FOV) is crucial toward that goal and the subject of much recent work. Here, we show that any wide-FOV lens system must have a minimal thickness, regardless of structural design and material composition. Such thickness bounds originate from the Fourier transform relation between space and angle. By analyzing the transmission matrices of ideal (aberration-free) lenses, we map out the minimal thickness as a function of the FOV, lens diamater, and numerical aperture. This bound is tight, as some inverse-designed multi-layer metasurfaces can approach such minimal thickness. Our transmission-matrix approach can also determine the thickness bounds of other systems beyond lenses. This work provides guidance for the design of future wide-FOV metasurfaces while establishing an intrinsic relation between angular diversity and spatial footprint in multi-mode systems.

The electron transfer kinetics are fundamental for various applications such as supercapacitors, molecular electronics and transistors and is typically studied by techniques like Cyclic Voltammetry CV and Electrochemical Impedance Spectroscopy EIS [1]. Although these classical techniques inherit important information about electron transfer kinetics of the full ensemble of molecules, many questions related to the charge dynamics at the solid-electrolyte interface at nanoscale are still difficult to answer. Therefore, Radio Frequency Electrochemical Scanning Tunneling Microscopy (RF-ECSTM) has been developed as a non-invasive heterodyne capacitive sensing method for electrochemical application with the ability to measure electron transfer dynamics and CV locally with reliable sensitivity to the current down to a few attoamperes (10^{<span style="font-size:10.8333px">-18</span>} A) [2]. The techniques is based on conventional ECSTM, where we additionally apply GHz and kHz frequencies to the conductive tip and substrate, respectively, to extract the local impedances of the sample. Here we report on RF-ECSTM as a tool for investigation of the rate constant of electrochemical reaction on ferrocene monolayers self-assembled on a gold Au (111) substrate. Local EIS spectra were recorded at the oxidation potential and outside the redox window to discard any parasitic impedance contribution from which we extracted the local rate constant of the faradic reaction to be K_ET ≈ 6 x 10^-5 s^-1. To conclude, RF-ECSTM is a versatile technique with the ability to measure the local K_ET at nanoscale with a minimum parasitic contribution comparing to other methods, it also provides a large spectrum of frequencies that might play an essential role in many applications i.e., molecular biophysics, electrochemical catalysis, sensors, transistors and batteries.
[1] Eckermann, A.L., Feld, D.J., Shaw, J.A. and Meade, T.J., 2010. Electrochemistry of redox-active self-assembled monolayers. Coordination chemistry reviews, 254(15-16), pp.1769-1802.
[2] Grall, S., Alić, I., Pavoni, E., Awadein, M., Fujii, T., Müllegger, S., Farina, M., Clément, N. and Gramse, G., 2021. Attoampere Nanoelectrochemistry. Small, p.2101253.

In the last decade, optical nanoantennas have revolutionized light manipulation and control at the nanoscale. Light absorption was initially considered a purely detrimental process, reducing the efficiency of optoelectronic devices. Recently, however, it has attracted growing interest, enabling novel light-energy conversion pathways and offering intriguing opportunities for reconfigurable systems.
In the first part of the talk, I will discuss self-induced optical heating effects in highly absorbing Silicon (Si) and Germanium (Ge) nanoresonators. In particular, I will show recent calculations demonstrating that, due to thermos-optical effects, self-heating can give rise to a complex, non-linear relationship between illumination intensity and temperature, even for moderate illumination intensities relevant for applications such as Raman scattering [1]. Subsequently, I will discuss how self-induced optical heating could be employed in optical devices and metasurfaces. In the second part of the talk, instead, I will discuss the opportunities of localized heating through metallic (plasmonic) nanoantennas. In particular, I will present our recent efforts in reducing the computational cost of photothermal energy dissipation in 3D arrays of nanoparticles and I will discuss the ensuing opportunities for material optimization.
[1] Tsoulos, Tagliabue, Nanophotonics 9 (12), 3849-3861

Tunable metasurfaces, especially with tunable color, are promising for many applications in adaptive optics and sensing when compactness and integration are necessary. Many techniques enable tunability – field-effect, structural phase change, mechanical stretching, electrochemistry, photochromic effects, and so on. However, the tunability with these approaches suffers from either being too slow or too small. For example, the state-of-the-art spatial light modulators (SLMs) using liquid crystals operate at less than 1 kHz. MEMS deformable mirrors are also limited to about 1 kHz. This limitation arises from the fact that the fundamental light-matter interaction in these approaches relies upon individual electrons interacting with photons. This limitation could be overcome by adopting a fundamentally new regime of light-matter interaction. In quantum materials or strongly correlated materials, a photon interacts with many ‘connected electrons.’ Thus, a stimulus such as light interaction can perturb not just a single electron, but the sea of ‘connected electrons’ to result in far greater tunability while still being fast. Here we propose 1T-TaS₂, a 2D quantum material with giant electrical and optical tunability at room temperature, for low-power, MHz-fast tunable meta-devices.
1T-TaS₂ is a quantum 2D material exhibiting a strong charge order even at room temperature. Previously, the electrical properties of 1T-TaS₂ have been shown to greatly change extensively change with various stimuli such as strain, chemical environment, and electrical bias. However, the optical properties remained little known. Recently, we studied the optical properties of 1T-TaS₂ under electrical bias and different illumination conditions. We reported fast tunable optical properties of this material arising from the reorganization of charge density wave (CDW) stacking. Our experimental investigation suggested that CDW domains in 1T-TaS₂ respond to light or electrical bias by switching their stacking configuration in a sub-microsecond timescale. The change in stacking configuration results in a unity-order change in refractive index at room temperature. Such huge tunability at such low-level stimulus (100 mW/cm² ~ 1 Sun) makes 1T-TaS₂ promising for tunable nanophotonic applications, especially on mobile platforms.
The giant index change observed in 1T-TaS₂ results in a huge change in bulk properties when combined with plasmonic/dielectric structures. Plasmon resonances induce strong vertical fields that interact with the change in c-axis permittivity of 1T-TaS₂ and thereby enhance the effects of tunability of bare 1T-TaS₂ films. We experimentally demonstrate a tunable meta-grating fabricated on 1T-TaS₂. The blazed grating shows 100% modulation depth at just 5 mW/cm² intensity of 514 nm wavelength laser. The grating has a bandwidth of 1 MHz and operates reliably in ambient air for at least two continuous days. Further, we designed a Fano-resonant metasurface on 1T-TaS₂ that exhibits a large change in its reflectance spectrum with different illumination intensities. The reflectance change corresponds to the blue and red colors for the 5000 K illumination source. Such MHz-fast, color-changing metasurfaces are promising for many applications including LiFi and 3D displays.

We reveal an unexplored form of tunneling for electromagnetic waves which features opposite behaviors for the electric and magnetic fields, with one turning into a growing field, and the other a decaying field, in a medium that exhibits both ε-µ-zero and bianisotropy. Our work provides a new mechanism for manipulating electromagnetic waves for novel device applications.
Quantum tunneling is one of the most fundamental phenomena in quantum mechanics, which underlies many important phenomena and applications that include photosynthesis, nuclear fusion, quantum computing, and scanning tunneling microscopy. Inside a barrier, the quantum wave loses its spatially oscillatory feature, and becomes a monotonically decaying wave. It generally leads to a transmission that drops exponentially with the height and the width of the barrier. Tunneling is not limited to quantum systems, but a universal phenomenon that governs all forms of waves, including electromagnetic waves, which is a vector wave that contains time-oscillating electric and magnetic fields.
Here, by engineering the complex electromagnetic responses of metamaterials, we show that an incident electromagnetic wave can be manipulated to experience anomalous tunneling accompanied by a growing electric or magnetic field. This phenomenon is based on the combination of two unique electromagnetic properties enabled by metamaterials, ε-µ-zero (EMZ) that decouples the electric field and magnetic field, and bianisotropy that leads to exponential behaviors of the two fields with opposite trends (decay and growth), respectively. when two BEMZ slabs of the same thickness but opposite κ are joined together, depending on the order of their combination, one of the fields (either electric field or magnetic field) can be selectively enhanced at the interface while the other one would be at its minimum, with the matched impedance at the incidence. This observation greatly enriches our understanding of tunneling of waves in electromagnetic systems. Importantly, the concentration of either electric or magnetic field individually provides more freedom for the field manipulation. The proposed modal can be applied to higher dimensions, such as the cylindrical and spherical coordinates for 2D/3D field focusing.

Recent advancements of optically thin metasurfaces enable us to explore and devise the critical limit of light-matter interaction. With the abrupt phase change at the interface of two media at the subwavelength scale, the metasurface can dictate light’s chirality, polarization, beam steering, resonance coupling, and plasmonic super focusing. Out of diverse design, GSP metasurface has gained much attention due to plasmon-enhanced scattering, absorbing, and polaritonic functionality in the optoelectronic device. Many research have been done to tune this metal-dielectric metasurface configuration, yet there has not been a full understanding of how the plasmonic effect of a structured metal and dielectric cavity control evanescent field towards Surface Wave (SW) propagation, Electro-magnetically Induced Absorption (EIA), and SPP generation, especially visible wavelength. In this work, we investigated the coupling effect of the GSPs modes in the case of phase gradient metasurface which consists of the anisotropic metal nanoantenna. With Finite Difference Time Domain (FDTD) simulation, we studied how simple geometric variation of nanoantenna’s length, this asymmetric structure could channel light from broadband (650-850) anomalous beam steering to SW and generate SPP for the normal incidence light at visible wavelength. We also study the band structure and dispersion relation for the unit cell to detect the localized and propagating optical modes. We further relate the result of the wavelength-dependent complex pattern of symmetric and asymmetric SPP mode excitation in the different regions of nanoantenna that originate due to asymmetric structure. The hybridization SPP mode by coupling across the dielectric cavity opens more possibilities of exploring the net effect of surface wave propagation in various dielectric cavity structures and tuning lots of interesting near field phenomena in plasmonic metasurfaces.

The new physics of magic-angle effects in twisted bilayer graphene motivated vast investigation into the flat bands hosted by van der Waals moiré structures. Considering the unique correspondence between condensed matter systems and photonic systems, it is important to study the photonic counterparts of twisted bilayer graphene with potential applications including Bose-Einstein condensation. However, the correlation between photonic flat bands and bilayer photonic moiré systems lacks in-depth formulation, impeding further development of moiré photonics. Here, we report a theoretical model of low-angle twisted bilayer honeycomb photonic crystals with a subwavelength interlayer separation (as a close analogy of twisted bilayer graphene). A coupled mode theory is used to describe the crosstalk among photonic units, followed by a continuum model of optical modes. Using our theoretical description, we discover magic-angle photonic flat bands with spiky photonic density of states. A phase diagram is constructed to correlate the twist angle and interlayer separation dependencies to the photonic magic angles. Full-wave simulation is conducted to verify the theoretical model, resolving that non-Anderson-type light localization exists in the AA regions at the magic angles. Our findings reveal a salient correspondence between fermionic and bosonic moiré systems and pave the avenue toward novel applications through advanced photonic band or state engineering.

As a distinct type of nanocapsules, hollow superstructures of inorganic nanoparticles have attracted increasing attention due to their controllable permeability, convenient functionalization, and efficient surface utilization. Conventionally, they are produced by assembling nanoparticles against expensive sacrificial templates. Herein, a general emulsion-based method is reported to assemble colloidal nanoparticles into submicron hollow superstructures, involving first co-assembly of colloidal nanoparticles with organic additives to form clusters, then overcoating the clusters with a polymer shell, and finally removing the organic additives and redispersing nanoparticles by exposing to a good solvent. The key to the success of this process is the reassembly of nanoparticles against the polymer shells as driven by the capillary force during solvent evaporation, producing hollow superstructures. Such a space-confined assembly process can be well controlled by choice of solvents and their evaporation rate. This general technique provides an open and low-cost platform for creating hollow superstructures of various inorganic nanoparticles, offering many opportunities for exploring unique applications that can take advantage of the collective properties of the constituent nanoparticles and the permeable nanoshell structures.

Metasurfaces offer unique opportunities for light control through dispersion engineering and optical nonlinearities. Here recent advances in metaoptics for pulse shaping (M. Ossiander et al. Nature Comm. 12, 6518 (2021) and spatial light modulation (I.C. Benea-Chelmus et al, Nature Comm. 12, 5928 (2021) are reported.
Transparent materials do not absorb light but have profound influence on the phase evolution of transmitted radiation. One consequence is chromatic dispersion causing ultrashort laser pulses to elongate in time while propagating. We experimentally demonstrated ultrathin nanostructured coatings that resolve this challenge: we tailored the dispersion of silicon nanopillar arrays such that they temporally reshape pulses upon transmission using slow light effects and act as ultrashort laser pulse compressors. The coatings induce anomalous group delay dispersion in the visible to near-infrared spectral region around 800 nm wavelength over an 80 nm bandwidth. We characterized the arrays' performance in the spectral domain via white light interferometry and directly demonstrate the temporal compression of femtosecond laser pulses. Applying these coatings to conventional optics renders them ultrashort pulse compatible and suitable for a wide range of applications.
Tailored nanostructures also provide at-will control over the properties of light using nonlinear optics, with applications in imaging and spectroscopy. Nanomaterials with χ(2) nonlinearities achieve highest switching speeds. Current demonstrations typically require a trade-off: they either rely on traditional χ(2) materials, which have low non-linearities, or on quantum well heterostructures that exhibit a high χ(2) in a narrow band. We have shown that a thin film of organic electro-optic molecules JRD1 in polymethylmethacrylate combines desired merits for active free-space optics: broadband record-high nonlinearity (10-100 times higher than traditional materials at wavelengths 1100-1600 nm), a custom-tailored nonlinear tensor at the nanoscale, and engineered optical and electronic responses. We demonstrated a tuning of optical resonances by △λ = 11 nm at DC voltages and a modulation of the transmitted intensity up to 40%. We realize 2 x 2 single- and 1 x 5 multi-color spatial light modulators and demonstrated their potential for imaging and remote sensing. The compatibility with compact laser diodes, the achieved millimeter size and the low power consumption are further key features for laser ranging or reconfigurable optics.

Chiral structure controlled at nanoscale provides a new route to achieve intriguing optical properties such as polarization control and negative refractive index. However, asymmetric structure control with nanometer precision is difficult to accomplish due to limited resolution and complex processes of conventional methods. In this regards, utilizing chirality transfer occurring at organic-inorganic materials offers viable route to overcome these limitations. Previously we developed a unique synthesis strategy that characteristic of molecule is transferred to gold nanoparticle morphology. Based on the system, here, we demonstrated novel chiral gold nanostructures exploiting chirality transfer between peptide and high-Miller-index gold surfaces. Enantioselective adsorption of peptides results in unequal development of nanoparticle surface and this asymmetric evolution leads to highly twisted chiral element in single nanoparticle making unprecedented 432 helicoid morphology. The synthesized helicoid nanoparticle showed strong optical activity (dissymmetry factor of 0.2 at 622 nm) which was substantiated by distinct transmittance color change of helicoid solution under polarized light. Modulation of peptide recognition and crystal growth enabled diverse morphological evolution and the structural alterations provided tailored optical response, such as optical activity, handedness, and resonance wavelength. We believe that our peptide directed synthesis strategy offers a truly new paradigm in chiral metamaterial fabrication and will be beneficial in the rational design of chiral nanostructures for use in novel applications.

Nanoparticles (NPs) of high refractive index materials supporting Mie resonances in the visible to near infrared range have been attracting much attention as a versatile platform for manipulating light-matter interaction at the nanoscale. By properly assembling high-index NPs, the optical response can be tailored via the near field coupling between NPs. The coupling provides accessible electric and magnetic hot spots in an assembly, and modulates the directionality of the light scattering.
A NP assembly can be produced by top-down microfabrication technology and bottom-up self-assembly technology. The former one has high accuracy and high reproducibility, but reducing the gap between NPs below 10 nm, which is crucial for near field coupling of the resonance modes, is still not straightforward. On the other hand, an assembly of almost touched NPs can be easily produced by the latter process. The research on self-assembly of NPs has been preceded by plasmonic NPs and a variety of structures have been produced with high controllability and reproducibility.
In this work, we develop a process to fabricate linear and zigzag arrays of silicon NPs by using a template-assisted capillary-assembly method. For the successful application of the method for controlled formation of a silicon NP assembly, a high-quality precursor, i.e., an agglomeration-free solution of silicon NPs with uniform size and shape, is crucial. We have developed a solution of perfectly spherical silicon NPs dispersible in alcohol by thermal disproportionation of silicon monoxide at very high temperature (>1475°C) and chemical etching, and by applying density gradient centrifugation process for the size separation [1]. For different length linear arrays of almost touched silicon NPs about 140 nm in diameter produced by the process, we measured angle- and polarization-resolved scattering spectra. We show that coupling of the electric dipole and magnetic dipole resonances between NPs strongly modifies the scattering spectra. For example, coupling shifts the resonance energy of the electric dipole mode, resulting in almost perfect overlap of the electric and magnetic resonances in a linear oligomer of silicon NPs. This appears as the Kerker-type forward scattering near the peak of the magnetic dipole resonance. We will also discuss the dark modes in coupled silicon NP arrays.
[1] Sugimoto, H. et al., Adv. Opt. Mater., 8 (12), 2000033. (2020)

Fluorophores and fluorescent species have been noted in literature to couple to plasmonically active metal nanoparticle films when these species are in their excited state. This coupling event leads to a directly detectable increase in fluorophore intensity known as Metal Enhanced Fluorescence (MEF), which has been noted in both literature by our group as well as others worldwide.
Contemporary research recently has shown the relationship between these coupling event to go beyond just visual enhancement, with a portion of the excited state energy of the fluorescent species being donated to the plasmon, which in turn through a process known as electron hopping is able to transfer that to adjacent particles until a detectable current can be noted across the surface of the film. This process known as Fluorophore Induced Plasmonic Current (FIPC) serves as a means for detection of a fluorescent species without the need for traditionally optical detection schemes.
One of the governing factors determining the possibility of this detectable current is the composition of the metal nanoparticle film. Preliminary research has shown the use of noble metals such as gold and silver as suitable candidates, with the strongest signals being noted from fluorophores that share a strong spectral overlap between the emissive properties of the fluorophore and the absorptive properties of the Film.
My research stems off of this preliminary research and looks at previously unused metals for this FIPC research, such as Aluminum and Copper nanoparticle films, as well as the introduction of mixed metal films that are able to use the properties of multiple nanoparticles to have a strong spectral overlap for a variety of fluorophores. These mixed metal substrates are shown to have a larger working area than single metal films alone, and give a better understanding of the reaction mechanics between this energy transfer.
These mixed metal substrates provide an avenue for the development of various biological and ecological assays that can be performed using fluorescence indicators, but are able to be performed outside of a traditional laboratory setting, providing real-time feedback for testing of biological and environmental samples.

The purpose of this study is to demonstrate plasmonic color modulation for dynamic full-color tuning devices. Active plasmonic color tuning is a technology for generating various colors continuously and instantaneously on a single plasmonic substrate and is expected to be applied to variable color filters, strain sensors, and anti-counterfeiting tags. In this study, we focused on the crystalline Ag nanocube monolayer for active color tuning because the resonance wavelength of hybridization plasmon mode shifts by controlling the inter-cube distance [1]. Moreover, the nanocube shape provides higher plasmon resonance due to the smaller dispersion of electron vibrations than the nanosphere shape, leading to a narrow-band spectrum and vivid color. Here, we proposed the active color tuning by continuously controlling the inter-cube distance of the monolayer using a PDMS stretchable and transparent substrate [2].
Crystalline Ag nanocubes were chemically synthesized by the polyol process [3], and these nanocubes were assembled in monolayer and high density by the Langmuir-Blodgett (LB) method [4]. The average size of the Ag nanocubes was 78.3 ± 4.6 nm, and the average cube density of the monolayer was 57.9 cubes/μm². The monolayer was fabricated in a PDMS stretchable substrate, and the transmission spectra were measured as biaxial stretching the substrate under random polarization. The color of the transmitted light changed continuously from magenta, orange, to yellow with stretching from 0 % to 20 %. We experimentally demonstrated the active plasmonic color tuning of the Ag nanocube monolayer by stretching the substrate. Furthermore, FDTD (Finite-Difference-Time-Domain) simulations were performed to investigate the dependence of the plasmon resonance wavelength on the inter-cube distance of the monolayer. The inter-cube distance of the monolayer increased linearly from 5.8 ± 5.4 nm to 24.8 ± 16.1 nm with 20 % substrate stretching by comparing the experimental results of resonance wavelength shift with FDTD simulation results.
We are currently considering an electrical stretching of the substrate based on MEMS (Micro Electro Mechanical Systems) technology for the practical use of active plasmonic color tuning. The technique that combines the PDMS stretchable substrate with MEMS has already been reported as polymer-silicon hybrid nanophotonic MEMS [5]. By applying this technique, the development of a device to fine control the stretching of the substrate in two dimensions is expected. In this presentation, we would like to show the results of active plasmonic color tuning and the recent progress in electrical stretching of the substrate based on MEMS.
[1] A. Mizuno, et al., Appl. Surf. Sci. 480, 846-850 (2019).
[2] A. Mizuno, et al., ACS Appl. Nano Mater. 4, 9, 9721-9728 (2021).
[3] Y. Sun, et al., Science 298, 5601, 2176-2179 (2002).
[4] J. A. Zasadzinski, et al., Science 263, 5154, 1726-1733 (1994).
[5] Steven C. Truxal, et al., 31st Annual International Conference of the IEEE EMBS Minneapolis, pp. 6693-6695.

The production of solar fuels from H₂O, sunlight, and CO₂ is an intriguing approach to achieve a sustainable society. Surface plasmon resonances (SPR) in metal nanostructures that can harvest sunlight and generate non-equilibrium hot carriers capable of catalyzing chemical reactions offer an appealing material platform for solar fuel generators. Harvesting hot holes in plasmonic-semiconductor heterostructures to drive oxidation reactions and balance the reduction reaction has been especially promising. We realized a Au/p-GaN heterostructure based plasmonic device which can perform photocatalytic water oxidation reaction and CO₂ reduction to CO with high selectivity under solar illumination, without external bias, in a gas phase operation condition. The balanced redox reaction pathways were validated with photoreductive deposition and labeled isotope experiments. We also studied the effect of interfacial layer and co-catalysts incorporation. Extended carrier life time with oxide passivation of surface trap states indicating efficient hot hole injection from Au nanoparticles into p-GaN is supported by transient absorption measurement. Co-catalysts facilitating charge transfer and utilization for catalytic reaction is also examined. An optimized device composed of plasmonic light absorber (Au), hot hole extractor (p-GaN), ultrathin interfacial passivation layer (Al₂O₃), and efficient co-catalysts (Cu) all together result in over 300% enhancement of CO generation rate than the Au/p-GaN case. This result as an outlook shows the plasmonic heterostructure platform can be further improved for self-sustaining artificial photosynthesis.

In this talk, I'll introduce the manufacturing method, allowing for rapid prototyping of optical Fourier surfaces and volumes (OFSs & OFVs). Both surface and volume gratings have long acted as a key role in various AR/VR displays. From a Fourier optical point of view, the sinusoidally modulated surface- and volumetric-index variants can be considered as the ideal requirements for these surface and volume gratings. Recently, surface and volume gratings satisfying these requirements have been conceptualized by OFSs and OFVs, respectively. Unfortunately, conventional manufacturing methods including monolithic lithography are intrinsically unable to address such requirements for OFSs and OFVs. This is because most lithographic methods are only compatible with the developments of the digitated “all” or “nothing” structures. By contrast, the holographic inscription can be a practical platform for the perfect OFSs and OFVs. The directions and degrees of soft mass migrations can be exactly guided by the holographically defined Fourier potentials. As such, the sinusoidally formulated Fourier potentials of the holographically mixed light can be implemented into the inscriptions of surface and volumetric gratings. Since mere illumination of the holographically mixed light is all the required process for the developing OFSs and OFVs, the available throughput can become much beyond what could be achieved with the conventional lithographic ways.

In recent years, it has become possible to manipulate light spectrally and spatially on demand at wavelength scale. Manipulations of ultrashort laser pulses and generation of high-angular momentum light at-source are challenging. In this talk, I will report our recent developments and applications of dielectric metasurfaces in compression of ultrashort laser pulses [1] and generation of high-purity and high-angular orbital angular momentum states [2].
We slow down light propagating in the out-of-plane direction of a dispersion optimized silicon nanopillar array to achieve a coating that compensates the material dispersion of fused silica glass up to ~ 6000 times as thick as the coating itself [1]. The coatings induce anomalous group delay dispersion in the visible to near-infrared spectral region around 800 nm wavelength over an 80 nm bandwidth. We characterize the arrays’ performance in the spectral domain via white light interferometry and directly demonstrate the temporal compression of femtosecond laser pulses. The resulting device operates in a broad spectral band around a tunable central wavelength and shows a significant advance compared to existing bulky pulse compression setups.
In the other hand, we reveal a metasurface-enhanced laser that overcomes a key limitation in lasers: conjugate symmetry of light’s angular momentum. We demonstrate new high-purity orbital angular momentum (OAM) states with quantum numbers reaching 100 and non-symmetric vector vortex beams that lase simultaneously on independent OAM states as much as 90 apart [2]. Our laser conveniently outputs in the visible, offering a compact and power-scalable source that harnesses intra-cavity structured matter for the creation of arbitrary chiral states of structured light.
References:
[1] M. Ossiander et al., Nature Communications 12, 6518 (2021).
[2] H. Sroor et al., Nature Photonics 14, 498–503 (2020).

Spectrally selective devices in the infrared spectral region are key components for the development of modern spectroscopic applications such as infrared sensors, thermal emitters, biomedical analysis or thermophotovoltaic devices among others [1,2]. Nowadays, the technological demands of some of these applications require narrowband devices with a very well-defined spectral resolution. In order to ensure the accurate performance of these devices, an intense research was carried out on tunability mechanisms to control their optical response, leading to mechanical, electrical, optical and thermal tunability approaches. Among these, thermo-optical phase-change materials such as the widely investigated VO₂ or Ge₂Sb₂Te₅ (GST) [3,4], are promising candidates for the development of spectrally selective tunable devices due to the distinctive optical properties they exhibit in their different thermodynamic phases. However, the lossy character that these thermochromic materials display in the infrared spectral region hinder the high spectral resolution of final devices. In this context, alternative low-loss adaptive thermo-optical materials are expected to improve the spectral resolution and outperform devices based on VO₂ and GST. As an example, here we show how the application of specific thermal treatments to narrowband photonic structures based on a material as ubiquitous as Si can lead to the development of highly-accurate spectrally-selective tunable devices with narrow bandwidth operation.
In this work, we fabricated photonic structures that rely on interference effects arising from multiple reflected beams on planar resonant cavities loaded with distributed Bragg reflectors (DBR) [5]. The architecture can be summarized as (Si-SiO₂)-DBR/SiO₂-cavity/LaB₆. By using low-loss materials such as Si and SiO₂ for the DBR, the electromagnetic dissipation losses in the structure can be reduced, thus ensuring devices with narrow spectral bandwidths. Additionally, since the optical properties of the materials play crucial roles in DBR-based structures, by monitoring the thermal-induced changes of phase from amorphous to polycrystalline Si, a continuous modification of the optical properties of the DBR structure can be achieved, resulting in the fine tuning of the resonant wavelength of the final device. This strategy can be applied to many other amorphous materials as an alternative to lossy thermochromic materials. This way, by using low-loss temperature-responsive materials, it is possible to simultaneously achieve both improved spectral resolution and fine spectral tunability for the development of spectrally selective devices capable of answering the highly-demanding spectral requirements of modern spectroscopic applications.
REFERENCES
[1] T. Yokoyama et al, Adv. Opt. Mater. 4, 1987 (2016).
[2] T. D. Dao et al, Adv. Sci. 6, 1900579 (2019).
[3] K. K. Du et al, Light Sci. Appl. 6, e16194 (2017).
[4] S. J. Kim et al, Nanophotonics 10, 713 (2021).
[5] A. T. Doan et al, Opt. Express 27, A725 (2019).

Metallic nanostructures can source, detect and control light through surface plasmons with applications ranging from photocatalysis, biochemical sensors, to light trapping in thin-film solar cells. Although the commonly used plasmonic materials such as gold and silver have shown great optical properties, they experience significant ohmic losses that severely limit the device performances. Sodium is predicted to be an ideal plasmonic material with much lower loss than gold and silver across the whole ultraviolet to near-infrared wavelength regime. Here we describe how sodium nanoparticles can exhibit high-quality plasmonic resonances through the excitation of surface lattice plasmons. We investigate both the sodium nanoparticles (size, array period, unit structure) and the excitation light (polarization, angle of incidence) to manipulate how light interacts with sodium nanoparticles. Specifically, by exciting in-plane and out-of-plane surface lattice plasmons, we obtain resonances with extremely narrow linewidths and localized electromagnetic field with amplified intensities for sodium nanoparticles. Sodium, as a low-cost and low-loss plasmonic material, provides an affordable alternative in a range of plasmon-enhanced applications such as chemical sensing, thin-film solar cells, and photonic circuits.

Background
Effective and accurate screening of oncological biomarkers in peripheral blood circulation plays an increasingly vital role in diagnosis and prognosis. High-sensitivity assays can effectively aid clinical decision-making and intervene in cancer in a localized status before they metastasize and become unmanageable. Unfortunately, current clinical screening methodologies can hardly simultaneously attain sufficient sensitivity and specificity, especially under resource-restrained circumstances. To circumvent such limitations, particularly for cancer biomarkers from early-onset and recurrence, we aim to develop a universal plasmonic platform for clinical applications, which macroscopically amplifies multiplexed fluorescence signals in a broad spectral window readily adapts to current assay setups without sophisticated accessories or expertise at low cost.
Methods
The plasmonic substrate was chemically synthesized in situ at the solid-liquid interface by rationally screening a panel of reducing monosaccharides and tuning the redox reactions at various catalyst densities and precursor concentrations. The redox properties were studied by Benedict’s assay and electrochemistry. We systemically characterized the morphologies and optical properties of the engineered plasmonic Ag structures by scanning electron microscopy (SEM) and spectroscopy. The structure-fluorescence enhancement correlation was explicitly explained by the finite-difference time-domain (FDTD) simulation and a computational model for gap distribution. Next, we established an enhanced fluoroimmunoassay (eFIA) using a model biomarker for prostate cancer (PCa) and validated it in healthy and PCa cohorts. Prognosis was explored in patients subject to surgical and hormonal interventions following recommended PCa guidelines.
Results
The monosaccharide-mediated redox reaction yielded a broad category of Ag structures, including sparsely dispersed nanoparticles of various sizes, semi-continuous nanoislands, and crackless continuous films. Optimal broad-spectral fluorescence enhancement from green to far-red was observed for the inhomogeneous, irregularly-shaped semi-continuous Ag nanoisland substrate (AgNIS), synthesized from a well-balanced redox reaction at a stable rate mediated by mannose. In addition, different local electric field intensity distributions in response to various incident excitations were observed at the nanoscale, elucidating the need for irregular and inhomogeneous structures. AgNIS enabled a maximized 54.7-fold macroscopically amplified fluorescence and long-lasting photostability. Point-of-care availability was fulfilled using a customized smartphone prototype with well-paired optics. The eFIA effectively detected the PCa marker in cell lines, xenograft tumors, and patient sera. The plasmonic platform rendered a diagnostic sensitivity of 86.0% and a specificity of 94.7% and capably staged high-grade PCa that the clinical gold standard test failed to stratify. Patient prognosis of surgical and hormonal interventions was non-invasively monitored following efficient medical interventions. The assay time was significantly curtailed on the plasmonic platform upon microwave irradiation.
Conclusions
By investigating the effects of monosaccharides on the seed-mediated chemical synthesis of plasmonic Ag structures, we deduced that potent multiplexed fluorescence enhancement originated from both an adequate reducing power and a steady reduction rate. Furthermore, the inhomogeneous structure with adequate medium gap distances afforded optimal multiwavelength fluorescence enhancement, thus empowering an effective eFIA for PCa. The clinically validated diagnostic and prognostic features, along with the low sample volume, point-of-care feasibility with a smartphone, and microwave-shortened assay time, warrant its potential clinical translation for widespread cancer biomarker analysis.

Phase change materials (PCMs), featuring various structural and electronic transitions, play an important role in realizing dynamically tunable metadevices. Owing to their large changes in optical properties, GeSbTe (GST) alloys and vanadium dioxide (VO₂) have become the most common PCMs used for tunable optical metadevices. In this work, we present several electrically-tunable active metasurfaces and plasmonic devices based on the phase transition of VO₂ [1-3], which include electrically tunable broadband infrared waveplates with dynamic metasurfaces, continuously and reversibly electro-tunable optical nanoantennas, and electrically-tuned flexible active absorbers. First, we demonstrate the electrically-tunable broadband infrared waveplates by combining phase-change material (VO₂) and dispersion-free metasurface [3,4]. The polarization states are modulated through the electrically-induced phase transition of vanadium dioxide, where the output polarization state can be continuously tuned from horizontal polarization to vertical polarization, or from circular polarization to linear polarization. In the second, we experimentally demonstrate a continuously and reversibly electro-tunable optical nanoantenna, by integrating an asymmetric gold nanodisk dimer array with a vanadium dioxide film. Thirdly, we have demonstrated a metal-flexible PCM(VO₂)-metal infrared meta-absorber with mechanical flexibility and electrical tunability. The investigations here can be applied in dynamic digital displays, optical data storage, imaging sensors, and active wearable devices.
References:
[1] Jia-Nan Wang, Bo Xiong, Ru-Wen Peng, Cheng-Yao Li, Ben-Qi Hou, Chao-Wei Chen, Yu Liu, and Mu Wang, “Flexible Phase Change Materials for Electrically-tuned Active Absorbers”, Small 17, 2101282 (2021).
[2] Jia-Nan Wang, Bo Xiong, Yu Liu, Chao-Wei Chen, Dong-Xiang Qi, Ben-Qi Hou, Ruwen Peng, and Mu Wang, “Continuously and reversibly electro-tunable optical nanoantennas based on phase transition of vanadium dioxide ", New Journal of Physics 23 (2021) 075002.
[3] Fang-Zhou Shu, Jia-Nan Wang, Ru-Wen Peng, Bo Xiong, Ren-Hao Fan, Ya-Jun Gao, Yongmin Liu, Dong-Xiang Qi, and Mu Wang, “Electrically tuning broadband infrared waveplates with dynamic metasurfaces”, Laser & Photonics Reviews 15, 2100155 (2021).
[4] Ren-Hao Fan, Bo Xiong, Ru-Wen Peng, and Mu Wang，“Constructing Metastructures with Broadband Electromagnetic Functionality”, Advanced Materials 32, 1904646 (2020).

There has been plenty of interest and investigation in plasmonic metasurfaces to realize high-performance flat optical components and polarization converter devices [1]. Although, these highly-efficient meta-devices are either mainly pivoted on lossless microwave regimes or focused on a reflection scheme at optical frequencies [2]. Most reported plasmonic metasurfaces are optimized under the realm of highly-radiative lossy electric and magnetic multipoles, which in fact limit their transmission efficiency [3]. As a result, plasmonic metasurfaces seem not very ideal for real applications in particular after introducing their dielectric counterparts. Nonetheless, it has been shown that dielectric metasurfaces cannot interact with the incident light as strongly as plasmonic structures [4]. The requirement of a high aspect ratio in dielectric metasurfaces further devaluates their practical applications.
In this work, we draw our attention to modifying the model that is commonly used for designing a plasmonic metasurface for decades. We innovate a strategy to enhance the transmission efficiency of plasmonic metasurface to the highest attainable level [5]. We take the advantage of Fano coupling between toroidal dipole and toroidal quadrupole that introduces the least radiative loss and strong field confinement as inherently existing in toroidal resonance. With the assistance of relative low-loss toroidal multipoles, a giant cross-polarization converter with a transmission efficiency of 22.9% in a single-layer plasmonic metasurface comparable to the theoretical bound (25%) has been experimentally demonstrated which is the highest available to date. The success of this work paves the way for highly-efficiency plasmonic metasurface components and systems that could be potentially developed for low-profile applications in the real world.
References
[1] A. Ahmadivand, B. Gerislioglu, R. Ahuja, Y. K. Mishra, Laser Photon. Rev. 2020, 14, 1900326.
[2] X. Ding, F. Monticone, K. Zhang, L. Zhang, D. Gao, S. Nawaz Burokur, A. De Lustrac, Q. Wu, C. W. Qiu, A. AlùProf, Adv. Mater. 2015, 27, 1195.
[3] M. Li, L. Guo, J. Dong, H. Yang, J. Phys. D: Appl. Phys. 2014, 47, 185102.
[4] Y. Li, Z. Huang, Z. Sui, H. Chen, X. Zhang, H. Guan, W. Qiu, J. Dong, W. Zhu, J. Yu, H. Lu, Z. Chen, Nanophotonics 2020, 9, 3575.
[5] A. Hassanfiroozi, P.-S. Huang, S.-H. Huang, K.-I. Lin, Y.-T. Lin, C.-F. Chien, Y. Shi, W.-J. Lee, P. C. Wu, Adv. Optical Mater. 2021 https://doi.org/10.1002/adom.202101007

We will present a design approach for complex-amplitude orbital angular momentum metasurfaces and its flexible implementation via 3D direct laser writing. Specific examples include orbital angular momentum holography allowing direct reconstruction of videos via encoding each frame into a different orbital angular momentum mode. A second implementation will focus on the combinatiioin of 3D-printed metasurfaces on the end facets of optical fibres, allowing broadband focusing of optical pulses, as well as optical trapping of nanomaterials and bacteria with a single fibre. The wider potential of such metasurfaces for applications in sensing and energy conversion will be discussed.

Broadband perfect absorbers in the visible region have attracted considerable attention in many fields, especially in solar thermophotovoltaic and energy harvesting systems. However, developing light absorbers with high absorptivity, thermal stability, and a broad bandwidth remains a great challenge. Here, we theoretically and experimentally demonstrate that a titanium nitride metasurface absorber exhibits broadband absorption with an average absorption of more than 92 % over a wavelength range of 400 nm to 750 nm. The increase in absorption is attributed to the localized surface plasmon resonance (LSPR). We demonstrate the plasmon-enhanced visible-light-driven hydrogen production from water using a polymer photocatalyst integrated with a TiN metasurface absorber. A 300 % increase in the hydrogen evolution rate was observed due to the LSPR that enhances the rates of light absorption, carrier separation, and hot-carrier transfer in polymer photocatalyst. These results enable a new approach to preparing high-efficiency solar energy harvesting systems. The concept is extensible to other types of photocatalyst such as 2D materials. In the end, we will also discuss the outlook for refractory metasurface in applications of solar energy harvesting systems.

Meta-lens is a new type of optical device that has risen in recent years.1 Meta-lenses compose of artificial nanostructures can manipulate the electromagnetic phase and amplitude. Meta-devices show excellent performance and novel applications to meet the optical demands2,3. The great advantages of meta-lenses are their new properties, lighter weight, small size, high efficiency, better performance, broadband operation, lower energy consumption, data volume reduction and CMOS compatibility for mass production. The meta-devices will come into our real-life soon because novel advantages of meta-devices are their novel properties, customizable, small size, lightweight, high efficiency, better performance, broadband operation, lower energy consumption, and Semiconductor process compatibility for mass production.
We inspired and learned from nature to develop the meta-lens array for intelligent imaging and sensing. The design, fabrication, and applications of the intelligent meta-lens array are reported in this talk. A 60 × 60 broadband achromatic optical meta-lens array is designed and fabricated for working in the whole visible range. We developed the meta-lens array based light field imaging system for digital focusing, full-color imaging, depth sensing for static and dynamic objects4, and 1D to 3D edge detection5. The achromatic meta-lens array can collect high dimensional light field information of objects in the scene. The imaging, depth sensing, edge information can be extracted by the imaging computing of the light field raw image formed by the achromatic meta-lens array. The high dimensional optical information of objects can be reconstructed slice by slice from a series of rendered images with different depths of focus. This research shows the importance of optical meta-devices for next-generation optical imaging and sensing. We believe this is an important milestone of the research on intelligent meta-devices opens up an avenue for future applications of optical devices in micro-robotic vision, unmanned-vehicle sensing, virtual and augmented reality, drones, and miniature personal security systems.
(1) Chen, M. K.; Wu, Y.; Feng, L.; Fan, Q.; Lu, M.; Xu, T.; Tsai, D. P. Principles, Functions, and Applications of Optical Meta-Lens. Advanced Optical Materials 2021, 9 (4), 2001414.
(2) Wang, S. M.; Wu, P. C.; Su, V. C.; Lai, Y. C.; Chen, M. K.; Kuo, H. Y.; Chen, B. H.; Chen, Y. H.; Huang, T. T.; Wang, J. H.et al. A broadband achromatic metalens in the visible. Nature Nanotechnology 2018, 13 (3), 227.
(3) Wang, S.; Wu, P. C.; Su, V.-C.; Lai, Y.-C.; Chu, C. H.; Chen, J.-W.; Lu, S.-H.; Chen, J.; Xu, B.; Kuan, C.-H. Broadband achromatic optical metasurface devices. Nature Communications 2017, 8 (1), 1.
(4) Lin, R. J.; Su, V.-C.; Wang, S.; Chen, M. K.; Chung, T. L.; Chen, Y. H.; Kuo, H. Y.; Chen, J.-W.; Chen, J.; Huang, Y.-T. Achromatic metalens array for full-colour light-field imaging. Nature Nanotechnology 2019, 14 (3), 227.
(5) Chen, M. K.; Yan, Y.; Liu, X.; Wu, Y.; Zhang, J.; Yuan, J.; Zhang, Z.; Tsai, D. P. Edge detection with meta-lens: from one dimension to three dimensions. Nanophotonics 2021.

Monolayer transition metal dichalcogenides (TMDs) are promising materials for applications in optoelectronic devices. However, due to the atomic-thin nature, the absorption in the monolayer TMDs has become a challenge in the access to high-performance optoelectronic devices. Unlike using the common plasmonic materials such as silver, gold, and aluminum to generate localized surface plasmon resonance (LSPR) to enhance the photoresponse of the monolayer TMDs. Here, we demonstrated 2D plasmonic phototransistors using a monolayer MoS₂ integrated with hafnium nitride (HfN) plasmonic metasurfaces. By using the designed plasmonic nanostructures via Finite-difference time-domain (FDTD) calculations, we have the ability to manipulate the coupling between exciton and surface plasmon resonance. In the reflection spectra, we observed the localized surface plasmon resonance (LSPR) of HfN metasurface could be tuned by controlling the geometry of the HfN nanodisk arrays. In this work, we designed a HfN nanodisk arrays with 430 nm in diameter and a period of 550 nm, the LSPR wavelength from the designed structure is at 667 nm (A exciton in MoS2). Hence, with the tunable LSPR in the HfN metasurfaces, we can further investigate the resonance-induced photoresponsivity of phototransistors. In the end, we will also discuss the potential applications of 2D plasmonic phototransistors based on the HfN metasurfaces.

Cellulose can be made reflective and transparent by varying its structure, forming an attractive platform for optical management from solar to thermal wavelengths.[1] By reversible wetting with common liquids, such as water, its transparency can further be dynamically controlled, which expands its use for a wider range of applications. Herein, we present a cellulose Janus structure with self-adaptive solar-induced drying and thermal regulation. The structure comprises a highly reflective (>90%) porous cellulose surface overlayed on a strongly absorbing (>95%) cellulose layer. On wetting, the top reflective surface turns transparent and allows the absorptive bottom surface to access direct solar radiation. This enables the bottom layer to absorb solar light and generate heat, which drives water evaporation and dries the structure, making it reflective again. Through in-situ reflection, temperature, and weight-loss (water loss) measurements, we have identified intriguing relationship between optical properties and drying process in the cellulose Janus structure.
[1] Gamage et al., Reflective and transparent cellulose-based passive radiative coolers, Cellulose volume 28, pages9383–9393 (2021)

We have developed an extremely compact and efficient spectrometer based on metasurface structures. The spectrometer consists of a mobile phone image sensor and dielectric nano post-embedded Fabry-Perot bandpass filters are directly integrated on top of the image sensor pixels. Each filter has a different peak transmission wavelength in the near IR region. Spectrum information is directly retrieved by intensity map of image sensor, i.e, image captured by sensor.

Embedded metasurface structures greatly simplify monolithic fabrication processes in constructing multiple bandpass filters having different transmission spectra. Also, metasurface provides superior optical performance of spectral filters since the dependence of incident angle onto the filter is reduced that the peak wavelength is not changed much compared to conventional Fabry Perot Filter. The spectrometer shows good transmission efficiency and excellent spectral resolution up to 1nm.
We applied the spectrometer technology to 1) Raman spectroscopy applications and 2) hyperspectral imaging.
For Raman spectroscopy application, we built handheld and smartphone-attachable Raman modules with some external optics to collect and transport Raman scattered photons onto the spectrometer chip. We will present applications of meat and vegetable freshness sensing. Also application for a drug classification will be shown . For hyperspectral imaging, optimized optical bandpass filters are pixel-wisely integrated on the image sensor so snapshot hyperspectral imaging method is possible in the mobile devices including smartphones. We will present applications of material identification for agriculture and skincare.
The spectrometer chip can be directly combined into smartphone and used as portable spectral analyzer. This new device will open new business opportunities and research fields for diverse applications.

Superconducting photon detectors have a low dark count rate and short timing jitter; thus, they provide outstanding detection performance. However, most reports focus on optimization within IR telecom wavelengths. Rarely do researchers focus on the optimization within the visible spectrum range. On the one hand, superconducting nanostrip single-photon detectors have been widely studied in the past decades because their intrinsic detection efficiency is higher than ones with microstrip. On the other hand, microstrip photon detectors have lower kinetic inductance and well fiber coupling efficiency that worth developing.
In this work, we deposited niobium nitride (NbN) thin film by ultrahigh vacuum radio-frequency (RF) magnetron sputtering on MgO (100) substrate at 800 °C with high crystalline quality. The superconductivity of the NbN film can be modified by different growth parameters, such as the argon/nitrogen flow rate, target, RF power, growth temperature, and growth substrate during sputtering. Thus, we optimized the metallic and superconductivity properties (with T_c ~15.5 K) of NbN film by adjusting the parameters above. We characterized the material properties of the NbN film by ellipsometry, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), transmission electron microscopy (TEM) X-ray diffractometer and superconducting quantum interference device (SQUID).
To increase photodetectivity, we deposited 5 nm-Al₂O₃ and Ag nanocube on NbN film, where the Ag nanocube was designed as nanoantenna, which induced gap plasmon resonance in the visible range, further to engineer the optical response for the superconducting devices. The superconducting states broke down by localizing strong electromagnetic fields such that the photodetectivity of the device was promoted. To design plasmonic nanostructures, we calculated the electromagnetic field distribution of the Ag nanocube/Al₂O₃/NbN structure by finite-difference time-domain (FDTD). We discovered a localized plasmonic resonance, which resonated at 532 nm wavelength, at the edge of Ag nanocube with 40 nm long and 30 nm thick. Hence, we can enhance the photon response of the detector to the visible range which can attribute to the gap plasmon resonance. Noted that the lowest power of light can be detected is 4.4 nW. In the end, we discuss the potential for NbN superconducting single-photon detector applications, such as large active area and polarization independence, etc.

All-dielectric metamaterials consist of high-index, sub-wavelength arrays that allow manipulation of electromagnetic permitivitty and permeability, and have a much lower loss at optical frequencies than metallic metamaterials. In this article, we propose a planar periodic Al₂O₃/Ge/ITO/SLG multilayer structure of dielectric interlayers to achieve a perfect absorber of mid and long-infrared (IR) radiation. Our numerical modeling findings prove that the permittivity data of each layer of material is nearly compatible with the ideal data. To demonstrate a crucial role in the maximum value of the absorption and bandwidth of the absorption, we performed a computational study as a function of the dielectric layer imaginary part (or loss tangent of tan(δ)) and Q factor. The dielectric structure possess unique importance as thermal application when considering the IR signature reduction in the long wavelength band and the reduced radiated energy dissipation along with the undetected band and the requirements for IR camouflage. This feature of dielectric metamaterials is associated with their inherent selective absorption/emission. Technically, the absorption is the same at the wavelength specified as the emission based on Kirchhoff's law, and the emission in the other band remains low. The temperature difference may occur not only from the properties of the surfaces but also from some material's optical properties and environmental conditions. In this study, we examine the possibility to obtain thermal camouflage at different background temperatures using a well-engineered all-dielectric metamaterial. The results showed that the camouflage material substantially reduces the contrast between the target and the background. Beyond that, our assessments validated that the contrast in the resulting short tips is due to the differences in the reflective properties of the material and the background. Experimental observations followed by numerical analysis, in which the significance of the absorption feature compared to the reflection was confirmed.

Modern image sensors consist of systems of cascaded and bulky spherical optics for imaging with minimal aberrations. While these systems provide high quality images, their improved functionality comes at the cost of increased size and weight. One route to reduce a system’s complexity is via computational imaging, in which much of the aberration correction and functionality of the optics is shifted to post-processing in software. Alternatively, a designer could miniaturize the optics by replacing them with diffractive optical elements, which mimic the functionality of refractive systems in a more compact form factor. Meta-optics are an extreme example of such diffractive elements, in which quasiperiodic arrays of resonant subwavelength optical antennas impart spatially varying changes on a wavefront. While separately both computational imaging and meta-optics are promising avenues toward simplifying optical systems, a synergistic combination of these fields can further enhance system performance and facilitate advanced capabilities.

In this talk, I will present a method to combine these two techniques to enable ultrathin optics for performing full-color and varifocal imaging across the whole visible spectrum as well as high precision depth sensing. I will also discuss the use of computational techniques for designing meta-optics with exotic behaviors lacking any intuition-informed design, as well as for performing computation on incident light, with applications in optical information processing, and object detection. By combining meta-optics and software backend, we can realize compact imaging systems with unprecedented functioncalities, including broadband aberration-free imaging, depth sensing and optical computing. We believe such hybrid digital-optical system will create a new research field on “Software Defined Optics”, akin to Software Defined Radio, where the software is used to simplify the hardware.

A variety of nanoscale phenomena are driven by quantum fluctuations, including spontaneous emission and the Casimir effect (i.e. the generation of forces between charge neutral objects at the nanoscale). By engineering these phenomena, we can develop next generation light sources, detectors, and microscale actuators that are not conceivable with classical physics. In this talk, I will describe our use of epsilon-near-zero (ENZ) materials to engineer these quantum effects and how they can be applied to various technologies. Specifically, we will show how ENZ materials can be used to control the spontaneous emission rate of quantum emitters and present the concept of electromagnetic bandgaps for nanoparticles comprising an ENZ material. Further, we will show how tunable ENZ materials can be used to convert electrical bias into mechanical motion through the modification of the allowed modes within a cavity composed of an ENZ material. Finally, we will discuss the outlook for using the concepts in future device architectures.