Symposium F.NM01—Nanophotonics—Emerging Hybrid Platforms, Materials and Functions

Light scattering from particles show unique angular scattering patterns, which are strongly dependent on the particle size to incident wavelength ratio and the refractive index contrast between host medium and the particles. Multiple scattering becomes important when the particle concentration increases, which complicates the angular scattering distributions. In many cases, empirical phase functions have to be applied. A novel empirical phase function was introduced to describe the angle-dependent distribution of scattered light from a collection of micro/nanoparticles inside a thin layer. Angular dependence of light scattering from a polymer host containing sub-micron dielectric and metallic particles was measured. The method gives an excellent approximation to the angular effects of multiple scattering arising from aggregation, non-spherical shape and surface roughness. In addition, it is also a good approximation in the single scattering regime, since it closely reproduces scattering phase functions of particles in the Rayleigh, Mie and geometrical optics regimes. The feasibility of our approach was demonstrated by its ability to fit experimental data on the forward and backward scattering profiles of plasmonic and dielectric submicron particle composite layers. The robust and wide applicability of our method is expected to attract a board interest from researchers in physics and materials science.

Arylazopyrazoles (AAP) show significant potential as a new family of molecular photoswitches owing to their efficient reversible trans-to-cis photoisomerization behavior and the high thermal stability of their metastable cis-isomer. Herein, we report on the design and synthesis of new arylazopyrazole based molecular switches with different spacer length and functional (OH, COOH) groups. The new AAPs were used for the fabrication of solid-state photoswitchable polydimethylsiloxane (PDMS) based composite films. The thin films were prepared by using PDMS as a polymer matrix and different concentrations of AAPs as chromophores at room temperature via spin-coating. The photoswitching behavior of the PDMS-AAP blend films and the neat AAP samples induced by irradiation with specific wavelength of light were investigated. We found that the as prepared PDMS-AAP blend films showed rapid and near-quantitative reversible trans-to-cis isomerization rates upon alternating irradiation with UV (l = 365 nm) and green (l = 525 nm) light which is comparable to the isomerization behavior of the neat AAP chromophores in solution. This indicates that the excellent photoswitching properties of AAPs are preserved in the PDMS-AAP composite films. Additionally, the as prepared PDMS-AAP films in the cis-form possess outstanding thermal stability and long-term optical stability. The results also show that the optical properties of the PDMS-AAP composite films can be tuned by using different ratios of the AAP chromophores and exposure to UV irradiation. This methodology provides a new approach for exploring the fabrication of polymers with solid-state photoswitching functionality. The effect of concentration of the AAPs on the reversible trans-to-cis isomerization processes of the PDMS-AAP composite films will be discussed in this presentation.

Recently, ultraviolet photodetectors (UVPDs) have been used on large scale applications such as environmental monitoring, flame detection, space communication, biomedical applications, pharmaceutical, and chemical analysis. Besides, advances in internet technologies such as the internet of things (IoT) endorse the integration of sensors more rapidly in every aspect of our lives. This trend demands the fabrication of smaller, lighter, less complicated, and low power devices. Therefore, designing low cost, self-powered, highly sensitive, flexible UVPD nanodevices are becoming a hot research area for near future optoelectronics. Silicon carbide (SiC), a wide bandgap semiconductor, is an excellent contender to fulfill those expectations. In addition to having characteristics of one-dimensional nanostructure such as high surface to volume ratio, high crystalline quality, SiC nanowires also possess superior material attributes like high breakdown voltage, high thermal conductivity, high drift velocity, excellent chemical and physical stability. In this study, we propose a very cost-effective, flexible, self-powered single SiC nanowire ultraviolet photodetector (SiCNW-UVPD) fabricated on a polyvinylchloride (PVC) substrate.
The self-powered flexible SiCNW-UVPD was fabricated via a commercially available transmission electron microscopy (TEM) grid (physical mask) and a sputter coater system. Gold (Au) electrodes of 110 µm x 110 µm and a gap of 4 µm were successfully achieved without the use of complicated and expensive methods like photolithography. Two-probe photocurrent-time (I-t) and I-V measurements were performed to reveal photoresponse characteristics such as the photo-to-dark current ratio (PDCR), sensitivity, and responsivity under 254 nm wavelength UV light at 0 V bias. A shallow dark current around 0.087 pA and a good PDCR value of 18 are obtained. Moreover, sensitivity and responsivity values are calculated as 1756 and 170 mA/W, respectively. In addition, the specific detectivity calculations, the effect of bias, and vigorous bending test results are discussed as well. Our work provides the following unique advantages: (i) to the best of our knowledge, the first demonstration of a single SiC nanowire PD on a flexible substrate with great performance, (ii) a very cost-effective fabrication method and readily applicable to any substrate, (iii) exhibiting high sensitivity and responsivity values under UV light at 0 V bias. Therefore, our SiCNW-UVPD device presents an opportunity for low-cost, easy to fabricate, self-powered photodetectors for upcoming future technologies such as wearable sensors, electronic skins, and the internet of things.

As scaffold materials for tissue engineering applications, hydrogels can mimic many important functions of extracellular matrices found in living tissues. Local temperature gradients can greatly impact the bioactivity of the matrix and cellular processes such as adhesion, migration and differentiation [1,2], while the knowledge of local temperature is essential understanding and modulating the cell properties and their responses. Various methods have been developed and applied for thermal measurements at the nano- and micro-scales, relevant to living tissues, such as infrared thermography [3], micro-thermocouples [4], AFM with thermocouple [5] and fluorescence-based thermography [6]. However, only a limited number of successful applications have been reported regarding three-dimensional localized temperature measurements as the first three methods can only be applied in two-dimensional cases [3-5]. In this contribution, we report a versatile platform to realize localized, simultaneous heating and temperature readout in both two and three dimensions. Localized heating was implemented in hydrogels by functionalizing them with gold nanorods (GNR) with high absorption around 785 nm, while the local temperature is reported by the incorporated Rhodamine B (RhB)-loaded silica particles or quantum dots. For these thermal nano-probe, the fluorescence intensity decreases when temperature increases. In combination with the nanomaterials, a versatile optical stimulation and readout platform was developed. We demonstrate that plasmonic NIR heating could lead to localized, up to 10 degrees temperature changes in hydrogel-based cellular matrices that can be monitored in situ by fluorescent nanoprobes, both in 2D and 3D.

Funding: This research was funded by the KU Leuven C1 projects, grant numbers C14/16/063 OPTIPROBE, C14/18/061 and the FWO research grant, grant number G0947.17N.

References:
[1] F. Rico et. al Biophys. J., 2010, 99, 1387-1396
[2] A.A. Khalili et.al Int J Mol Sci, 2015, 16(8), 18149–18184
[3] C.W. Meyer et.al Front. Phys., 2017, 8
[4] F. Yang et.al Sci Rep 2017, 7, 1721
[5] J. Rho et.al RSC Adv., 2018,8, 27616-27622
[6] K.Okabe et.al Pflugers Arch - Eur J Physiol 2018, 470, 717–731

This talk introduces you to all the existing and new journals in the family of Nature. It covers the detailed information and guidelines on scientific manuscript preparation and submission. Tips for writing a good abstract and a good paper will be provided. Editorial and peer-review processes will be discussed.

We present our research controlling light at the nanoscale for infectious disease diagnostics, including detecting bacteria at low concentration, sensing COVID antibodies and antigens, and visualizing in-vivo inter-cellular forces. First, we combine Raman spectroscopy and deep learning to accurately classify bacteria by both species and antibiotic resistance in a single step. We design a convolutional neural network (CNN) for spectral data and train it to identify 30 of the most common bacterial strains from single-cell Raman spectra, achieving antibiotic treatment identification accuracies exceeding 99% and species identification accuracies similar to leading mass spectrometry identification techniques. Our combined Raman-CNN system represents a proof-of-concept for rapid, culture-free identification of bacterial isolates and antibiotic resistance.

Second, we describe resonant nanophotonic surfaces, known as “metasurfaces” that enable multiplexed detection of SARS-CoV-2 antibodies and antigens. Our metasurfaces utilize guided mode resonances excited in high refractive index nanostructures. The high quality factor (“high-Q”) modes (Q > 1000) produce a large amplification of the electromagnetic field intensities near the nanostructures that increase the response to minute refractive index changes from targeted binding of an antibody; simultaneously, the optical signal is beam-steered for multiplexed detection. We describe how this platform can be manufactured at scale for portable, low-cost assays.

Finally, we introduce a new class of in vivo optical probes to monitor biological forces with high spatial and temporal resolution. Our design is based on upconverting nanoparticles that, when excited in the near-infrared, emit light of a different color and intensity in response to nano-to-microNewton forces. The nanoparticles are sub-30nm in size, do not bleach or photoblink, and can enable deep tissue imaging with minimal tissue autofluorescence. We present the design, synthesis, and characterization of these nanoparticles both in vitro and in vivo, focusing on the forces generated by the roundworm C. elegans as it feeds and digests its bacterial food. Chronic cytotoxicity assays are used to confirm biocompatibility. Our force measurements are coupled with electrical measurements of muscle contractions in both wild-type and mutant animals, providing insight into the interplay between mechanical, electrical, and chemical signaling in vivo.

Hundreds of thousands of patients in the United States are diagnosed with cancer annually. While the accurate determination of disease stage is closely linked to the probability of patient survival, current diagnostic technology presents patients and physicians with trade-offs between duration, precision and cost. Here, we present an all-optical, label-free technology for quantitative, real-time cancer tissue diagnostics on a single, clinically-compatible chip. Periodically-arranged sub-wavelength dielectric nanostructures, known as metasurfaces, are patterned into dielectric layers on glass microscope coverslips, where biopsied tumor tissue sections can be deposited following routine clinical procedure. Specifically, our technique maps the anisotropy and orientation of collagen fibers, a quantitative marker of cancer stage in tissue, onto metasurface structural color.

We pattern a sub-wavelength rectangular lattice of rhombohedral perturbations into stacked layers of silicon nitride (215 nm) and silicon dioxide (75 nm), where < 10 nm bandwidth guided-mode resonances yield a high-purity chromaticity in reflectance, with a polarization-sensitive colorimetric response. With full-field numerical simulations, we demonstrate how selective tuning of the guided-mode resonances maps the orientation angle of collagen fibers onto blue-to-green chromaticity, while changes in refractive index are separately mapped onto red-to-green chromaticity as the sensing medium varies from air to biological tissue. For the example case of breast cancer diagnostics, where the presence and orientation of collagen fibers at the tumor margin can distinguish early-stage from metastasized disease, we quantify the performance of our metasurface via the 1976 CIE Lab color discrimination. We find that our metasurface outperforms polarized light microscopy by unambiguously mapping tissue orientation onto structural color at higher color discrimination values, while enabling the distinction of orientation angles which otherwise yield an identical response. This improved performance is attributed to engineered symmetry-breaking of the metasurface geometry by Stokes parameter analysis of the metasurface-reflected fields. Finally, we experimentally characterize the anisotropic optical properties of planar sheets of ordered synthetic collagen scaffolds using both conventional polarimetry and our metasurface device. Working at the interface of nanoscale optics and medicine, our colorimetric metasurface platform has the potential to set a new benchmark for rapid, quantitative and cost-effective cancer tissue diagnostics.

Biomaterials are of increasing interest for photonics applications, including for biophotonics. The size of bioparticles used for nanophotonics can be similar to that of plasmonic nanoparticles; e.g., in the range of 50-500 nm. Some virus simulants scatter light and have been detected by highly sensitive photonic crystals.

Cephalopods are known to have important coloration and camouflage properties. The chromatophores in squid skin can change shape and influence light scattering and color; they are responsible for the bulk color changes (red, yellow, brown) observed in cephalopods. Underneath the chromatophore lie iridocyte cells, multilayered Bragg reflectors that reflect color and can cause iridescence. The chromatophore contains xanthommatin (Xa) pigments confined in nanostructured granules with sizes in the range 300-600 nm and achieves both pancake-like, expanded states and spherical, punctate state. The diameters of these granules are well-sized, and perhaps optimized for nanophotonic scattering of visible light, especially blue-green wavelengths; not absorption. Recently, researchers have succeeded in mimicking chromatophore color and in particular, the light scattering (diameter ~ 500 nm) granules within them, using artificial materials and chemically-synthesized Xa[1]. These novel material platforms can be used for more efficient control of light at the nanoscale.

In the past, our group has discovered that the Xa pigment in cephalopod chromatophores has a very high refractive index (n < 1.92), using fixed-wavelength refractometry (589 nm) and cavity ring-down spectrometry [2]. The latter experiment required drying aerosols. Here we report comprehensive broadband ellipsometric measurements of broadband (300 nm – 3200 nm) optical indices (n and absorption coefficient k) of Xa pigment, using a novel method for extracting natural Xa pigment from the squid’s chromatophores and forming smooth, high-quality films. These measurements give a more complete description of Xa’s fundamental optical properties, than previously known. We report a new short wave infrared absorption resonance and its possible origin. These important findings will assist in the design of new lightweight optical components, such as lenses, that can change in response to external stimuli and perhaps even image underwater. Most lenses are designed for a broad spectrum of light, where it is essential to know (n, k) across the entire wavelength range of interest, in order to predict optical system performance.

In this presentation, with a more advanced and quantitative understanding of squid skin optics, we improve on earlier results, that made important breakthroughs but did not have experimental optical constants, to extract scattering and absorption parameters. We use a 4-flux optical model (specular and diffuse light) of the chromatophore, optically coupled to the iridocyte. Armed with fundamental values of n and k, we employ direct experimental data for Xa to better quantitatively assess the optical response (e.g. scattering, reflectivity, transmission, and absorption) of the combined chromatophore-iridocyte optics.

These fundamental findings are important and will aid in the control of nanoscale light, using the novel Xa biomaterial, which can enable new scattering and coloration functionalities due to its unusually high n. Such a high-index material, which may be possible to synthesize in an environmentally-friendly way, may enable new designs of lightweight optical systems that can adapt and change shape, perhaps with an electrical trigger, and may aid in underwater detection or imaging. We discuss possible new designs and new nanophotonic systems, using the interesting Xa biomaterial.

[1] Adv. Opt. Mat. 6 1701369 2018.
[2] J. Phys. Chem. Lett. 8 313 2017.

Shortwave infrared (SWIR, 900-1700nm) enables in vivo imaging with high spatial resolution and penetration depth due to the reduced photon scattering and suppressed tissue autofluorescence at long wavelengths. Recently, small organic SWIR dyes have drawn great interest because SWIR dyes have excellent biocompatibility as compared to their inorganic counterparts. However, the application of SWIR dyes for in vivo imaging is hampered by their low quantum yield and photostability. To resolve these issues, in this work, the SWIR dyes are placed in close proximity to gold nanorod (AuNR) for surface plasmon enhanced SWIR fluorescence. The plasmon enhancement is optimized by varying the dye-to-AuNR number ratio and a maximum enhancement factor of ~ 45-fold is achieved. In addition, it is found that the highest dye-to-AuNR number ratio gives the maximum emission on the weight basis and this ratio is used for synthesizing the plasmon enhanced SWIR imaging probes. The outmost surface of the imaging probes is modified with different ligands. Their effect on the targeting capability and intratumor distribution of the imaging probes is investigated and compared in two orthotopic ovarian cancer models. Lastly, it is demonstrated that the plasmon enhanced SWIR imaging probes are able to detect submillimeter-sized ovarian cancer nodules, making them excellent candidates for real-time fluorescence imaging-guided surgery.

As of June 1st 2020, COVID-19 has impacted 213 countries with over 6 million cases globally. Antibody tests identify individuals who have been infected with SARS-CoV-2 and have developed an immune response; they also identify individuals with anti-SARS-CoV-2 antibodies who are potential donors of convalescent plasma. However, current serologic antibody tests are either qualitative or slow, are unable to correlate viral loads to antibody type and level, and do not offer real-time information about antibody-antigen binding on-rates, off-rates, or affinity constants. Here, we design and fabricate high quality factor (Q) silicon metasurfaces to sensitively detect SARS-CoV-2 antibodies and monitor antigen-antibody interaction dynamics at the few-protein level. Our metasurfaces utilize guided mode resonances excited in high refractive index nanostructures. The high Q modes (Q > 1000) produce a large amplification of the electromagnetic field intensities near the nanostructures that increase the response to minute refractive index changes from targeted binding of an antibody. Using a Si-on-Sapphire platform, we fabricate biperiodic array metasurfaces with quality factors exceeding 2500. Next, we functionalize our metasurfaces with the spike receptor-binding domain (RBD) to bind with IgM, IgG, and IgA, and their subtypes. Importantly, our zwitterionic and PEGylated matrix functionalization minimizes non-specific adsorption and enables oriented antibody attachment. On dried samples, transmission spectroscopy reveals metasurface resonant shifts exceeding the resonant full-width-half-max with varied antibody concentrations from clinical samples. A fluidic platform also allows real-time interrogation of metasurface resonant shifts as the serological samples are introduced. We describe how this platform can be manufactured at scale for portable, low-cost assays, and also how it can be modified to enable point-of-care antigen testing.

Efforts to examine large optical nonlinear response in structured and novel electromagnetic materials are ongoing. This work is partly motivated by the promise of leveraging structural resonances or regions of exotic materials response, such as the epsilon near zero or ENZ regime to amplify the underlying nonlinear response of component materials. This nonlinear response can in turn be leveraged to explore new device response when incorporated into metasurfaces and metamaterial structures. The mechanisms for this nonlinear response are varied and span the range of traditional nonlinear materials. Leveraging such resonances or narrow response regimes however, leads to limitations in the band width of the response as has been reported previously. In this presentation, we will explore the use of multilayered structures composing so-called hyperbolic metamaterials as nonlinear media with large and broad band optical response. Such structures have been proposed as nonlinear materials as well as for a variety of other applications. We examine the spectrally and temporally-resolved response of multilayer Arsenide semiconductor materials which are constructed to possess hyperbolic metamaterial properties. By controlling the composition and doping of the materials, and then examining their response during optical excitation of free carriers, we can examine the material, structure, and optical effects on the nonlinear response. To analyze ultrafast changes in carrier concentration we use a pump-probe experiment. Pump and probe laser pulses are 85 fs in duration and their center wavelength can be tuned separately with an optical parametric amplifier (OPA) and difference frequency generator (DFG) in either path. Typical wavelengths used were 1300 nm for the pump and 4, 5, or 5.5 µm for the probe. The differential intensity measurements were taken by appropriately positioned detectors to record ΔR/R for reflection or ΔT/T for transmission. We have explored the photo-induced change in optical properties of a multilayer semiconductor system constructed so as to generate hyperbolic dispersion under photoexcitation using pump-probe spectroscopy. The response shows characteristics wavelength and polarization dependent features that support the emergence of hyperbolic dispersion at pump excitation that induce sufficient photoinduced carrier responses. This response can serve as a basis for highly nonlinear systems and as a material from which highly nonlinear metasurfaces can be made.

The optical response of epsilon-near-zero (ENZ) materials has been a topic of significant interest in the last few years as the electromagnetic field inside media with near-zero permittivity has been shown to exhibit unique optical properties, including strong electromagnetic wave confinement, non-reciprocal magneto-optical effects, and abnormal nonlinearity.

This talk will review our recent development on the linear and nonlinear properties of conducting oxide epsilon-near-zero materials [1-6]. I will present a method to engineer the nonlinear refraction coefficients and the nonlinear absorption coefficients of Al-doped zinc oxide (AZO) ENZ thin films synthesized by atomic layer deposition (ALD) technique. Our results suggest that ENZ nonlinear refraction and nonlinear absorption properties of AZO films are enhanced by excitation of the ENZ modes and can be engineered by changing the mode-strengths via control over either the excitation angles or by ALD engineering of material dispersion/ film thickness. Measured values as large n₂^(eff)≈10^-9 mm²/W and β₂^(eff)≈-10^-5 mm/W are obtained along with approximately an order of magnitude tunability.

In addition, I will also discuss the first experimental demonstration of optically confined ENZ resonance excitation in a side-polished optical fiber waveguide uniformly coated with AZO nanolayer. We observed a transmission resonance dip in the ENZ regime with on and off resonance difference of ~20 dB in an ENZ-optical fiber with a 30 nm-thick AZO layer, which is attributed to the coupling between the fundamental mode of the optical fiber and ENZ mode supported by the ultrathin AZO film. We also demonstrate the active nonlinear optical and electrical control of the hybrid ENZ resonances in the AZO ENZ optical fibers. The ENZ-optical fiber provides a unique excitation platform of hybrid ENZ resonance with relatively long light-interaction length and eases the complexity of optical configuration without required sophisticated prism/grating coupling, which has potential applications in nonlinear and quantum zero index photonics, in-fiber optical sensing, lasers, and dispersion control.

This work was supported in part by the National Science Foundation (grant number: 1752295), Air Force Office of Scientific Research (AFOSR, Award number: FA9550-19-1-0274; FA2386-18-1-4099), and Defense Advanced Research Projects Agency (grant number N66001-17-1-4047).

Reference:
(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)A. Anopchenko, S. Gurung, L. Tao, C. Arndt, H. W. Lee, “Atomic Layer Deposition of Ultra-thin and smooth Al-doped ZnO for Zero-Index Photonics”, Materials Research Express, 5, 014012 (2018).
(3)S. Gurung, A. Anopchenko, S. Bej, J. Joyner, J. Myers, J. Frantz, H. W. Lee, “Atomic layer engineering of epsilon-near-zero ultrathin films with controllable field enhancement,” Accepted in Advanced Materials Interfaces (2020).
(4)K. Minn, A. Anopchenko, J. Yang, H. W. Lee, “Excitation of epsilon-near-zero resonance in ultra-thin indium tin oxide shell embedded nanostructured optical fiber,” Nature Scientific Reports 8, 2342 (2018).
(5)J. Yang, K. Minn, A. Anopchenko, S. Gurung, H. W. Lee, “Coupling to Epsilon-near-Zero Mode on Ultrathin Atomic Layer Deposited Conducting Oxide Film in Optical Fiber”, submitted (2020).
(6)A. Anopchenko, S. Gurung, S. Bej, H. W. Lee, “Field Enhancement of Epsilon-near-Zero Modes in Realistic Ultra-Thin Absorbing Films,” submitted (2020).

Periodic arrays are an exceptionally interesting arrangement for plasmonic nanostructures due to their ability to support strong collective lattice resonances, which arise from the coherent multiple scattering enabled by the array periodicity. Thanks to these exceptional properties, periodic arrays are being exploited in a wide variety of applications, including ultrasensitive biosensing, nanoscale light emission, and color printing, to cite a few. In this talk, we will discuss the response of arrays with multi-particle unit cells using an analytical approach based on plasmon hybridization, which provides a simple and efficient way to design periodic arrays with engineered properties [1]. We will pay particular attention to bipartite arrays and show how, depending on the relative position of the particles within the unit cell, these systems can support super- or subradiant lattice resonances [2]. We will also discuss how the interplay between the response of the individual constituents and the collective interaction determines the ultimate limits of the field enhancement provided by a periodic array [3]. We will finish by addressing the effect that the presence of edges as well as disorder have on the response of these systems [4].

[1] Baur, S.; Sanders, S.; Manjavacas, A. ACS Nano 2018, 12, 1618.
[2] Cuartero-Gonzalez, A.; Sanders, S.; Zundel, L.; Fernandez-Dominguez, A. I.; Manjavacas, A.; submitted 2020.
[3] Manjavacas, A.; Zundel, L.; Sanders, S. ACS Nano 2019, 13, 10682.
[4] Zundel, L.; Manjavacas, A. J. Phys: Photonics 2019, 1, 015004.

Optical researchers have long sought to develop materials and systems that enable more efficient nonlinear processes. From semiconductors, polymers, organize, and metamaterials, many promising materials, and approaches have been investigated, yet the primary platforms for nonlinear phenomena remain bulk crystals and optical fiber. A new entry into the area of nonlinear materials is epsilon-near-zero materials. In the past few years, these films have been shown to enhance various nonlinear processes from extreme refractive index tuning, phase conjugation, harmonic generation, and adiabatic frequency shifts in films with a thickness on the order of the wavelength due to unique intrinsic and extrinsic enhancement of the nonlinear response. Despite a variety of experimental successes, the theory of the effect has largely lagged behind. Here we outline a theoretical approach to describing extreme nonlinear index tuning in Drude epsilon-near-zero materials such as transparent conducting oxides. Our approach provides an analytical description suited to estimate the order of magnitude of the nonlinearity given basic properties, as well as a full numerical approach capable of predicting the strength, dispersion, and intensity dependence of the nonlinearity for multiple materials without fitting parameters. We utilize this model to investigate the impact of loss on the nonlinear response and develop a figure of merit to evaluate the performance of up-and-coming material platforms. Ultimately we believe our approach will aid the development and optimization of the promising epsilon-near-zero platform for both free-space and on-chip nonlinear applications.

Optical metamaterials are engineered structures that exhibit electromagnetic properties not found in naturally occurring materials. These structures, which can exhibit properties including near-zero or negative refractive indices, very large electric permittivities, and negative permeabilities, may enable new cloaking, metatronic nanocircuitry, and optical computing technologies. While there have been significant advances in the design and fabrication of two-dimensional (2D) metasurfaces, planar structures create nontrivial angular and polarization sensitivities, making omnidirectional operation impossible. Although three-dimensional (3D), isotropic metamaterials have been proposed, including ones that exhibit unnaturally high refraction, they have yet to be made. Here, we report the use of DNA-mediated programmable assembly to prepare isotropic 3D metacrystals from oligonucleotide-modified Au nanocubes. We use finite-element modeling to show such structures could have an effective index as large as ~8 in the mid-infrared, far greater than that of natural materials, and for the first time, experimentally observe multipolar Mie resonances in 3D metacrystals with well-formed habits. Specifically, we synthesized cubic superlattices with effective indices of ~4.7, comparable to the previous record in the optical regime for 2D metasurfaces. These sub-micron cube-shaped metacrystals exhibit mid-infrared Mie resonances, which were characterized by synchrotron infrared microspectroscopy.

While the noble/coinage metals have dominated chemistry research in nanomaterials that support surface plasmons, more recently the quest for sustainable and inexpensive plasmonic media has begun to focus on Aluminum. The nucleation and growth chemistry of Aluminum is unique, however, and few lessons learned from aqueous-based metal nanoparticle nucleation and growth apply to this metal. Aluminum nanocrystal growth is catalyst-driven, and the nucleation event is also strongly influenced by solvent molecules that complex with the catalyst-alane precursor. Studies of this system has led us to discover paths toward size and shape control dependent on the catalyst properties. With both size and shape dependence come new properties, which include new roles as optical antennas in plasmonic photocatalyst complexes. We show that for reactions that are driven by a hot electron mechanism, there is a strong dependence of reaction rate upon nanoparticle shape, for particles with the same plasmon resonant energy.

VO₂ is a phase change material that undergoes a first order crystalline phase transition at a critical temperature (T_c = 68^oC), resulting in significant changes in intrinsic electrical and optical properties, especially in the infrared. Optical changes with this phase transition are of particular interest as passive and active components of optoelectronic devices, specifically for thermal regulation and modulated signaling. Realizing this type of device often requires the integration of thin, conformal VO₂ films with complex, non-planar structures (like metamaterials). Thus, atomic layer deposition (ALD) is the ideal deposition method in these cases.
Traditional metal-based plasmonic materials suffer from high optical losses, which has promoted research towards alternative low-loss materials that can support plasmonic-like effects. One such approach employs phonon-mediated collective-charge oscillations (surface phonon polaritons, SPhPs) that are supported by nanostructured polar dielectric materials (SiC, AlN, etc), which inherently are low-loss. Geometric design of the nanostructures enables spectral tuning of resonant features between the longitudinal and tranverse optical phonons of the polar material, typically in the infrared regions. However, the spectral position and amplitude of these resonances remain fixed after fabrication. Integrating phase change materials with these structures provides a way to achieve active modulation of resonances.
In this work, we will discuss the ALD process optimization and characterization of thin VO₂ films itself and subsequent integration into different optoelectronic structures for active modulation. In particular we will present a case study where nanopillar arrays were etched into SiC and AlN to create narrowband resonances in the long-wave infrared region. These structures were subsequently coated with ALD VO₂ films with different thicknesses (8-75nm). As-deposited VO₂ films are highly conformal and amorphous, and cause the resonances to shift and broaden due to the different dielectric environment. However, after annealing the films at 525°C in 6x10^-5 Torr, the VO₂ films crystallize resulting in sharper resonances and spectral locations close to the initial uncoated structures. Temperature-dependence reflectance and emission measurements show that by heating through the VO₂ transition temperature, the amplitude of the resonances can be modulated. Full signal modulation (ie. on/off) requires at least a 16nm VO₂ film. This work shows the ability to actively tune surface phonon polariton resonances using ALD phase change materials.

Wurtzite Gallium Phosphide (GaP) nanowires have been intensively studied due to the possibility of finding a new direct band gap material in the green spectral range. However, recent works have suggested a pseudodirect band gap instead. Here, we investigate the optical properties of Au-catalyzed GaP nanowires, obtained by chemical beam epitaxy and nanoparticle crawling-assisted vapor-liquid-solid growth. We discuss how this phenomenon impacts nanowire morphology, and leads to the formation of an asymmetric GaP nanostructure. Using large volume (micron-sized) GaP structures in the wurtzite phase, we were able to carry out absorption measurements to elucidate the exact value of the fundamental band gap, excitonic emissions and valence band splitting energies of the wurtzite phase. Our data indicates the existence of a dipole forbidden (pseudo-direct) band gap at low temperature at (2.19 +/- 0.02) eV. We also discuss how to control the incorporation of unintentional impurities and its effect on the luminescence of this material. Furthermore, we show that high densities of large volume, wurtzite GaP structures (up to 40micra in length and over 2micra in diameter), with sidewalls presenting roughness in nanometer scale, can easily be obtained with our nanoparticle crawling-assisted growth method for antireflective coating in device applications.

Biosynthesis, as an alternative method, sparked an attractive attention in the research world due to its biocompatibility concerning production potential antibacterial agents. However, compared to chemical and physical methods, the main drawback associated with the biological synthesis is the inability to get the size and / or shape desired nanoparticles with a good yield.
In order to overcome this granulometry problem associated with a biological approach, this research work consisted, first step, to synthesize zinc oxide nanoparticles (ZnO NPs), by an entirely ecological chemistry process, simple and profitable using the extract from the dry husk of corn (Zea mays) as an effective chemical reducing agent without the addition of any standard acid or basic component. The physical properties such as structure, size, morphology and elementary composition, as well as the optical properties of the bio-synthesized nanoparticles were determined using the following different characterization techniques: High resolution scanning electron microscopy (HR -MEB), high resolution transmission electron microscopy (HR-MET), X-ray diffraction (DRX), energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy and Raman scattering, UV-Visible spectroscopy, as well as photoluminescence (PL) spectroscopy.
In a second step, we therefore synthesized zinc oxide nanofilms (ZnO NFs) in a biological approach assisted by spin coating technique, and thus studied its main physical properties using the same investigation techniques as mentioned above through a comparative study with the properties of bio-synthesized ZnO nanoparticles (ZnO NPs).
After investigation, the results show that the size of ZnO nanoparticles has been reduced by 20%, thus suggesting that the spin coating technique is an appropriate high quality method for improving the biosynthesis process of oxide-based nanomaterials, by overcoming the size phenomenon linked to this biological approach.

The electronic and optical properties of colloidal quantum dots (QDs) are significantly affected by impurities and doping. Crucial to efficient electronic doping of QDs is suitable charge compensation, but current compensation methods based on surface chemical modification and substitutional impurities have limited controllability and feasibility, respectively. Here, we describe an electronic n-type doping method where charge compensation is achieved with Li⁺ ions occupying interstitial sites. By controlling the electrochemical potential, Li⁺ ions are intercalated into CdSe QDs, allowing compositional control of Li_xCdSe, x < 0.3. At high lithiation levels, the Fermi energy is shifted into the QD conduction band, resulting in n-type degenerate doping characteristics, including bleaching of the band edge optical absorption, activation of intraband transitions, and blue-shifting of the photoluminescence spectrum. By reversing the sweep of the electrochemical potential, the original optical features of undoped QDs are partially recovered, indicating the reversibility of Li⁺ ion intercalation. Overall, this work establishes electrochemical interstitial doping as a reversible method for tuning the optical properties of colloidal QDs.

Titanium nitride (TiN) thin films on Si are very attractive for TiN-based integrated plasmonic devices. However, the heretofore reported TiN films on highly coveted Si substrates typically exhibited only moderate plasmonic properties due in large part to substantial defect densities caused by the large lattice mismatch between Si and TiN (21.9%). Therefore, smaller lattice mismatched substrates such sapphire and bulk MgO have been more commonly used. In this work, we report a comparative study on the structural and optical properties of TiN thin films grown by atomic layer deposition on different substrate materials including Si (001), MgO-buffered Si (001), c-sapphire, and bulk MgO (001). High-resolution X-ray diffraction (XRD), high-resolution scanning transmission electron microscopy (STEM), and variable angle spectroscopic ellipsometry were used to characterize the TiN films on these substrates. We observed that the TiN films grown on Si (001), MgO-buffered Si (001), and bulk MgO (001) substrates are all (001)-oriented possessing four-fold in-plane symmetry, while the films grown on c-sapphire substrate exhibit (111) orientation with six-fold in-plane symmetry. Insertion of a thin (~10 nm) MgO buffer led the intensity of XRD reflections from the TiN films grown on Si (001) to increase significantly, and cube-on-cube epitaxial growth was confirmed by high-angle annular dark-field imaging analysis of STEM. To characterize the plasmonic quality of the TiN films grown on different substrates, we used the traditional figure of merit (FOM) that equals to -ε'/ε", which quantifies the tradeoff between dielectric losses (ε") and metallicity (ε'). The peak FOM values are 2.4, 2.7, 2.8, and 2.9 for TiN films grown on Si (001), c-sapphire, MgO-buffered Si (001), and bulk MgO (001), respectively. As a result of the improved crystalline quality, use of a thin MgO buffer layer enhances the plasmonic quality of TiN films on Si (001) nearly to the level of TiN on bulk MgO (001) substrate. This thin MgO interlayer makes TiN thin films on MgO-buffered Si (001) an attractive material for applications in integrated plasmonic devices such as modulators and resonators based on the silicon platform.

Titanium nitride (TiN) is a promising material for plasmonic and metamaterials due to its temperature stability, chemical stability, low surface energy, mechanical robustness, and zero-crossover wavelength in the visible region. In this work we demonstrate the reactive magnetron sputtering of heteroepitaxial TiN on (0001)-Al₂O₃, (001)-MgO and (0001)- LiNbO₃ substrates. Additionally, we demonstrate the incorporation of TiN in metal-dielectric stacks. We discuss the effect of substrate material and deposition conditions on the TiN growth, stoichiometry and crystallinity. Coupled x-ray diffraction (XRD) show high quality epitaxial TiN growth on all three substrates. Pendellosung fringes on the (111)-TiN diffraction peak for the coupled XRD of TiN on c-plane sapphire and LiNbO3 and the (001)-TiN diffraction peak on MgO indicate uniform d-spacing and a pristine interface. Further structural characterization, however, reveals differences in crystal defects, strain and surface morphology based on substrate crystal structure and lattice mismatch. A 6-fold symmetry is seen in the pole figure XRD of the TiN grown on sapphire, indicating the presence of stacking faults. These domains, which are also evident in atomic force microscopy (AFM) of the TiN surface, are attributed to different stacking within the TiN domains. Cross-hatching features similar to those on the bare MgO substrate seen in the AFM of the TiN surface. X-ray photoelectron spectroscopy was used to investigate compositional differences between films grown with different deposition conditions. The real (ε₁) and imaginary (ε₂) permittivity for TiN was measured using variable angle spectroscopic ellipsometry. The TiN behaves metallic on all substrates for wavelengths above 470-490 nm. We calculate a quality factor (QLSPR = - ε_1/ε₂ ) of 3.34, 3.9 and 4.2 for TiN on LiNbO₃, MgO and Al₂O₃ respectively, for a wavelength of 1550 nm.

Phase-change materials (PCMs) can introduce reconfigurable behavior into metamaterials and optical devices that otherwise have fixed properties. Vanadium dioxide (VO₂) is outstanding among PCMs due to its near-room-temperature transition (T_c~68°C), sharp change in optical properties, and ultrafast (tens of femtoseconds) switching speeds. It has served as the active material in passive thermal control coatings, ultrafast photonic modulators, tunable ring resonators, and reconfigurable hyperbolic metamaterials. Many avenues for tailoring the optical properties and transition behavior of thin-film VO₂ have been explored—doping, strain, and defects being among the most prominent—but adjusting T_c without loss of contrast between the metal and insulating phases remains a challenge. Tungsten doping, for example, can reduce T_c; but heavy doping severely reduces the contrast and degrades the sharpness of the transition, before T_c is even below room temperature.
In contrast to thin films, VO₂ nanoparticles, doped with tungsten and embedded in a silica matrix, can achieve T_c ~ -25°C without significant loss of contrast (1). Here, we investigate the formation of self-assembled VO₂ nanoparticles by solid-state dewetting of VO₂ thin films, and study how the phase transition behavior of those nanoparticles depends on particle size, W-doping, and silica encapsulation. Nanoparticle formation by solid-state dewetting, much simpler and faster than lithographic techniques, is driven by surface energies and statistical mechanics of particle aggregation. The resulting nanoparticles are remarkably consistent across different film deposition techniques (pulsed laser deposition, atomic layer deposition, RF sputtering, and vapor transport); a comparison of results on silicon shows that the average nanoparticle radius is directly proportional (slope ≈ 2.8) to the initial film thickness, regardless of the deposition method. We show how this size dependence, as well as the nanoparticle size distribution, can be predicted with a simple model.
The effect of nanoparticle size on hysteresis in the metal-insulator transition has been studied previously, usually by measuring large groups of particles in aggregate and considering the average particle size. By employing scattering-type scanning near-field optical microscopy, we observe the phase transition in individual nanoparticles with increased sensitivity, spatial resolution, and temperature resolution (across the region of strong correlation) relative to previous work (2). Simultaneously investigating multiple individual nanoparticles allows us to observe precise size and shape effects and to understand the degree of intrinsic particle-to-particle variation. Finally, by comparing the effects of W-doping and SiO₂-encapsulation on these nanoparticles and on corresponding thin films, we elucidate how particle-size effects, strain resulting from encapsulation, and/or a synergistic combination of the two can lead to greater tunability of T_c without loss of switching contrast. Together, these results represent an advancement in our understanding of how VO₂ nanoparticles form via solid-state dewetting and in our ability to tailor their size, shape, T_c, and hysteretic behavior to suit the needs of individual applications, thus further increasing the versatility of VO₂ as an active component for “smart” optical devices.

1. Lopez R, Haynes TE, Boatner LA, Feldman LC, Haglund RF. 2002. Temperature-controlled surface plasmon resonance in VO2 nanorods. Opt. Lett. 27:1327-9
2. Donev EU, Lopez R, Feldman LC, Haglund RF. 2009. Confocal Raman Microscopy across the Metal-Insulator Transition of Single Vanadium Dioxide Nanoparticles. Nano Lett. 9:702-6

Degenerately doped metal oxides derive a high free charge carrier population from charge-compensated defects. Spatial control of the dopant distribution within each nanocrystal allows tuning of the potential energy landscape for charge carriers in a way not possible for conventional metals. Here, we employ a slow-addition approach to nanocrystal synthesis that enables control of radial distribution of substitutional tin dopants within an indium oxide lattice. When dopants are segregated either to the core or surface of spherical indium oxide nanocrystals, the potential established by space-charge build up at the interface between indium oxide and tin-doped indium oxide prevents charge carriers from distributing homogeneously throughout the nanocrystal. The doped region maintains a higher carrier concentration than the undoped region. The dielectric permittivity then varies radially, resulting in two distinct spectral modes in the plasmonic optical response. Spectra reported previously for spherical plasmonic semiconductor nanocrystals have shown hints of this dual-mode response, but the reason for its appearance has remained largely uninvestigated. We developed a simple model employing a core-shell Maxwell-Garnett effective medium approximation that can reliably fit dual-mode spectra and can explain the evolution of the secondary mode in our dopant-segregated system.

Two sample series were synthesized and investigated: one with an undoped core and doped shell of increasing thickness and another with a doped core and undoped shell of increasing thickness. After shell growth of sufficient thickness, a second absorption mode manifested in both series. The effective medium model allowed us to extract intrinsic material properties, in particular the the carrier concentration of both the core and the shell, that supported the experimentally observed trends in both series. Lastly, we proved that the sensitivity of the plasmonic response to the surrounding refractive index depends on dopant distribution, underlining the impact that intra-nanocrystal carrier distribution can have on the interaction between plasmonic nanocrystals and their environment.

Solid-state defect qubit systems with viable spin-photon interfaces show promise for single-photon generation, metrology and quantum information applications. Photon collection efficiency presents a major challenge for efficient utilization of defect qubits in high refractive index host materials like diamond and silicon carbide. Here we present a flexible inverse-design optimization framework that can reconcile a broad range of design constraints and generate planar dielectric photonic structures operating near the theoretical limit. Our optimized device utilizes a hybrid gallium phosphide (GaP) on diamond structure for photoluminescence (PL) extraction from synthesized NV centers in diamond. The enhancement is fairly insensitive to spatial localization and efficiently extracts photons from all dipole orientations. Additionally, we constrain patterning to the GaP layer, to minimize perturbations of the NV environment due to diamond etching. We fabricate the optimized device and observe efficient PL extraction from shallow (~100 nm) single NVs and near-surface (<100 nm) NV ensembles created by nitrogen ion implantation and annealing. These versatile devices exhibit a broadband PL enhancement for wavelengths in the measured range of 575 nm to 800 nm. We measure up to fourteen fold enhancement of free-space PL collection rate from device coupled NVs over non-device NVs and quantify the effect of fabrication and device integration on defect charge state and spectral stability. We expect such inverse-design hybrid devices will enable realization of scalable arrays of single-photon emitters, rapid characterization of new quantum emitters, high sensitivity metrology and efficient heralded entanglement schemes.

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 3D dielectric elements, designed to be placed on top of the pixels of image sensors, that sort and focus light based on its color and polarization with efficiency significantly surpassing 2D absorptive and diffractive filters. The devices are designed via iterative gradient-based optimization to account for multiple target functions while ensuring compatibility with existing nanofabrication processes, and they are experimentally validated using a scaled device that operates at microwave frequencies. 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 further discuss capabilities enabled by volumetric meta-optical devices for compact on-chip polarimetry and discuss implementations using multi-layer nanofabrication.

Atomically thin van der Waals (vdW) materials have generated strong interest in the community due to its unusual physical properties and possibility of creating high-performance devices. However, the ultrathin nature of these materials limits the achievable light-matter interaction. One promising way to increase such interaction is via nanophotonic confinement of light. In fact, researchers have demonstrated various nanophotonic devices using such vdW materials, including optically pumped lasing, second harmonic generation, cavity enhanced electroluminescence, and exciton-polaritons. In this talk, I will explore the possibility of going to quantum optical regime using this material system integrated with nanophotonic resonators. I will discuss the role of exciton-phonon interaction in the cavity photoluminescence spectrum and present a quantum optical model of the coupled system in presence of the phonons. I will then report the observation of the exciton-polaritons using transition metal dichalcogenides (TMD) coupled with a metasurface structure. This hybrid light-matter system can be used to reach single photon nonlinear regime via further engineering the nanophotonic structure and patterning the TMD material.

Resonant dielectric metasurfaces offer unparalleled degrees of freedom to design flat-optics and study fundamental aspects of light-mater interaction, due to the freedom to use optical resonances of electrical and magnetic character and different symmetries, extremely low loss when operating below the semiconductor band gap, and controlled coupling and interactions between the meta-atoms. Additionally, transient sub-picosecond behavior can be imparted to the metasurface using optical pumping and controlling of carrier dynamics and decay. In this talk I will overview some of our work based on such metasurfaces made from III-V semiconductors. In particular, I will focus on ultrafast switching and diffraction, transient frequency conversion and perfect absorbing metasurfaces for THz emission and detection.

Here I discuss our recent demonstrations of ITO-based opto-electronic devices and introduce the concept of 'Strainoptronics'. The presentation includes the following concepts: 1) Here we demonstrate a spectrally broadband, gigahertz-fast Mach–Zehnder interferometric modulator exhibiting a miniscule Vπ L of 95 V-um, deploying a subwavelength short electrostatically tunable plasmonic phase shifter based on heterogeneously integrated indium tin oxide thin films into silicon photonics. This GHz-fast broadband integrated modulator bears relevance since ITO is a foundry-compatible material. Unlike the crystal- orientation-sensitive LN, ITO optoelectronics is synergistic to enhancing electrostatics known from transistor technology. 2) Strain-engineered high-responsivity MoTe2 photodetector for silicon photonic integrated circuits In integrated photonics, specific wavelengths such as 1,550 nm are preferred due to low-loss transmission and the availability of optical gain in this spectral region. For chip-based photodetectors, two-dimensional materials bear scientifically and technologically relevant properties such as electrostatic tunability and strong light–matter interactions. However, no efficient photodetector in the telecommunication C-band has been realized with two-dimensional transition metal dichalcogenide materials due to their large optical bandgaps. Here we demonstrate a MoTe2-based photodetector featuring a strong photoresponse (responsivity 0.5 A/W) operating at 1,550 nm in silicon photonics enabled by strain engineering the two-dimensional material. Non-planarized waveguide structures show a bandgap modulation of 0.2 eV, resulting in a large photoresponse in an otherwise photoinactive medium when unstrained. Unlike graphene-based photodetectors that rely on a gapless band structure, this photodetector shows an approximately 100-fold reduction in dark current, enabling an efficient noise-equivalent power of 90 pW/Hz^0.5. Such a strain-engineered integrated photodetector provides new opportunities for integrated optoelectronic systems.

Miniaturized optical systems with planar form factors and low power consumption have many applications in consumer electronics, imaging and spectral sensing systems, and as integral parts of medical and industrial equipment. Flat optical devices based on dielectric metasurfaces introduce a new approach for the realization of such systems at low cost using conventional nanofabrication techniques. In this talk, I will present an overview of our work on the development of various high-performance optical dielectric metasurface components and systems. In particular, I will discuss multi-layer dielectric metasurfaces that enable complete control of wavefront at multiple wavelengths. I will present an on-chip integration platform enabled by vertical or lateral integration of multiple metasurfaces and active optoelectronic components such as laser arrays and image sensors. I will also introduce a novel technique for engineering chromatic dispersion by cascading and discuss various optical systems implemented in this platform and a single-snapshot multispectral imager.

The novel property of optical metasurface which consist of nano-antenna made by artificial nanostructures, has attracted a lot of attention lately. Nano-antenna at the interface can behave like secondary radiation sources after interactions of the incident electromagnetic wave. Various optical devices based on the specific designs of meta-surfaces are called optical meta-devices. The great advantages of meta-devices are their new properties, lighter weight, small size, high efficiency, better performance, broadband operation, lower energy consumption, and CMOS compatibility for mass production. Given the demands of photonics, many optical meta-devices for the applications of controlling of the incident light are developed, such as for beam deflection and reflection, polarization control and analysis, meta-holography, second-harmonic generation, meta-laser, tunability, imaging, absorption, color display, focusing of light, multiplexer of colour routing and light-field sensing, etc. Considering the flexibility, the metasurface with integrated functionalities may be promising for multi-focusing microscopy, high-dimension quantum technology, hyperspectral microscopy, micro robotic vision, automobile sensing, virtual and augmented reality (VR and AR), drones, and miniature personal security systems [1-7].
References
1. S. M. Wang, P. C. Wu, V.-C. Su, Y.-C. Lai, C. H. Chu, J.-W. Chen, S.-H. Lu, J. Chen, B. B. Xu, C.-H. Kuan, T. Li, S. N. Zhu and D. P. Tsai, Nature Comm. 8, 187 (2017).
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Micro- and nanostructures have been widely applied to enhance the performance of optical components and systems. Structures whose characteristic dimensions are greater than the wavelength of interest, l, can effectively manipulate the scalar properties of light, while nanostructures with dimensions << l/2 can manipulate the vector properties of light. By restricting the photonic functionality to a thin plane (i.e., 2.5D photonics), it’s possible to create very thin lightweight imaging lenses,¹ spectrum-splitting solar concentrators,² and integrated-photonics devices.³ In this presentation, we will highlight examples of each such device that overcome limitations of conventional implementations (if they exist at all). For example, flat multi-level diffractive lenses (MDLs) have recently shown achromatic bandwidth,^4,5 depth-of-focus⁶ and efficient high-numerical aperture focusing⁷ far better than their conventional counterparts, all while reducing weight and thickness by several orders of magnitude.
Computational techniques, such as nonlinear optimization, coupled with electromagnetics modeling can drive the inverse design of novel optical and photonic components and systems. When guided by manufacturing constraints, such techniques can result in highly practical, low-cost, ultra-compact (on the order of l x l) and multi-functional integrated-photonics components, such as polarization beam-splitters,³ wavelength splitters,⁸ couplers,⁹ waveguide bends,¹⁰ etc. We’ll also describe a nanophotonic cloak that enables two devices to be placed closer together than is otherwise feasible, leading to an increase in integration density.¹¹
When coupled with advanced machine-learning or linear-algebraic methods, 2.5D photonics can engines of information manipulation, enabling highly non-intuitive forms of optical systems such as photography with no lenses^12,13 or microscopy with only a surgical needle^14,15 or multi-spectral imaging with a diffractive element.¹⁶

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A photonic MOSFET includes a low resistance laser or LED in the MOSFET drain region, and a photon sensor or avalanche photo diode (APD) in the well region. MOSFET, laser and APD are fabricated as one integral device. When a voltage is applied to the MOSFET gate, and a voltage is applied to the drain, both MOSFET and laser are on. Light emitted from the laser is absorbed by the APD or photon sensor in the channel / well region right next to the laser, and a light current is triggered. The large light current flows into the drain to contribute to the total output drain current. When the MOSFET is turned off, both laser and APD are switched to off immediately. Photonic CMOS includes a photonic NMOSFET and a photonic PMOSFET.

There are two approaches to fabricate photonic CMOS transistors: (1) The MOS transistor is built in a silicon wafer. Lasing semiconductor films (III-V or II-VI) are selectively deposited in the drain region with low temperature selective CVD or epitaxy. (2) A thin silicon film is deposited on a GaAs (or GaN) substrate. CMOS is built in this thin Si layer. In the drain region, the Si layer is selectively etched with RIE (Reactive Ion Etch or "Dry Etch"). Lasing semiconductor films are selectively deposited on the opened GaAs substrate. Photon sensors or APD are fabricated in the substrate prior to the CMOS process. The process integration is simplified and low cost - compatible to the existing sub-7nm advanced CMOS process flow, including FINFETs.

Nonlinear optical materials are fabricated in the CMOS isolation regions, or buried oxide (BOX) for silicon on insulator (SOI), and in the optical waveguides connected with the photonic CMOS. The nonliear feature enables the photonic CMOS for laser communications, and optical computing. When an external IR laser beam reaches a photonic CMOS, it is converted to a higher frequency light, and processed by the built-in photon sensor or APD in the photonic CMOS. The may also be laser communications among the photonic CMOS transistors.

Since silicon is an indirect bandgap material and can not emit light, a hybrid platform, or hybrid silicon laser, needs to rely on lasers in GaAs, GaN or other II-VI materials bonded or fabricated with silicon. The bonding process often comes with higher resistance, lower speed, and additional costs (due to the system on chip configuration and more expensive packages). Forming a substrate with both Si and GaAs (InP, GaN..etc.) results in complicated process integration, and is not compatible to advanced CMOS ULSI or ASIC. If a laser is built with GaAs, then connected with a nearby silicon circuit, the process is quite sophisticated and performance may be compromised.

For high power lasers, the hybrid approach could cause more instability and vulnerable to temperature variations.

All such issues can be bypassed or resolved with the photonic CMOS technology. Lasing films are selectively deposited in the drain regions of CMOSFETs. CMOS and lasers are combined for higher speed, drive current, and better performance.

In this paper we would like to present I-V characteristics of photonic CMOS, and the effects of laser quantum efficiency and photon sensor absorption rate on the photon CMOS performance. We will also show several process integration schemes for nonlinear optical effects. Other potential products with photonic CMOS, such as new SRAM, circuit designs, and CMOS imaging applications will be discussed.

Currently, burning fossil fuels all over the world causes excessive emission of CO₂ accelerating climate change and causing detrimental harm to the environment. The capture of CO₂ and converting it into a high-value fuel has the potential to close the carbon cycle and mitigate harm to the climate in the future. Methanol is widely used in the chemical industry for producing chemicals, such as olefins, formaldehyde, methyl tert-butyl ether, and acetic acid. The reaction which produces methanol by CO₂ hydrogenation favors strict reaction conditions, like low temperature and high pressure. By using indium oxide as catalyst under ambient atmosphere, we found there is the fastest reaction rate at 250C with limited methanol desorption and the fastest methanol desorption at 500C with limited methanol production. Here, we design a two temperature processes for CO₂ hydrogenation reaction to produce methanol by shifting temperature between 250C and 500C. Indium oxide can absorb light from a blue LED efficiently, and allows quick temperature flip from these two temperatures. When using a blue LED to increase the temperature from 250C to 500C, the temperature increases more rapidly with the help of photothermal effect than pure heating, which can save about 1 min in the total desorption process. The study of these two temperature processes and using light as a heating source are investigating the energy efficiency between the pure heating process and the light plus heating process to optimize the energy input in the chemical reaction.

Plasmonic nanostructures feature extraordinary absorption cross sections, highly tunable optical responses, and capability of concentrating the excitation energy in subwavelength volumes. They are furthermore attractive for driving sustainable photochemistry such as selective CO₂-reduction and N₂-fixation under mild conditions due to the presence of an active catalytic surface and the participation of hot electrons and/or phonons upon light excitation. Although Au and Ag nanoparticles have a strong visible range plasmonic response, they are relatively unstable in high-demanding photocatalytic environments due to thermal deformation and oxidation (for Ag). In contrast, the metallic refractory material hafnium nitride (HfN) offers a desirable combination of a visible range plasmonic response, high chemical inertness, catalytically active surface, and an extremely high melting point. Although the merits are clear, the synthesis of nanostructured HfN and the detailed photophysical characterization are still relatively unexplored.

In our work we use ultrafast transient absorption to characterize the photophysical behavior of crystalline, water-soluble HfN nanoparticles with a plasmonic response around 550 nm. We find that the ultrafast response is distinctly different from the noble metals due to the extremely short lifetime of plasmonic hot electrons in HfN. Using a dual-parabolic two-step model, we completely justify the response at short and longer timescales, and predict a phonon outburst directly after pulsed light excitation. Our results reveal that HfN is highly promising for thermoplasmonic and photocatalytic applications, where material robustness is essential.

Recently, exciting new physics of plasmonics has inspired a series of key explorations to manipulate, store and control the flow of information and energy at unprecedented dimensions. For example, analogous to the familiar description of optical energy and linear momentum, non-local symmetry transformations of photons lead to new degrees of freedoms with respect to optical spin and optical helicities that are controllable at sub-wavelength scales thanks to the emerging nanohelical structures and networks. In this talk I will introduce versatile 3D shape transformations of nanoscale structures by deliberate engineering of the topography-guided stress equilibrium of gold nanostructures. By using the topography-guided stress equilibrium, rich 3D shape transformation such as buckling, rotation, and twisting of nanostructures is precisely achieved, which can be predicted by our mechanical modeling. Benefiting from the nanoscale 3D twisting features, giant optical chirality is achieved in an intuitively designed 3D pinwheel-like structure, in strong contrast to the achiral 2D precursor without nano-kirigami. High-contrast cross-polarized diffractions are successfully observed in both linear and radial configurations, resulting from constant phase difference between the pinwheel structures of opposite handedness. We further demonstrate an on-chip and electromechanically reconfigurable nano-kirigami with optical functionalities. The nano-electromechanical system is built on a CMOS-compatible Au/SiO2/Si substrate and operated via attractive electrostatic forces between the top gold nanostructure and bottom silicon substrate. Large-range nano-kirigami like 3D deformations are clearly observed and reversibly engineered, with scalable pitch size down to 0.975 μm. The demonstrated nano-kirigami, as well as the exotic 3D nanostructures, could be adopted in broad nanofabrication platforms and could open up new possibilities for the exploration of functional micro-/nanophotonic and mechanical devices. Nanoscale control of optical spin and optical helicities with nano-kirigami are promising for numerous technological applications, such as chiral molecular sensing, separation and synthesis, super resolution imaging, nanorobotics, and ultrathin broadband optical components for chiral light.

Metamaterials and metasurfaces have offered exciting platforms for engineering light-matter interaction that can provide desired functionalities. Over the past several years, numerous features and exciting applications of metastrucures have been explored and developed. One of the interesting possibilities in using metamaterials is the development of “metamaterial computing machines”, which we have been exploring in my group. In such paradigm, structures are designed in order to perform mathematical operations when waves propagate through them. Effectively, materials become computing machines. We have been investigating various scenarios in which metastructures can solve integral and differential equations and can invert matrices as incoming waves interact with them. In addition to using materials platform for this purpose, we have also investigating how collections of the Mach-Zehnder interferometers (MZIs) can provide another platform for solving equations and doing mathematics with light. In this talk, I will discuss some of our ongoing research projects on this topic and will present an overview of our most recent results.

Nanophotonic design spaces are often highly nonlinear; the large number of parameters (both geometric and material properties) will result in an exceptionally complex and high dimensionality design space that renders an exhaustive parameter sweep computationally insurmountable. Additionally, the design of a full nanophotonic device involves optimization at multiple levels, including material selection/design, device design, and system level design. In this talk, I will discuss multiple optimization frameworks for different design problems, including device level design for multispectral filters and phase modulation devices and system level design for beamsteering devices and other active metasurfaces. At the device level, we have (1) employed NOMAD (non-linear optimization using mesh adaptive direct search), a hybrid (global +local), derivative free optimization method, and (2) developed a new Bayesian optimization algorithm, MF-MI-Greedy, which uses multiple levels of fidelity, to optimize various nanophotonic structures including dielectric metasurfaces and multispectral filters. At the system level, we have (1) developed a new iterative genetic algorithm and (2) employed neural nets to optimize antenna arrays of active metasurfaces with non-ideal phase and amplitude characteristics. These studies suggest that selection of the correct optimization algorithm at a given design level can significantly reduce computational time for design and improve overall device performance.

Introducing thin, light-weight and high efficiency photovoltaics will ensure solar cells to be integrated in urban landscapes, small gadgets or even in space. While thinning down of absorber materials is a good strategy to overcome these restrictions, it comes down to the expense of poor light absorption. By contrast, nano-structuring is an effective approach to tackle flexibility, material and weight reduction without compromising absorption through the exploiting of their strong interactions with light.
In this talk I will describe the power of semiconductor nanostructures to capture and trap sunlight, enabling a wide range of new solar cells designs. First, I will introduce the wave guiding effects in semiconductor nanowires. The absorption cross-section in semiconductor nanowires can reach up to x100 their geometrical footprint, so that nanowire arrays can reach to the same absorption as in bulk films with a small fraction (< 3%) of the material. We have seen experimental evidence for the strong extinction in vertically standing GaAs and InAs nanowires, using 3D confocal fluorescence microscopy. We observe how light is perturbed up to few microns away from the nanowires for a broad range of wavelengths. We demonstrate that by probing ordered arrays with this method, the effective absorption coefficient and cross-section can be obtained without the need of a transparent substrate. Because of the absorption cross-section in nanowires is ruled by the efficient incoupling of light to the waveguide modes, absorption is relatively narrow-band (~50 nm) with the central wavelength depending on the nanowire diameter. The coloured appearance in sparse arrays are actually the perfect framework to generate a new class of semi-transparent photovoltaics, that can be used as windows and other urban spaces. We find that the multi-mode absorption in the arrays, result into higher absorbance as compared to the thin film counterpart with the same average visible transparency. Second, I will show that the diffractive power of nanowire arrays can be modulated by the wavelength-selective 1D wave guiding in the nanowires. Using the tailored structural properties in the array, we show that a nanowire array can serve as both solar cell and light in-coupler to a thin film cell underneath. The dual waveguiding effects in GaAs nanowire-Si thin film tandem architectures give rise to a 4-fold light absorption increase in the Si ultrathin bottom cell. This is a significant light trapping scheme that is obtained “for free” when using a nanostructured top cell. As a final remark, I will touch upon emerging fabrication methods for nanostructured semiconductor and conducting metamaterials. In particular, electrochemical deposition is a convenient growth method that allows for control over the growth conditions through combinations of solvents, additives, and pH, in addition to conventional parameters such as heat and pressure. By combining scalable imprint lithography with electrochemical deposition, we demonstrate large-area plasmonic lattices of In,Ga metals that can be electrochemically transformed to dielectric (In,Ga)As.
References
[1] R. Frederiksen, G. Tutuncuoglu, F. Matteini, K. L. Martinez, A. Fontcuberta, and E. Alarcon-llado, “Visual Understanding of Light Absorption and Waveguiding in Standing Nanowires with 3D Fluorescence Confocal Microscopy,” ACS Photonics 2017.
[2] N. Tavakoli, E. Alarcon-Llado, "Combining 1D and 2D waveguiding in an ultrathin GaAs NW/Si tandem solar cell”, Optics Express 2019
[3] Valenti, M. et al. "Grain size control of crystalline III–V semiconductors at ambient conditions using electrochemically mediated growth”, J. Mat. Chem. A 2020

In this work we examine principles of laser propulsion for spaceflight in the solar system. We discuss regimes of laser operation, system requirements on acceleration distance and sail-craft dimensions and weight. We show that with laser propulsion small satellites may be injected into various trajectories on a path to other planets. We show that travel times to planets may be as small as one month. We analyze spacecraft trajectory for various laser illumination regimes. Lastly, we discuss sail-craft materials and photonic design. We show that with a proper choice of low-loss materials and photonic engineering highly reflective sails may be crafted to ensure efficient momentum transfer. We further study pathways for radiative heat dissipation to ensure sustainable spacecraft operation and propulsion. We discuss several specific design concepts and demonstrate the feasibility and promise of such laser propulsion with today’s technology. Our work shows that laser propulsion may offer an alternative for fast transit time and high velocity missions beyond the reach of conventional chemical rockets and plasma engines.

As we approach the physical speed and power consumption limits of standard microelectronics processors, new strategies for computing are required. Among the different explored routes, analog optical computing has gained significant attention for its capability to process large amounts of data at negligible energy per bit and high speeds. Moreover, recent advances in nanofabrication techniques have enabled unprecedented control over light propagation and manipulation via subwavelength thickness metasurfaces. This, in turn, opens entirely new avenues in the field of optical analog computing.
Here, we propose a metasurface-based platform for analog computing that solves the Fredholm integral equation of the second kind for free-space radiation
Φ(x) = I₀(x) + ∫^b_a K(x,t)Φ(t)dt (1)
where Φ(x) is the unknown solution of Eq.(1), K(x,t) is a function of two variables that acts as the kernel of the integral operator, and I₀(x) is an arbitrary input function. When the kernel of such an integral equation does not match that of an integral transform for which we have an inversion formula (e.g. Fourier transform) and is not separable, the last resort to solve Eq. (1) is to exploit the Neumann series. The latter is a successive approximation method: the initial guess Φ₀(x) = I₀(x) is fed into the integral in Eq. (1) obtaining a second approximation Φ₁(x). By repeating this procedure iteratively, our guess Φ_n(x) approaches the solution Φ(x) as n→∞.
Following the same reasoning, it is possible to physically implement this method in an analog fashion by means of a metasurface coupled to a feedback system. In fact, Eq. (1) can be discretized by sampling its terms over N points in the interval [a, b] thus being equivalent to an NxN matrix equation. Next, it is possible to think of the N sampling points as N physical modes addressing the system and to consider the integral operator as a scattering matrix which performs matrix multiplication between sets of these modes. In our proposed scheme, the modes are the discrete diffraction channels characterizing a 2D metagrating, determined by its periodicity, while the discretized integral operator is given by the metasurface scattering matrix S. An arbitrary discretized input I₀(x) (where x is a vector of length N) is multiplied by the metasurface S-matrix upon reflection and it is fed back by a semi-transparent mirror for the next iteration. Intuitively, this system performs an analog Neumann series.
The core requirement of this procedure is the possibility of designing a metasurface that has a prescribed S-matrix. To this end, we optimize the metagrating unit cell by means of the adjoint method. In order to prove the generality of this method, we choose a random passive and reciprocal S-matrix with N=3 as our objective. The resulting optimized unit cell consists of a complex silicon nanostructure on a sapphire substrate. The unit cell is embedded in a SiO₂ superstrate capped on top by a silver semi-transparent mirror. The design wavelength is =710 nm while the unit cell is 825 nm wide and 415 nm deep. These dimensions are chosen such that only 3 diffraction order exists in the superstrate (where the Neumann series is performed) while 3 plus 4 extra “dump” channels diffraction order are present in the substrate to act as energy dumps in the case of a non-unitary kernel. The optimized metagrating can approximate rather well the desired S-matrix achieving a figure of merit FOM = ∑_i,j |S_ij - S_opt,ij|², where S_opt,ij is the simulated optimized S-matrix and S_ij is the prescribed one.
Next, we compared the simulated response of the entire structure (including the mirror) to the ideal solution of Eq. (1) showing a nice agreement. At the conference, we will show our experimental progress and optical measurements.

Phase-contrast imaging is an optical method invented by Nobel Laureate Frits Zernike to visualize unstained living cells. As cells are highly transparent, their structural information is hidden in the phase profile of the transmitted light field, where different micro-organisms inside the cell induce different amounts of phase delay. The underlying key principle of the phase-contrast technique is to interfere the light scattered by a specimen with reduced, π/2 phase-retarded unscattered light (DC component). As a result, the original phase undulation of the electric field is transformed into an enhanced amplitude modulation and thus the contrast in the image is maximized. Conventionally, phase-contrast imaging is achieved with a set of bulky Fourier optics components for filtering and selective phase retarding elements, limiting its use in certain applications. Due to the physical diffraction by these elements, conventional phase-contrast suffers from a limited range of enhancement and various artifacts, including halos and shade-off.

Here, we propose to replace the bulky optics in conventional phase contrast imaging by nanopatterned cover slip that can conveniently be inserted into any bright field microscope to achieve phase-contrast imaging. The cover slip is patterned with a 1 mm-by-1 mm area of a visually transparent guided-mode resonance structure. More specifically, we fabricated a 90-nm-thick, 210-nm-wide Si₃N₄ gratings on top of 136-nm-thick Si₃N₄ slab waveguide with a lattice constant of 390 nm. The structure can be used to perform phase-contrast microscopy by interfacing the normally-incident planewave to the guided-mode resonance at a 630 nm illumination wavelength. The DC incident light incurs a 98% amplitude reduction and a π/2 phase pickup while the scattered, non-DC Fourier components of the scattered light from a specimen pass through with near-unity transmission. We deposit the Si₃N₄ waveguide on an amorphous quartz substrate by plasma-enhanced chemical vapor deposition (PECVD) and pattern the grating with electron beam lithography and reactive ion etching (RIE). We characterize the optical dispersion properties of the fabricated samples and verify the phase contrast imaging capability with various phase objects such as cells, nanofabricated samples and 2D material flakes. We observe a 10 fold contrast enhancement for a wide range of objects with reduced artifacts as compared to Zernike’s method. We expect this opens a promising avenue for compact, quantitative, and high-accuracy phase-contrast microscopy.

Light can exert an optical force upon its interaction with an object due to the momentum exchange between massless photons and the object. The optical force has been harnessed for a variety of applications including trapping, accelerating and binding microscopic particles as well as pressurizing thin-films and two-dimensional materials. It has also been envisioned as a propulsion mechanism for space applications, leading to the advent of light sailing technology. Light sails gain momentum from the radiation pressure due to illumination from a light source - the Sun or laser beams - to provide the thrust for driving satellites in an obit or driving spacecrafts in certain trajectories without reliance on reaction mass thus allowing for continuous acceleration and greater longevity. In this talk, we will demonstrate how photonic metasurfaces can be used as light sails offering an unprecedented control over motion degrees of freedom via local manipulation of optical forces and torques through anomalous reflection and refraction. We will present a comprehensive modeling of motion dynamics of large-area meta-sails consisting of graded pattern nanostructured metasurfaces by linking the macroscopic optical response evaluated via a ray tracing approach based on the generalized Snell's law to the mechanical response governed by Newton-Euler equations, through Maxwell's stress tensor method. We will discuss the required conditions for the stability of a meta-sail riding on a high-power laser beam toward achieving relativistic velocities for deep space exploration and interstellar voyage, in terms of phase gradient and center of mass location. We will show an all-dielectric metasurface design featuring ultralow areal density while offering a high reflectivity and wide phase agility in a broad bandwidth which enables efficient acceleration with sustainable beam-riding stability for target relativistic velocities up to 20% speed of light despite relativistic Doppler broadening. In particular, we will disuss how the chromatic dispersion of the metasurface affects the stability and acceleration due to the relativistic Doppler shift in the frame of the accelerating meta-sail. We will present the estimated motion trajectories of the meta-sail driven by a 100 GW laser beam reaching to 20% speed of light in less than 12 minutes and illustrate that the Doppler-induced effects can be used for damping the amplitude of bounded oscillations through an engineered chromatic dispersion to yield a radiative loss in the reflection channel, toward minimizing the residual motion of the meta-sail and achieving a high-precision trajectory en route to a far destination.

The terahertz (THz) band of the electromagnetic spectrum falls in the region between microwaves and infrared, spanning from 0.1 to 30 THz. This is an information-rich frequency regime, where many molecular vibration/rotation modes and collective excitations occur in materials, providing THz “fingerprints”. Hence, THz spectroscopy and imaging are considered powerful tools for materials characterization and identification [1], being particularly beneficial to the biomedical field (e.g. diagnostics) and defense (e.g. security screening) due to their ionization-free nature and transparency to non-conducting materials. Furthermore, pulsed THz spectroscopy and imaging are particularly powerful for studying dynamic processes in materials with femtosecond time resolution. To date, the vast majority of THz pulsed systems are based on generation of single-cycle linearly polarized THz pulses, with a detection technique that can only measure one polarization component over one real-time scan. This is problematic when exploring complex material systems (e.g. birefringent materials) with anisotropic properties that could be affected by surface topography, crystal structure, stress and magnetic fields.
In this work, we have designed, fabricated and characterized polarization-sensitive THz detectors based on orthogonally-crossed semiconductor nanowire networks [2], and realized a single monolithic device that allows simultaneous measurements of the orthogonal components of the electric field of THz pulses without crosstalk. This device architecture is inspired by our pervious findings [3] that both single semiconductor nanowires and bow-tie THz antenna exhibit extremely high polarization selectivity to THz radiation. Single-crystal indium phosphide nanowires were used as the active material, grown by selective area epitaxy in a metalorganic chemical vapor deposition system, and were fabricated with a double-bowtie electrode (antenna) geometry design. A nanoscale ‘Transfer Print’ technique was developed [4], enabling rapid and accurate integration of nanowires onto the devices with well-defined location and orientation. The fabricated nanowire devices were incorporated into a THz time-domain spectroscopy system for performance tests. Moreover, the capabilities of the detector in metamaterials and birefringent materials analysis were demonstrated. Our nanowire detectors are derived from photoconductive antennas, and thus share the same operating mechanism and data analysis technique, which are the most popular detector type employed in many custom-made and commercial THz pulsed systems, suggesting an ease of direct implementation in both scientific and industrial settings. This detector type can be miniaturized down to the nanoscale size for on-chip THz spectroscopic and imaging applications.

Reference
[1] S. S. Dhillon et al., Journal of Physics D: Applied Physiscs 50, 043001 (2017).
[2] K. Peng et al., Science 368, 510-513 (2020).
[3] K. Peng et al., Nano Letters 16, 4925-4931 (2016).
[4] B. Guilhabert et al., ACS Nano 10, 3951-3958 (2016).

Mid-infrared (MIR) spectroscopy is a powerful technique for sensing through identifying the vibrational fingerprints of the analyte molecules. However, due to the large mismatch between the nanometric sizes of the molecules and the wavelengths of MIR light, a sufficient amount of samples is a necessity to produce long enough optical paths so that signals from the intrinsically weak light-molecule interactions can accumulate to the sensitivity threshold of the detectors. Conventional characterization techniques, such as Fourier-transform infrared (FTIR) spectrometers, require either samples with a thickness of at least some hundreds of nanometers or complex apparatus, such as attenuated total reflection (ATR) accessories. As a consequence, high-sensitivity identification of molecules with minute samples is still a challenge in MIR spectroscopy.
Nanophotonic resonators offer unprecedented opportunities to boost the optical fields at the subwavelength scale. Coupling the analyte molecules with resonant building blocks has been proven an effective approach to enhance light-molecule interactions. However, as the wavelength extends to the MIR region, plasmonic resonances gradually fade out and become less intense compared with in the visible region, making them not ideal candidates for sensing purposes. Here, taking advantage of the high-quality silicon microspheres as lossless Mie resonators, we exploit the possibility of using high-index dielectric antennas for enhanced molecular sensing.
High-index dielectric particles are well-known for their capability to support Mie resonances. For example, spheres sized a few hundreds of nanometers can scatter visible light in a highly directional manner due to the interference between electric and magnetic dipole modes. By using FTIR microspectroscopy, we manage to characterize infrared optical properties of single silicon microspheres and identify the first 14 modes, from magnetic dipole up to the electric 128-pole, with the help of Mie theory. Then, to get the optima conditions for the sensing purpose, we study the evolution of the resonance modes when the particles are placed on a mirror, i.e., at different locations of a standing wave. It is found that at MIR wavelengths, the fundamental magnetic and electric dipole modes, which spectrally overlap the molecular vibrations, can be selectively enhanced by altering the design of the substrate. We demonstrate ultra-sensitive measurements of sub-10 nm Poly(methyl methacrylate) (PMMA) under different conditions, evidenced by the pronounced spectral feature corresponding to the C=O bond vibration near 1722 cm^-1. In comparison, measurements in the absence of the Si antennas or an optimized spacer do not record any meaningful sensing signals. Our results provide a possible solution to high-sensitivity MIR molecular sensing based on a miniaturized photonic platform with minute analytes.

The ultra-thin optical structures of high-refractive-index materials, such as semiconductors and III-V compounds, can be used for control of light at the subwavelength scale [1]. We envision that metasurfaces with Mie resonances have a high potential in being utilized for control of coherent light states. The integration of light-emitting devices, such as VCSELs and VeCSELs, and dielectric metasurfaces has been shown as a viable technique to solve the issues of an arbitrary beam-shaping light-emitting device with programmable controllability as well as laser profiles and self-collimation [2]. The monolithic approach based on direct sculpturing of emitting surfaces into metasurfaces provides an authentic new degree of freedom of beam-shaping VCSELs without compromising the device performance.
Nanopillar metasurface at the back-side surface of a bare VCSEL is used as polarization insensitive meta-atoms, and each nanopillar works as an independent nanoscale resonator with a low-quality factor. The larger the effective refractive index, the more drastic change in the transmitted phase delay can be induced. Simultaneously, the bigger diameter of nanopillars, the better the transverse confinement of light is achieved. The integrated III-V compound metasurface operates as a passive element shaping the laser beam at the emission surface rather than adjusting the laser cavity. Programmable laser emission is controlled by the metasurface properties with the employment of arbitrary beam profiles.
We examine the scattering properties as well as reflection, transmission, and absorption of various high-index nanopillar supporting Mie resonances. We also analyze the effect of the thickness of the intermediate layer between the nanopillars and oxide substrate. We report on the scattering features of the metasurface illuminated normally from either substrate or superstrate side. We demonstrate that adding the high-index intermediate layer can shift desired resonances along the spectrum.
[1] H. Ahmed and V.E. Babicheva, "Nanostructured Tungsten Disulfide WS2 as Mie Scatterers and Nanoantennas," MRS Advances 5, 1819-1826 (2020).
[2] Y.-Y. Xie, et al., "Metasurface-integrated vertical cavity surface-emitting lasers for programmable directional lasing emissions," Nat. Nanotechnology 15, 125–130 (2020).

Topology plays a fundamental role in contemporary physics and enables new information processing schemes and wave device physics with built-in robustness. Recently, significant efforts have been devoted to transposing topological principles to bosonic systems. In the first part of this talk, I will discuss our invention of the first topological laser, a non-reciprocal light source capable of coupling stimulated emission to selected waveguide output in a controllable manner. I will also discuss unique optical devices based on this platform. In the second part of the talk, I will discuss our recently proposed scheme to systematically implement singularities known as exceptional points in passive plasmonics. I will discuss the new scheme and how we overcame current immuno-assay nano sensing record with plasmons by more than two orders of magnitude.

We use cathodoluminescence imaging spectroscopy (CL) as a powerful tool to characterize optical metasurfaces at deep-subwavelength spatial resolution.[1] In CL, a 5-30 keV electron beam is raster-scanned over the surface while the emitted radiation in the optical and near-infrared spectral range is detected. In the coherent excitation mode the electron beam passing through the metasurface creates a femtosecond electric field oscillation that couples strongly to polarizable electrons in the material, providing a spectrally broadband nanoscale probe of the local optical density of states.

We use CL to image localized modes of resonant plasmonic and dielectric nanostructures and reconstruct their scattering wavefronts using CL holography. We determine the phase and amplitude of wavefronts scattered by single-crystalline Au nanocubes and nanoholes, and derive from that the dominant scattering dipoles.[2] We discuss how the electron wavepacket may collapse as it generates a coherent superposition of surface plasmon polaritons and plasmonic transition dipole radiation that interfere in the far field.

We correlate CL data with photon-induced near-field electron microscopy (PINEM) of plasmonic nanotips in Au nanostars. As shown before,[3] strong plasmonic near fields can dress the high-energy electron energy spectrum into a ladder of coherent harmonics, shaping the electron quantum wavepacket. We will show that this enables probing of the plasmon charge distribution and the corresponding near-field intensity of plasmonic nanotips at the true nanoscale. We introduce how specially tailored optical metasurfaces enable entirely new ways to shape electron wavepackets in space and time.

[1] Polman, A., Kociak, M. & García de Abajo, F.J. Electron-beam spectroscopy for nanophotonics, Nature Mater. 18, 1158 (2019)
[2] Schilder, N., Agrawal, H., Garnett, E.C. & Polman, A. Phase-resolved surface plasmon scattering probed by cathodoluminescence holography, ACS Photonics, https://doi.org/10.1021/acsphotonics.0c00209 (2020)
[3] Feist, A., Echternkamp, K.E., Schauss, J., Yalunin, S.V., Schafer, S. & Ropers, C. Quantum coherent optical phase modulation in an ultrafast transmission electron microscope, Nature 521, 200 (2015).

Plasmonic, photonic and excitonic states at interfaces govern the optical response of tunable nanophotonic structures such as active metasurfaces, photodetectors and photocatalysts. I will discuss how we can harness plasmonic and excitonic states at interfaces to enable power-efficient phase modulation of active metasurfaces and discuss implications for time-modulated flat optics. Specifically, I will discuss avenues for increasing the power density and power efficiency of plasmonic conducting oxide metasurfaces for temporal phase and amplitude modulation and requirements for time-modulated photonic applications, such as multi-frequency generation and realization of non-reciprocal metasurfaces. I will also report on how time-resolved measurements of plasmonic and excitonic states at interfaces can yield insights about fundamental light-matter interactions in these systems. Specifically, I will discuss i) how electron-hole correlations affect the dynamics of hot carrier relaxation in plasmonic Au/GaN photodetectors and photocatalysts and ii) how time-resolved measurements of excitons generation inform our understanding of carrier dynamics and transport in transition metal dichalcogenides such as WS₂ and MoS₂.

Topological insulators (TIs) are materials with a bulk bandgap crossed by surface states that exhibit linear dispersion and spin-momentum locking. Electrons occupying these surface states have low mass and large Fermi velocity, while the spin-momentum locking leads to a reduction in scattering, since a change in momentum requires a spin flip. Among other interesting properties, two-dimensional Dirac plasmons can be excited from these surface electrons. These plasmons are expected to exhibit resonances in the terahertz (THz), a frequency range of interest for chemical identification, imaging, and communication. The TI plasmons should inherit the unusual properties of their constituent electrons: the excitations should be two-dimensional and massless, similar to those found in graphene. Unlike graphene, TI plasmons should also exhibit spin-momentum locking, leading to a reduction in scattering and an increased plasmon lifetime.
In this talk, I will describe our efforts to excite localized plasmon polaritons in TI nanoribbons, propagating plasmon polaritons in TI thin films, and complex plasmon phonon polariton modes in TI multilayer stacks. In thin films, a plasmon can be excited on the top and bottom of the film simultaneously. These plasmons couple to each other, resulting in an acoustic and optical mode. When the film is etched into stripes, we can excite localized plasmon polaritons. By changing the stripe width and film thickness, we can map out the dispersion of these modes, demonstrating extraordinarily high mode indices and long plasmon lifetimes. We can also apply a metallic grating to an unpatterned TI thin film and couple into propagating plasmon polariton modes. I will discuss our efforts to map the propagating plasmon polariton dispersion by changing the grating parameters. Finally, using molecular beam epitaxy, we can grow stacks comprising alternating layers of TI material and trivially-insulating band insulator material. These stacks house multiple topological surface states, and the plasmons residing in these states can couple to one another and to the phonons in the system. The multilayers thus contain complex coupled plasmon phonon polariton modes. We can understand the properties of these modes by changing the various structural parameters in these films. By growing a stack comprising many alternating TI/trivial insulator layers, we can move toward creating a THz Dirac metamaterial.

Thin layers of van der Waals (vdW) materials, ranging from insulators to semiconductors, semi-metals and superconductors, exhibit novel optical, electronic, orbital, magnetic, topological and superconducting properties. Varying the interfacial stacking order of vdW homo/heterostructures further yields Moiré bands and new physical phenomena. Nanoscale strain-engineering of vdW materials (e.g., graphene, transition-metal dichalcogenides, TMDs) can induce novel quantum/topological phenomena, such as giant pseudo-magnetic fields (PMF) and flat bands associated with quantized Landau levels in monolayer graphene, and strain-dependent energy gaps and exciton lifetimes for monolayer 2H-phase TMDs. Quantum confinement due to reduced spatial dimensions and broken symmetry is known to induce new quantum states that are non-existent in the bulk. Spin-orbit coupling of light by optical vortices (a.k.a. twisted light, Laguerre-Gaussian beam) and plasmonic vortices produces photons with nontrivial spin/orbital-angular-momentum, which can impart integer and fractional topological charges onto condensed matter and atomic/molecular systems through light-matter interactions. Therefore, investigating and controlling the light-matter interactions in vdW nanomaterials/heterostructures/metastructures represents a pathway to explore new quantum phenomena and to advance opto-electronic/valleytronic/spintronic applications.

We employ scalable CVD and PECVD fabrication techniques to synthesize a variety of vdW nanomaterials and their heterostructures, including monolayer and bilayer graphene,^[1-3] quasi one-dimensional (1D) graphene nanostripes,^[1,4] h-BN,^[5] and monolayer-to-multilayer TMDs MX₂ and MX_2-yX′_y (M = Mo, W; X, X′ = S, Se, Te; 0 ≤y ≤2).^[6] We investigate their properties by means of spatially resolved and polarization-dependent photoluminescence (PL), time-resolved PL (TRPL), Raman spectroscopy, x-ray photoelectron spectroscopy (XPS), Kelvin-probe force microscopy, conductive atomic force microscopy (c-AFM), scanning tunneling microscopy/spectroscopy (STM/STS).^[6,7] We perform nanoscale strain engineering on monolayer graphene to control the strain-dependent PMF and electronic correlation, and to achieve valley/spin polarization;^[7] and on TMDs to control the energy gaps and exciton lifetimes. We develop plasmonic vortices on TMDs by metasurfaces together with circularly polarized light to control the light-matter interaction and valley/spin polarization in TMD and the topological charge. Our approach leads to various interesting findings, including: valley splitting, quantum oscillations, spontaneous symmetry breaking and spin polarization, quantum valley-Hall and quantum anomalous-Hall effects in valley Hall transistors based on nanoscale strain-engineered monolayer graphene;^[7] exceedingly long and temperature independent optically excited carrier lifetimes (~ 10 ns) as well as broadband (from IR to UV) photoluminescence in quasi-1D graphene nanostripes; much enhanced valley polarization in monolayer WS₂ by combining plasmonic vortices with circularly polarized light; and compositional dependent opto-electronic/valleytronic properties in WS_2-yTe_y (0 < y < 2). Based on these experimental findings, we discuss potential applications of van der Waals nanomaterials/heterostructures/metastructures to opto-electronic/valleytronic/spintronic devices.

References:

[1] N.-C. Yeh et al., Nanotechnology 30, 162001 (2019).
[2] D. A. Boyd et al., Nat. Commun. 6, 6620 (2015).
[3] Y.-C. Chen et al., Carbon 156, 212-224 (2020).
[4] C.-C. Hsu et al., Carbon 129, 527 (2018).
[5] W.-H. Lin et al., Chem. Mat. 29, 4700 (2017).
[6] W.-H. Lin et al., ACS Nano 14, 1350-1359 (2020).
[7] C.-C. Hsu et al., Sci. Adv. 129, 527 (2018).

Non-classical states of light are important resources for scalable quantum technologies, such as quantum information processing, networking, and metrology. Entangled photon pairs, in particular, have applications in solid-state quantum repeaters, a crucial component of long-distance quantum networking that overcomes transmission loss by leveraging the effects of entanglement swapping and quantum teleportation. Despite the diverse applications for such non-classical states of light, methods for generating them deterministically remain limited. Here, we propose a scheme to deterministically generate entangled photon pairs from dipole-coupled pairs of defects in solid-state materials [1], which already have wide applicability in quantum technologies as quantum memories because they combine the favorable coherence and nonclassical emission properties of isolated atoms with the scalability and stability of solid-state technologies. Based on this scheme, we predict spectroscopic signatures and quantify the entanglement with physically realizable system parameters. We find that the entanglement measures of the emitted photons are robust to relative differences in frequency between the intermediate states. Importantly, the Bell state fidelity can be optimized by e.g. tuning the defect transition dipole moments. A defect-based entangled photon pair source would offer numerous advantages including flexible on-chip photonic integration and tunable emission properties via external fields, electromagnetic environments, and defect selection.

[1] Wang, D., Neuman, T., & Narang, P., arxiv:2004.13725 (2020).

Vortex beams of light have recently attracted great attention because of their ability to sensitively probe symmetries and topologies of materials. At the same time, electron beams are known to excite special states of light, for example by the generation of transition radiation (TR) or surface plasmon polaritons (SPPs). Here we bring together these two worlds, and present a plasmonic optical metasurface which under excitation with 30 keV electrons generates a collimated light beam with well-defined orbital angular momentum in the visible-near-infrared spectral range.
We fabricate spiral-shaped geometries composed of arrays of holes in a 40-nm Au film on a Si₃N₄ membrane by Au⁺⁺ focused ion beam milling. We excite the spiral geometries with 30 keV electrons almost at the center and collect the coherent cathodoluminescence and observe clear light vortex beams in the far field. We fabricate spiral arrays with left-and right-handed geometries and different winding numbers and find that the angular momentum of the generated TE- and TM- polarized Laguerre-Gauss-like light beams matches the number of spiral arms in our structure. Finite-difference-time-domain (FDTD) simulations are used to determine the evolution of electric and magnetic field components over time and space, and provide detailed insights in the interference effects that lead to the build of the vortex beams over time.
We describe the observation of the vortex CL beams with well-defined topological charge by a model in which 30 keV electrons create: (1) TR at the point of impact and (2) SPPs that travel over the metal surface and scatter out by generating magnetic dipoles in the plasmonic holes. Interference of these two scattering contributions in the far field then leads to the vortex beams. Propagation of SPPs along the spiral shaped hole arrays contributes strongly to the creation of the vortex beam. In our geometries, the diameter of the holes was varied from 50-200 nm along the spirals, which enabled the creation of vortex beams over a broad spectral range from 380-900 nm.
The generation of electron-beam induced light beams with tailored topology, spectrum, and temporal profile enables femtosecond pump-probe spectroscopy in which material excitations are created by electron excitation and probed with tailored light pulses that arrive at well-defined times.

On macroscopic scale, solar cell efficiency can be increased with concentrating lenses, that focus the sunlight on the cell. An equivalent effect can be achieved with nanostructures that make the light emitted by the solar cell directive towards the sun, while eliminating some of the drawbacks of macroscopic concentration. We have developed an evolutionary algorithm to design dielectric nanostructures for directional emission, leading to structures with a directivity as high as 306. In our previous work^[1] the experimentally achieved value was significantly lower than the modeled directivity, due to several limiting factors. In this work we overcome these limitations, by using a newly build measurement setup and a specially designed material system. By combining a Fourier microscope with an integrating sphere, we are able to measure emission into the full 4π solid angle, which allows us to measure full directivity. The predicted directivity is calculated for a point source at the center of the nanolens. In the experiment the point source is resembled by small clusters of emitting particles, which are placed in an ordered array to facilitate the proper alignment. This is achieved with a recently developed technique of direct patterning of CsPbBr₃ perovskite nanocrystals by e-beam lithography, by which the ligands of the nanocrystals are directly crosslinked upon exposure. Subsequently the dielectric nanolenses are fabricated on top of the clusters by 2-photon lithography.

[1] Johlin, E.; Mann, S. A.; Kasture, S.; Koenderink, A. F.; Garnett, E. C. Broadband highly directive 3D nanophotonic lenses. Nature Communications 2018, 9, 1-7.

The confinement of electromagnetic radiation in hybrid modes of light and collective excitations in solids (polaritons) promises to open new ways of dynamical control of matter, such as manipulation of radiative electronic transitions and of redistribution of vibrational energy. Two-dimensional (2D) materials have become the primary platform for polaritonic applications, where hexagonal boron nitride is the gold standard for exhibiting 2D phonon polaritons that confine light to volumes several orders of magnitude smaller than in free space [1,2].

Here, we show that phonon polaritons in the recently synthesized monolayers of oxide perovskites [3] can keep up with the benchmarks set by hexagonal boron nitride and confine electromagnetic radiation to extremely small volumes in the technologically important low mid-infrared and terahertz spectral regions. Specifically, we use a combination of first-principles calculations and microscopic phenomenological modeling to compute polaritonic figures of merit, such as the confinement and propagation quality factors of monolayers of SrTiO₃, KTaO₃, and LiNbO₃. Our results combined with the extraordinary tunability of oxides suggest that monolayers of oxide perovskites will become an emergent platform for 2D phonon polaritonics in the near future.

[1] Siyuan et al., Adv. Mater. 1806603 (2019)
[2] Rivera, Christensen, and Narang, Nano Lett. 19, 2653 (2019)
[3] Dianxiang et al., Nature 570, 87 (2019)
[4] Juraschek and Narang, in preparation (2020)

Effective control over the emission properties is of great importance to nanoemitters in order to realize optimized functionalities and to reduce undesired losses. However, challenges exist in both efficiency and integration when extra optical components are coupled at the nanometer scale. High-index dielectric nanoantennas hold particular promise to tackle these challenges, owing to their multipolar Mie resonances and low material loss. Uniquely, the mutual interference of the size-dependent magnetic and electric modes can lead to effective directivity modulation of the far-field radiation pattern. Since coupling with nanoantennas can significantly modify the emission properties of emitters via Purcell effect and sophisticated interference, positioning such dielectric resonators in the near field of a nanoemitter is also a potential route for directional emission. Moreover, a combination of the directivity modulations of both excitation and emission can provide more degrees of freedom to tune the emission properties in total.
Here, we propose a universal platform for dipole nanoemitters, based on single lossless hydrogenated amorphous Si nanospheres (a-SiNS:H), to modulate the directivity of both the excitation and emission at visible wavelengths. Thanks to its truly subwavelength nature, a-SiNS:H can strongly modify the emission of coupled emitters and make the radiation pattern highly directional. As a zero-dimensional (0D) structure, it also provides high spatial resolution so that multiple functions can be realized under an ultra-compact design, which fits well with the compactness of nanoemitters. Specifically, rigorous analytical theory for dipole-sphere hybrids is derived, along with numerical simulations, to predict the performance under a variety of conditions. Controllable forward-to-backward intensity (F/B) ratios as predicted by theory are experimentally proved with 125 samples of 532 nm laser excitation and 635 nm exciton emission from a monolayer WS₂. Moreover, flexible emission property engineering is demonstrated for different nanoemitters (e.g. WS₂ and MoS₂) and excitation wavelengths, thanks to the facile sphere size control.
In the derivation, we consider an a-SiNS:H coupled with an electric dipole emitter, which is oriented tangential to the surface of a-SiNS:H and positioned out of it by a small distance, mimicking the exciton emission from the monolayer of transition-metal dichalcogenides (TMDs). Following a similar procedure as in the standard Lorenz-Mie theory, we expand the incident field, i.e., the exciton emission, and the scattered field, by vector spherical harmonics. The derived analytical solution in the form of expanded spherical harmonics then allows fast computation of the far-field optical properties for information to guide the directivity modulation. Also, it helps to understand the role of each resonance in constituting the total scattering and determining the directivity.
To demonstrate the controllable F/B ratios experimentally, we modify our microscope system to effectively separate the directivity modulations of the excitation and emission. A CVD-grown monolayer WS₂ is used as the nanoemitter. The comparison of the modulated emission (excitation) in the forward and backward directions is always made with fixed excitation (emission) conditions. The F/B ratio can be up to 2.5 and down to 0.5.
Finally, the proposed a-SiNS:H-based platform enables us to achieve the desired emission properties, such as enhanced/suppressed emission in the forward/backward direction. More importantly, the same a-SiNS:H-nanoemitter hybrid can perform different functions by tuning the excitation wavelengths with rational designs. This gives us two degrees of freedom, i.e., the a-SiNS:H size and excitation wavelength, to fit the wanted performances for any nanoemitters, leading to potential applications in ultra-compact and multiplexed integrated photonics.

Spin-orbit optical interactions in photonic systems exploit the analogy between the quantum mechanical description of electronic spin-orbit system and synthetic Hamiltonians derived for propagation of electromagnetic waves in dedicated spatial structures. In the presented work a two-dimensional system – a thin optical cavity filled with liquid crystal – was created, in which trapped photons have unusual properties. It was found that during the modification of the properties of the cavity by external voltage, they behaved like quasiparticles endowed with a magnetic moment, that is "spin", under the influence of an artificial magnetic field. We show that for the liquid crystal cavity different regimes can be obtained. First this unique system allows for tunability of TE-TM splitting and shows giant values of this splitting. One of the most interesting phenomena directly dependent on the TE-TM splitting in a cavity is the optical spin Hall effect. We show that thanks to the novel design and the unique possibility to tune the splitting even for zero incidence angle, we are able to observe typical for normal cavities quadrupole spin textures as well as never reported before patterns resembling: dipole, spin doughnuts and whirls [1]. Secondly, we investigate an artificial Rashba-Dresselhaus spin-orbit interaction (SOI) and synthetic magnetic field (Zeeman-like term) [2]. With the ability to manipulate the effective refractive indices for different polarizations of light it is possible to tune the energy splitting between cavity modes. When two linearly polarized modes of different parity are brought into resonance theoretical analysis of birefringent electromagnetic waveguide results in SOI effects of light, in the form of H_RD=−2ασ_zk_y, where σ_z is the Pauli matrix describing polarization (spin) of light and k_y is light’s direction of propagation. The new photonic system perfectly imitates electronic properties and leads to many surprising physical effects. It allows to control the spin state of photon making a new building block for the novel optoelectronic devices e.g., optical neural networks and performing neuromorphic calculations. It can be integrated with light emitters (like various dopants: quantum dots, dyes, thin layers of transition metal dichalcogenides) for room-temperature strong light-matter coupling and lasing. While, engineering of spin-orbit synthetic Hamiltonians in cavities opens the way to photonic emulators of quantum Hamiltonians with internal degrees of freedom.

References:
[1] K. Lekenta et al., Tunable optical spin Hall effect in a liquid crystal microcavity. Light Sci. Appl. 7, 74 (2018),
[2] K. Rechcinska et al., Engineering spin-orbit synthetic Hamiltonians in liquid-crystal optical cavities. Science 366, 727 (2019).

The ability to control the optical properties of photonic materials on demand is essential for the operation of a variety of devices ranging from active color filters and routers to switchable mirrors and sensors. Here we will present our latest work on actively controllable photonic devices using alloyed metal hydrides. Using our recently developed apparatus for in situ measurements of optical and mechanical response upon hydrogenation, we first show how pure metal hydrides containing Pd, Mg, Zr, Ti, and V can be used as tunable photonic elements throughout the visible and near-infrared [1]. A number of devices will be shown including structures that have five orders of magnitude change in reflectivity, as well as planar systems for physical encryption and counterfeit detection. Second, we will show how alloying can not only improve cyclability, but also lead to higher hydrogen fractions than previously obtained as a result of film stresses and microstructuring [2]. These results have led to new insights in the dynamic behavior of metal-hydrides and will help in the design of next-generation hydrogen sensors and tunable photonic devices.

[1] Kevin J. Palm, Joseph B. Murray, Tarun C. Narayan, and Jeremy N. Munday, “Dynamic Optical Properties of Metal Hydrides,” ACS Photonics, 5, 4677–4686 (2018)
[2] KJ Palm, JB Murray, JP McClure, MS Leite, JN Munday, “In Situ Optical and Stress Characterization of Alloyed Pd_xAu_1–x Hydrides” ACS applied materials & interfaces 11, 45057-45067 (2019)

Chalcogenide glasses have recently received attention for their potential use in dielectric metasurfaces for several reasons: They typically possess high linear refractive indexes, enabling high index contrast devices; some compositions exhibit exceptionally high optical nonlinearity (n₂>1000× that of silica) making them good candidates for nonlinear devices; they have wide transmission windows spanning from the visible through the longwave infrared; and some compositions undergo a structural phase transition, enabling devices with switching based on phase-change. We discuss a variety of approaches to achieve tunability in chalcogenide glass metasurfaces from our work and that of others. We review recent progress on the use of the Kerr nonlinearity in As₂S₃ films to achieve power-dependent behavior and the application of this effect for the manipulation of orbital angular momentum. We discuss the design and realization of chalcogenide-based photonic quasicrystals enabling simultaneous phase matching of an arbitrary number of nonlinear optical processes in a single engineered nanostructure. We discuss recent observations that a permanent index change can be induced in As₂Se₃ via thermal tuning and the use of this effect to achieve permanent one-way tuning of the optical resonance of a metasurface. We discuss work being done by a variety of groups on phase-change chalcogenide materials such as germanium-antimony-telluride and gallium-lanthanum-sulfide in order to realize switchable metasurfaces. Finally, we discuss future prospects for the use of chalcogenide glass metasurfaces for applications that require tuning of the optical response.

Tunable metasurfaces allow to control the phase, amplitude and propagation direction of electromagnetic waves. Integrating phase-change materials into metasurface design provide an opportunity to control metamaterial properties at the meta-atom length scale. This could enable full reprogrammability and post-fabrication adjustments of adaptive metasurfaces. A state of phase change material near a meta-atom can be switched by a local deposition of heat using optical, electrical pulses or hot scanning probes. Resulting phase transition process is intrinsically non-stationary and inhomogeneous. The phase transition from crystalline to amorphous state is accompanied by the drastic change of the material properties leading to time-varying and position-dependent refractive index, conductivity, absorption rate and heat conduction. In order to describe local phase transition in metamaterials, a self-consistent numerical treatment of electromagnetic, carrier transport, heat transfer and phase transition processes becomes of primarily importance. Understanding the phase transition process at the meta-atom length scale is essential for a purposeful design of programmable adaptive metasurfaces. Here, we report on a self-consistent multiphysics description of a local phase transition in metasurfaces. The developed model is applied to analyse local phase transition in different metasurfaces under the application of different mechanism of the local heat deposition.

While metasurfaces are on the path to revolutionize the way we realize passive optical components, they have the same potential to affect the field of nonlinear optics. The interplay between relaxation of phase matching, control over microscopic optical fields and ability to manipulate far-fields make nonlinear metasurfaces a new paradigm for this field. Metasurfaces can become nonlinear in different ways: they can be made from nonlinear optical materials or can be fabricated on top of such materials. In this tutorial I will cover the evolution of nonlinear metasurfaces starting from all metallic, metallic on different nonlinear materials, to dielectric and then using engineered nonlinearities using heterostructures of different materials.

Among many emerging technologies, energy and display are two essential technologies that can revolutionize our current society. Tunable nanophotonics holds the key to both these emerging technologies. While tunable meta-devices enable 3D displays and such other display applications, tunable thermal emitters enable efficient energy conversion. Phase transition phenomena that arise from the collective behavior of materials enables both these classes of nanophotonic devices. While an electronic phase transition leads to a huge optical tunability, an optical one results in highly selective thermal emitters necessary for efficient energy conversion. Here, we demonstrate tunable nanophotonic devices based on strong correlations in 1T-TaS₂ and frequency-selective thermal emitters based on non-Hermitian nanophotonics.

Optical systems with losses such as thermal emitters require non-Hermitian physics to describe them. Such a non-Hermitian description of thermal emitters allows new powerful design tools such as phase, symmetry, topology. Here, we demonstrate an optical phase transition in a hybrid plasmonic-photonic system exhibiting passive parity-time symmetry. We employ localized dielectric resonances in an array of silicon disks forming a quasi-bound state in-continuum for dielectric resonance. A tungsten bottom plane serves as a plasmonic resonator. The electric dipolar resonance in the silicon resonator is coupled to its image in the tungsten and exhibits passive parity-time symmetry. The thickness of the MgF₂ spacer controls the coupling between the two resonators. Using this hybrid plasmonic-photonic system, we show that the best of both plasmonic and dielectric resonances, i.e., high spectral selectivity and near-unity emissivity can be achieved. Further, we elicit the role of symmetry and topology on the spatial and spectral selectivity of the thermal radiation in this system.

Another type of phase transition arising from the collective behavior of electrons in solids results in a large optical tunability. In general, the speed and magnitude of optical tunability trade-off with each other. For example, MEMS devices are slow but highly tunable, while nonlinear materials are ultrafast but possess tiny tunability. However, a large tunability and a moderately fast response are simultaneously possible in strongly correlated materials exhibiting phase transition due to their collective behavior. 1T-TaS₂ is one such strongly correlated material that exhibits an electronic phase transition above room temperature. Previous studies have shown that many stimuli can tune the electrical properties of 1T-TaS₂. However, optical properties remained less explored. Here, we demonstrate that a unity-order index tunability at room temperature is possible in 1T-TaS₂ shows just by shining low-intensity incoherent white light. Upon incoherent illumination of intensity less than 1000 mW/cm², the c-axis permittivity of this material was observed to change by unity order. We show that the tunability arises from the change in stacking order of charge density wave-domains across layers of this material. The observed tunability has a sub-microsecond response time allowing a few MHz speed. Thus, 1T-TaS₂ is promising for tunable nanophotonic devices. We demonstrate an optically tunable meta-grating device that could potentially revolutionize the emerging display and imaging technologies.

Starting from the 1970s, great efforts have been made to miniaturize bulky optical devices. The progress accelerated significantly over the last decade due to the emerging field of metasurfaces. These planar nanophotonic devices, made from judiciously-engineered, subwavelength-thick optical nano-resonators, are capable of controlling the amplitude, phase, and spectral properties of light waves with subwavelength resolution. Some progress has also been made to create active metasurfaces, but it remains challenging to create large-scale devices that offer repeatable and fast dynamic manipulation of the light field. Dynamic tuning is challenging, as the semiconductors and noble metals used in metasurfaces generally display weak electrorefractive and electroabsorptive effects. On the other hand, materials which show larger tunability often only support low-quality optical resonances and/or are not chemically stable.

Here, we demonstrate a comprehensive active photonic platform that affords facile tuning of silicon (Si) nano-resonators by altering their isotropic dielectric environment with microfluidics. We show this platform enables broadband amplitude control, full-color spectral control, and precise manipulation of geometric phase elements. The tunability, speed and robustness meet the requirements of various display and phased array applications. Specifically, we show that the integration of a nanophotonic device with a microfluidic cavity can provide on-demand control over the refractive index (n=1-1.7) of the environment of the nanoresonators in a metasurface by flowing different liquids in real time.

The observed, broad tunability of our metasurfaces results from the strong dispersion of the resonances with the refractive-index of the surrounding medium. This dispersion allows us to manipulate the interference of the scattered light from the Si nano-resonators. By summing up the scattering contribution from all the excited multipoles with the same symmetry, the optical response of a Si nano-resonator array can be modeled as two effective optical planar cavities with symmetric and anti-symmetric radiation profiles. Because of the different electric field distribution inside the nanoresonators, the two cavities show different dispersion with the refractive index of surrounding medium. An intuitive temporal coupled mode theory is applied to calculate the reflection and polarization conversion (diffraction) efficiency spectra and they agree well with full-field simulations.

To show of our platform and design concept, we experimentally demonstrate both a broadband (~100 nm) large amplitude modulation and full-color spectral tuning (from blue to red, reflection peak shifts from 480 nm to 580 nm) for individual pixels. We stress that these numbers are measured under un-polarized white light illumination. A digital number display controlled by microfluidic circuitry is also fabricated and characterized to verify the possible use of these elements for display applications. We also demonstrate that a geometric phase metasurface can be turned “on” and “off” actively, leading to new applications such as temporal beam tapping. What’s more, multiplexing different resonator arrays enables the opportunity to switch the function of a metasurface phased array. For example, an achromatic active lens that can select which color of light be focused is demonstrated. We show that the dielectric environment can be programmed up to 20 Hz, satisfying a number of vision-related applications.

Altogether, these results demonstrate an unprecedented comprehensive active photonic platform driven by liquids. They can lead to many practical applications, ranging from temporal beam focusing, steering, to real-time holography. The successful integration of nanophotonics with mature microfluidic cavity technologies further paves the way towards actively controlled nanophotonic displays and phased arrays.

Subwavelength nanostructures with tunable compositions and geometries show favorable optical functionalities for the implementation of nanophotonic devices. Diverse colloidal particles are appealing building blocks to construct these functional nanostructures. So far, many optical techniques have been developed to manipulate, assemble, and print colloidal particles from aqueous solutions into desired configurations on solid substrates. However, these techniques operated in liquid environments usually suffer from pattern collapses, Brownian motion of nanoparticles, and challenges on reconfigurable assembly.

Here, we present an all-optical technique, termed optothermally-gated photon nudging (OPN), for the versatile manipulation and dynamic patterning of a variety of colloidal particles on a solid substrate at nanoscale accuracy. OPN takes advantage of a thin surfactant layer to optothermally modulate the particle-substrate interaction, which enables the manipulation of colloidal particles on substrates with optical scattering forces. Along with in situ optical spectroscopy, this non-invasive and contactless nanomanipulation technique provides a powerful platform to design active optical devices and study the coupling between colloidal structures. We further demonstrated the assembly of active dielectric chiral nanostructures on a solid surface, where the chiroptical responses are tailored on-demand via manipulating the silicon nanoparticle. The optical chirality originates from the coupling between the optical resonances of the silicon nanoparticle and the silicon nanowire, which is supported by both numerical simulations and coupled mode theory analysis. Our technique not only provides insights into designing functional nanomaterials but also kindles new strategies to develop adaptive devices for photonic and electronic applications.

Surface functionalization with photoswitches, organic molecules that reversibly change their conformations in response to light, introduces photoresponsivity to otherwise static nanomaterials. A promising class of nanomaterials for optoelectronics is mixed-dimensional heterojunctions (MDHJs) between quantum dots (QDs) and monolayered semiconductors such as molybdenum disulfide (MoS₂). I have developed ligands that can be used to introduce photoresponsivity to such materials through surface functionalization. The class of photoswitches I will discuss, fulgides, switch between twisted, non-conjugated states and planar, conjugated states on picosecond time scales. I designed and synthesized a fulgide QD ligand that can extract electrons from photoexcited QDs in one state but not the other (Figure 1). This photoswitchable charge extraction enables reversible control of QD photoluminescence, with possible applications in all-optical memory. I synthesized derivatives of these photoswitchable ligands that can functionalize MoS₂ monolayers and thereby serve as bridging ligands in MoS₂-QD MDHJs. In this role, the photoswitches could be used to modulate charge transfer in mixed-dimensional heterojunctions and thus enable reconfigurable optoelectronic devices.

Topological states of light have attracted a lot of attention recently due to the existence of counter-intuitive nontrivial boundary effects arising from bulk properties of optical systems. The corresponding edge states, having their origin in topological properties of a bulk, are protected from perturbations and defects, and therefore they are promising for a wide range of applications in photonic circuitries. Conventionally, the bulk-boundary correspondence relates an n-dimensional bulk to a (n-1)-dimensional boundary state [1]. Recently the bulk-boundary correspondence was generalized to describe higher-order effects such that an n-dimensional bulk defines its (n-m)-dimensional imbedded boundary states. Prominent examples are 0D topological corner states of light in 2D structures [2-4] realized so far at the micrometre scales. Such corner states, due to their tight confinement in all directions, provide a novel route towards facilitating light-matter interactions in novel type of topological cavities. However, the current systems supporting the corner states employ building blocks that are an order of magnitude larger compared to the wavelength of light, which cannot be used for applications in nanophotonics. Here we bring topological corner states of light to the nanoscale and employ them for enhancing nonlinear optical processes. Specifically, we design topologically nontrivial 2D dielectric metasurfaces with C6-symmetric honeycomb lattices supporting both 1D edge states and 0D corner states. We observe topology-driven nonlinear light generation from the nanoscopic corner states and compare topological and trivial localisation of light.

In the past, judiciously designed high-index dielectric nanostructures supporting both electric and magnetic Mie resonances have been successfully employed for conventional forms of bulk-boundary correspondence, such as 1D states in 2D structures [5] and 0D states in 1D structures [6]. Here we observe 0D corner states in a 2D nanophotonic topological insulator. In our system, nontrivial topology arises from topological insulator geometry of periodic photonic structures, and the corner states emerge in a cascaded process in which first a bandgap is opened in a spectrum of 2D bulk modes which is bridged by 1D topological edge modes; second, a mini bandgap is opened in 1D modes hosting 0D corner states. We realize this concept experimentally using resonant dielectric metasurfaces on a metal substrate. The metasurfaces consist of two arrays of silicon nanopillars hexamers —expanded and shrunken hexamers, characterized by distinct topological invariants. We fabricate our sample with standard EBL and employ a nonlinear imaging to excite a broad sample area with a short pulse, high peak power tuneable laser and detect nonlinear third-harmonic signal generated by Si hexamers. We observe experimentally 2D bulk modes, 1D edge states, and 0D corner states at different wavelengths. We study several types of topologically nontrivial corner states and compare them to trivial light localisations. Miniaturization of topological corner states to the nanoscale paves the way towards their on-chip applications in compact classical and quantum nanophotonic cavities.

[1] L. Lu et al., “Topological photonics,” Nat. Photon., 8, 821, 2014.
[2] J. Noh et al., “Topological protection of photonic mid-gap defect modes,” Nat. Photon. 12, 408 2018.
[3] A. El Hassan et al., “Corner states of light in photonic waveguides,” Nat. Photon, 13, 697 2019.
[4] S. Mittal et al., “Photonic quadrupole topological phases,” Nat. Photon.13, 692 2019.
[5] D. Smirnova et al., “Third-harmonic generation in photonic topological metasurfaces,”
Phys. Rev. Lett. 123, 103901, 2019.
[6] S. Kruk et al., “Nonlinear light generation in topological nanostructures,” Nat. Nanotech. 14, 126 2019.

We advanced the development of solution-phase approaches for the preparation of lithium niobate (LiNbO₃) nanoparticles with an average, tunable size from 7 to 100 nm. This solution-phase process results in the formation of crystalline, uniform nanoparticles of LiNbO₃ at a reaction temperature of 220 °C with an optimal reaction time of as short as 30 h. Advantages of these methods include the preparation of single-crystalline LiNbO₃ nanoparticles without the need for further heat treatment or without the need for using an inert reaction atmosphere. The growth of these nanoparticles began with a controlled agglomeration of nuclei formed during a solvolysis step. The reactions subsequently underwent the processes of condensation, aggregation, and Ostwald ripening, which remained the dominant process during further growth of the nanoparticles. These processes did produce single-crystalline nanoparticles of LiNbO₃, suggesting an oriented attachment process. Average dimensions of the nanoparticles were tuned from 7 to ~100 nm by either increasing the reaction time or changing the concentration of the lithium salts used in the solvothermal process. The nanoparticles were also confirmed to be optically active for second harmonic generation (SHG). These particles could enable further development of SHG based microscopy techniques.

VO₂ is a phase change material that undergoes a first order crystalline phase transition at a critical temperature (T_c = 68^oC), resulting in significant changes in intrinsic electrical and optical properties, especially in the infrared. Optical changes with this phase transition are of particular interest as passive and active components of optoelectronic devices, specifically for thermal regulation and modulated signaling. Realizing this type of device often requires the integration of thin, conformal VO₂ films with complex, non-planar structures (like metamaterials). Thus, atomic layer deposition (ALD) is the ideal deposition method in these cases.
Traditional metal-based plasmonic materials suffer from high optical losses, which has promoted research towards alternative low-loss materials that can support plasmonic-like effects. One such approach employs phonon-mediated collective-charge oscillations (surface phonon polaritons, SPhPs) that are supported by nanostructured polar dielectric materials (SiC, AlN, etc), which inherently are low-loss. Geometric design of the nanostructures enables spectral tuning of resonant features between the longitudinal and tranverse optical phonons of the polar material, typically in the infrared regions. However, the spectral position and amplitude of these resonances remain fixed after fabrication. Integrating phase change materials with these structures provides a way to achieve active modulation of resonances.
In this work, we will discuss the ALD process optimization and characterization of thin VO₂ films itself and subsequent integration into different optoelectronic structures for active modulation. In particular we will present a case study where nanopillar arrays were etched into SiC and AlN to create narrowband resonances in the long-wave infrared region. These structures were subsequently coated with ALD VO₂ films with different thicknesses (8-75nm). As-deposited VO₂ films are highly conformal and amorphous, and cause the resonances to shift and broaden due to the different dielectric environment. However, after annealing the films at 525°C in 6x10^-5 Torr, the VO₂ films crystallize resulting in sharper resonances and spectral locations close to the initial uncoated structures. Temperature-dependence reflectance and emission measurements show that by heating through the VO₂ transition temperature, the amplitude of the resonances can be modulated. Full signal modulation (ie. on/off) requires at least a 16nm VO₂ film. This work shows the ability to actively tune surface phonon polariton resonances using ALD phase change materials.

In this work we examine principles of laser propulsion for spaceflight in the solar system. We discuss regimes of laser operation, system requirements on acceleration distance and sail-craft dimensions and weight. We show that with laser propulsion small satellites may be injected into various trajectories on a path to other planets. We show that travel times to planets may be as small as one month. We analyze spacecraft trajectory for various laser illumination regimes. Lastly, we discuss sail-craft materials and photonic design. We show that with a proper choice of low-loss materials and photonic engineering highly reflective sails may be crafted to ensure efficient momentum transfer. We further study pathways for radiative heat dissipation to ensure sustainable spacecraft operation and propulsion. We discuss several specific design concepts and demonstrate the feasibility and promise of such laser propulsion with today’s technology. Our work shows that laser propulsion may offer an alternative for fast transit time and high velocity missions beyond the reach of conventional chemical rockets and plasma engines.

Miniaturized optical systems with planar form factors and low power consumption have many applications in consumer electronics, imaging and spectral sensing systems, and as integral parts of medical and industrial equipment. Flat optical devices based on dielectric metasurfaces introduce a new approach for the realization of such systems at low cost using conventional nanofabrication techniques. In this talk, I will present an overview of our work on the development of various high-performance optical dielectric metasurface components and systems. In particular, I will discuss multi-layer dielectric metasurfaces that enable complete control of wavefront at multiple wavelengths. I will present an on-chip integration platform enabled by vertical or lateral integration of multiple metasurfaces and active optoelectronic components such as laser arrays and image sensors. I will also introduce a novel technique for engineering chromatic dispersion by cascading and discuss various optical systems implemented in this platform and a single-snapshot multispectral imager.

Introducing thin, light-weight and high efficiency photovoltaics will ensure solar cells to be integrated in urban landscapes, small gadgets or even in space. While thinning down of absorber materials is a good strategy to overcome these restrictions, it comes down to the expense of poor light absorption. By contrast, nano-structuring is an effective approach to tackle flexibility, material and weight reduction without compromising absorption through the exploiting of their strong interactions with light.
In this talk I will describe the power of semiconductor nanostructures to capture and trap sunlight, enabling a wide range of new solar cells designs. First, I will introduce the wave guiding effects in semiconductor nanowires. The absorption cross-section in semiconductor nanowires can reach up to x100 their geometrical footprint, so that nanowire arrays can reach to the same absorption as in bulk films with a small fraction (< 3%) of the material. We have seen experimental evidence for the strong extinction in vertically standing GaAs and InAs nanowires, using 3D confocal fluorescence microscopy. We observe how light is perturbed up to few microns away from the nanowires for a broad range of wavelengths. We demonstrate that by probing ordered arrays with this method, the effective absorption coefficient and cross-section can be obtained without the need of a transparent substrate. Because of the absorption cross-section in nanowires is ruled by the efficient incoupling of light to the waveguide modes, absorption is relatively narrow-band (~50 nm) with the central wavelength depending on the nanowire diameter. The coloured appearance in sparse arrays are actually the perfect framework to generate a new class of semi-transparent photovoltaics, that can be used as windows and other urban spaces. We find that the multi-mode absorption in the arrays, result into higher absorbance as compared to the thin film counterpart with the same average visible transparency. Second, I will show that the diffractive power of nanowire arrays can be modulated by the wavelength-selective 1D wave guiding in the nanowires. Using the tailored structural properties in the array, we show that a nanowire array can serve as both solar cell and light in-coupler to a thin film cell underneath. The dual waveguiding effects in GaAs nanowire-Si thin film tandem architectures give rise to a 4-fold light absorption increase in the Si ultrathin bottom cell. This is a significant light trapping scheme that is obtained “for free” when using a nanostructured top cell. As a final remark, I will touch upon emerging fabrication methods for nanostructured semiconductor and conducting metamaterials. In particular, electrochemical deposition is a convenient growth method that allows for control over the growth conditions through combinations of solvents, additives, and pH, in addition to conventional parameters such as heat and pressure. By combining scalable imprint lithography with electrochemical deposition, we demonstrate large-area plasmonic lattices of In,Ga metals that can be electrochemically transformed to dielectric (In,Ga)As.
References
[1] R. Frederiksen, G. Tutuncuoglu, F. Matteini, K. L. Martinez, A. Fontcuberta, and E. Alarcon-llado, “Visual Understanding of Light Absorption and Waveguiding in Standing Nanowires with 3D Fluorescence Confocal Microscopy,” ACS Photonics 2017.
[2] N. Tavakoli, E. Alarcon-Llado, "Combining 1D and 2D waveguiding in an ultrathin GaAs NW/Si tandem solar cell”, Optics Express 2019
[3] Valenti, M. et al. "Grain size control of crystalline III–V semiconductors at ambient conditions using electrochemically mediated growth”, J. Mat. Chem. A 2020

The optical response of epsilon-near-zero (ENZ) materials has been a topic of significant interest in the last few years as the electromagnetic field inside media with near-zero permittivity has been shown to exhibit unique optical properties, including strong electromagnetic wave confinement, non-reciprocal magneto-optical effects, and abnormal nonlinearity.

This talk will review our recent development on the linear and nonlinear properties of conducting oxide epsilon-near-zero materials [1-6]. I will present a method to engineer the nonlinear refraction coefficients and the nonlinear absorption coefficients of Al-doped zinc oxide (AZO) ENZ thin films synthesized by atomic layer deposition (ALD) technique. Our results suggest that ENZ nonlinear refraction and nonlinear absorption properties of AZO films are enhanced by excitation of the ENZ modes and can be engineered by changing the mode-strengths via control over either the excitation angles or by ALD engineering of material dispersion/ film thickness. Measured values as large n₂^(eff)≈10^-9 mm²/W and β₂^(eff)≈-10^-5 mm/W are obtained along with approximately an order of magnitude tunability.

In addition, I will also discuss the first experimental demonstration of optically confined ENZ resonance excitation in a side-polished optical fiber waveguide uniformly coated with AZO nanolayer. We observed a transmission resonance dip in the ENZ regime with on and off resonance difference of ~20 dB in an ENZ-optical fiber with a 30 nm-thick AZO layer, which is attributed to the coupling between the fundamental mode of the optical fiber and ENZ mode supported by the ultrathin AZO film. We also demonstrate the active nonlinear optical and electrical control of the hybrid ENZ resonances in the AZO ENZ optical fibers. The ENZ-optical fiber provides a unique excitation platform of hybrid ENZ resonance with relatively long light-interaction length and eases the complexity of optical configuration without required sophisticated prism/grating coupling, which has potential applications in nonlinear and quantum zero index photonics, in-fiber optical sensing, lasers, and dispersion control.

This work was supported in part by the National Science Foundation (grant number: 1752295), Air Force Office of Scientific Research (AFOSR, Award number: FA9550-19-1-0274; FA2386-18-1-4099), and Defense Advanced Research Projects Agency (grant number N66001-17-1-4047).

Reference:
(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)A. Anopchenko, S. Gurung, L. Tao, C. Arndt, H. W. Lee, “Atomic Layer Deposition of Ultra-thin and smooth Al-doped ZnO for Zero-Index Photonics”, Materials Research Express, 5, 014012 (2018).
(3)S. Gurung, A. Anopchenko, S. Bej, J. Joyner, J. Myers, J. Frantz, H. W. Lee, “Atomic layer engineering of epsilon-near-zero ultrathin films with controllable field enhancement,” Accepted in Advanced Materials Interfaces (2020).
(4)K. Minn, A. Anopchenko, J. Yang, H. W. Lee, “Excitation of epsilon-near-zero resonance in ultra-thin indium tin oxide shell embedded nanostructured optical fiber,” Nature Scientific Reports 8, 2342 (2018).
(5)J. Yang, K. Minn, A. Anopchenko, S. Gurung, H. W. Lee, “Coupling to Epsilon-near-Zero Mode on Ultrathin Atomic Layer Deposited Conducting Oxide Film in Optical Fiber”, submitted (2020).
(6)A. Anopchenko, S. Gurung, S. Bej, H. W. Lee, “Field Enhancement of Epsilon-near-Zero Modes in Realistic Ultra-Thin Absorbing Films,” submitted (2020).

Tunable metasurfaces allow to control the phase, amplitude and propagation direction of electromagnetic waves. Integrating phase-change materials into metasurface design provide an opportunity to control metamaterial properties at the meta-atom length scale. This could enable full reprogrammability and post-fabrication adjustments of adaptive metasurfaces. A state of phase change material near a meta-atom can be switched by a local deposition of heat using optical, electrical pulses or hot scanning probes. Resulting phase transition process is intrinsically non-stationary and inhomogeneous. The phase transition from crystalline to amorphous state is accompanied by the drastic change of the material properties leading to time-varying and position-dependent refractive index, conductivity, absorption rate and heat conduction. In order to describe local phase transition in metamaterials, a self-consistent numerical treatment of electromagnetic, carrier transport, heat transfer and phase transition processes becomes of primarily importance. Understanding the phase transition process at the meta-atom length scale is essential for a purposeful design of programmable adaptive metasurfaces. Here, we report on a self-consistent multiphysics description of a local phase transition in metasurfaces. The developed model is applied to analyse local phase transition in different metasurfaces under the application of different mechanism of the local heat deposition.

Metamaterials and metasurfaces have offered exciting platforms for engineering light-matter interaction that can provide desired functionalities. Over the past several years, numerous features and exciting applications of metastrucures have been explored and developed. One of the interesting possibilities in using metamaterials is the development of “metamaterial computing machines”, which we have been exploring in my group. In such paradigm, structures are designed in order to perform mathematical operations when waves propagate through them. Effectively, materials become computing machines. We have been investigating various scenarios in which metastructures can solve integral and differential equations and can invert matrices as incoming waves interact with them. In addition to using materials platform for this purpose, we have also investigating how collections of the Mach-Zehnder interferometers (MZIs) can provide another platform for solving equations and doing mathematics with light. In this talk, I will discuss some of our ongoing research projects on this topic and will present an overview of our most recent results.

This talk introduces you to all the existing and new journals in the family of Nature. It covers the detailed information and guidelines on scientific manuscript preparation and submission. Tips for writing a good abstract and a good paper will be provided. Editorial and peer-review processes will be discussed.

Periodic arrays are an exceptionally interesting arrangement for plasmonic nanostructures due to their ability to support strong collective lattice resonances, which arise from the coherent multiple scattering enabled by the array periodicity. Thanks to these exceptional properties, periodic arrays are being exploited in a wide variety of applications, including ultrasensitive biosensing, nanoscale light emission, and color printing, to cite a few. In this talk, we will discuss the response of arrays with multi-particle unit cells using an analytical approach based on plasmon hybridization, which provides a simple and efficient way to design periodic arrays with engineered properties [1]. We will pay particular attention to bipartite arrays and show how, depending on the relative position of the particles within the unit cell, these systems can support super- or subradiant lattice resonances [2]. We will also discuss how the interplay between the response of the individual constituents and the collective interaction determines the ultimate limits of the field enhancement provided by a periodic array [3]. We will finish by addressing the effect that the presence of edges as well as disorder have on the response of these systems [4].

[1] Baur, S.; Sanders, S.; Manjavacas, A. ACS Nano 2018, 12, 1618.
[2] Cuartero-Gonzalez, A.; Sanders, S.; Zundel, L.; Fernandez-Dominguez, A. I.; Manjavacas, A.; submitted 2020.
[3] Manjavacas, A.; Zundel, L.; Sanders, S. ACS Nano 2019, 13, 10682.
[4] Zundel, L.; Manjavacas, A. J. Phys: Photonics 2019, 1, 015004.

Hundreds of thousands of patients in the United States are diagnosed with cancer annually. While the accurate determination of disease stage is closely linked to the probability of patient survival, current diagnostic technology presents patients and physicians with trade-offs between duration, precision and cost. Here, we present an all-optical, label-free technology for quantitative, real-time cancer tissue diagnostics on a single, clinically-compatible chip. Periodically-arranged sub-wavelength dielectric nanostructures, known as metasurfaces, are patterned into dielectric layers on glass microscope coverslips, where biopsied tumor tissue sections can be deposited following routine clinical procedure. Specifically, our technique maps the anisotropy and orientation of collagen fibers, a quantitative marker of cancer stage in tissue, onto metasurface structural color.

We pattern a sub-wavelength rectangular lattice of rhombohedral perturbations into stacked layers of silicon nitride (215 nm) and silicon dioxide (75 nm), where < 10 nm bandwidth guided-mode resonances yield a high-purity chromaticity in reflectance, with a polarization-sensitive colorimetric response. With full-field numerical simulations, we demonstrate how selective tuning of the guided-mode resonances maps the orientation angle of collagen fibers onto blue-to-green chromaticity, while changes in refractive index are separately mapped onto red-to-green chromaticity as the sensing medium varies from air to biological tissue. For the example case of breast cancer diagnostics, where the presence and orientation of collagen fibers at the tumor margin can distinguish early-stage from metastasized disease, we quantify the performance of our metasurface via the 1976 CIE Lab color discrimination. We find that our metasurface outperforms polarized light microscopy by unambiguously mapping tissue orientation onto structural color at higher color discrimination values, while enabling the distinction of orientation angles which otherwise yield an identical response. This improved performance is attributed to engineered symmetry-breaking of the metasurface geometry by Stokes parameter analysis of the metasurface-reflected fields. Finally, we experimentally characterize the anisotropic optical properties of planar sheets of ordered synthetic collagen scaffolds using both conventional polarimetry and our metasurface device. Working at the interface of nanoscale optics and medicine, our colorimetric metasurface platform has the potential to set a new benchmark for rapid, quantitative and cost-effective cancer tissue diagnostics.

Here I discuss our recent demonstrations of ITO-based opto-electronic devices and introduce the concept of 'Strainoptronics'. The presentation includes the following concepts: 1) Here we demonstrate a spectrally broadband, gigahertz-fast Mach–Zehnder interferometric modulator exhibiting a miniscule Vπ L of 95 V-um, deploying a subwavelength short electrostatically tunable plasmonic phase shifter based on heterogeneously integrated indium tin oxide thin films into silicon photonics. This GHz-fast broadband integrated modulator bears relevance since ITO is a foundry-compatible material. Unlike the crystal- orientation-sensitive LN, ITO optoelectronics is synergistic to enhancing electrostatics known from transistor technology. 2) Strain-engineered high-responsivity MoTe2 photodetector for silicon photonic integrated circuits In integrated photonics, specific wavelengths such as 1,550 nm are preferred due to low-loss transmission and the availability of optical gain in this spectral region. For chip-based photodetectors, two-dimensional materials bear scientifically and technologically relevant properties such as electrostatic tunability and strong light–matter interactions. However, no efficient photodetector in the telecommunication C-band has been realized with two-dimensional transition metal dichalcogenide materials due to their large optical bandgaps. Here we demonstrate a MoTe2-based photodetector featuring a strong photoresponse (responsivity 0.5 A/W) operating at 1,550 nm in silicon photonics enabled by strain engineering the two-dimensional material. Non-planarized waveguide structures show a bandgap modulation of 0.2 eV, resulting in a large photoresponse in an otherwise photoinactive medium when unstrained. Unlike graphene-based photodetectors that rely on a gapless band structure, this photodetector shows an approximately 100-fold reduction in dark current, enabling an efficient noise-equivalent power of 90 pW/Hz^0.5. Such a strain-engineered integrated photodetector provides new opportunities for integrated optoelectronic systems.

Micro- and nanostructures have been widely applied to enhance the performance of optical components and systems. Structures whose characteristic dimensions are greater than the wavelength of interest, l, can effectively manipulate the scalar properties of light, while nanostructures with dimensions << l/2 can manipulate the vector properties of light. By restricting the photonic functionality to a thin plane (i.e., 2.5D photonics), it’s possible to create very thin lightweight imaging lenses,¹ spectrum-splitting solar concentrators,² and integrated-photonics devices.³ In this presentation, we will highlight examples of each such device that overcome limitations of conventional implementations (if they exist at all). For example, flat multi-level diffractive lenses (MDLs) have recently shown achromatic bandwidth,^4,5 depth-of-focus⁶ and efficient high-numerical aperture focusing⁷ far better than their conventional counterparts, all while reducing weight and thickness by several orders of magnitude.
Computational techniques, such as nonlinear optimization, coupled with electromagnetics modeling can drive the inverse design of novel optical and photonic components and systems. When guided by manufacturing constraints, such techniques can result in highly practical, low-cost, ultra-compact (on the order of l x l) and multi-functional integrated-photonics components, such as polarization beam-splitters,³ wavelength splitters,⁸ couplers,⁹ waveguide bends,¹⁰ etc. We’ll also describe a nanophotonic cloak that enables two devices to be placed closer together than is otherwise feasible, leading to an increase in integration density.¹¹
When coupled with advanced machine-learning or linear-algebraic methods, 2.5D photonics can engines of information manipulation, enabling highly non-intuitive forms of optical systems such as photography with no lenses^12,13 or microscopy with only a surgical needle^14,15 or multi-spectral imaging with a diffractive element.¹⁶

References:
S. Banerji, et al., PNAS, 116(43) 21375 (2019).
P. Wang, et al., Prog. Photovolt. 23(9) 1073 (2015).
B. Shen, et al., Nat. Photon. 9, 378 (2015).
M. Meem, et al., arXiv:2004.13476 (2020).
M. Meem, et al., arXiv:2001.03684 (2020).
S. Banerji, et al., Optica 7(3) 214-2017 (2020)
M. Meem, et al., Optica 7(3) 252-253 (2020).
A. Y. Piggott, et al., Nat. Photon. 9, 374 (2015).
B. Shen, et al., Opt. Exp. 22(22) 27175 (2014).
B. Shen, et al., Opt. Lett. 40(24) 5750 (2015).
B. Shen, et al., Nat. Commun. 7, 13126 (2016).
G. Kim & R. Menon, Opt. Exp. 26(18), 22826 (2018).
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G. Kim , et al., Sci. Rep.. 7, 44791 (2017).
G. Kim, et al., Appl. Phys. Lett. 106, 261111 (2015).
P. Wang & R. Menon, Optica, 2(11) 933 (2015).

Recently, exciting new physics of plasmonics has inspired a series of key explorations to manipulate, store and control the flow of information and energy at unprecedented dimensions. For example, analogous to the familiar description of optical energy and linear momentum, non-local symmetry transformations of photons lead to new degrees of freedoms with respect to optical spin and optical helicities that are controllable at sub-wavelength scales thanks to the emerging nanohelical structures and networks. In this talk I will introduce versatile 3D shape transformations of nanoscale structures by deliberate engineering of the topography-guided stress equilibrium of gold nanostructures. By using the topography-guided stress equilibrium, rich 3D shape transformation such as buckling, rotation, and twisting of nanostructures is precisely achieved, which can be predicted by our mechanical modeling. Benefiting from the nanoscale 3D twisting features, giant optical chirality is achieved in an intuitively designed 3D pinwheel-like structure, in strong contrast to the achiral 2D precursor without nano-kirigami. High-contrast cross-polarized diffractions are successfully observed in both linear and radial configurations, resulting from constant phase difference between the pinwheel structures of opposite handedness. We further demonstrate an on-chip and electromechanically reconfigurable nano-kirigami with optical functionalities. The nano-electromechanical system is built on a CMOS-compatible Au/SiO2/Si substrate and operated via attractive electrostatic forces between the top gold nanostructure and bottom silicon substrate. Large-range nano-kirigami like 3D deformations are clearly observed and reversibly engineered, with scalable pitch size down to 0.975 μm. The demonstrated nano-kirigami, as well as the exotic 3D nanostructures, could be adopted in broad nanofabrication platforms and could open up new possibilities for the exploration of functional micro-/nanophotonic and mechanical devices. Nanoscale control of optical spin and optical helicities with nano-kirigami are promising for numerous technological applications, such as chiral molecular sensing, separation and synthesis, super resolution imaging, nanorobotics, and ultrathin broadband optical components for chiral light.

Nanophotonic design spaces are often highly nonlinear; the large number of parameters (both geometric and material properties) will result in an exceptionally complex and high dimensionality design space that renders an exhaustive parameter sweep computationally insurmountable. Additionally, the design of a full nanophotonic device involves optimization at multiple levels, including material selection/design, device design, and system level design. In this talk, I will discuss multiple optimization frameworks for different design problems, including device level design for multispectral filters and phase modulation devices and system level design for beamsteering devices and other active metasurfaces. At the device level, we have (1) employed NOMAD (non-linear optimization using mesh adaptive direct search), a hybrid (global +local), derivative free optimization method, and (2) developed a new Bayesian optimization algorithm, MF-MI-Greedy, which uses multiple levels of fidelity, to optimize various nanophotonic structures including dielectric metasurfaces and multispectral filters. At the system level, we have (1) developed a new iterative genetic algorithm and (2) employed neural nets to optimize antenna arrays of active metasurfaces with non-ideal phase and amplitude characteristics. These studies suggest that selection of the correct optimization algorithm at a given design level can significantly reduce computational time for design and improve overall device performance.

Thin layers of van der Waals (vdW) materials, ranging from insulators to semiconductors, semi-metals and superconductors, exhibit novel optical, electronic, orbital, magnetic, topological and superconducting properties. Varying the interfacial stacking order of vdW homo/heterostructures further yields Moiré bands and new physical phenomena. Nanoscale strain-engineering of vdW materials (e.g., graphene, transition-metal dichalcogenides, TMDs) can induce novel quantum/topological phenomena, such as giant pseudo-magnetic fields (PMF) and flat bands associated with quantized Landau levels in monolayer graphene, and strain-dependent energy gaps and exciton lifetimes for monolayer 2H-phase TMDs. Quantum confinement due to reduced spatial dimensions and broken symmetry is known to induce new quantum states that are non-existent in the bulk. Spin-orbit coupling of light by optical vortices (a.k.a. twisted light, Laguerre-Gaussian beam) and plasmonic vortices produces photons with nontrivial spin/orbital-angular-momentum, which can impart integer and fractional topological charges onto condensed matter and atomic/molecular systems through light-matter interactions. Therefore, investigating and controlling the light-matter interactions in vdW nanomaterials/heterostructures/metastructures represents a pathway to explore new quantum phenomena and to advance opto-electronic/valleytronic/spintronic applications.

We employ scalable CVD and PECVD fabrication techniques to synthesize a variety of vdW nanomaterials and their heterostructures, including monolayer and bilayer graphene,^[1-3] quasi one-dimensional (1D) graphene nanostripes,^[1,4] h-BN,^[5] and monolayer-to-multilayer TMDs MX₂ and MX_2-yX′_y (M = Mo, W; X, X′ = S, Se, Te; 0 ≤y ≤2).^[6] We investigate their properties by means of spatially resolved and polarization-dependent photoluminescence (PL), time-resolved PL (TRPL), Raman spectroscopy, x-ray photoelectron spectroscopy (XPS), Kelvin-probe force microscopy, conductive atomic force microscopy (c-AFM), scanning tunneling microscopy/spectroscopy (STM/STS).^[6,7] We perform nanoscale strain engineering on monolayer graphene to control the strain-dependent PMF and electronic correlation, and to achieve valley/spin polarization;^[7] and on TMDs to control the energy gaps and exciton lifetimes. We develop plasmonic vortices on TMDs by metasurfaces together with circularly polarized light to control the light-matter interaction and valley/spin polarization in TMD and the topological charge. Our approach leads to various interesting findings, including: valley splitting, quantum oscillations, spontaneous symmetry breaking and spin polarization, quantum valley-Hall and quantum anomalous-Hall effects in valley Hall transistors based on nanoscale strain-engineered monolayer graphene;^[7] exceedingly long and temperature independent optically excited carrier lifetimes (~ 10 ns) as well as broadband (from IR to UV) photoluminescence in quasi-1D graphene nanostripes; much enhanced valley polarization in monolayer WS₂ by combining plasmonic vortices with circularly polarized light; and compositional dependent opto-electronic/valleytronic properties in WS_2-yTe_y (0 < y < 2). Based on these experimental findings, we discuss potential applications of van der Waals nanomaterials/heterostructures/metastructures to opto-electronic/valleytronic/spintronic devices.

References:

[1] N.-C. Yeh et al., Nanotechnology 30, 162001 (2019).
[2] D. A. Boyd et al., Nat. Commun. 6, 6620 (2015).
[3] Y.-C. Chen et al., Carbon 156, 212-224 (2020).
[4] C.-C. Hsu et al., Carbon 129, 527 (2018).
[5] W.-H. Lin et al., Chem. Mat. 29, 4700 (2017).
[6] W.-H. Lin et al., ACS Nano 14, 1350-1359 (2020).
[7] C.-C. Hsu et al., Sci. Adv. 129, 527 (2018).

The ability to control the optical properties of photonic materials on demand is essential for the operation of a variety of devices ranging from active color filters and routers to switchable mirrors and sensors. Here we will present our latest work on actively controllable photonic devices using alloyed metal hydrides. Using our recently developed apparatus for in situ measurements of optical and mechanical response upon hydrogenation, we first show how pure metal hydrides containing Pd, Mg, Zr, Ti, and V can be used as tunable photonic elements throughout the visible and near-infrared [1]. A number of devices will be shown including structures that have five orders of magnitude change in reflectivity, as well as planar systems for physical encryption and counterfeit detection. Second, we will show how alloying can not only improve cyclability, but also lead to higher hydrogen fractions than previously obtained as a result of film stresses and microstructuring [2]. These results have led to new insights in the dynamic behavior of metal-hydrides and will help in the design of next-generation hydrogen sensors and tunable photonic devices.

[1] Kevin J. Palm, Joseph B. Murray, Tarun C. Narayan, and Jeremy N. Munday, “Dynamic Optical Properties of Metal Hydrides,” ACS Photonics, 5, 4677–4686 (2018)
[2] KJ Palm, JB Murray, JP McClure, MS Leite, JN Munday, “In Situ Optical and Stress Characterization of Alloyed Pd_xAu_1–x Hydrides” ACS applied materials & interfaces 11, 45057-45067 (2019)

Chalcogenide glasses have recently received attention for their potential use in dielectric metasurfaces for several reasons: They typically possess high linear refractive indexes, enabling high index contrast devices; some compositions exhibit exceptionally high optical nonlinearity (n₂>1000× that of silica) making them good candidates for nonlinear devices; they have wide transmission windows spanning from the visible through the longwave infrared; and some compositions undergo a structural phase transition, enabling devices with switching based on phase-change. We discuss a variety of approaches to achieve tunability in chalcogenide glass metasurfaces from our work and that of others. We review recent progress on the use of the Kerr nonlinearity in As₂S₃ films to achieve power-dependent behavior and the application of this effect for the manipulation of orbital angular momentum. We discuss the design and realization of chalcogenide-based photonic quasicrystals enabling simultaneous phase matching of an arbitrary number of nonlinear optical processes in a single engineered nanostructure. We discuss recent observations that a permanent index change can be induced in As₂Se₃ via thermal tuning and the use of this effect to achieve permanent one-way tuning of the optical resonance of a metasurface. We discuss work being done by a variety of groups on phase-change chalcogenide materials such as germanium-antimony-telluride and gallium-lanthanum-sulfide in order to realize switchable metasurfaces. Finally, we discuss future prospects for the use of chalcogenide glass metasurfaces for applications that require tuning of the optical response.

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 3D dielectric elements, designed to be placed on top of the pixels of image sensors, that sort and focus light based on its color and polarization with efficiency significantly surpassing 2D absorptive and diffractive filters. The devices are designed via iterative gradient-based optimization to account for multiple target functions while ensuring compatibility with existing nanofabrication processes, and they are experimentally validated using a scaled device that operates at microwave frequencies. 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 further discuss capabilities enabled by volumetric meta-optical devices for compact on-chip polarimetry and discuss implementations using multi-layer nanofabrication.