Symposium EQ05—Plasmonics, Nanophotonics and Metaphotonics—Design, Materials and Applications

Topological photonics mimic the topological condensed matter physics to transport the photons efficiently and robustly. In this poster presentation, we present spin-valley locking of edge states through staggered chiral photonic crystals in the two-dimensional honeycomb lattice. We demonstrate the bulk band dispersion with a bandgap in the spin-valley locked edge states and by using a coupled dipole method. And the spin-valley locking characteristics can be observed when the staggered chirality breaks the inversion symmetry.

References
Kim, M., Kim, Y., & Rho, J. (2020). Spin-valley locked topological edge states in a staggered chiral photonic crystal. New Journal of Physics, 22(11), 113022.

A grand challenge for nanophotonics is the realization of tunable metasurfaces enabling active control of the key constitutive properties of light – amplitude, phase, wavevector and polarization. Active metasurfaces that enable dynamic modulation of reflection amplitude, phase and polarization have been recently explored using several active materials and modulation phenomena, including carrier index in plasmonic ENZ structures, reorientation of liquid crystal molecules, electrooptic effects in quantum well heterostructures and index change in phase change materials. The rapid advances in understanding of exciton resonances in layered van der Waals materials has now stimulated thinking about active metasurfaces that exploit excitonic modulation phenomena to enable ‘van der Waals active metasurfaces’. As one example, I will describe recent advances in electrically reconfigurable polarization conversion across the telecommunication wavelength range in van der Waals layered materials, integrated in a Fabry-Pérot cavity. The large electrical tunability of the excitonic birefringence in tri-layer black phosphorus enables spectrally broadband polarization conversion over nearly half the Poincaré sphere. We observe both linear to circular and cross-polarization conversion with voltage, demonstrating dynamic access to polarization diversity. As a second example, we discuss the observed large gate tunability of the complex refractive index in monolayer MoSe₂ by Fermi level modulation near the A and B excitonic resonances for temperatures between 4 K to 150 K. By tuning the charge density, we observe both temperature and carrier dependent epsilon-near-zero response in the permittivity and transition from metallic to dielectric near the A exciton energy, and directly observe active phase modulation in monolayer MoSe₂ gated heterostructures, whose voltage dependence is consistent with our complex index measurements. These results have broad implications for the use of monolayer transition metal dichalcogenides in active metasurfaces, and I also will give a general outlook for the wide range of possibilities for active van der Waals metasurfaces.

Plasmonic and dielectric metasurfaces have been widely investigated as a means to tailor the wavefronts of externally incident light for passive device applications. At the same time, their direct integration within active optoelectronic devices such as light emitters and photodetectors is far less established. In this work, we report the design and development of plasmonic metasurfaces that can promote directional light emission and angle-sensitive photodetection from a nearby active material. These results are significant for the continued miniaturization and large-scale integration of optoelectronic devices and related systems, and as a way to create new functionalities and enable novel applications. Specifically, the use of metasurface directional photodetectors for image sensing and processing will also be presented, including the demonstration of lensless compound-eye vision and optical spatial filtering for edge detection.

Nanostructured photonic surfaces are a promising platform for shaping the radiation pressure of light for optical manipulation beyond the limits of conventional approaches. Here, we present a framework for self-stabilizing optical manipulation of freestanding metasurfaces in three dimensions [1]. Our framework reveals that the challenging problem of self-stabilization against translational and rotational perturbations in three dimensions can be transformed to a simpler scattering analysis of the metasurface unit cell in two dimensions. We derive universal analytical stiffness coefficients applicable to arbitrary three-dimensional radial metasurfaces and radial beam intensity profiles. These derivations are confirmed with ray tracing simulations, eigenvalue analysis of the Jacobian, and numerical solutions of the full nonlinear equations of motion.

The implications of this reduced problem dimensionality are twofold. First, the framework is universally applicable to arbitrary embedded element profiles and/or radial beam variations. Second, our formalism enables efficient design of optomechanical metasurfaces as well as light beam configurations. We present examples of non-conventional phase-gradient profiles and beam intensity variations with substantial (e.g., order-of-magnitude) improvement in longitudinal propulsion force while maintaining self-stability.

These results facilitate the discovery of macroscale photonic objects for stable manipulation in collimated, but otherwise unfocused light beams. This ability to manipulate macroscopic objects is in stark contrast with traditional methods of optical tweezing which rely on focused laser beams and external feedback. Such use of metasurfaces for mechanical stabilization could open up new perspectives for long-range manipulation of macroscopic objects, with potential for terrestrial and space applications.

[1] Kumar, Kindem, Ilic, https://arxiv.org/abs/2106.11278.

Ultrafast nanophotonics aims at the development of nanodevices for light modulation with unprecedented speed [1]. A promising approach exploits the optical nonlinearity of nanostructured materials to modulate their effective permittivity by exploiting intense ultrashort optical pulses [2]. In plasmonic nanostructures such pulses trigger a delayed third-order nonlinear response ruled by internal relaxation of the metallic nanosystems, thus allowing all-optical light modulation. Currently, challenge in ultrafast pulse manipulation remains the feasibility of all-optical modulation of light over a broad bandwidth, with a full recovery over sub-picosecond timescale. Among different kinds of all-optical functionalities, ultrafast polarization switching has recently attracted a huge interest for advanced applications in photonics and beyond [3,4].
Here we combine quantitative numerical model with polarization-resolved pump-probe spectroscopy to predict and observe that photoinduced spatio-temporal transients can break the symmetry of a plasmonic metasurface, even if composed of highly symmetric metaatoms. Symmetry breaking generates a broadband dichroic response with ultrafast recovery of the isotropic configuration well before the complete relaxation of the nanostructure [5].
The plasmonic metasurface used is made of symmetric gold nanocrosses, with polarization independent static transmittance with a broad dip around 800 nm, due to the degenerate longitudinal plasmonic resonances of the nanocross two arms. Such a degeneracy can be broken by the resonant absorption of a pump pulse with linear polarization along the direction of one of the arms. Photoabsorption creates a highly inhomogeneous near field. This asymmetric absorption pattern in each metaatom locally affects the electronic population of gold, inducing a non-uniform out of equilibrium hot-carriers distribution which anisotropically modifies the metal permittivity. The fingerprint of the ultrafast pump-induced symmetry breaking is a transient transmission anisotropy, which can be revealed in a polarization-resolved pump-probe experiment.
The quantitative numerical model describes the ultrafast spatial transients in terms of spatial and temporal variables linked by a set of three coupled equations, referred to as the Inhomogeneous Three-Temperature Model (I3TM). The I3TM extends the popular 3TM [6], by including the spatial dependence of these variables.
We employ finite-element method numerical analysis in the frequency domain to calculate the transmission spectrum of the metasurface for a linear polarization parallel and orthogonal to the pump, for a set of permittivity configurations obtained by sampling the gold permittivity distribution at different pump-probe delays. Such spatio-temporal electronic transients at the nanoscale translate into an anisotropic local permittivity distribution, and subsequent ultrafast dichroic optical response of the metasurface, dominated by non-thermal hot-electrons dynamics. This modulation is broadband and returns to the isotropic configuration within few hundreds of fs. Thus, the main contribution to this phenomenon arises from non-thermal carriers while thermalized carriers contribute less. This is remarkable given that the electronic temperature relaxation takes place on a much longer time scale, far beyond the symmetry breaking window.
Our results pave the way to all-optical control of light at TeraHz speed, focused on ultrafast polarization management.
[1] A. V. Kildishev, et al, Science 339, 1289 (2013).
[2] N. Yu, F. Capasso, Nature Materials 13, 139 (2014).
[3] Y. Yang, et al. Nature Photonics 11, 390-396 (2017).
[4] L. H. Nicholls, et al. Nature Photonics 11, 628-633 (2017).
[5] A. Schirato, M.Maiuri et al. 14 723-727 Nature Photonics (2020).
[6] C. K. Sun,et al.Phys. Rev. B 50, 15337 (1994).

The ability to selectively interact with one circular polarization of light has wide ranging applications, including stereoscopic displays, document security, photonic circuits, biological detection, and others. Unfortunately, most molecular chiral materials only possess a weak circular dichroism (CD) response. Top-down fabricated pseudo-chiral metasurfaces have exhibited significantly stronger interactions with circularly polarized light (CPL), however it is often difficult and cost prohibitive to fabricate these lithographically patterned metasurfaces over large areas. In this presentation, we explore the effects of surface termination on the CD response of self-assembled, 3-D network-structure, optical metamaterials.
The gyroid is a triply periodic, minimal surface that contains screw axes in the [100], [110], and [111] directions and can be fabricated via block copolymer self-assembly. The single gyroid has been well described as a photonic crystal, where it has been shown to support both polarization dependent and complete photonic bandgaps. More recently, linear and circular dichroism have been detected in metal infilled gyroid optical metamaterials. In these structures both the bulk and surface structures affect the resulting optical properties; Dolan et al. recently showed that linear dichroism in a [110] oriented gyroid metamaterial arises from the anisotropic surface termination.
Here we use finite-element time-domain simulations to show that termination dependent resonances also play a dominant role in the CD response of [100], [110], and [111] oriented, silver infilled gyroid metamaterials. Films consisting of 4-unit cell thick gyroids with sharp interfaces and a unit cell size of 65 nm support CD resonances in the visible and near infrared on the order of 10 %, with large g-factors of 0.2. [100] oriented gyroids possess a termination dependent differential transmission of left- and right- CPL that repeats every quarter unit cell. Differential reflectance consists of strong, redshifted resonances that also occur every quarter unit cell, but alternate in handedness. These redshifted resonances arise from an antenna effect on the surface, where nearly touching nanostructures concentrate the incident electric fields by more than 100-fold. Varying the volume fraction of the silver infill results in different termination dependent CD responses, but the common theme of strong, redshifted resonances every quarter unit cell remains.
In the [110] direction, single gyroid metamaterials display both termination independent and termination dependent CD. The termination dependent resonance is observed in differential reflection and arises from a similar antenna effect. However, in contrast to the [100] direction, the surface terminated resonances of the two enantiomeric single gyroids are shifted by a quarter unit cell. Because of this, we find that [110] oriented double gyroids, which are achiral in the bulk, can also exhibit either a strong positive or negative CD response depending on the termination exposed.
Lastly, while [111] oriented gyroid metamaterials cannot generate the antenna structures discussed above, they do support a variety of hexagonally packed, triangular metasurfaces that repeat every third of a unit cell. Depending on termination selected, [111] oriented gyroid metamaterials can either selectively resonate at the top or bottom surface or support metasurface resonances at both surfaces simultaneously.
In conclusion, chiral gyroid metamaterials support strong, termination dependent CD responses in each of the high-symmetry directions. This presentation will explore the mechanism of this termination dependent resonant response.

Optical metasurfaces that provide spectral and spatio-temporal control of thermal radiation, particularly that in the near-to-mid infrared temperature regime, have promising applications in chemical fingerprinting, infrared imaging, and in thermophotovoltaics. Here we demonstrate a narrowband thermal emitter platform that is tunable via the application of a gate voltage. The emitter consists of a 1D photonic crystal supporting a guided mode resonance that couples to a localized surface plasmon mode of a proximitized graphene ribbon array. We show a narrowband resonance at 7μm which can be tuned into high and low emissivity states. The on/off ratio between the high emissivity and low emissivity modes of operation is over 60, and the Q-factor of the resonant on-state is greater than 5000. We investigate the performance of the device under realistic material conditions, and excitation schemes. Furthermore, the resonant absorption characteristic is well-modeled by a coupled-modes theory. Using this theory, we predict the response of the emitter over a large range of geometric parameters, in both the near-field coupled and the far-field coupled regime.

Metasurfaces are leading to the emergence of new optical components that circumvent the limitations of standard refractive and diffractive one by enabling dispersion engineering, which also leads to entirely new functionalities based on the local control of phase amplitude and polarization. Dispersion engineering has led to the demonstration of metalenses with correction of monochromatic aberrations and to achromatic metalenses and hybrid refractive/diffractive doublets across the visible spectrum. The planarity of flat optics will lead to the unification of semiconductor manufacturing and lens-making; recent industrial advances in this direction will be discussed. Polarization optics, polarimeters and polarization sensitive cameras without moving parts and conventional birefringent optics will be presented. Metasurfaces also offer fresh opportunities for structuring light as well as the dark. I will discuss spin to total orbital angular momentum (OAM) converters and OAM lasing, as well as flat devices that enable light’s spin and OAM to evolve, simultaneously, from one state to another along the propagation direction. Finally, the demonstration of 2D phase and polarization singularities and the unique applications that they will open will be discussed.

For efficient nonlinear processes, the engineering of the nonlinear optical properties of media becomes an important task. The most well-known technique for spatially engineering nonlinear optical properties is the quasi-phase matching scheme for second-order processes like second harmonic generation. However, the widely used technique of periodic polling of natural crystals only provides a binary state for the nonlinear material polarization, which is equivalent to a discrete phase change of π of the nonlinear polarization. The continuous tailoring of the phase of the nonlinear susceptibility would greatly enhance flexibility in the design and reduce parasitic effects. In this tutorial, we will discuss nonlinear metamaterials with a continuously controllable phase of the local effective nonlinear polarizability. We will focus on plasmonic metasurfaces with various designs for the meta-atom geometry together with different polarization states of the light. In particular for circular polarization states, the controllable nonlinearity phase results from the phase accumulation due to the polarization change along the polarization path on the Poincare Sphere (the so-called Pancharatnam-Berry phase) and depends therefore only on the spatial geometry of the metasurface. By using a fixed orientation of the meta-atom, the nonlinear phase can be spatially arbitrarily tailored over the entire range from 0 to 2π. In contrast to the quasi-phase matching scheme, the continuous phase engineering of the effective nonlinear polarizability enables complete control of the propagation of harmonic generation signals, and therefore, it seamlessly combines the generation and manipulation of the harmonic waves for highly compact nonlinear nanophotonic devices. We will discuss the concepts of enhancing nonlinear processes with simultaneous phase engineering for the manipulation of second- and third-harmonic generation from metasurfaces and the restrictions with respect to symmetry and geometry of meta-atoms. Nonlinear metamaterials have fundamental significance in nonlinear optics and for tailored nonlinearities, as they provide a further degree of freedom in the design of nonlinear materials.

Progress in understanding resonant subwavelength optical structures has fueled a worldwide explosion of interest in both fundamental processes and nanophotonic devices for imaging, sensing, solar energy conversion and thermal radiation control. For most nanophotonic materials, the optical properties are encoded and fixed permanently into the nanoscale structure at the time of fabrication. Achieving electronic tunability of the optical properties is an emerging opportunity to bring metamaterials and metasurfaces to life as dynamic objects composed of tunable nanoscale resonators and antennas. Gated field effect tuning of the carrier density in conducting oxides and two-dimensional materials enables the optical dispersion of individual structures to be altered from dielectric to plasmonic, yielding active nano-antenna arrays with electrically tunable absorption, radiative emission and scattering properties.

The dissolved gas concentration in a reactor has significant importance in the chemical and biological processing industries as mass transfer from the injected gas to an aqueous solution must occur for the gas to be usable. As the solubility of the gas in the solution is low and there are no probes for measuring dissolved gas concentration, a volumetric mass transfer coefficient is selected as a criterion of design for bio-chemical reactors. However, it is difficult to accurately predict the non-equilibrium state dissolved gas distribution using only the volumetric mass transfer coefficient. In this study, we predict a local concentration of dissolved carbon monoxide based on surface enhanced Raman spectroscopy and a distribution of the concentration in the reactor in a real time using computational fluid dynamics and machine learning. Through these techiniques, biological C1 conversion reaction will be monitored in real-time without offline sampling.

Acknowledgement: This work was supported by a National Research Foundation of Korea (NRF) under grant(No. 2020R1A2C2010986);C1 Gas Refinery Program through the National Research Foundation of Korea (NRF) under grant(No. 2018M3D3A1A01055759).

Plasmonic nanostructures facilitate the interaction of light and matter with applications in photodetection, photocatalysis, biochemical sensing, photoelectrochemistry, and energy harvesting. In particular, the overlap of plasmon resonances with the solar spectrum makes them well suited for renewable energy technologies. A central concept in plasmonics is the strong electric field enhancement when plasmons are squeezed into nanogaps between adjacent particles. Field enhancement is directly related to physical processes including hot carrier generation. Electric fields can be enhanced by using high quality plasmonic materials such as Cu, Ag, and Au and engineering nanogaps with dimensions approaching 1 nm. Tuning plasmonic resonances to match lattice diffraction modes can further boost electric fields. Additional enhancements are possible by adding electric functions to plasmonic devices, but few studies have successfully integrated electrical interconnects with plasmonic nanostructures, and scalable fabrication of arrays with sub-10 nm nanogaps is challenging due to limits on conventional lithography. In this paper, we present a study of the optical properties of large arrays of interconnected plasmonic nanostructures with sub-10 nm nanogaps for strong field enhancements.

We use a nanofabrication approach based on selective area atomic layer deposition to modify lithographically defined nanostructures to create large arrays of plasmonic dimers with nanogaps 1-5 nm. Our plasmonic dimers are nanorod shaped with dipolar resonances tuned at 800 nm. Copper is deposited selectively on palladium coated gold nanorods and the optical resonances are monitored as nanogaps are narrowed to less than 5 nm. Palladium is used as a seed layer to promote copper growth, while gold provides high quality plasmonic resonances. Compared with pure gold, the addition of palladium increases the FWHM from 86 nm to 158 nm due to the inferior plasmonic properties of palladium. However, after copper growth to create Cu/Pd/Au layered nanorods, resonances improve and FWHM decreases to 141 nm. At early stages of deposition, plasmon resonances blue shift, which is unexpected. Control experiments show that blue shifts are due to heat-induced rounding of features during the first cycles of growth. Single particle resolution AFM and high-resolution SEM data confirm feature rounding and reduced nanorod aspect ratios. Further deposition leads to red shifts due to narrowing of nanogaps between nanorods, as well reduction of FWHM from the improved plasmonic properties of Cu. Overall, resonance shifts are less than expected due to competing effects from deposition and heat induced shape changes.

Results for interconnected plasmonic devices are qualitatively similar to plasmonic dimers, but with more attenuation of the optical resonances. We investigate a dipole-line configuration where arrays of nanorods are interconnected and linear features are used as counter electrodes. Again, there are dominant blue shifts during the first few deposition cycles due to heat induced shape changes of the interconnected rods, but compared with control experiments, deposition adds more broadening. Continued deposition to reduce nanogaps to less than 10 nm causes expected red shifts and sharpening of the resonances, but compared to unconnected plasmonic dimers, the optical resonances are significantly broadened. Analysis of high-resolution SEM images shows that the electrical contacts contribute to reshaping of the dipole antennas more extensively than for unconnected devices. The reduced aspect ratios from reshaping leads to broader resonances. Strategies to improve the quality of resonances will be discussed. In this paper, we also discuss surface lattice resonances in the extended arrays, which can be used to reduce FWHM and increase local electric field intensity for hot carrier devices.

Plasmonic metal nanoparticles have emerged as particularly attractive exogenous contrast agents for a wide variety of biomedical imaging techniques because their unique properties, including intense light absorption/scattering induced by the localized surface plasmon resonance (SPR), excellent biocompatibility, and lack of toxicity, hold great potential to improve these imaging techniques for detecting early-stage diseases with high sensitivity and specificity.

In this presentation, we first describe a facile and green method of fabricating chain-like gold nanoparticle (CGNP) clusters with narrow gap distances and a red-shifted longitudinal SPR that is tunable from 590 nm to 710 nm. These CGNP clusters are fabricated via self-assembly of gold NP monomers generated by a method of pulsed laser ablation (PLA) of the bulk target in deionized water. These PLA-generated gold NP monomers have unique capping agent-free surface and high purity, which allow precision surface engineering with different types of ligands in order to self-assemble them into highly linear nanochains [1,2].

In the second part of this presentation, the application of CGNP clusters as contrast agents for enhanced in-vivo photoacoustic microscopy (PAM) molecular imaging of eye diseases, named choroidal neovascularization, in live rabbits is explained. The synthesized CGNP clusters show excellent biocompatibility and photostability. These CGNP clusters are conjugated with linear arginine–glycine–aspartate (RGD) peptides (CGNP clusters- RGD) for molecular imaging. After intravenous injection of CGNP clusters-RGD, they bind to CNV in rabbit eyes, resulting in a selective PAM imaging of CNV only at 650 nm due to minimal tissue absorption at that wavelength. In addition, from the comparison of PAM signals before and after intravenous injection of CGNP clusters-RGD, it has been observed that CGNP clusters provide up to a 17-fold increase in PAM signal at 650 nm [3].

References
[1] W. Qian, M. Murakami, Y. Ichikawa, Y. Che, J. Phys. Chem. C., 115, 23293-23298, (2011).
[2] A. McLean, M. Kanetidis, T. Gogineni, R. Ukani, R. McLean, A. Cooke, I. Avinor, B. Liu, P. Argyrakis, W. Qian, R. Kopelman, Chemistry of Materials, 33, 2913-2928. (2021).
[3] V. P. Nguyen, W. Qian, Y. Li, B. Liu, M. Aaberg, J. Henry, W. Zhang, X. Wang, Y. M. Paulus, Nature Communications, 12:34 (2021).

Toxic organophosphorus pesticides and chemical warfare agents (CWAs) create health hazards for both civilian and military personnel, necessitating the design of materials that offer protection against the target molecules. Traditionally, activated carbon is used in this role, but high-surface area carbons typically only adsorb CWAs; the intact molecules persist on the material surface. Materials that adsorb and then degrade CWAs into nontoxic products are a desirable alternative as they negate the risk of re-exposure and lessen disposal burdens for spent materials. Oxide aerogels decorated with metal nanoparticles (NPs) provide ideal platforms to achieve this degradative sorption. Oxide aerogels provide a high surface area, mesoporous network that is stable under ambient conditions, while the addition of metal NPs offers additional chemical pathways and improved kinetics for CWA degradation. In this talk we compare the activity of metal NP–decorated oxide aerogels (Au/TiO₂, Cu/TiO₂, and Cu/CeO₂) for degradation of CWA simulants (DMMP), as well as live agents such as sarin, under a wide variety of environmental conditions. Generally, oxide aerogels adsorb these molecules intact under dark and dry conditions, but drive hydrolysis under humid and aerobic conditions and initiate photooxidative degradation upon excitation of the semiconducting bandgap with broadband light. Metal NP–decorated aerogels expand these capabilities by providing highly reactive basic OH sites at the interface between metal NP and networked oxide NPs to achieve rapid degradation under dark conditions. This arrangement further accelerates degradation under illumination by leveraging plasmonic absorption of visible light. Our results reveal oxide aerogel–supported metal NPs to be promising materials for decontamination of organophosphorus compounds under a wide variety of environmental conditions.

Quantum dots (QD) form an emerging class of photo- and electroluminescent materials. Their narrow and tunable emission spectrum offers the benefits of a broad excitation-energy range with an environmentally stable, narrow emission. QD-based LED is a strongly active research field. In the typical device structure, the QD electroluminescent layer is stacked within hybrid heterostructures. Holes and electrons, injected from opposite sides through the application of a DC voltage, recombine radiatively in the QD layer, leading to light emission [1]. These devices suffer from a major drawback: the materials (their energy configuration and refraction index) of the stacked layers should be carefully chosen so as to optimize the energy-band diagram of the system for efficient electron-hole recombination in the photoactive layer [2] and minimize internal light reflection.
A further way exists to obtain emission of light from an electroluminescent QD layer, without any charge injection. It consists in sandwiching such a layer between two transparent dielectric layers and applying an alternate voltage. The presence of an electric field inside the photoactive region induces a relative shift between the energy levels of nanoparticles at different positions. This allows, above a threshold field, the creation of electron-hole pairs and their transport within the active layer. The recombination of an electron/hole pair on the same QD is at the origin of the observed light emission [3].
Previous QD-based devices present several limitations: the use of toxic Cd-based QD [3,4,5], the necessity of a complex multilayer structure [4,5] or the necessity of a combination of QD and bulk layers to obtain light emission [6].
We present a device overcoming all these limitations. Our device is based on an active layer of spin-cast non-toxic Mn-doped ZnS nanoparticles, sandwiched between two dielectric layers. By exploiting one-pot surfactant-free microwave-assisted polyol synthesis, we obtain high-crystalline nanoparticles, with enhanced phosphorescence emission due to Mn ions [7] at the solid state, which can be closely packed in the emitting layer, allowing an emission mechanism due to local charge creation and transport with only three layers. This new synthesis route paves the way to far less expensive paths for the active layer deposition such as spin coating or inkjet printing.
More specifically, our device is based on a glass substrate covered with a conductive and transparent thin layer of Indium Tin Oxide (ITO), followed by a dielectric layer deposited by Atomic Layer Deposition (ALD), a spin-cast ZnS:Mn nanoparticle film, and a second dielectric layer. Finally, metallic contacts are deposited by thermal evaporation.
Our device is characterized structurally by means of Scanning Electron Microscopy, showing an active layer with a very compact granular structure. We observe electroluminescence from the device only above a voltage threshold, coherently with previous observations. A direct comparison between electroluminescence and the photoluminescence spectra clearly shows that the observed luminescence corresponds to the emission coming from the transition taking place in Mn ions.
Monte Carlo simulations allow to show that the excitation mechanism cannot be linked to any impact triggering. Moreover, an effective-medium description of the active layer, along with a capacitance measurement on the device, allowed us to highlight a high packing of the QD inside the active layer. Moreover, the calculated potential fall per QD is compatible with an interpretation of light emission in terms of field-induced charge transfer and recombination.
References
[1] X. Dai et al., Nature 515, 96 (2014)
[2] V. Wood et al., ACS Nano 3, 3851 (2009)
[3] V. Wood et al., Nano Lett. 11, 2927 (2011)
[4] Z. Yang et al., Mat. Today 24, 69 (2019)
[5] Y. Shirasaki et al., Nat. Photonics 7, 20 (2013)
[6] V. Wood et al., Nano Lett. 9, 2367 (2009)
[7] D. Bera et al. Materials 3, 2260 (2010)

The detection of volatile organic compounds (VOCs) at trace levels (ppb to ppm) is of extreme importance in the field of environmental monitoring for health related matters. The classical method for such measurements is gas chromatography, which is time consuming, expensive, and not portable. Spectroscopic techniques in the mid infrared (3–20 μm) are able to detect directly the unique fingerprints of molecular transitions, therefore offering a valid alternative in terms of versatility and selectivity. The target substance with an active IR vibration can be detected from the unique fingerprint of its vibrational transitions, without the need for an intermediate transducer, or any prior knowledge. However, light–matter interaction is extremely weak in the IR, due to the small absorption cross-section (∝ a³/λ), with a being the molecule characteristic size. Long optical paths are needed to boost the signal to noise ratio for applications where detection at ppb levels is required. To keep a small footprint, it has been proposed to coat mid-IR waveguides or optical fibers with materials able to adsorb species of interest, thereby locally increasing the concentration. However, this approach often lacks sensitivity that is required in many applications. One example is the monitoring of benzene at ppb or sub ppb levels, that remains challenging even if a separate pre-concentration step is employed.
Surface-enhanced infrared absorption (SEIRA) has proven effective in detecting solid state monolayers via coupling resonant nano-antennas to vibrational transition of target molecules (Neubrech et al. Chem. Rev. 2017). However, for this to work, the target molecules have to be very close to the antenna surface, where the field enhancement is the highest. For low concentrated gases the probability of a molecule transiting within the strong field enhancement region can be very low, hampering the use of SEIRA for the detection of low concentrated volatile compounds.
In this talk we will present a hybrid system composed of a plasmonic nano-antenna array (gold nanorods) coupled with nanosized zeolite (Awala et al. Nat. Mat. 2015) coating for detection of volatile organic compounds in the near field of the resonators, enabling to extend the SEIRA toward the detection of gases at very low concentrations. We will show that the concerted action of the FAU zeolite coating and the resonant nano-antennas enables us to attain performance equivalent to a ≈120 m long optical path, and led to the detection record traces of benzene (25 ppb) within 10 min (Nunez et al. Adv. Func. Mat., 2021). To the best of our knowledge this sets a record for benzene detection using IR spectroscopy without external pre-concentration steps, and represent a great enhancement with respect to the low ppm range detection limit reported in the literature by using IR spectroscopy or IR tunable lasers.
Then, we will show detailed characterisation of the system properties (SEM, EDX, optical), compare to numerical simulations (FDTD) and assess performance as a function of IR light polarisation. Finally, we will show that we can discriminate benzene from other VOCs such as toluene.
This approach may ultimately allow the fabrication of a compact system for rapid detection of pharmaceutical and biocompounds with high sensitivity and high selectivity.

During de last decades photonic crystals (PC) have been widely studied. Most of these studies have been focused on the theoretical behaviour of these structures among almost all their configurations. Nevertheless, their experimental performance has been discussed but in a narrower number of configurations mostly focusing on the 1D and 2D structures based on a holed slab. In this work, we want to present both simulation and nanofabrication results of 2D PC based on silicon nanopillars on silicon-on-insulator (SOI) substrates. The use of this reverse configuration has been scarcely studied and brought to a successful experimental performance. The addition of waveguides (WG) and cavities within the structure will allow light control through it. The potential application of these structures will be the development of a force sensor, which will measure horizontal forces. This system will operate in the Near Infrared (NIR) wavelength range.
The design of the structures has been carried out using the MEEP Python library what uses the Finite-Difference Time-Domain (FDTD) method to solve Maxwell’s Equations over time and using eigenmode and eigenfrequencies solver. The determination of the best configurations has been done with a thorough study of the band diagrams of the structures both in 2D and 3D and their Transmission-Reflection spectra. Both strategies have shown good agreement between them and with literature. These simulations have shown the opening of a photonic bandgap (PBG) between 1.3 and 1.9µm for Transversal Magnetic (TM) polarised light. These results have been obtained for a triangular structure using pillars of 100nm radius and 500nm interpillar distance. The introduction of a WG shrinking de radii of one line of pillars has given rise to the introduction of a mode within the PBG raising from the bottom bands. Finally, the insertion of cavities within the WG has promoted the apparition of resonance peaks.
The nanofabrication of these structures has been carried out inside an advanced clean-room facility. A multi-step process has been studied and optimised for the successful fabrication of nanopillars with radii from 100nm down to 60nm. SOI substrates have been used to ensure the vertical confinement of light. The fabrication process uses an electron beam lithography (EBL) exposure, followed by a lift-off, and ending with an anisotropic dry etch of the silicon layer. The EBL process consists of the deposit of 180nm CSAR-P6200.09 photoresist, followed by a 20nm-thick layer of thermally-evaporated aluminium to ensure the correct discharge of the sample during the exposure. A JEOL JBX-9500FSZ e-beam writer, using a 100kV electron beam, is used for the exposure. The thermal aluminium layer is removed using TMAH, while the photoresist is developed using AR 600-546 developer. The lift-off process is carried out using Remover 1165 after the e-beam deposition of 20nm of aluminium. In this manner, the remaining photoresist will be removed leaving only the aluminium that was directly in contact with the substrate. This aluminium will work as a mask material during the etching step. The latest consists of a reactive ion etching (RIE) process which uses a combination of SF₆:C₄F₈ chemistries resulting in an anisotropic etch of the uncoated silicon and giving rise to the nanopillars.
Discussion about the similarities and differences between the simulated and the fabricated structures will be carried out and, if different, proposals of improvement in the nanofabrication process will be brought up.

InGaAs is an ideal material for photodetection at telecommunications wavelengths thanks to its direct bandgap, which is tunable by changing the relative In and Ga content. However, InGaAs devices typically require InP substrates, which are costly and difficult to process. Moreover, the In content of the InGaAs layer is limited by the requirement for lattice matching.[1] Nanowires (NWs) are a promising platform for heteroepitaxy of InAs and InGaAs on more common substrates like Si and GaAs thanks to their inherent strain relaxation. NWs also offer several performance advantages over bulk devices.[2] As an example, their thin, anisotropic geometry enables polarization-dependent absorption and fast carrier collection, making them suitable for multiplexed applications.[3] NWs also benefit from a significant reduction in the amount of scarce materials, such as In, required to make a device, and strain relaxation machanisms allow a greater degree of freedom over their chemical composition. NWs are most often grown vertically, perpendicular to the substrate, such as by the vapor-liquid-solid or vapor-solid techniques. However, vertical NWs are difficult to incorporate into devices, usually requiring transfer of a single NW to a secondary substrate, a process that is not scalable. A horizontal configuration is therefore preferred for many applications. Recently, horizontal networks of high-quality In(Ga)As NWs have been demonstrated by growing them on top of GaAs nanomembrane buffers using selective-area epitaxy on III-V substrates.[4,5]

In this work we investigate the growth of horizontal In(Ga)As NWs on two different crystal orientations of GaAs substrates by molecular beam epitaxy. For growth on (111)B substrates, we demonstrate InGaAs NWs positioned atop high aspect ratio GaAs nanomembranes and describe how such NWs might be used for advanced photodetector architectures. We also show that the In content of these NWs can be tuned by controlling growth conditions, but pure InAs NWs are not achievable.[6] Next, we demonstrate In(Ga)As NWs grown on (100) substrates. The NWs grow conformally on low-aspect-ratio GaAs nanomembranes, whose morphologies are dominated by either {111} or {110} facets, depending on their alignment on the substrate. We demonstrate the ability to control the In content of the InGaAs NWs with basic growth parameters and characterize the homogeneity of the alloy composition as a function of the NW geometry. We also show that pure InAs can be achieved with just a narrow intermixed region near the interface. Finally, we present preliminary electrical characterization from basic four-point photoconductive devices based on NWs grown under different conditions. The charge transport properties are largely determined by the crystal quality. Correspondingly, the presence of a surface passivation layer has a significant influence on the overall photoresponse. We compare NWs with and without a passivating shell and characterize the photoconductive response for both single NWs and ensembles in an array. Insights from these preliminary results will guide design choices for future photodetector devices based on these NWs.


¹ R. LaPierre et al., J. Phys. D: Appl. Phys. 50 (2017)
² C. L. Tan et al., Nanophotonics. 7 (2018)
³ A. Dorodnyy et al., IEEE J. Sel. Top. Quant. Electr. 24 (2018)
⁴ M. Friedl et al., Nano Lett. 18 (2018)
⁵ F. Krizek et al., Phys. Rev. Mat. 2 (2018)
⁶ M. Friedl et al., Nano Lett. 20 (2020)

Funding from Swiss National Science Foundation, NCCR QSIT

Within one hour, the sun supplies our planet with energy sufficient for humanity’s yearly energy consumption¹. Nanowire solar cells are a sustainable complementary technology to silicon solar cells, as they have the potential to reach the efficiencies of world-record III-V solar cells while only using about 10 % of the material². Although nanowire solar cells have demonstrated high efficiencies^3-6 and proven to have benefits compared to their bulk counterparts such as superior radiation tolerance⁷ the next step in nanowire solar cell technology – a tandem-junction solar cell that combines different band gaps within the same nanowire – has been elusive up to now.
Here, we report on single nanowire tandem junction solar cells based on both InP/GaInP and InAsP/InP. These lattice mismatched combinations of materials are accessible because of elastic relaxation at the lateral free surface of nanowires⁸. The development of the nanowire tandem junction photovoltaics was supported by a setup that combines a nanoprobe system with electron beam induced current (EBIC) measurements and a light emitting diode (LED) inside a scanning electron microscope⁹. By the use of the nanoprobe system we show voltage addition of the two sub-cells. We are able to study both sub-junctions independently with EBIC, by either using a light bias from the LED setup to saturate the bottom-junction, or by using a voltage bias to saturate the top-junction. We believe that this type of characterization will be helpful to further optimize the efficiency of nanowire tandem junction solar cells, and to develop nanowire multi junction solar cells using more materials similar to the six-junction solar cell which has the record efficiency of 47.1 %¹⁰.

1. Morton, O. Nature 2006, 443, (7107), 19-22.
2. Anttu, N. Acs Photonics 2015, 2, (3), 446-453.
3. Wallentin, J.; Anttu, N.; Asoli, D.; Huffman, M.; Åberg, I.; Magnusson, M. H.; Siefer, G.; Fuss-Kailuweit, P.; Dimroth, F.; Witzigmann, B.; Xu, H. Q.; Samuelson, L.; Deppert, K.; Borgström, M. T. Science 2013, 339, (6123), 1057-60.
4. van Dam, D.; van Hoof, N. J.; Cui, Y.; van Veldhoven, P. J.; Bakkers, E. P. A. M.; Gomez Rivas, J.; Haverkort, J. E. ACS Nano 2016, 10, (12), 11414-11419.
5. Åberg, I.; Vescovi, G.; Asoli, D.; Naseem, U.; Gilboy, J. P.; Sundvall, C.; Dahlgren, A.; Svensson, K. E.; Anttu, N.; Björk, M. T.; Samuelson, L. IEEE Journal of Photovoltaics 2016, 6, (1), 185-190.
6. Hrachowina, L.; Zhang, Y.; Saxena, A.; Siefer, G.; Barrigon, E.; Borgström, M. T. In Development and Characterization of a bottom-up InP Nanowire Solar Cell with 16.7% Efficiency, 2020 47th IEEE Photovoltaic Specialists Conference (PVSC), 15 June-21 Aug. 2020, 2020; pp 1754-1756.
7. Espinet-Gonzalez, P.; Barrigón, E.; Otnes, G.; Vescovi, G.; Mann, C.; France, R. M.; Welch, A. J.; Hunt, M. S.; Walker, D.; Kelzenberg, M. D.; Åberg, I.; Borgström, M. T.; Samuelson, L.; Atwater, H. A. ACS Nano 2019, 13, (11), 12860-12869.
8. Glas, F. Physical Review B 2006, 74, (12).
9. Barrigón, E.; Hrachowina, L.; Borgström, M. T. Nano Energy 2020, 78, 105191.
10. Geisz, J. F.; France, R. M.; Schulte, K. L.; Steiner, M. A.; Norman, A. G.; Guthrey, H. L.; Young, M. R.; Song, T.; Moriarty, T. Nature Energy 2020, 5, (4), 326-335.

Size-dependent changes in the electronic band structure is one of the key features of nanoparticles in the quantum confinement region. CuFeS₂ nanoparticles have a strong absorption feature in the visible region that has, controversially, been described as neither an excitonic transition nor a free carrier plasmon oscillation. Instead, the absorption feature in CuFeS₂ nanoparticles has been attributed to quasi-static optical resonances from inter-band transitions between the valence, intermediate, and conduction bands. Previous work has reported conflicting accounts of the existence of this absorption band, the size-dependent optical properties, and the cation oxidation states in this CuFeS₂.
In this work we answer these outstanding questions and determine the origin of the plasmonic behavior in CuFeS₂ by characterizing the nucleation and growth stages of the reaction through a series of ex-situ and in-situ probes (e.g., x-ray absorption spectroscopy (XAS) and x-ray emission spectroscopy (XES)), and by characterizing a size-series of nanoparticles through experiment and theoretical calculations and models. We found that CuFeS₂ nanoparticle growth is initiated by Cu_2-xS seeds formation and then gradual Fe cation addition. Moreover, the growth of the quasi-static resonance feature during synthesis is driven by the band structure modification from Fe²⁺ incorporation into the nanocrystals. As opposed to the pure Cu¹⁺/Fe³⁺ predicted by DFT studies, a mixed oxidation state of Cu¹⁺/Cu²⁺ and Fe²⁺/Fe³⁺ is observed, indicating that the chalcopyrite phase (CuFeS₂) encompasses a range of oxidation states depending on the synthesis conditions. Moreover, by examining a size-series of particles we find the quasi-static optical resonance redshifts while the absorption edge blueshifts as the particle size decreases. Through density functional theory (DFT) and tight binding modeling, we elucidate the size dependence of the band structure, especially focusing on the change in the intermediate band. Using a Lorentzian oscillator optical model to simulate the absorption spectrum with inputs from the DFT calculated band structure and band shifts from the tight-binding model, we find the size-dependent shifts of the optical resonance peak position in CuFeS₂ is due to a tri-band quantum confinement effect that results in both the valence band to intermediate band and intermediate band to conduction band bandgap expansion that accompanies a decrease in particle size. Moreover, the linear optical Lorentzian model predicts the optical resonance peak is tunable across the visible range by partially filling the intermediate band, lowering the conduction band, or expanding the intermediate band.
Overall, in this study, we provide fundamental understandings about the origin of the visible quasi-static resonance in CuFeS₂ nanoparticles, and elucidate the correlation between the absorption feature shift and the band structure. These results are relevant to other quantum-confined intermediate band systems, which have potential use in optical applications as well as photovoltaic devices.

Image processing has become a critical technology in a variety of science and engineering disciplines. While most image processing is performed digitally, optical analog processing has the advantages of being low-power and high-speed though it requires a large volume. Meta-optics provide the advantage of thin form factor optics while also allowing complex transfer functions to be employed. In this talk, I will discuss the use of meta-optics for applications in image processing. Specifically, I will discuss meta-optic pre-filters including edge filters as well as filters for identifying higher level spatial features. These meta-optics are designed in conjunction with the digital system and I will discuss how co-design can make the hybrid optical / digital system more tolerant to deterministic and stochastic noise. These analog optical operations can be used to replace, or augment, digital processes for increasing speed while reducing power consumption.

Bulk optics have been used for various optical instruments such as microscope or camera, but it has limitations in size and thickness because of accumulation of various lenses. Metalenses are an emerging field as one of the next-generation optical technology for miniaturization of optical system. Subwavelength thick nanostructures on metalens control the amplitude, phase, absorption, emission, and transmittance of incident light. Focal length can be control by adjusting phase shift from 0 to 2 according to the change of shape, size, and period of nanostructures. Many kinds of metalens such as GaN nanopillar, Si nanodisk, or TiO₂ nanofin have been studied recently, but chromatic aberration correction problem remains as the limitation in metalenses. Some researches such as group delay dispersion have been conducted to correct chromatic aberration, low focusing efficiency ( ~ 10 %) is unresolved limitation.
In this work, we suggested a new method to design achromatic metalens for 400, 600 and 800 nm wavelengths focusing. Spatial multiplexing method can correct chromatic aberration very simply. First, three metalenses are designed for each wavelengths. In the next step, meta-atoms of each metalenses are interleaved in one metalens. The coupling between three kinds of meta-atoms is small, therefore we can expect three wavelength focusing in one metalens. We used finite-difference time-domain (FDTD) simulation to investigate field distribution and focusing efficiency of each metalenses. By using this method, we demonstrated achromatic metalens with numerical aperture of 0.89, which focus light at 400, 600, and 800 nm to same focal spot. Focusing efficiency of achromatic metalens for 400, 600 and 800 nm are 38, 44 and 54 %, respectively. This design method can attend significant role to demonstrate desired achromatic metalens.

Laser-driven optical near-fields mediate stimulated emission and absorption of photons by free electrons [1,2], providing unique control over the electron wave front in both space and time [3-5]. Besides a large field amplitude, efficient energy exchange in this process requires a match between the phase velocity of the optical field and the electron velocity – a condition that can be only fulfilled in the presence of matter [6,7]. Here, we explore how optical near fields can be tailored to efficiently manipulate slow non-relativistic electrons at the nanoscale. We design a thin-film metallo-dielectric metasurface perforated by a hexagonal array of circular holes, ensuring transparency to electrons while enabling diffractive coupling of resonant lattice modes to an external light field. The layer periodicity is optimized such that the normal component of the optical field inside the holes is dominated by a single Fourier component at a spatial frequency ω/v, determined by the electron velocity v and the optical pump frequency ω.

Experiments are performed in a scanning electron microscope (SEM) at a design electron energy of 5 keV. The metasurface is illuminated by a ns-pulsed near-infrared pump laser synchronized with sub-ns electron pulses that are generated by an ultrafast electrostatic beam blanker. To measure the energy distribution of electrons transmitted through the hole array, we use an electrostatic retarding field analyzer (RFA) with an energy resolution on the order of 0.1 eV. The employed Schottky field emission source provides an intrinsic electron energy spread as low as ΔE < 0.9 eV, allowing us to fully resolve discrete electron energy-gain and -loss sidebands that are evenly spaced at multiple integers of the pump photon energy of 1.2 eV. The strength of the electron-near-field coupling can be quantified by the total width of the measured electron energy distribution, with its complex shape revealing the intriguing quantum nature of the electron-near-field interaction process. We show spatially resolved measurements of the electron-near-field interaction strength across the hole array and discuss how in-plane variations can be controlled to tailor the temporal and transverse phase profile of the incident electron beam.

Our work combines concepts of nanophotonics with a growing research field on coherent electron wave front shaping, while demonstrating SEM as a versatile and robust platform to study new quantum optical phenomena at the nanoscale. For the future, we plan to gain deeper insights into the wave/particle duality of free electrons by probing the emission of coherent cathodoluminescence and correlating it with the temporal shape and spatial phase profile of pre-defined electron quantum states.
References
[1] B. Barwick et al., Nature 462, 902-906 (2009).
[2] A. Feist et al., Nature 521, 200-203 (2015).
[3] K. E. Priebe, et al., Nat. Photonics 11, 793-797 (2017).
[4] Y. Morimoto and P. Baum, Nat. Phys. 14, 252-256 (2018).
[5] G.M. Vanacore et al., Nat. Commun. 9, 2694 (2018).
[6] F.J. Garcia de Abajo and M. Kociak, N. J. Phys. 10, 073035 (2008).
[7] S.T. Park et al., N. J. Phys. 12, 123028 (2010).

Metasurfaces have paved the way for high-performance wavefront shaping and beam steering applications. Phase-gradient metasurfaces (PGM) are of high importance owing to the powerful and relatively systematic tool they offer for manipulating electromagnetic wavefronts and achieving various functionalities. Herein, we numerically present a novel unit cell known as bipodal Cylinders (BPC), made of Silicon (Si) and placed on a Silicon dioxide (SiO₂) substrate to be compatible with CMOS fabrication techniques and to avoid field leakage into a high index substrate. Owing to its geometrical structure, the BPC structure provides a promising unit cell for electromagnetic wave manipulation. We show that BPC offers a way to shift the electric dipole mode to a frequency higher than that of the magnetic dipole mode. We investigate the effect of varying different geometrical parameters on the performance of such unit cell. Building on that, a metasurface is then presented that can achieve efficient electromagnetic beam steering with high transmission of 0.84 and steering angle of 15.2^o; with very good agreement with the theoretically predicted angle covering the whole phase range from 0 to 2.

Fano resonance arises via interference between discrete and continuum bands. It undergoes a dramatic change in line shape shifts with any small perturbations in the system, thus proving its potential application in active tuning for display and sensors applications. Here, we explore and demonstrate the generation of Fano resonance from the coupling of symmetric or asymmetric-shaped Metasurfaces with Bragg reflectors. We have utilized a scalable route of soft-nanoimprinting lithography for the generation of Metasurfaces and incorporated layer-by-layer assembly of slide-ring-based materials to offer desirable elasticity and strain-tunable optical properties in the integrated device. Furthermore, we study the active tuning of the device by stretching the array in the x- and y-axis and the reflector in z-direction which allows for wide color tuning ability with narrow bandwidth in the colorimetric response. Our experimental findings agree well with simulations depicting high feature fidelity of Metasurfaces. By stretching the resulting device over 40% strain we demonstrate the mechano-chromic sensing abilities in a span of wavelengths.

Nanoscale laser designs have seen major advances over the last decade, with breakthroughs in cavity architectures and gain materials. Plasmonic lasers based on nanoparticle lattices are of special interest because the cavity structure can be tuned at the unit-cell level, which then affects the type of near-field optical feedback contributing to lasing action. Moreover, depending on whether the gain materials are molecular or nanoparticulate in nature, new optical modes can contribute to the amplification process. This talk will describe different classes of plasmonic nanoparticle lasers that can produce reversible and switchable lasing. We will focus on both static and dynamic approaches to achieve tunable color and polarization at multiple, different wavelengths. Finally, we will discuss prospects for white-light lasing.

Despite wide investigation on molecular imaging contrast agents, there are still strong unmet medical needs to enhance their signal-to background ratio, brightness, photostability, and biocompatibility with multimodal imaging capability. Here, we assessed the feasibility of fluorescent nanodiamonds (FNDs) as carbon based photostable and biocompatible materials for molecular imaging applications. Because FNDs have negatively charged nitrogen vacancy (NV) centers, they can emit bright red light. FNDs were conjugated to hyaluronate (HA) for the target-specific molecular imaging. HA is a biocompatible, biodegradable, and linear polysaccharide with abundant HA receptors in the liver, enabling the liver targeted molecular imaging. In vitro cell viability test revealed the biocompatibility of HA-FND conjugates and the competitive cellular uptake test confirmed their target-specific intracellular delivery to HepG2 cells with HA receptors. In addition, in vivo fluorescence lifetime (FLT) assessment revealed the imaging capability of FNDs and HA-FND conjugates. After that, we could confirm the statistically significant liver-targeted delivery of HA-FND conjugates by in vivo imaging system (IVIS) analysis and ex vivo biodistribution tests in various organs. The renal clearance test and histological analysis corroborated in vivo biocompatibility and safety of HA-FND conjugates. All these results demonstrated the feasibility of HA-FND conjugates for further molecular imaging applications.

Optical nanoantennas have become ubiquitous tools to enhance the spontaneous emission of atoms, molecules and quantum dots. In order to design practical sources with high power, it is necessary to move from a single emitter to an ensemble of emitters.

We can therefore use layers of emitters distributed on top of planar arrays of antennas of metallic or dielectric nanostructures to build light-emitting metasurfaces. They are designed to control the phase and amplitude of the electromagnetic modes in the plane. They can be engineered to emit light in specific directions and for specific wavelengths and polarization. However, metasurface designs rules that have been established over the years are still based on the understanding that optical antennas operate through the Purcell effect. Models based on this paradigm fail to fully explain spectral and spatial features observed in many experiments, such as the emergence of spatial coherence or spectral modifications of emitters.

In a first part, we start by discussing the differences between emission by a single emitter and emission by a thermalized assembly of quantum emitters to show that a statistical framework is required to understand their interactions with optical antennas. We then introduce a model of light emission by thermalized ensembles of emitters, based on fluctuational electrodynamics and the fluctuation-dissipation theorem [1]. A generalized form of the Kirchhoff's law, can be derived and extended to photoluminescence and electroluminescence. Light emission by the structure can be connected to the local absorptivity rate within the structure [2].

In a second part, we use our modeling procedure in the context of two experimental and theoretical studies of light emitting systems. Firstly, we focus on the properties of light emitted by a patterned ensemble of quantum dots consisting in a lamellar grating on top of a slab. This system exhibits spatial and spectral features, such as bright, directional and polarized emission. Our model accurately reproduces and predicts all features of the system’s photoluminescence. It also addresses modification of the optical properties of the resonator by the pumping process, allowing to explore regimes of intense pumping of the emitters.

Secondly we use our approach to explore the origin of spatial coherence in light-emitting structures. We consider a system consisting in layers of molecular dyes deposited on top of a silver metallic layer. We start by introducing a general coherence-absorption relation that connects the coherence of the fields at two positions in the system to the power absorbed by the system when illuminated by two sources [3]. This relation shows the degree of spatial coherence can be deduced from experimental measurements or numerical codes simulating absorption in the layers. We find that surface waves and delocalized modes play a major role in building the spatial coherence. Our model is a key asset in the design of directional light sources based on photoluminescence.

Our approach of light emitting systems involving ensembles of emitters provides simple and efficient design tools relying on the determination of absorptivity of the structure. It accounts for the interplay between the electromagnetic properties of the emitter (resonances, antenna effect…) and the matter excitations which are included in the model through the imaginary part of the permittivity of the material and the subsystem’s temperature and chemical potential.

References :

[1] Light emission by a thermalized ensemble of emitters coupled to a resonant structure, L. Wojszvzyk, H. Monin, J.J. Greffet, Adv. Optical Mater. 20, 1801697 (2019)
[2] Light emission by nonequilibrium bodies: local Kirchhoff law, J.J. Greffet, P. Bouchon, G. Brucoli, F.Marquier, Phys.Rev.X 8, 021008 (2018).
[3] General relation between spatial coherence and absorption, D. Tihon, S. Withington, E. Bailly, B. Vest, and J.-J. Greffet, Opt. Express 29, 425-440 (2021)

Optical fiber is a well-established waveguide that has been utilized in a variety of applications such as data communications, spectroscopy, imaging, remote sensing, surgeries, and seismic activity monitoring, etc. However, the optical properties of an optical fiber (for instance, mode intensity and propagation loss) are fixed after drawing fabrication due to the static property of the dielectric glasses. Therefore, integrating active materials and nanostructures into optical fibers is promising for enhancing processing and transmission capabilities and novel functionalities. Zero-index materials have been extensively studied recently due to their unique and extreme optical properties [1]. Ultrathin transparent conducting oxide (TCO) epsilon-near-zero (ENZ) layer with the real part of permittivity approaches zero can support ENZ mode with enhanced and highly confined optical field [2]. Furthermore, the ENZ properties of TCO can be dynamically tuned by nonlinear optical pulse excitation [3-4].
In this work, we demonstrated the excitation of hybrid ENZ mode in a side-polished optical fiber deposited with an ultrathin layer of aluminum-doped zinc oxide (AZO). The 40-nm-thick AZO layer on the side of the fiber, fabricated by the atomic layer deposition technique, has ENZ properties at a near-infrared wavelength of ~1550 nm. The confined ENZ mode in the AZO thin film is excited due to the evanescent field coupling between the fundamental mode of the fiber and the ENZ mode supported by the AZO layer. Transmission measurement shows a resonance at a wavelength of 1600 nm with resonant strength of 6 dB in an ~1cm long AZO-coated side-polished optical fiber, which agrees with full-wave electromagnetic simulation and phase-matching condition. We further demonstrated that the highly confined ENZ mode could be used for developing efficient tunable in-fiber devices such as ultrafast optical modulators and switches. Due to the giant nonlinearity at ENZ frequency, the mode propagating in the ENZ optical fiber can be dynamically manipulated by femtosecond laser excitation. Our preliminary results show that the modulation strength of ~9.6 dB/mm can be achieved by modulating the hybridized ENZ resonance under femtosecond laser pulse excitation with an intensity of 200 GW/cm². The dynamic control of hybridized ENZ-optical fiber mode offers a compact and efficient method of manipulating the ENZ property and functionality in the optical fiber, which can find potential applications for ultrafast optical switches, polarization controllers, and other active optical fiber devices.
This work was supported by the Air Force Office of Scientific Research (Award number: FA9550-21-1-02204).

References
1. Liberal, I.; Engheta, N. Nature Photonics, 11(3), 149 (2017).
2. Campione, Salvatore, Igal Brener, and Francois Marquier. Physical Review B 91.12, 121408 (2015).
3. Alam, M. Z.; Schulz, S. A.; Upham, J.; De Leon, I.; Boyd, R. W. Nature Photonics, 12, (2), 79 (2018).
4. Kinsey, N., DeVault, C., Kim, J., Ferrera, M., Shalaev, V. M., & Boltasseva, A. Optica, 2, (7), 616-622 (2015).

Discovering unconventional optical designs via machine-learning promises to advance on-chip circuitry, imaging, sensing, energy, and quantum information technology. In this talk, photonic design approaches and emerging material platforms will be discussed showcasting machine-learning-assisted topology optimization for optical metasurface designs for applications in thermophotovoltaics, reflective optics and more.

CdO is one of the most promising low-loss infrared (IR) plasmonic materials to date. Incorporation of a donor dopant such as Y, In, or F has demonstrated tunable carrier concentration between 10¹⁹-10²¹ cm^-3, corresponding to mid-IR plasmonic modal frequencies. In addition, high-power impulse magnetron sputtering (HiPIMS) consistently produces epitaxial films with high mobilities between 300-500 cm²/V-s, minimizing optical losses. We have shown how manipulating donor and acceptor defect chemistry through controlling deposition and post-processing conditions enables one to engineer epsilon-near-zero (ENZ) modes spanning a broad range of the mid- to near-IR (1650-5325 cm^-1).

While we have largely explored light confinement in CdO via electron oscillations, we now aim to further manipulate light through magnetism by Gd doping to create a dilute magnetic semiconductor. In this presentation, we show the excellent structural and morphological properties of Gd-doped CdO across a broad doping range, enabling fine permittivity control with ENZ behavior spanning the mid- to near-IR. Further, we will explore the effect of the ENZ condition on magneto-optic properties.

The opportunity to employ sub-wavelength nanostructures to tune light-matter interaction has triggered tremendous interest in the last decade, thanks to the plethora of optical properties that can be achieved in metamaterials or metasurfaces, that are often not seen in nature. Among those examples, split-ring resonators can display optically-induced magnetic responses with unique possibilities to enhance absorption in the visible and near IR. Tuning the geometric parameters of the nanorings enables for control of the absorption band at optical frequencies, with implication in multiple fields, from thermal solar, to photovoltaic (selective absorbers, anti-reflections coatings, etc.) and optoelectronics (optical coatings, sensitive photodetectors, optical stealth, etc.) [1] [2].

However, whereas design strategies are well established, experimental techniques to fabricate such materials at a large scale and low cost are still in their infancy; they mainly rely on e-beam lithography for the fabrication of small area, high-resolution metasurfaces, or on nanoimprint lithography for larger area. In both cases, several fabrication steps have to be undertaken, including etching steps and metal evaporation steps, that are not easily scalable. In addition, the materials obtained are typically polycrystalline, with negative impact on their optical quality.

The objective of the research is to provide a first proof of concept of tunable silver circular split-rings entirely made by scalable bottom-up approaches, showing enhancement of absorption at optical frequencies.

First, FDTD simulations were performed to realize a parametric study of silver nanoring design in order to optimize its geometric parameters for absorption enhancement (inner, outer diameters, thickness, split-ring gap, array periodicity). This showed full control on resonance peaks and absorbance enhancement in the visible and near IR.

Next, we will show the realization of circular silver split-ring arrays on a large area via bottom-up techniques. Silver single-crystalline nanocubes (synthesized in solution via the polyol process) are self-assembled with high yield (>90%) in PDMS templates previously nanostructured with a pre-defined split-ring motif. Epitaxial nanowelding techniques at low temperature (Sciacca et al. Advanced Materials, 2017) are then employed to connect the individual nanocubes together to obtain a continuous quasi-monocrystalline material. We will show electron microscopy characterization at different steps of the fabrication process to show the high crystalline quality and support our claims.

Finally, we will show measurements of the macroscopic (5 mm²) optical properties of the split-ring arrays, and compare to those calculated via FDTD simulations.

Our work shows for the first time the realization of a high optical quality metasurfaces at a large scale fully obtained with bottom-up techniques.

[1] T. P. Otanicar, et al., Filtering light with nanoparticles: a review of optically selective particles and applications. Adv. Opt. Photonics, vol. 8, no. 3, pp. 541–585 (2016)
[2] A. F. Koenderink et al., Nanophotonics: Shrinking light- based technology. Science 348 (2015)

Noble metal nanoparticles exhibit localized surface plasmon resonance (LSPR) when interacting with light. This property has led to the study of these particles for numerous applications, such as catalysis, biosensing, gas detection, and chemical sensing. One commonly used material in hydrogen sensing is palladium, which can dissociate hydrogen gas and form a hydride, resulting in a shift of the LSPR absorption. The kinetics of hydrogen absorption reaction and desorption in palladium are relatively fast, which is useful for gas sensing in hydrogen economy applications. However, some applications require long-term detection of total hydrogen levels, where fast desorption is not desired. Core-shell nanoparticles consisting of a noble metal core and a shell material, such as a transition metal oxide, could provide a material system for hydrogen detection with a more permanent plasmonic shift during exposure to hydrogen. In this work, we show the synthesis of various noble metal core-shell nanoparticles, including gold-copper (I) oxide and gold-palladium, with spherical and spiked structures and evaluate their ability to detect hydrogen in colloidal suspensions. In addition, we will discuss electrochemical methods to synthesize core-shell nanoparticles and spectroelectrochemistry to characterize the nanoparticles during growth.

Design and fabrication of large-area metamaterials is an ongoing challenge for the material science and technological communities. In the present work, we propose a scalable and low-footprint strategy to design large-area, frequency-selective Cu-Sn disordered network metamaterials that exhibit nearly perfect absorption in a given spectral range. These nano-modulated structures exhibit a broadband spectral response, similar to connected wire mesh metamaterials.
We demonstrate that chemical engineering applied to the thin film materials allows the production of nano-modulated networks and allows tuning the absorption frequencies of the disordered meta-networks to a particular spectral region spanning the near-infrared - UV diapason.
Our approach relies on the establishment of disordered networks of a particular morphology by employing chemical etching to co-sputtered Cu-Sn-Al thin films with the purpose to remove aluminum from the film volume. As a result, the films adopt a particular topological and phase configuration that yield characteristic absorption signals. Moreover, tuning the initial composition of the Cu-Sn-Al thin film during the film production enables the control over the phase state and the spatial modulation period of the porous network which in turn defines the particular set of the plasmon absorption frequencies activated in the system.
Experimental verification of the plasmonic response of the produced networks is carried out in our work by employing electron energy loss spectroscopy (EELS) in the low-loss region. We resolve highly confined gap surface-plasmon (GSP) modes activated at the surfaces of the porous structures due to the near periodic nano-sized topology of the network.

Photocatalysis can be regarded as part of the push towards renewable and greener energy production. Water splitting is a suitable example, where the ideal semiconductor has a bandgap of 1.9 – 2.3 eV. However, semiconductors with such a bandgap are usually unstable during photocatalytic water splitting [1]. Extending the absorption spectrum of a catalyst with larger band gaps can be achieved utilizing localized surface plasmon resonance (LSPR). The ability to tune the dominant frequency of the metal particles by changing their geometry and size makes LSPR even more intriguing [1]. The plasmon absorption peak energies of certain metal nanoparticles (NPs) are in the range of visible light, potentially allowing their energy to drive chemical reactions with sunlight or contribute to photovoltaic applications [2]. The plasmonic metal NPs can utilize the energy directly and act as photocatalysts, they can be used in an antenna-reactor setup with catalytically active metallic NPs, or they can transfer the energy to the semiconductor surrounding them [1-3]. The chemical and physical properties of these metal NPs, as well as of the substrate define the type of energy transfer and consequently chemical reaction possible.

In this work NPs of nickel (Ni), copper (Cu), and ruthenium (Ru) were created within strontium titanate (STO) by exsolution in a hydrogen/argon mix (HArmix, 5%) at elevated temperatures (600 °C to 900 °C ). Subsequently, the exsolved Ni was replaced by gold (Au) to different degrees by galvanic replacement. The versatility of the technique was demonstrated by using A-site excess and A-site deficient STO, as well as galvanic replacement by platinum (Pt). While Ni, Ru, and Pt are only weakly plasmonically active, their catalytic properties may be used in an antenna-reactor setup. Thin films and powders were synthesized by pulsed laser deposition and solid state reactions methods, respectively. Particles of pure Au, ladybug-like Au decorated Ni particles on STO, and flower-like Au decorated Ni particles on STO, as well as potential antenna-reactor setups, were obtained. The main difference between the ladybug-like and flower-like samples was the size of the Au particles decorating the Ni. In general, shorter galvanic replacement times led to spherical NPs, while prolonged ones yielded (bimetallic) NPs with complex geometries.

Scanning Transmission Electron microscopy (STEM) and energy dispersive X-ray dispersion (EDX) were utilized to determine the structure and chemical environment of the particles. STEM and Electron Energy Loss Spectrometry (EELS) in the low electron energy loss regime were employed to study the plasmonic response on the nm-length scale, directly observing “hot-spots” in the electric field. The STEM-EELS/EDX experiments were performed with a monochromated 300 kV STEM probe with Zero-Loss Peak (ZLP) full-width at half maximum of 110 meV on a FEI Titan G2. The Au nanoparticles exhibited peaks in the EELS spectra between 1.50 and 2.40 eV. While the 2.40 eV peak is well known for Au nanoparticles, a Finite Difference Time Domain (FDTD) approach was used to investigate them further, determining that peaks towards 1.50 eV originate in parts of the particles near STO or with complex geometries. Geometries and the chemical environment of the particles can be used to explain the splitting and shifting of the peaks. Moreover, the PEC performance of samples with partially of fully galvanically replaced Ni NPs by Au, showed a clear improvement compared to samples without NPs.

The Research Council of Norway is acknowledged for the support to the Norwegian Center for Transmission Electron Microscopy (NORTEM, no 197405/F50), the NANO2021 researcher project FUNCTION (no. 287729) and FRINATEK project PH2ON (288320).

[1] N. Wu, Nanoscale 10(6), (2018), 2679-2696.
[2] Zhang, Q. et al., J. Materiomics 3(1), (2017), 33-50
[3] Zhou, L. et al. Nat. Energy, 5, (2020), 61-70

Changing the composition of alloy nanostructures can be a way of tuning their plasmonic properties. Rational tuning for a desired purpose requires a method that provides both prediction and the fundamental understanding of the predicted properties. We present an approach based on density functional theory (DFT) calculations coupled with electrodynamics simulations that meets the aforementioned requirements. Our results show the importance of the chosen exchange-correlation functional on the quality of prediction for bulk alloys that becomes critical in the case of nanostructures. Furthermore, we show the value that the fundamental insights provided by DFT have by explaining the causes for predicted low energy interband transitions as an emergent property of alloying. Lastly, we compare our approach to other state-of-the-art methods for computing hot carrier energy distribution and study the possibility of tuning the hot carrier distribution by alloying.

Local Surface Plasmon Resonances (LSPR) on metal nanoparticles have been found to enhance photocatalysis with the largest enhancement by noble metals. They turn out to have properties which results in a strong LSPR response [1]. However, the catalytic properties of the noble metals are poor due to low adsorption of reactant molecules on their surface [1]. A solution to this issue is to use the LSPR in the noble metal to enhance the photocatalytic abilities of a supporting semiconductor. The semiconductor can be enhanced through different mechanisms including local electromagnetic field enhancement, resonant energy transfer and the generation of hot carriers from LSPR decay [2]. In this contribution, we investigate the latter possibility by calculating the absorption in Ni and Au nanoparticles (NPs) embedded on a SrTiO₃ (STO) surface. The hot carrier injection is especially interesting since it enables light absorption below the band gap of STO (E_g^STO = 3.25 eV).
The simulations were performed using the finite difference time domain method. The simulated geometries were modeled after the structure of Ni-doped STO thin-films grown by pulsed laser deposition, where subsequent exsolution gave socketed Ni NPs on the surface of the thin-films [3]. Some samples were afterwards treated to galvanically replace the Ni with Au [4]. To investigate the thin-film samples computationally, we compared the plasmonic response of Au and Ni spherical NPs (5 nm ≤ radius ≤ 15 nm) embedded on the surface of STO in the energy range below the band gap of STO.
The simulations confirmed that the Au NPs have a larger absorption cross section (σ_abs) than Ni NPs as expected [5] and validated the galvanic replacement as a method to increase the plasmonic effect in the thin-film samples. In addition, we found that, for both materials, the σ_abs increased when the NPs were embedded deeper into the substrate. The increase is probably due to the high refractive index of STO compared to vacuum which redshifts the LSPR peak away from interband transitions at 2.4 eV for Au and 4.7 eV for Ni [6,7]. The redshift therefore works to decrease damping. The σ_abs per geometrical cross section (σ_geo) peak height seems to increase linearly with the redshift in energy. For Au, a LSPR energy at 2.05 eV gives an σ_abs peak height equal to the σ_geo and at 1.9 eV σ_abs is almost 7σ_geo.
For Ni, most of the absorption spectra have the largest σ_abs around the interband transitions at 4.7 eV and a smaller peak around 2.5 eV, with a few exceptions. The exceptions have both a large size and are deeply embedded in STO which are two things that redshift the LSPR energy. However, the smaller LSRP peaks, which lie below the band gap of STO, seem to follow the same linear trend as for Au. We observe that with a LSPR energy of 2.55 eV the σ_abs peak height is equal to the σ_geo and at 2.4 eV σ_abs is around 1.4σ_geo. If we look at the σ_abs per volume, the linear trend is not as clear. The smaller particles have a larger σ_abs per volume compared to the larger ones, but the absorption still increases with redshift in LSPR energy.
In conclusion, we have calculated the absorption in Ni and Au NPs embedded on a STO surface, as a first insight into hot electron injection as a way to enhance the photocatalytic abilities of STO. We found that the embedding of the particles deeper into the semiconductor increased their σ_abs, and that the σ_abs/σ_geo peak height is linearly proportional to the redshift in LSPR energy. These results are a starting point to assess this attempt to photocatalytically enhance STO.

[1] K. Sytwu, M. Vadai, and J. A. Dionne, Adv. Phys.-X 4, UNSP 1619480 (2019).
[2] N. Wu, Nanoscale 10, 2679 (2018).
[3] D. Neagu et al., Nat. Commun. 6, 8120 (2015).
[4] K. G. Both et al., (unpublished).
[5] U. Kreibig and M. Vollmer, Optical Properties of Metal Clusters, 1st ed. (Springer, Berlin, 2013).
[6] P. E. Hopkins, J. Heat Transf. 4 (n.d.).
[7] T. Pakizeh, J. Phys. Chem. C 115, 21826 (2011).

We present a study of possible variations of fluorescence spectrum between individual ultrabright silica nanoparticles (SiNPs) at the level of single nanoparticles. The extremely high fluorescent brightness of these particles has been created by encapsulating fluorescent dye molecules inside the nanoporous silica matrix of nanoparticles. A specific environment around the dye molecules allows attaining a very high concentration of the dye molecules without quenching of fluorescence. Different dyes can be mixed within each nanoparticle, thereby creating a platform for fluorescent sensors. The ultrabright nature of these particles allows doing measurements using single nanoparticles. For a quantitative study, it is paramount to demonstrate that the fluorescence spectra collected from individual nanoparticles are substantially similar. Encapsulation of multiple dyes within the same particle exhibits Forster Resonance Energy Transfer (FRET), which may cause differences in fluorescence spectra because of the random location of the encapsulated dye molecules inside each particle. .
In this study, direct measurements and simulations were performed to study the possible variation in the fluorescence spectra of individual SiNPs. A confocal fluorescent microscope attached with a single-photon camera was used for the direct measurements. These results were compared to the simulation of fluorescent spectra, which took into account the multiple random positioning of dye molecules inside each particle.
Overall, the direct measurements and simulation were in very good agreement with each other and showed that the spectral variation between single SiNPs was rather small. We can conclude that individual ultrabright SiNPs can be used in a variety of applications as taggants and sensors, including multiplexed biomedical imaging, flow cytometry, and ratiometric fluorescence sensors.

Dynamic control of wavefront shaping is essential for optical technologies spanning communication, computation, and sensing. Reconfigurable metasurface devices are especially important for lightweight and compact applications, including LidAR, LiFi, and AR/VR systems. However, achieving reconfigurable devices that demonstrate efficient modulation remains an outstanding challenge, due in part to the low electro-optic coefficient of most materials and weak light-matter interactions afforded by traditional metasurfaces. Current reconfigurable metasurface designs optimize for phase contrast and thus generally operate close to absorption peaks, leading to low efficiencies and poor contrast between diffraction orders. Here we present fully reconfigurable, high-efficiency wavefront shaping, exemplified by both high reflection efficiency and a large dynamic tuning range, achieved via high-Q silicon-on-lithium niobate metasurfaces. Our metasurface consists of subwavelength nanobars of silicon, 500 nm wide and 220 nm tall, atop 100 nm thick lithium niobate. Periodic perturbations to the constituent nanobars allow guided modes to couple to free space, with strongly enhanced electromagnetic fields localized in the lithium niobate. These guided mode resonances achieve Q factors exceeding 30,000, enabling strong electromagnetic field confinement without compromising the metasurface transfer function; these high-Q resonances can make the constituent nanoantennas more sensitive to subtle changes in refractive index. Using full-field simulations, we show how applying a voltage across individual nanoantennas shifts the spectral position of the high-Q resonance, modifying the antenna phase and amplitude, and achieving nearly a full 2 pi phase variation with reflectance above 91%. With the ability to individually address metasurface elements through transparent conducting oxide contacts on each antenna, we construct phase gradient transfer functions defined by the applied field, rather than varying the size, spacing, or geometry of the constituent nanoantennas. By changing the nanoantenna biasing configuration, we show a single metasurface can be tuned to switch between multiple transfer functions and steer light to different angles with high diffraction efficiency. We demonstrate how an applied field can modulate the metasurface to act as reflective beamsplitting or beamsteering devices, depending on the biasing conditions,with varying angles and efficiencies as high as 85%. Near-continuous beam-steering can be achieved by modifying the bias across each antenna; for example, voltages of +14.5, 0, and -14.5 steer to 31 degrees, while applied voltages of +18.6, +2.7, -2.7, and -18.6 and +21.1, +5, 0, -5, and -21.1 steer to 23 degrees and 18 degrees, respectively. Our presentation will discuss not only the design but also fabrication and characterization en route to a versatile metasurface platform that can reconfigurably achieve a variety of transfer functions.

Optical limiters are optical devices that utilize optical nonlinearities to achieve decreasing transmittance versus increasing incident optical intensity, which is useful for protecting sensitive detectors or human eyes against high-intensity illumination. Most optical limiters utilize nonlinear absorption to absorb the light when the incident intensity increases significantly. However, such absorptive designs may be suboptimal because the absorption of high-power light may result in damage to the limiter via melting, sputtering, or ablation. Therefore, a more-ideal solution is to make optical limiters that are reflective (i.e., they reflect excess light) rather than absorptive. There have been limited efforts to realize reflective optical limiters, and existing designs have limitations in terms of spectral and angular bandwidth, speed, etc.

In our talk, we will present our design of an ultrathin, broadband reflective optical limiter based on dynamically tunable resonant transmission through metal aperture antennas using thin-film vanadium dioxide (VO₂), an electron-correlated material that features a dramatic change in optical properties across its insulator-metal phase transition (IMT). Specifically, we incorporated gold (Au) frequency-selective surfaces (FSSs), which are metallic arrays of aperture antennas that feature resonant transmission, onto VO₂ thin films on transparent substrates (here, GaAs). The FSS-VO₂ optical limiter can feature high resonant transmission when the VO₂ is in the insulating phase (open state of the device), but reflects light when the resonant transmission is detuned or extinguished by the IMT of VO₂ (limiting state of the device).

We designed and optimized the FSS-VO₂ structure using finite-difference time-domain (FDTD) simulation (implemented in Lumerical FDTD), primarily targeting three figures of merit: a) high open-state transmittance, b) low limiting-state transmittance, and c) low limiting-state absorption. Via varying the FSS geometries, our optimized design can feature a broad open-state transmission band (FWHM > 2 µm) centered at any desired wavelength in the infrared from ~2 to 11 µm and essentially zero transmittance (~0.008) for all wavelengths in the limiting state. Because of the deep-subwavelength thickness and symmetry of the FSS structure, our design works for a large angular bandwidth (up to angle of incidence of 50°) for any polarization.

We used e-beam lithography to fabricate a design that has an open-state transmission band centered at ~10 µm. We measured the transmittance spectrum of the fabricated device for both states using a Fourier-transform infrared (FTIR) microscope, and the results agreed well with our simulations. The limiting performance of the fabricated device was characterized using our intensity-dependent transmission measurement setup with a continuous-wave CO₂ laser operating at λ= 10.6 µm. Note that we needed to preset the device at a higher temperature using a heat stage because the CO₂ laser we used could not provide sufficient power to photothermally trigger the IMT of VO₂ from the room temperature. Our experiments showed that the device was able to feature limiting performance for different incident-intensity ranges by varying the preset temperature. Moreover, the experimental results were well captured by our opto-thermal simulation (implemented in COMSOL Multiphysics) which also provided us insight to the limiting performance of our design regarding the limiting threshold and the device speed. We also used this opto-thermal simulation to explore opportunities for unidirectional limiting (i.e., nonlinear optical isolators).

Photonic crystal cavities are ideal for enhancing weak light-matter interactions by simultaneously increasing the residence time of an optical mode while shrinking its mode volume. In addition, these structures can act as high extinction and narrow-band filters due to the optical bandgap formed. This can be used for on-chip filtering and dispersion engineering, both of which are necessary components in integrated photonic circuits. Lithium Niobate has emerged as a strong candidate for integrated photonics due to the ability to fabricate low-loss waveguides and high-quality factor ring resonators on a chip. In addition, its non-centrosymmetric structure allows for second order nonlinear optical effects like three-wave mixing phenomena and the electro-optic effect. Lithium Niobate’s strong electro-optic effect is particularly useful, as it allows for tunability and optical modulation at microwave frequencies. Combining photonic crystals in integrated lithium niobate photonics is of great interest for optical technologies spanning nonlinear classical and quantum photonics.

Here, we explore Fabry-Perot cavities integrated in a Lithium Niobate on Insulator platform for tunable on-chip photonics. The mirrors are formed by photonic crystal sections on either side of low-loss waveguides. To reduce unwanted scattering losses, we utilize a “nanofin”-type geometry with alternating regions of wide and thin waveguide sections. We design our devices on uncladded 300 nm and 600 nm Lithium Niobate with underlying silicon dioxide. For 300 nm Lithium Niobate devices, a waveguide width of 1 micron, photonic crystal period of 480 nm, and alternating photonic crystal waveguide widths of 1600 nm and 400 nm forms an optical band gap near 1525 nm. We experimentally demonstrate bandgaps exceeding 80 nm in bandwidth at telecom frequencies and extinction exceeding 50 dB with a photonic crystal less than 30 microns long. By designing mirror segments with tapered sections to reduce impedance mismatch, we further demonstrate Fabry-Perot cavities with quality factors exceeding 150,000. We have also designed and tested devices on 600 nm Lithium Niobate with quality factors exceeding 75,000 in strongly overcoupled cavities, suggesting that much higher quality factors are achievable without sacrificing on transmission efficiency. We characterize the potential loss mechanisms by varying the cavity length and observe linear increases in the quality factor with cavity length, suggesting that mirror reflectivity is the dominant limiting factor in our devices, rather than scattering from waveguide or mirror imperfections. Future designs suggest that quality factors approaching 1 million are possible in devices with mode volumes smaller than typical ring resonator structures.

Coupled with Lithium Niobate’s strong electro-optic effect (r₃₃ ~ 30 pm/V), the high-Q nature of these devices lend themselves strongly to tunable nanophotonics and nonlinear photonics. We envision these devices in a variety of integrated optical technologies, including optical sensing and detectors, microwave-frequency modulation, and integrated quantum photonics.

Vanadium dioxide (VO₂) has been widely investigated for tunable and reconfigurable photonic devices because it features dramatic changes in its optical properties across its insulator-metal phase transition (IMT) at ~68 °C. As a strongly correlated material, the IMT temperature of VO₂ is determined by the stability of the electron hybridization, which is sensitive to the strain environment in the thin film, providing opportunities for engineering of the IMT. Most existing methods for modulating the IMT temperature of VO₂ either use different substrate materials or orientations to result in different strain environments in the film, or introduce dopants via physical or chemical processes during growth. Therefore, such methods need to be developed particularly depending on the choice of synthesis method, and the IMT characteristics are predetermined during the growth and fixed in the post-growth films.

In our talk, we present a method to modulate the IMT characteristics (e.g., the IMT temperature, or the temperature range of the IMT, which we refer to as IMT width) by introducing defects into VO₂ films via a post-VO₂-growth ion-implantation process. Our method is applicable for VO₂ films synthesized using different methods, and we demonstrated that such modulation in the VO₂ properties can be achieved even after a nanostructured device has been patterned on top of the VO₂ film.

Under irradiation by high-energy ions (Ar ions in our experiments), vacancies of V and O result from the collision cascade between the impinging ions and atoms of V and O, resulting in changes in IMT characteristics due to the induced changes in the strain and stoichiometry of the VO₂. In our experiments, we used 75-keV Ar ions to irradiate a ~100-nm VO₂ film grown on a sapphire substrate. We irradiated different areas of the film with different ion fluences ranging from 1x10¹³ to 2x10¹⁴ cm^-2. The IMT temperature and width of all the irradiated regions were characterized using temperature-dependent Fourier-transform infrared (FTIR) reflectance measurements. We also quantitatively extracted the temperature-dependent refractive index throughout the IMT for all irradiated regions using a combination of ellipsometry analysis and effective-medium calculation.

Based on our experiments, we conclude that the IMT characteristics of VO₂ can be grouped into distinct behaviors with respect to irradiation fluences: When irradiated by ion fluences of <6x10¹³ cm^-2, the IMT temperature can be continuously lowered by up to ~20 °C with no substantial changes either in the IMT width or in the refractive-index contrast between the insulating and metallic phases. In contrast, for the higher irradiation fluences, though the IMT temperature can be further reduced, it is accompanied by a broadened IMT width and less refractive-index contrast.

The low-irradiation-fluence modulation is generally favorable for reconfigurable photonic designs because it can simply tune the IMT temperature (i.e., device triggering threshold) without trading off the optical contrast between device states. As a demonstration, we fabricated an optical limiter that incorporated gold aperture antennas onto a VO₂ thin film. The device featured a high transparency window centered at λ~4 µm when the VO₂ is in the insulating phase and low transmittance for all wavelengths when the VO₂ is triggered to the metallic phase at 68 °C. By irradiating this device with an ion fluence of 6x10¹³ cm^-2, we reduced the switching temperature to 52 °C.

We will discuss some recent ideas and developments on how plasmonics and machine learning can be employed to dramatically speed up quantum process to make them immune to decoherence and how they can significantly improve the performance of quantum photonics devices and systems.

Graphene plasmonic resonances have been broadly studied in the terahertz and mid-infrared (MIR) ranges because of their electrical tunability and large confinement factors, which can enable exploration of extreme light-matter interactions, among other optoelectrical applications. Plasmonic resonances occur in graphene when it is patterned into nanostructures (e.g. ribbons) which confine the graphene plasmons and enables coupling to free-space radiation. But an upper frequency limit (MIR) is set by the limits of electron beam lithography, which can make graphene nanostructures as small as 15 nm. In this talk, I will show that nonlocal and quantum effects become significant for graphene nanostructures less than 15 nm. Those changes blue-shift the frequencies of the graphene plasmons, pushing their operational range from the MIR into the short-wave infrared (SWIR). These effects are realized in centimeter-scale arrays of graphene ribbons as narrow as 12 nm created using a novel, bottom-up block copolymer lithography method that allowed for the creation of plasmonic cavities with resonant wavelengths as short as 2.2 μm. The confinement factors of these cavities can reach 135, which, while exceedingly large, is less than what has been theoretically predicted – a result of strong nonlocality. The combined SWIR response and large confinement factors observed in these graphene nanoribbon arrays – combined with the scalable fabrication method used to create them – makes these nanostructures an attractive platform to explore next-generation optoelectrical devices and strongly enhanced spontaneous emission in the SWIR.

Composites lie at the core of nanophotonics. Many devices in fields such as photosensing, photochemistry and thermophotonics are based on disordered nanomaterials with arbitrary shaped and randomly distributed structures, where their dispersion, filling fraction and size need to be finely tuned to achieve optimal performance. Although simulations are critical in this task, predicting the optical response of these materials is challenging, given the random orientation and anisotropic morphology of the structures involved. The radiative transfer equation (RTE), while being specifically formulated to describe light transport in random scattering media, becomes computationally expensive to solve when the structures are non-spherical.

In this talk, I will discuss our newly developed universal theory of average light scattering of randomly oriented scatterers (particles or collections of particles), and a practical method for radiative transfer simulations of composite with scatterers of arbitrary morphology. First, I will introduce our fluctuational-electrodynamics-based theory to compute the average absorption cross section, scattering cross section, and asymmetry parameter of randomly oriented scatterers of arbitrary shape. The three parameters can be directly integrated into traditional RTE solvers, enabling computationally inexpensive radiative transfer simulations. The accuracy of this modeling framework is demonstrated against optical measurements of polymer composites with metal-oxide crystals of irregular morphology.

As I will discuss by the end of this talk, the modeling framework could provide a better understanding of the interrelation between nanoscale electromagnetic interactions and the macroscopic response of disordered nanomaterials, such as the effects of agglomeration of plasmonic nanostars in optical absorption, or the impact of multiple scattering in the light trapping of heterostructured photocatalysts.

Plasmon catalysis is an interesting technology concept for powering chemical processes with sunlight. It makes use of the localized surface plasmon resonance (LSPR) of catalytically active metallic nanoparticles. Upon illumination, a coherent electron oscillation occurs in these nanoparticles, which rapidly dephases and generates hot electrons. These can then either be transferred into an electron accepting orbital of an adsorbed molecule, or thermalize due to electron-electron and electron-phonon scattering leading to an increased temperature of the catalyst. Both from a fundamental and application perspective, differentiating between the non-thermal and photothermal contribution is essential.
Recently, we reported the sunlight powered conversion of carbon dioxide and green hydrogen to methane and water – the so-called Sabatier reaction – using alumina supported Ru nanocatalysts [1,2]. Here, we report the sunlight powered conversion of carbon dioxide and green hydrogen to carbon monoxide and water – the so-called reverse water gas shift (rWGS) reaction – using a titania supported plasmonic Au nanocatalyst. The rWGS reaction is endothermic (ΔH_298K = 41.2 kJ mol^-1) and limited by equilibrium. In the conventional thermal rWGS process, catalysed by e.g. Cu, Rh or Pt, temperatures above 800°C are applied to produce CO with an acceptable selectivity (about 80%). In our study, we demonstrate that CO can be formed with high selectivity using only sunlight and without external heating of the reactor. Using mildly concentrated artificial sunlight (1.44 W cm^-2 equals 14.4 suns) we realized a selectivity of 98% and a CO production rate of 429 mmol g_Au^-1 h^-1. Temperature measurements performed with a thermocouple positioned in contact with the bottom side of the catalyst bed indicated a catalyst temperature of 140°C. We expected the top layer of the catalyst bed to have a higher temperature since it is directly exposed to light. Using a simple theoretical model based on Kirchhoff’s law and Stefan-Boltzmann’s law for macro scale objects, we estimated that the collective temperature at the top surface of the catalyst reached 300°C upon illumination with 14 suns. We furthermore demonstrated that the relationship between the reaction rate and the light intensity is exponential, which points to a strong photothermal component contributing to this reaction.
In reference reactions performed in dark with the same catalyst methane was formed in large quantities, while upon illumination CO was formed with a selectivity of 98% and CH₄ formation was largely suppressed. We propose the promotion of H₂O and CO desorption through charge transfer of plasmon generated charges to explain this difference.
In conclusion, we have demonstrated that plasmon catalysis can be applied to convert the greenhouse gas CO₂ into the valuable chemical intermediate CO using sunlight as sustainable energy source. Further studies on quantification of photothermal and non-thermal contributors and other aspects involving the reaction mechanism as subject of research in our labs.

[1] R. Grote, R. Habets, J. Rohlfs, F. Sastre, N. Meulendijks, M. Xu, M. A. Verheijen, K. Elen, A. Hardy, M. K. Van Bael, T. den Hartog, Pascal Buskens, Collective photothermal effect of Al₂O₃ supported spheroidal plasmonic Ru nanoparticle catalysts in the sunlight powered Sabatier reaction. ChemCatChem 2020, 12, 5618-5622.
[2] Francesc Sastre Calabuig, Nicole Meulendijks, Jessica Rodríguez-Fernández, Jorgen Sweelssen, Caroline Versluis, Ken Elen, Marlies K. van Bael, Tim den Hartog, Marcel A. Verheijen, Pascal Buskens, “Sunlight Fueled, Room Temperature Conversion of CO₂ and H₂ to CH₄ with an Anisometric Plasmonic Ru Nanocatalyst”, ACS Omega 2019, 4, 7369-7377.

The development of dynamic self-assembly (nano)systems is still a young research area (1) with promising future applications due to the ability of actively rearranging building blocks on the nanoscale. An in-depth investigation of the assembly and disassembly processes is of great importance to gain insights into the intermediate assembly states. Here we show the detailed picture of temperature-driven reversible clustering of gold nanoparticles (gold nanospheres(2) and gold nanorods).
Using in-situ UV-VIS spectroscopy, we revealed the emergence of hysteresis during the cyclic temperature changes. By varying nanoparticles diameter and their surface charge as well as the rate of applied stimulus we were able to describe qualitatively the contribution of thermodynamic and kinetic hysteresis.
A particularly intriguing case is the reversible clustering of gold nanorods.They adapt to transient mutual configurations, such as tip-to-tip or side-to-side, enriching the hysteretic response. In addition, the large absorption cross-section of gold nanorods in the infrared spectral range allowed us for taking advantage of their efficient light to heat conversion. Therefore by introducing light as stimulus, instead of an external heating source, we were able to control locally the change of temperature which in turn altered the colloidal stability of the nanorods.
Then, we designed a more sophisticated colloidal system and integrated feedback loops, leading to oscillatory and self-sustained clustering under continuous energy flux. We found that the presence of hysteresis in reversible aggregation is the prime condition to invoke oscillatory clustering otherwise the system adapts to its equilibrium steady state without oscillations.

We foresee that the presented experimental framework offers an exciting playground to study the nature of meta-stable assemblies, and enables possible applications in spatiotemporal catalysis, thermo-mechanical nanotransducers, smart windows, among others.


REFERENCES
(1) Grzelczak, M.; Liz-Marzán, L. M.; Klajn, R. Stimuli-responsive self-assembly of nanoparticles. Chem. Soc. Rev. DOI: 10.1039/C8CS00787J.
(2) Kruse, J.; Merkens, S.; Chuvilin, A.; Grzelczak, M. Kinetic and Thermodynamic Hysteresis in Clustering of Gold Nanoparticles: Implications for Nanotransducers and Information Storage in Dynamic Systems. ACS Appl. Nano Mater. 2020, 3 (9), 9520–9527. DOI: 10.1021/acsanm.0c02249.

Photo-Induced Enhanced Raman Spectroscopy (PIERS) is a new surface enhanced Raman spectroscopy (SERS) modality with an order-of-magnitude Raman signal enhancement of adsorbed analytes over that of typical SERS substrates. Despite the impressive PIERS enhancement factors and explosion in recent demonstrations of its utility, the detailed enhancement mechanism remains undetermined. Using a range of optical and X-ray spectroscopies, supported by density functional theory calculations, we elucidate the chemical and atomic-scale mechanism behind the PIERS enhancement. Stable PIERS substrates with enhancement factors of 106 were fabricated using self-organized hexagonal arrays of TiO2 nanotubes that were defect-engineered via annealing in inert atmospheres, and silver nanoparticles were deposited by magnetron sputtering and subsequent thermal dewetting. We identified the key source of the enhancement of PIERS vs. SERS in these structures as an increase in the Raman polarizability of the adsorbed probe molecule upon photo-induced charge transfer. A balance between crystallinity, which enhances charge transfer due to higher electron mobility in anatase-rutile heterostructures but decreases visible light absorption, and oxygen vacancy defects, which increase visible light absorption and photo-induced electron transfers, was critical to achieve high PIERS enhancements.

On-demand polarization control of light plays a critical role in a variety of classical and quantum optics-based applications such as holography, imaging, dichroism sensing and polarization encoding of qubits. We demonstrate electrically reconfigurable, high dynamic range, broadband polarization conversion with atomically thin van der Waals heterostructures, using tri-layer black phosphorus (BP) integrated in a Fabry-Pérot cavity, at telecom wavelengths. Here, we leverage the exciton-dominated near-unity birefringence in naturally stacked three atomic layers of BP (~1398 nm) which enables a broadband operation window of the entire telecom E, S and C bands (1360-1580 nm). The inherent optical anisotropy of BP, further enhanced by integration into a Fabry-Pérot resonant cavity, allows generation of polarization states over the entire Poincaré sphere via spectral and input azimuthal angle tuning. Furthermore, the extreme polarizability of the exciton to an external electric field provides near-complete suppression of its dipole oscillator strength, enabling active control of the polarization state across nearly half the Poincaré sphere. Specifically, both linear to circular and linear cross-polarization conversion schemes, among numerous other trajectories, were achieved (for example at ~1444 nm) via electrical doping (up to ~7x10¹²/cm²) demonstrating superior control over the reflected ellipticity and azimuthal angles. Our findings highlight the potential of BP in future chip-based polarization-sensitive electro-optic platforms such as spatial light modulators, variable phase retarders, and sensors.

High refractive index dielectrics are emerging as a novel platform to shape and control light at the nanoscale by engineering electric and magnetic Mie-resonances in sub-wavelength nanostructures. Between two-dimensional (2D) semiconductors, the family of transition metal dichalcogenides (TMDs) exhibits exceptional optical properties, such as tightly bound excitons and single-photon emitting centers, and unique mechanical properties leading to favorable integration in current nano-photonic architectures. In our work, we integrated single and double layer TMDs WSe₂ with Mie-resonant gallium phosphide (GaP) dimer nano-antennas and demonstrate the enhancement of the light-matter interaction of 2D excitons and native single-photon emitters. At room temperature, we observe an enhancement of the PL up to 10⁴, compared to WSe₂ on planar GaP, and show this is a result of increased absorption, enhanced spontaneous emission rate, and emission redirection [1]. Moreover, the nano-structures displace the semiconducting layers in the out-of-plane direction, allowing the tuning of the local WSe₂ bandstructure via strain [2]. At cryogenic temperatures, the strain-induced potential acts as a trap for photo-generated excitons, leading to the nano-scale positioning of bright single-photon emitters in monolayer WSe₂ [3]. We compare the optical properties of quantum emitters on GaP nano-antennas to those positioned on non-resonant SiO₂ nano-pillars. In agreement with our numerical simulations, we observe more than two orders of magnitude enhancement in the collected PL intensity of nano-antenna coupled SPEs, further exhibiting long PL lifetimes - up to 200 ns - and ultra-low PL saturation thresholds. We show this is related to an increased quantum efficiency of the single-photon emission when the TMD layer is coupled to the nano-antenna near-field. We report quantum efficiency with values up to > 80% on GaP nano-antennas, compared to an average of 4% when positioned on SiO2 nano-pillars. Our results highlight Mie resonances in dielectric nanostructures as a novel nanophotonic platform to engineer light-matter interaction in two-dimensional materials and single-photon emitters.

[1] Sortino, L. et al. Enhanced light-matter interaction in an atomically thin semiconductor coupled with dielectric nano-antennas. Nat. Commun. 10, 5119 (2019)
[2] Sortino, L. et al. Dielectric nano-antennas for strain engineering in atomically thin two-dimensional semiconductors. ACS Photonics 7, 2413–2422 (2020)
[3] Sortino, L. et al. Bright single photon emitters with enhanced quantum efficiency in a two-dimensional semiconductor coupled with dielectric nano-antennas. arXiv: 2103.16986 (2021)

The ability to arbitrarily stack two-dimensional (2D) crystals have led to new opportunities to design and assemble artificial lattices without the constraints of epitaxy. However, these opportunities are limited by difficulties in controlling the thickness of exfoliated van der Waals (vdW) layers, which prevents the realization of repeat units with high fidelity. Recently, the development of metal-organic chemical vapor deposition (MOCVD) synthesis has allowed the creation of uniform, wafer-scale samples with thickness control. This allows the production of stacks of disparate 2D layers with multiple repeating units, which permits the engineering of electronic and optical dispersions in these heterostructures. We have engineered the optical dispersion in a superlattice structure by alternating layers of 2D chalcogenides and dielectric insulators. We demonstrate 94% narrowband absorption in <6 nm active layer excitonic absorber medium at room temperature through the design of the unit cell parameters. This is accompanied by an enhancement in the photoluminescence of square-centimeter-sized samples. The TMDC-insulators also show evidence of strong light-matter coupling and exciton-polariton formation with a Rabi splitting up to 190 meV. These results demonstrate that it is possible to engineer optical properties in 2D heterostructures and suggest approaches to create a broad class of scalable, designer optical metamaterials from atomically thin layers.

Weyl semimetals are a class of materials with unique properties as a results of a single touching point of valence and conduction band and a linear electronic dispersion, where the electrons behave as quasi-massless fermions, comparable to the Dirac cone in graphene. This leads to the formation of Fermi arcs and topologically protected edge states on the surface of Weyl semimetals and gives them a generally high electron mobility, a large magnetoresistance and effects such as the Adler-Bell-Jackiw anomaly. Furthermore, the Weyl semimetal family of transition metal monopnictides, were shown to have outstanding optical properties. Among others, the glass coefficient of TaAs in the near-infrared was measured to be the highest reported so far[1] and the second-order nonlinear coefficient has been measured to be two magnitudes larger than the one for GaAs[2]. These properties make Weyl semimetals promising candidates for optical applications, however, most previous works were limited to bulk crystals that are naturally limited in means of flexibility and device applications.
In this work, we perform first steps towards nanophotonics with Weyl semimetals by characterizing the linear and nonlinear optical response of recently realized epitaxially grown NbP thin films[3,4]. By spectral ellipsometry, we determined the linear refractive index and extinction coefficient with good agreement to theoretical predictions and thereby confirm the semimetallic character of the material. Despite the strong absorption over the visible spectrum, we show strong third-harmonic generation that reaches a maximum efficiency of 10^-5 % when exciting in the near-infrared. Comparing this to the signal detected from a bulk crystal of the same material, the nonlinear response of the thin film is stronger by more than a factor two. Furthermore, we performed non-degenerate pump-probe spectroscopy with 10 fs temporal resolution. We observe that the ultrafast optical response of the bulk NbP crystal is dominated by electronic absorption processes and subsequent scattering dynamics in the Weyl cone, in good agreement with previous studies on comparable materials. On the other hand, the NbP thin film shows a probe wavelength independent response that is dominated by the nonlinear optical Kerr effect and an ultrafast modulation depth of close to 1%, again a factor of two stronger than the bulk crystal. Undergoing a complete relaxation within less than 50 fs, the NbP film would allow all-optical switching with a full-width half-maximum bandwidth of 10-20 THz. We ascribe the boosted nonlinear performance to the reduced volume that contains only 10-20 unit cells of the material and therefore limits the absorption and electron-electron interactions.
In conclusion, our results show that the epitaxial grown NbP thin films inherit the intrinsic optical properties of the bulk crystal but significantly reduce the absorption due to the reduced dimensionality, which further boosts the nonlinear optical interactions. Together with the fact that thin films enable common top-down nanofabrication techniques, this paves the way for various promising applications of Weyl semimetals in the field of nanophotonics.



REFERENCES:

[1] G. B. Osterhoudt et al., “Colossal mid-infrared bulk photovoltaic effect in a type-I Weyl semimetal,” Nat. Mater., vol. 18, no. 5, pp. 471-475, May 2019.
[2] L. Wu et al., “Giant anisotropic nonlinear optical response in transition metal monopnictide Weyl semimetals,” Nat. Phys., vol. 13, no. 4, pp. 350–355, Apr. 2017.
[3] A. Bedoya-Pinto et al., “Realization of Epitaxial NbP and TaP Weyl Semimetal Thin Films,” ACS Nano, vol. 14, no. 4, pp. 4405-4413, Apr. 2020.
[4] A. Bedoya-Pinto et al., “Large Fermi-Energy Shift and Suppression of Trivial Surface States in NbP Weyl Semimetal Thin Films,” Adv. Mater., vol. 33, no. 21, p. 2008634, May 2021.

Squeezing light down to sub-wavelength scales allows the emergence of a host of fascinating phenomena in non-linear and quantum optics. These find applications in photonics and quantum information processing to name a few. While such a feat has been realized using conventional plasmonic nanoantennas, the losses inherent to these systems limit the scope of applications by lowering their damage threshold and damping quantum coherences. To this end, epsilon near-zero (ENZ) thin films, with their vanishing real permittivity and relatively low imaginary permittivity, are attractive candidates to tightly confine light with little losses. These have been used in the past in conjunction with plasmonic nanoantennas to reach high levels of confinement. We sought to integrate these materials in dielectric nanoantennas to achieve comparable levels of confinement while maintaining losses low.

We achieved sub-wavelength light confinement with photonic gap antennas (PGAs) consisting of a thin film of indium tin oxide (ITO), an ENZ material, embedded in an amorphous silicon nanopillar. PGAs of various sizes were fabricated with a multilayer deposition process followed by pseudo-Bosch photolithography. The antennas’ scattering spectrum was measured using dark field scattering spectroscopy and the resonant character of the light confinement was assessed through third harmonic generation. We interpreted these results in the framework of coupled mode theory and show that the resonances responsible for the light confinement arise from the hybridization of the silicon pillar modes with the epsilon near-zero mode of the ITO thin film. This work opens up the possibility for high efficiency non-linear photonics with high damage thresholds, as well as strong light-matter interaction for the generation of quantum light and ultrastrong light-matter interaction.

The engineered material dispersion of the thin-film transparent conductive oxide indium tin oxide (ITO) has been proven to exhibit extremely large epsilon-near-zero (ENZ) and low index-driven nonlinear responses. This has enabled the rapid development of compact and integrated CMOS-compatible devices that perform efficient nonlinear operations such as sub-picosecond all optical modulation, ultra-fast pulse shaping through frequency translation, and optical time reversal. Recent theoretical studies indicate that both bound electrons in the lattice and hot electron contributions significantly drive the nonlinear response of ITO materials. These results strongly motivate the optimization of material structure beyond dispersion engineering to boost nonlinear enhancement using material lattice contributions. Specifically, crystallographic texture describes the tendency for the grains of a polycrystalline films to avoid random distributions in favor of preferred orientations and can profoundly affect the nonlinear optical material properties. We show how modified growth and post-deposition conditions can dramatically modify ITO crystallographic texture and impact the nonlinear properties of ITO films characterized using Z-scan and four-wave mixing experiments.

Two-dimensional transition metal carbides and carbonitrides are promising plasmonic materials for surface-enhanced Raman spectroscopy (SERS), allowing sensitive detection of analyte molecules. Their high electronic conductivity and rich surface chemistries enable both SERS electromagnetic and chemical enhancements. We have synthesized seven most common MXenes – Nb₂C, Mo₂C, Ti₂C, V₂C, Ti₃C₂, Mo₂TiC₂, and Ti₃CN with the transversal plasmon resonances ranging from the visible region to NIR. We were able to detect the 10^-7 M concentrations of Rhodamine 6G dye on MXene substrates, reaching the enhancement factors of 10⁵-10⁶. The results were compared to the commercially available substrates and showed advantageous performance of Ti₃C₂ compared to gold nanoparticles. We tested the best performing MXene with a variety of dyes, suggesting the versatility of MXene SERS substrates. Due to high charge carrier densities and tunable surface chemistries, MXenes show potential for SERS sensors that are inexpensive, easy to fabricate, and optimize for specific applications.

Interest in flexible photonic layers made of polymeric materials is growing as they can find applications to create reflective surfaces for IR reflection, UV reflection, etc. for applications such as thermal management since they can have tunable heat dissipation or shielding properties. One-dimensional photonic crystals, specifically Distributed Bragg Reflectors (DBR) made of alternating high and low refractive index polymeric materials, have attracted research attention for the past few decades in this regard. However, very few materials exhibit the optical properties to function as polymeric DBRs. Major problems encompassing polymer-based DBRs include the low refractive index of the polymer, incompatible optical properties of the polymer, and casting of uniform thickness of alternating layers without damaging the underlying layers. Herein, we explore creating a DBR with high reflective properties developed by applying an additive directly to the high refractive index polymeric material to increase the refractive index difference of the two alternating layers from 0.2 to 0.5. Such enhancements helped us achieve 100% better reflective properties for near and far-infrared regions of the electromagnetic spectrum, with broader reflection bands requiring only half the number of layers than most common DBRs used in the literature. We hope such findings would make polymeric DBR materials useful for defense vehicles and clothing for shielding purposes and heat dissipation as they are flexible and readily integrate into many surfaces.

We acknowledge DoD Grant #W911QY2190015 for support of this project.

Nanowire solar cells have the potential to reach the same efficiencies as the world record III-V solar cells by the use of a fraction of the material^{1, 2}. For solar energy harvesting, large area nanowire solar cells have to be processed but so far high efficiency nanowire solar cells have been demonstrated only on small areas less than 1 mm^{2 3-5}.
In this work, we demonstrate the synthesis of epitaxial, photovoltaic InP nanowire arrays on a 2” wafer. We used talbot displacement lithography and placed black plastic stripes on the phase shift mask to block the UV light in order to define five array areas with different nanowire diameters on the same wafer by sequentially varying the exposure dose. We use a photoluminescence (PL) mapper to characterize the sample with both steady-state and time-resolved PL maps. Subsequently, we compare it to a reference wafer that was exposed homogenously. Furthermore, by the use of a white light source inside the PL mapper instead of the laser diode used for PL, we measure reflectance (R) maps. We created a database for modeled R(λ,D,L) for varying λ, D, and L at a fixed period of 500 nm for the hexagonal array of nanowires⁶. Then, we simultaneously extract the diameter and length of the nanowires from the several thousand experimentally measured R spectra over the full wafer, using the modeled R database. The extracted knowledge of large-scale nanowire synthesis will be crucial for the upscaling of nanowire based solar cells, and the demonstrated wafer-scale characterization methods will be central for quality control during manufacturing.

1. Wallentin, J.; Anttu, N.; Asoli, D.; Huffman, M.; Åberg, I.; Magnusson, M. H.; Siefer, G.; Fuss-Kailuweit, P.; Dimroth, F.; Witzigmann, B.; Xu, H. Q.; Samuelson, L.; Deppert, K.; Borgström, M. T. Science 2013, 339, (6123), 1057-60.
2. Anttu, N. Acs Photonics 2015, 2, (3), 446-453.
3. van Dam, D.; van Hoof, N. J.; Cui, Y.; van Veldhoven, P. J.; Bakkers, E. P. A. M.; Gomez Rivas, J.; Haverkort, J. E. ACS Nano 2016, 10, (12), 11414-11419.
4. Åberg, I.; Vescovi, G.; Asoli, D.; Naseem, U.; Gilboy, J. P.; Sundvall, C.; Dahlgren, A.; Svensson, K. E.; Anttu, N.; Björk, M. T.; Samuelson, L. IEEE Journal of Photovoltaics 2016, 6, (1), 185-190.
5. Hrachowina, L.; Zhang, Y.; Saxena, A.; Siefer, G.; Barrigon, E.; Borgström, M. T. In Development and Characterization of a bottom-up InP Nanowire Solar Cell with 16.7% Efficiency, 2020 47th IEEE Photovoltaic Specialists Conference (PVSC), 15 June-21 Aug. 2020, 2020; pp 1754-1756.
6. Anttu, N.; Heurlin, M.; Borgström, M. T.; Pistol, M. E.; Xu, H. Q.; Samuelson, L. Nano Lett. 2013, 13, (6), 2662-7.

A luminescent solar concentrator (LSC) photovoltaic (PV) system is a solar light harvesting device that absorb the sunlight by large LSC panel and concentrate photoluminescent light on PV cell placed at the edge of LSC panel to convert it into electricity. The nano-sized inorganic-organic cluster complex (dMDAEMA)₄[Re₆S₈(NCS)₆] (refers as RMC, where dMDAEMA is 2-dimethyl amino ethyl methacrylate) is a good candidate for LSC luminophore due to largely downshifted broad photoluminescence suitable to PV cell. However, the low quantum yield (QY) of RMC limit the performance of LSC. Here, Zn doped CuGaS/ZnS core-shell quantum dots (ZQD) as a Stokes shift engineered donor with high QY was used to improved performance of LSC. These two metal chalcogenide luminophores, RMC and ZQD, are chemically suitable to be dispersed in amphiphilic polymer matrix making clear waveguide with low reabsorption and optically well matched emission resulting high power conversion efficiency (PCE) and optical efficiency (η_opt). We achieved maximum η_opt of 3.47 % and PCE of 1.26 %, while maintaining high transparency of over 80 % in visible wavelength range. This dual-dye LSC system exhibits reabsorption free and increased emission by isolation of dyes in polymer matrix and energy transfer from ZQD to RMC. This LSC system suggests as a promising candidate for up-scaling building-integrated PVs (BIPV) for use in windows.

Photonic crystals describe a class of periodic dielectric materials that exhibit a photonic bandgap (i.e., rejection wavelength) corresponding to specific wavelengths of light where propagation through the crystal is forbidden. In the past few decades, particular interest has focused on photonic crystals composed of crystalline colloidal arrays (CCAs), which exhibit iridescent structural colors similar to that of the precious opal. These biomimetic colloids are composed of closed-packed, electrically charged particles that self-assemble into three-dimensional periodic arrays due to their repulsive Coulombic interactions. While a CCA does not exhibit a complete photonic bandgap due to a low refractive index contrast, a CCA possesses a rejection wavelength in the visible regime that can be described by Bragg’s equation (λ = 2n_cd_hklsin(θ)). The CCA’s rejection wavelength (λ) can be shifted across the full visible spectrum by changing the interplanar spacing (d_hkl), changing the refractive index (n_c), or a combination of the two parameters.

X-ray radioluminescent CCAs with highly tunable emission colors are synthesized using an emulsion polymerization procedure. An anthracene methyl methacrylate derivative (AMMA) is copolymerized within a poly(styrene-co-propargyl acrylate) (PS-PA) basis to generate an x-ray radioluminescent PS-PA-PAMMA CCA (87 nm), emitter 1 (E₁). Due to the electrostatic repulsive forces between the colloidal nanoparticles, E₁ spontaneously self-assembles into an FCC structure. E₁ possesses a rejection wavelength in the visible regime that can be shifted across the visible spectrum by a change in the interplanar spacing, which is achieved by the addition or removal of deionized water. Typical anthracene radioluminescence spanning 400 – 515 nm, with a maximum at 420 nm, is detected when there is no rejection wavelength interference. However, a decrease in spontaneous emission at the rejection wavelength is observed when the rejection wavelength is shifted within E₁’s radioluminescence. The radioluminescence decrease corresponding to the rejection wavelength can be shifted across the entire radioluminescence of E₁. In this way, the CCA’s radioluminescence can be manipulated by photonic means. Furthermore, the CCA’s color characteristics can be adjusted across the visible spectrum by x-ray induced sequential Förster Resonance Energy Transfer (FRET). Energy transfer is achieved by copolymerizing one or two additional organic fluorophores that form FRET pairs with AMMA and each other. A naphthalimide methyl methacrylate derivative (NMMA) that absorbs in the blue region where AMMA emits is incorporated within E₁ to generate a PS-PA-PAMMA-PNMMA CCA (125 nm), emitter 2 (E₂). E₂ emits in the green region of the visible spectrum, with a maximum at 535 nm, when pumped with an x-ray source. Additionally, a rhodamine B methyl methacrylate derivative (RMMA) that absorbs in the green region where NMMA emits is incorporated within E₂ to generate a PS-PA-PAMMA-PNMMA-PRMMA CCA (122 nm), emitter 3 (E₃). E₃ emits in the red region of the visible spectrum, with a maximum at 630 nm, when pumped with an x-ray source. In this way, x-ray induced FRET or x-ray induced sequential FRET is achieved, respectively. Furthermore, the rejection wavelength of E₂ and E₃ can be coupled with their radioluminescence to fine-tune the color characteristics of the liquid systems.

Chiral inorganic nanostructures have attracted growing attention due to their potential applications and technological prospects. Even though various strategies have been developed for fabricating chiral inorganic nanostructures, the complex processes and expense of chiral biomolecules and their derivatives remain formidable challenges to realize mass production and large-area fabrication with simple and cost-effective processes. Here, we suggest a generalized protocol for the fabrication of chiral inorganic nanostructures including Au, Ag, Pd, and TiO₂ nanostructures based on achiral templates consisting of block copolymer inverse micelles and racemic mixtures. Self-assembly of poly(styrene-b-4-vinyl pyridine) (PS-b-P4VP) diblock copolymer inverse micelles containing inorganic precursors and DL alanine additives is shown to exhibit chiral properties in a specific environment in solution, a prerequisite element for the evolution of chirality. Notably, we found the pyridine groups of P4VP interacted more favorably with D-alanine than with L-alanine via hydrogen bonding in non-polar solvents, leading to the chirality transfer from D-alanine to the D-alanine-bonded P4VP micellar cores. In-situ synthesis of nanoparticles (NPs) in the micellar core by a mild reducing agent enabled strong electronic interaction between induced chiral micellar cores and in-situ formed NPs. Therefore, we achieve outstanding chiroptical properties with high anisotropy factors of 8 10^-4 although chiral reagents and scaffolds were not involved in any step. Our design concept pinpoints forwarding steps to construct an extended library of viable chiral inorganic nanostructures, ensuring a substantial development in the field of chiroptics.

At present deep space exploration is limited by transit time and overall mission costs. Conventional spacecraft make use of chemical or electric engines, which have fundamental limitations on the possible speed one can achieve. Light driven propulsion offers a conceptually different approach to space travel. In this talk I will overview our work on solar and laser driven propulsion for space exploration.

First, I will survey our work on laser driven propulsion. I will outline key requirements and challenges associated with Breakthrough Starshot mission. I will then discuss implications of utilizing smaller and more affordable systems for near-term space exploration. I will show that with relatively moderate laser powers (~1 MW) conceptually new regimes for spacecraft maneuvering and orbiting on interplanetary and Earth orbital missions emerge. Next I will discuss nanophotonic designs that optimize the tradeoff between mass, reflectance and a need for a radiative cooling. I will also summarize challenges and opportunities of laser driven propulsion for space exploration.

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

A crystalline colloidal array (CCA) is a photonic crystal made of self-assembled colloidal nanoparticles, in which a periodic dielectric structure prohibits the propagation of a particular wavelength of light throughout the material. The blocked wavelength is known as the rejection wavelength, or stopband, of the material. Iridescence is observed in CCAs because of the rejection wavelength located within the visible light spectrum, causing the liquid system to resemble a precious opal. CCAs utilize the long-range electrostatic repulsive interactions between the colloidal particles to induce self-assembly into a crystal-like structure. Bragg’s equation can be used to estimate the observed rejection wavelength based on the angle of incident light, interplanar spacing, and refractive index of the material. This dependence on interplanar spacing indicates that the rejection wavelength can be manipulated by adjusting the particle spacing. Incorporating a scintillator into the CCA results in the system exhibiting the typical emission of the scintillator, given that the rejection wavelength does not overlap the range of emission. If the rejection wavelength is located within the emission range of the scintillator, the light at that wavelength is prohibited from propagating and the signal can be blocked. In this work, an emulsion copolymerization was used to synthesize monodisperse, polystyrene-based radioluminescent terpolymer nanoparticles that self-assembled into a face-centered cubic (fcc) structure. Anthracene is an organic scintillator that emits blue light when excited with an x-ray source. During the copolymerization, anthracene was covalently incorporated into a poly(styrene-co-propargyl acrylate) (PS-PA) basis in the form of anthracen-9-ylmethylmethacrylate (AMMA) to form a poly(styrene-co-propargyl acrylate-co-anthracen-9-ylmethylmethacrylate) (PS-PA-PAMMA) liquid CCA. Because liquid CCAs are susceptible to ionic impurities and mechanical stress, the CCA was encapsulated in a hydrogel network. The hydrogel network was photopolymerized in situ with the CCA by mixing the CCA with poly(ethylene glycol) methacrylate (PEGMA), poly(ethylene glycol) dimethacrylate (PEGDMA), and diethoxyacetophenone (DEAP) and exposing the mixture to ultraviolet light. Encapsulating the liquid CCA in a hydrogel network stabilized the crystalline structure and created a more mechanically robust film which was preserved by storing in deionized (DI) water. The hydrogel-encapsulated CCAs exhibited similar rejection wavelength and emission properties to the liquid CCAs. Multiple methods can be used to shift the rejection wavelength of the CCAs. Diluting the liquid CCA with DI water increased the interparticle spacing, red-shifting the rejection wavelength. The rejection wavelength of the hydrogel-encapsulated CCA was tuned by swelling and drying the film. When excited with an x-ray source, the encapsulated CCA exhibited the radioluminescence characteristic of anthracene with a maximum emission at 425 nm. The radioluminescence of the hydrogel-encapsulated CCA decreased when the rejection wavelength was shifted to overlap the characteristic emission. The overall emission of the system can be controlled by coupling the rejection wavelength of the CCA with the emission of the scintillator.

On-chip nanophotonics devices such as those possible on Silicon or GaAs for near-infrared frequencies have been challenging to achieve in the visible frequency regime. III-Nitride based materials have excellent material properties for visible frequency nanophotonics such as broad gain regime (e.g., for lasing) and large refractive index for waveguiding visible light. Surface emitting lasers have been demonstrated from photonic crystal (PhC) nanowire arrays [1,2]. Nevertheless, fabricating devices such as cavities, modulators and lasers that can be integrated for on-chip photonics requires light transport to be confined to a submicron sized active layer. This can be done with a low refractive index optical cladding layer separating the active layer from the high refractive index substrate layer. This is achieved in a relatively straightforward manner in Si by using low index SiO₂ buffer layer (silicon-on-insulator) or in GaAs by oxidizing an AlGaAs buffer layer to a low index AlGa_xO_1-x layer. Such options are not available for III-nitrides making on-chip integrated photonics in the visible. One approach that can address this challenge is by creating a nanoporous layer of GaN with a reduced effective index between the active region and the substrate. Nanoporous GaN layer produced by electrochemical etching have been used as a low refractive index layer to fabricate a distributed Bragg reflector (DBR) mirror [3] that can be utilized for VCSELS. We will describe fabrication of membrane type PhC cavities using nanoporous GaN as cladding from the high refractive index substrate. We will describe the effects of the porosity on the PhC cavity quality factor and cavity emission wavelength.
Sandia National Laboratories is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

[1] Wright, J. B.; Liu, S.; Wang, G. T.; Li, Q.; Benz, A.; Koleske, D. D.; Lu, P.; Xu, H.; Lester, L.; Luk, T. S.; Brener, I.; Subramania, G., Multi-Colour Nanowire Photonic Crystal Laser Pixels. Sci. Rep. 2013, 3, 2982.
[2] Anderson, P. D., Koleske, D. D., Povinelli, M. L. & Subramania, G. Improving emission uniformity and linearizing band dispersion in nanowire arrays using quasi-aperiodicity. Optical Materials Express 7, 3634 (2017).
[3] Mishkat-Ul-Masabih, S.; Luk, T. S.; Rishinaramangalam, A.; Monavarian, M.; Nami, M.; Feezell, D., Nanoporous distributed Bragg reflectors on free-standing nonpolar m-plane GaN. Applied Physics Letters 2018, 112 (4), 041109.

Effcient solar energy conversion requires broadband visible light harvesting which is not provided by most semiconductor photocatalysts. The engineering of Mie-Resonances in nanostructures of these materials offers a route for trapping light and expansion of the absorption throughout the visible spectrum. Anapole excitations and associated absorption engineering in dielectric nanoresonators are therefore the key tool of our research due to the intricate combination of nonradiating modes and strong electromagnetic field confinement in the underlying material. We present our studies on engineered nanoantennas composed of photocatalytic dielectrics and demonstrate increased light-harvesting capabilities in otherwise weakly absorptive spectral regions.

We employ anapole excitations in single nanodisks of oxygen-vacancy-rich TiO_2-x , a prominent photocatalyst that provides a powerful platform for exploring concepts in absorption enhancement. The arising photocatalytic effect is monitored on the single particle level using the well-established silver reduction reaction on TiO₂ . With the freedom of changing the optical properties of TiO₂ through tuning the abundance of oxygen vacancies, we discuss the interplay between cavity damping and the anapole-assisted field confinement for absorption enhancement.

On the route towards upscaling, collective anapole-anapole interactions are investigated in rectangular arrays of nanodisks of GaP, as a high refractive index of the resonator material emerges as favourable. Our experimental findings show that maximum visible light extinction by the array and maximum absorption in the GaP are not achieved by the densest packing of resonators. Counterintuitively, increasing the array periodicities such that collective effects spectrally overlap with the anapole excitation of a single particle leads to an absorption enhancement of up to 300% compared to a single disk. An analysis of coupling in one- and two-dimensional arrays with polarization-dependent measurements and numerical simulations allows us to discriminate between coupling interactions parallel and perpendicular to the polarization axis and evaluate their strengths.

Based on our findings we fabricate large-scale nanoimprinted photoelectrodes that make use of the investigated photonic effects, allowing for the generation of quantifiable catalytic yields. We share our latest results of wavelength dependent photoelectrochemistry on these electrodes and demonstrate a photocurrent enhancement by a factor of 5.7 compared to a planar film under hydrogen evolution reaction conditions.

Our results provide a fundamental understanding of tailored light absorption in coupled anapole resonators and reveal important design guidelines for advanced metasurface approaches in a wide range of energy conversion applications.

Tailoring light-matter interactions with precise yet scalable fabrication of hybrid structures, only few nanometers in size, is essential to advance nano-optics and plasmonics. Conventional top-down fabrication techniques though lack the resolution necessary to achieve such dimensions. Alternatively, colloidal nanoparticles, with controlled size, shape, composition, and crystallinity, can reach unprecedented resolutions while also enabling unique functionalities. However, their current integration suffers from stochastic processing and assembly into passive structures, lacking the positional accuracy and structural complexity essential for functional devices and systems. In this work, we present a fabrication platform in which bottom-up directed assembly is combined with dry-phase transfer printing to achieve deterministic, large-area, and high-throughput patterning of colloidal metallic nanoparticles onto arbitrary surfaces with single particle resolution. This approach provides a versatile platform for colloidal particle integration into next-generation optoelectronic devices. Utilizing gold nanocubes as an example, we discuss fabrication of scalable nanopatch antenna arrays with deterministic and easily tailorable optical response on-demand. By further integrating these structures with molecular emitters, we present controlled enhancements leveraging precise engineering of light-matter interactions towards realizing functional building blocks for diverse quantum optical technologies.

Raman spectroscopy is a widely used characterization technique in material science. It is a non-destructive tool with relatively simple instrumentation, and provides intrinsic qualitative information of analytes by probing their vibrational modes. In many cases, Raman enhancement is essential for detecting low-intensity signals in high-noise environments, spectrally unresolved features, and hidden modes. Here we present optical and Raman spectroscopic characterization of fullerene C 60 in a gold microcavity. The fabrication of single-layered gold mirrors is facile, low cost and direct but was proven to give considerably significant enhancement. The findings of this work demonstrate the cavity resonance as a powerful tool in obtaining tunability over individual peak for selective enhancement in the tuned spectral range. The PL of the material within the cavity has demonstrated a red shift assumed to be caused by the low-energy transitions. These transitions are induced by virtual low-energy states generated by the cavity. We further observe that adopting this principle enables resolution of active Raman modes that until now were unobserved. Finally, we assigned the new experimentally observed modes to the corresponding motions calculated by DFT.

There is an incontrovertible need for low dimension materials that can provide reliable, high performance microelectronics. Single-walled carbon nanotubes (SWNTs) are one-dimensional materials that are ideal for microelectronic applications due in part to their unique optical and electronic attributes. SWNTs possess superior charge carrier mobilities when compared to conventional microelectronic materials such as Si and GaAs, and their π-conduction pathways are impervious to electromigration which is an increasing source of failure as device size decreases. Nanotube based technologies are hindered by the current standards of production, which yield heterogeneous distributions of SWNT diameters or chiralities. These irregular compositions of electronic structures often preclude their inclusion in practical applications such as field-effect transistors (FETs) or optical sensors. To isolate homogeneous SWNT samples with unique electronic structure, the aqueous two-phase (ATP) separation method gives access to electronically sorted SWNT chiralities that were previously inaccessible. We have adapted and optimized the ATP purification method to isolate a highly enriched (11,11) metallic SWNTs (m-SWNTs). Additionally, we have developed novel compositions of matter that have unique electronic and structural characteristics: SWNTs that are helically wrapped in a single chain fashion by chiral semiconducting polymers. These assemblies provide robust organic and aqueous solvent soluble superstructures featuring exceptional electronic and morphological homogeneity.
Disruption of the highly symmetric electronic structure of (11,11) m-SWNTs through polymer wrapping shifts the allowed wave vectors away from the K and K’ Dirac points in reciprocal space, resulting in a metallic to semiconducting phase transition in these m-SWNT assemblies. We have recently demonstrated metal to semiconductor phase transitions in polymer-wrapped m-SWNTs through a number of optical and potentiometric methods in both aqueous solution and the solid state. Results indicate that symmetry breaking of the SWNT electronic structure is accomplished through chiral single-chain helical polymer wrapping at a fixed pitch length that give rise to newly allowed low energy electronic transitions. These noncovalently wrapped polymer/m-SWNT hybrid assemblies do not introduce chemical defects and therefore are practical for applications leveraging electronically homogeneous nanotubes that require high reproducibility, or for devices where band gap magnitudes are tuned on the fly through modulation of electrical or optical fields. These new low energy band gap polymer/m-SWNT compositions provide a new means to manipulate optical and electro-optical function in hybrid nanoscale assemblies.

The use of optical antennas has been the subject of an increasing number of scientific studies as they can improve the efficiency of a wide range of near-field optical technologies, including light transmission and reception, spectroscopy, sensing, and imaging. Antennas are designed primarily to facilitate efficient energy transfer between a localized source and free space while serving as impedance matching elements. In this regard, most previous studies have focused on antennas made of metals since their surface plasmon polaritons can effectively confine light within deep subwavelength boundaries to enhance the coupling between the source and the antenna. However, plasmonic antennas suffer from ohmic losses, which can lead to considerable heat generation, resulting in melting or irreversible structural alteration of the antenna and thermochemical destruction of the nearby matter. An alternative way to mitigate ohmic losses is to make antennas from dielectrics instead of metals. Indeed, all-dielectric antennas have several advantages over plasmonic antennas, including excellent photostability, low heating, and high radiative quantum efficiency. Unfortunately, the relatively weak spatial confinement offered by the volumetric modes in conventional dielectric antennas makes it difficult to achieve efficient coupling between the source and the antenna itself.

We present here an all-dielectric multilayer antenna configuration that supports strongly confined modes while maintaining unity antenna quantum efficiency. Our antenna design consists of a high-index pillar structure with a transverse gap filled with low-index material. The index contrast between the gap material and the pillar material allows for strong localization of the field within the gap. We provide a detailed explanation of the operation principle of such Photonic Gap Antennas (PGAs) based on Fabry-Perot-like resonances of horizontal slot-waveguides with symmetrically and asymmetrically placed gap layer. We calculate the mode volumes of the PGA based on quasinormal mode theory and obtain mode volumes in the range of 10^-4 λ₀³ that are on par with plasmonic antennas. We demonstrate that, when acting as a transmitter, the PGA can improve the spontaneous emission rate of a point dipole by ~1300 times that of its emission rate in free space. Reciprocally, when acting as a receiver, the PGA can lead to a near-field intensity enhancement by a factor of ∼3000 with comparison to the intensity of the incident plane wave. Moreover, we show that the PGA can operate over a broad spectral bandwidth (△λ ≈ 300 nm at λ = 1.25 μm), while providing unidirectional out-of-plane radiation.

Confining and guiding light on a deep sub-wavelength scale is a key goal in nanophotonics. Plasmonic materials are used in this respect, but are often limited by large Ohmic absorption losses, resulting in undesirable heating effects and short propagation distances in waveguides. Dielectric materials, on the other hand, have substantially lower losses than metals, but do not confine light on the same sub-wavelength scale. However, by combining these two classes of materials into a single nanostructure, it should be possible to achieve simultaneous low-loss and deep sub-wavelength modes by confining plasmonic resonances in a low-loss dielectric. Here, we show that a simple geometry – a silicon (Si) nanowire (NW) with a uniform gold (Au) shell – can confine NIR light in a NW just 210 nm in diameter. Furthermore, the hybrid dielectric environment produces two unique effects: the high-index Si core allows for the Au plasmon resonances to exist in a much smaller structure and at a substantially lower energy than they would normally occur, and the Si core's Mie resonances blueshift and adopt a Fano lineshape due to coupling with the plasmons. Core diameter and shell thickness are easily tunable and can dramatically shift the positions of the hybrid plasmonic resonances. We combine bottom-up vapor-liquid-solid SiNW growth with metal sputtering to synthesize a vertically aligned, hybrid Si/Au NW - a structure that is not accessible by more conventional lithographic or solution-phase techniques. To measure optical properties, we employ a custom-built supercontinuum laser microscope for single-NW extinction measurements. The tunability of the materials and optical properties of this structure make it an exciting candidate for a range of photonic applications, from sensing to optical computing.

Self-assembly of geometrically complex structures has been accomplished in dispersions but not on the surfaces, which is needed for further studies and applications of their intriguing electromagnetic/optical properties. Their successful assembly can be accomplished using intrinsic chirality of the nanostructure and/or exerting local chiral bias. That could be translated to a variety of technologies, from medicinal chemistry to advanced manufacturing. Here, we show that the strong optical activity of plasmonic materials under circularly polarized light (CPL) affords photon-to-matter chirality transfer, fabricating centimeter-scale chiral 3D superstructures. Illuminating CPL on substrate immersed in silver salt solutions induces the formation of nanoparticles (NPs) with 15–20 nm in diameter and subsequent asymmetric deposition/assembly to construct chiral superstructures on the substrate. Circular dichroism (CD) spectra vividly showed opposite polarity after exposure to photons with left- and right circular polarization. The wavelength of the light source determined the size of the final superstructure as well as their surface plasmonic resonance (SPR) and CD band wavelength. Electron microscopy images for its intermediate growth stages showed vivid handedness of the rotational assemblies of NPs. The experimental evolution of CD peaks in different growth stages correlated well with those calculated for assembled nanostructure models with complex geometry, as established by the prediction of deposition/assembly pattern by electromagnetic simulations. The chiral patterns were fabricated on polydimethylsiloxane (PMMA) stamp by CPL exposure time to demonstrate the simplicity and universality of these photosynthetic routes to direct construction of metallic 3D chiral metamaterials on arbitrary substrates with controlled chirality. These simple and versatile architected metamaterials can be used for extensive applications, including but not limited to photonic, optoelectronic, and electromechanical devices as well as enantioselective catalytic and sensing systems.

The physical speed and power consumption of conventional microelectronics processors are reaching their limits, and consequently, novel paradigms and approaches for computing and information processing are desired. One such approach, which has gained significant renewed interest recently, is analog optical computing capable of processing large amounts of data with high speed and low energy consumption. Moreover, the tremendous recent advances in the field of metamaterials and metasurfaces have yielded the possibility of shaping optical fields in extreme ways over subwavelength thicknesses. This, in turn, has unlocked new opportunities for all-optical computing strategies and opened entirely new avenues in the field of analog signal processing and computing with waves.
Here, we demonstrate, theoretically and experimentally, a Si-metagrating-based optical platform that combines a tailored scattering matrix design and a feedback system to enable the solution of Fredholm integral equations of the second kind from the far-field
g(u)=I_in (u)+∫_a^b K(u,v) g(v)dv, (1)
where g(u) is the unknown solution of the integral equation, K(u,v) is a function of two variables that acts as the kernel of the integral operator, and I_in (u) is an arbitrary input function. This problem can be discretized by sampling the variables u and v over N points in the interval [a,b] to form two vectors, u and v, and recast into a matrix equation

g =I_in + K g (2)

where g =g(u), I_in = I_in (u) and K=K(u,v)*(a-b)/N where u and v are evaluated as N discrete points. The solution g can be thought of as the result of the Neumann series g = ∑_n (K)^n I_in= (I_N - K)^(-1) I_in where I_N is the NxN identity matrix. Next, it is possible to think of the N mathematical sampling points as N discrete physical modes, and thus g is a vector representing the complex amplitude of these modes on a given plane with a chosen direction. The integral operator can then be represented by a scattering matrix that performs matrix-vector multiplication between these sets of modes and can be mapped onto the reflective part of the scattering matrix of a Si metagrating. Finally, a semi-transparent mirror feeds the signal back into the system mimicking an analog Neumann series.
By means of the adjoint method, it is possible to inverse-design the unit cell of the Si metagrating (not including the mirror) in order to achieve a prescribed S-matrix. The unit cell is embedded in a SiO₂ superstrate. The design wavelength is λ=706 nm while the unit cell is 825 nm wide and 400 nm deep. Next, we include a semitransparent mirror, and hence the feedback system, in the simulation and check how that results in the inversion of the designed S-matrix. From the transmission block of the S-matrix of the entire stack (metagrating, SiO₂ spacer, and semitransparent mirror) it possible to read out the analog solution provided by the structure. At the conference, we will present the comparison between the metasurface-based analog solution and the ideal solution demonstrating good agreement for all the inputs, both in terms of the real and imaginary parts.
To demonstrate these concepts experimentally, the metasurface was fabricated with e-beam lithography on Si-on-sapphire substrate. Next, the sample was planarized with a layer of SiO₂ solgel and an extra layer of sputtered SiO₂. Finally, 15 nm of Au were evaporated on the sample using an organic adhesion monolayer. At each step, the transmission spectrum of the sample was measured and shows a good match with the simulated spectrum. At the conference, we will show the agreement between optical measurements and simulations for the ideal structure that solves the integral equation.
Finally, future applications of these concepts could include TMDCs two-dimensional materials to explore to dynamically tune the encoded mathematical operation, paving the way for all-optical reconfigurable computing circuitry solving problems of further enhanced complexity.

Building-integrated photovoltaics (BIPV) has received much attention recently to minimize the use of land for PV systems, decrease the overall system and installation costs due to its multifunctional use, and make PV systems more aesthetically appealing. The luminescent solar concentrator (LSC) is such a building-integrated photovoltaic device, consisting of a glass or plastic waveguide with embedded luminophores and integrated solar cells along the edges or in a matrix. Therefore, the LSC can directly be used as a (vertical) facade or roof and offers more aesthetic options due to its semi-transparent and/or colored surface.
Even though the LSC concentrates sunlight in the waveguide and guides it to solar cells smaller than the top surface area, it cannot surpass the detailed balance limit for a single-junction solar cell. In short, the LSC remains at 1-sun illumination intensity, and the potential V_OC increase due to increased J_SC (concentration factor) is offset by the coincident increase in the J₀. Thus, the power conversion efficiency (PCE) of the LSC is limited to the typical 34%, while the world record remains at 7.1%, indicating both the poor performance of LSCs so far, but also the significant potential gain. One of the main loss mechanisms in an LSC is the emission of photons through the escape-cone, accounting for about 25% of all lost photons. This is a direct consequence of isotropic luminophore emission and the escape-cone angle of a glass/plastic waveguide in air.
In this work, we use anisotropic luminophore emission to decrease the escape-cone losses in an LSC. We do so by embedding CdSe-CdZnS core-shell nanoplatelets, the luminophores, into high-index TiO2 nanocylinders that exhibit Mie-like resonances. An array of TiO2 cylinders is then incorporated into the waveguide. The shape and size of the cylinders are optimized such that sunlight with energy higher than the luminophore bandgap is resonantly absorbed in the out-of-plane direction, while the Stokes-shifted luminophore emission is resonantly directed in-plane. We use FDTD simulations to find the optimal nanostructure and identify the Mie-like modes that are excited by the incoming sunlight and couple to the luminophore emission. The simulations predict that the escape cone losses are reduced from 25% to 12%.
Arrays of TiO2 nanocylinders were fabricated on top of a glass substrate. First, a multilayer stack of TiO2, CdSe-CdZnS core-shell nanoplatelets, and TiO2 was created. Using e-beam lithography and subsequent lift-off, we created a chromium hard-mask for reactive ion etching of the multilayer. After removal of the chromium, we obtained an array of TiO2 cylinders with 450 nm diameter, 120 nm height, and 1800 nm pitch, with an embedded nanoplatelet layer of 30 nm thick. SEM images and spatially resolved photoluminescence measurements show that the luminophores are only located inside the cylinders as designed.
To demonstrate the directional emission, we measure the luminophore emission in a Fourier-microscopy setup. To emulate the incorporation of the cylinder array inside the waveguide, we used an immersion-oil objective, which also enables the collection of luminophore emission under grazing angles. Besides the use in LSCs, directional luminophore emission has potential applications in LEDs and up- and down-conversion for photovoltaics.

Silicon light funnels are three-dimensional subwavelength structures in the shape of inverted cones with respect to the incoming illumination. Light funnel (LF) arrays can serve as efficient absorbing layers on account of their light trapping capabilities, which are associated with the presence of high density complex Mie modes. Specifically, light funnel arrays exhibit broadband absorption enhancement of the solar spectrum. In the current study, we numerically explore the optical coupling between surface light funnel arrays and the underlying substrates. We show that the absorption in the LF array-substrate complex is higher than the absorption in LF arrays of the same height (~10% increase). This, we suggest, implies that a LF array serves as an efficient surface element that imparts additional momentum components to the impinging illumination, and hence optically excites the substrate by near-field light concentration, excitation of traveling guided modes in the substrate, and mode hybridization.

Future photonic technologies underpinning high-performance communications, ultrafast computations and compact biosensing will rely on densely packed reconfigurable optical circuitry based on nanophotonics. For many years, plasmonics was considered as the only available platform for subwavelength optics, but the recently emerged field of Mie-resonant metaphotonics provides more practical alternatives for nanoscale optics by employing resonances in high-index dielectric nanoparticles and their structures such as metasurfaces. This talk aims to discuss recent advances and future emerging directions in the physics of dielectric Mie-resonant nanostructures with high quality factors (Q factors) for efficient spatial and temporal control of light by employing multipolar Mie resonances and bound states in the continuum (BIC), with applications of these concepts to nonlinear optics, active photonics, and topological lasers.

In particular, we discuss the physics of photonic BICs and their applications to metadevices, including enhancement of nonlinear response, light-matter interaction, and development of active nanophotonic devices. We discuss how BIC-empowered dielectric metastructures can be used to generate efficiently high-order optical harmonics from bulk and to boost the intrinsic nonlinearity of transition metal dichalcogenide (TMDC) flakes. We explore TMDC resonators composed of structured dielectric arrays and individual nanoparticles for strong light-matter coupling phenomena. We discuss the extension of metasurface functionalities for biosensing applications in biomarker detection and quantum information processing with entangled photons. Finally, we demonstrate how tunability of BICs in the momentum space can be used to realize a novel type of efficient lasing based on a finite-size cavity with a small footprint.

In addition, we introduce a novel platform for plasmonic metasurfaces empowered by optical BICs, for engineering high-Q optical resonances. Our plasmonic metasurfaces are composed of arrays of vertically oriented pillar meta-molecules covered by a thin layer of gold. We suggest and realize experimentally anisotropic plasmonic metasurfaces supporting high-Q resonances governed by quasi-BIC collective resonant modes. With this platform, we demonstrate several other effects including novel plasmonic metasurfaces for full Stokes polarization perfect absorption in the mid-infrared. In this latter case, the metasurface unit cell consists of coupled anisotropic meta-atoms forming a diatomic metamolecule. The designed plasmonic metastructures provide a strong field enhancement by at least one order of magnitude higher than conventional perfect absorbers.

Optical nanoantennas play a central role for enhanced light-matter interactions in modern nanophotonics. Integration of a quantum emitter into a nanoantenna that is capable of increasing local density of optical states (LDOS) results in the enhanced spontaneous emission rate through the Purcell effect. In the past decade, the main focus of the Purcell enhancement has been placed on the manipulation of an electric dipole transition, and less attention has been paid to that of the magnetic dipole and higher order optical transitions. Dielectric nanoantennas have been suggested for manipulating spontaneous emission of a magnetic dipole through the magnetic Purcell effect. However, the experimentally observed enhancement of magnetic dipole emission has been marginal and much smaller than the theoretical prediction. In this work,[1] we develop a composite system, that is, a Si nanosphere decorated with Eu³⁺ complexes, in which magnetic dipole emission of Eu³⁺ is efficiently coupled to the magnetic Mie modes of the nanosphere. By means of single particle spectroscopy, we systematically investigate the light scattering and photoluminescence spectra of the coupled system with diameters of Si nanospheres from 130 to 245 nm. We demonstrate that by tuning the magnetic quadrupole Mie resonance to the ⁵D₀-⁷F₁ magnetic dipole transition of Eu³⁺, the branching ratio between the magnetic and electric dipole transitions is enhanced up to 7 times. We also show the magnetic Purcell enhancement in an ensemble of Si nanospheres with size distribution. The observed magnetic Purcell enhancement driven by colloidal Mie resonators offers an opportunity not only to develop novel fluorophores with enhanced magnetic dipole emission, but also to control the excited state of the matter in photochemistry.
[1] H. Sugimoto, M. Fujii ACS Photonics, in press (2021) DOI:10.1021/acsphotonics.1c00375

Chalcogenide phase change materials (PCMs) have shown many exciting and interesting characteristics, such as non-volatile structure transition, up to trillions of reversible cycles[1], high stability, and large optical and electric differences between their two distinct structure phases. These make them not only been used in existing electric storage[2], but also a potential candidate for reprogrammable photonics devices, micro-electromechanical systems, and tunable radio frequency devices.
An accurate model to describe the PCM switching behavior is necessary for designing programmable photonic devices. The optical properties of PCMs are determined by their crystal microstructures and phase transitions which depend on the heating and quenching conditions. To understand the evolution of a PCM photonics device's optical response during switching, and the conditions that can lead to multi-level optical devices, it is important to know not only the initial and final states, but also how the optical constants evolve during nucleation, crystal growth, and melt-quenching. Therefore, a model that accurately predicts the optical properties during and after phase transition will be a key enabler of successful photonic device designs, and in particular, phase change multi-level devices.
The existing models, such as the JMAK method and density functional theory, can not satisfy the need of modeling a PCM device in accuracy, cost, time consumption and device size. However, semi-empirical methods can have a good compromise between accuracy, scaling, and performance. Recently, Meyer et al developed a multiphysics model based on phase field method to describe the optical properties of a phase change metasurface perfect absorber but no experiments was conducted to verify the multiphysics model[3].
Herein, we developed an accurate multiphysics framework based on the Gillespie cellular automata (GCA) method to quantitively simulate the evolution of PCM crystal microstructures and optical response of PCM photonics devices. The multiphysics framework self-consistently combines the nanosecond non-isothermal laser heating, non-linear and probabilistic GCA, effective medium theory and the Fresnel Equations together. To prove the accuracy, the framework was applied to a programmable thin-film optical stack, and laser switching experiments and transmission electron microscopy were performed. Comparison between simulation and experimental results shows that the model can accurately predict not only the final optical state, the crystallographic microstructure evolution and the transient optical response, but also multilevel optical reflectivity and transmissivity which corresponds to partial crystallization of the PCM film. Therefore, we foresee the model will be widely applied to optimizing the interconnection weights in all-optical neural network schemes, controllable metamaterial phase arrays, and displays. For this reason, the code is publicly available at the GitHub website[4], and we encourage others to use it.
1. Kim, W., et al. ALD-based confined PCM with a metallic liner toward unlimited endurance. in 2016 IEEE International Electron Devices Meeting (IEDM). 2016.
2. Hoddeson, L. and P. Garrett, The discovery of Ovshinsky switching and phase-change memory. Physics Today, 2018. 71(6): p. 44-51.
3. Meyer, S., Z.Y. Tan, and D.N. Chigrin, Multiphysics simulations of adaptive metasurfaces at the meta-atom length scale. Nanophotonics, 2020. 9(3): p. 675-681.
4. Wang, Y., et al. Multiphysics GCA codes. 2021; Available from: https://github.com/YunzhengWang/multiphysics_GCA.

Significant photoinduced electric effects observed in nanostructured metal systems are clearly associated with plasmonic excitations. In order to better understand the origin and be able to predict the effect in various materials and geometries, we perform theoretical calculations for the effective "plasmonic pressure" and "plasmonic striction" ("gradient") forces based on the electro-magnetic hydrodynamic momentum loss approach (EHML) in various plasmonic materials, including gold, silver, aluminum and TiN. We show that the wavelength dependence for the pressure force is expected to be weak for most of the optical and near infra-red range with a rapid increase toward the short wavelengths. The opposite (a decrease with decreasing wavelength) is predicted for the striction force. A direct comparison of experimental data to the theoretical calculations offers new insight into the nature of the plasmon drag effect and promises a conclusive answer to which of the proposed forces has a larger impact on the magnitude of the plasmon drag effect. It is the hope of the authors that this more complete understanding of the phenomenon will contribute to the design on devices that will more efficiently utilize the plasmon drag effect for a myriad of applications from environmental sensors to opto-electric interfaces.

Nanoparticles can exhibit exotic physics, display quantum phenomena, and offer opportunities in multiple disciplines and applications. Plasmonics is one field in which nanoparticles has made a large impact, for example in biosensing, the pharmaceutical industry, and quantum optics. Particles are typically chemically synthesized and can be tailored to meet specific plasmon resonance requirements based on the chemical recipe. Resonances of nanoparticles can extend from the near infrared (NIR) to the ultraviolet (UV) spectral regions and can even be spectrally tunable based on the level of incorporated dopants [1]. However, once synthesized, the plasmon response cannot be dynamically modified. Here we report use of the electron beam in a scanning transmission electron microscope (STEM) to modify plasmon resonances of doped semiconductor nanocrystals in both space and energy.
Using the framework of a self-assembled system of nanoparticles, we direct the electron beam at higher-than-normal operating currents in order to intentionally modify individual nanoparticles. Previous work related to the use of an electron beam for modification of structures has shown precise cutting and welding of metallic an silicon nanowires [2], positioning of atoms and dopants within two dimensional materials [3], and sculpting 1D nanowires in 2D materials [4], however, here we focus on the application to nanoparticle plasmonics, and attempt to understand the mechanisms responsible for modification.
We demonstrate that near atomic-resolution sculpting of nanoparticles is possible by selectively removing atomic columns with a converged electron beam in nanoparticles whose thicknesses range from 10-15 nm. Entire particles can also be removed from the array, allowing for plasmonic hotspots to effectively be patterned by the electron beam. The modification to the plasmonic response occurs in both space and energy domains and we explore these modifications on-the-fly with electron energy loss spectroscopy (EELS). Not surprisingly, the altered geometry supports additional localized plasmons, e.g., creation of a hole within a nanoparticle allows for existence of edge plasmons on the edge of the hole. We also observe spectral shifting of plasmon resonances upon electron irradiation in different conditions, and attribute this to element-specific removal of atoms from core-loss EELS measurements. The ability to adjust the particle response on the fly significantly increases the design possibilities for nanostructure development and is key for nanophotonic design.

[1] S. H. Cho et al., J., Chem. Mater. 2019 31, 2661.
[2] S. Xu et al. Small 2005 1, 1221.
[3] O. Dyck et al. Nat. Rev. Mat. 2019 4, 497.
[4] J. Lin et al. Nat. Nanotechnol. 2014 9, 436-442

This effort is based upon work supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), Materials Sciences and Engineering Division (K.M.R., A.R.L. S.V.K.). S.H.C acknowledges (NSF, CHE-19052631609656, CBET-1704634, NASCENT, an NSF ERC EEC-1160494, and CDCM, an NSF MRSEC DMR-1720595), the Welch Foundation (F-1848), and the Fulbright Program (IIE-15151071).

Control and enhancement of spontaneous emission of magnetic and electric dipole transitions can be achieved by use of plasmonic structures. In our work, we study modification in the emission spectra, polarization and radiation patterns of Eu³⁺ ions in very close vicinity of plasmonic films, gratings and cavities. We compare effects observed in dense and diluted arrangements of emitters, and show that on the contrary to theoretical predictions, reduced quenching is observed in monolayers of closely packed EuTTA molecules. Significant enhancement of p-polarized emission is observed at certain angles in gratings for both dense and dilute Eu systems, which is ascribed to decoupling of surface plasmon polaritons excited by the emitters with a different efficiency for electric and magnetic emitters. Effects are considered theoretically based on the Green's function approach. The effects of roughness of metal and mutual emitter coherence of emitters are discussed.

We report on the design of plasmonic nanostructures that enhance the hot-electron generation and can be used as nanoelectrodes. The advantage of the nanostructure is brought about by the periodic arrangement of nanoantennas and the excitation of collective resonances [1,2]. These collective multipole resonances result in a significant field enhancement in the proximity to nanoantenna and, therefore, a more efficient generation of hot electrons. We analyze the case when hot electrons are injected into the aqueous environment and the nanoantennas have a high aspect ratio supporting multipole resonances along the antenna. We show that by selecting structure dimensions and periodic arrangement comparable to the wavelength of Rayleigh anomalies, one can achieve a significant enhancement in the localized electromagnetic field and enhanced hot-electron generation.

D.B. acknowledges the support from grant N00014-18-1-2739 (the Navy HBCU-MI program) and grant N00014-19-1-2117 (the office of Naval Research). V.E.B. acknowledges the support from The University of New Mexico Research Allocations Committee, award RAC 2022, for the computational resources.

References:
[1] D. Bosomtwi, M. Osinski, and V. E. Babicheva, “Mode Coupling and Rabi Splitting in Transdimensional Photonic Lattices,” 2020 IEEE 20th International Conference on Nanotechnology, 107-110 (2020).
[2] D. Bosomtwi, M. Osinski, and V. E. Babicheva, “Lattice Effect for Enhanced Hot-Electron Generation in Nanoelectrodes,” under consideration (2021).

Nanoparticle phase transformations and their transient states underlie many technologies in energy storage, memory, and catalysis. However, modifying these intermediate states requires bridging the length-scale gap between the atomic-scale structural features (such as atomic coordination number and surface strain) that influence dynamics and the macroscale extrinsic parameters (e.g. temperature, chemical environment) that can be controlled. Optical excitation of localized surface plasmon resonances (LSPRs) offers a solution for overcoming this size mismatch. Here, we use in situ environmental transmission electron microscopy coupled with optical illumination to demonstrate how LSPRs enable spatially-modified phase transformation dynamics in nanoparticles. As a model reaction, we study the gas-phase photocatalytic dehydrogenation of a variety of Au-Pd systems, including Au-Pd crossed-bar systems, and AgPd bimetallic nanoparticles, where [Ag,Au] acts as a plasmonic light absorber and Pd serves as the catalyst. Under controlled hydrogen pressures, temperatures, and illumination conditions, we study the study the kinetics of the hydrogen absorption and desorption reactions triggered by the optical excitation of plasmons. We find that plasmons control photochemistry in four ways. First, plasmons modify the rate of distinct reaction steps differently, increasing the overall rate more than ten-fold. Secondly, plasmons open new reaction pathways that are not observed without illumination. Third, reaction nucleation occurs at electromagnetic hot-spots – even when those hot-spots do not occur in the preferred nucleation site. Finally, through atomic-resolution imaging, we show how plasmons modify sub-surface hydrogen within the first few atomic layers of the nanoparticle. We link our single particle results to reactor-scale nanoparticle performance in reactive hydrogen environments, showing how catalyst sub-surface hydrogenation impacts not only product yield but also product-selectivity.

Most materials interact weakly with light, which is why we treat light as an external perturbation. There has been growing interest in systems where light-matter interaction is much stronger so that it becomes comparable to other characteristic energies of the system. In the regime of ultrastrong and deep strong coupling the coupling energy approaches the bare excitation energy of a material. This regime leads to exotic effects like the population of the ground state with virtual photons, phase transitions, and a breakdown of the Purcell effect. Here we discuss how extreme light-matter coupling is systematically achieved in materials. We introduce densely packed supercrystals of gold nanoparticles that have coupling energies corresponding to the visible spectral range.[1,2] We present measurements of their plasmon-polaritons, confirm the breakdown of the Purcell effect as an example of the effects predicted for deep strong coupling, and show how to use such materials as enhancing agents in surface-enhanced vibrational spectroscopy.[1-4] We discuss a universal model to treat light-matter coupling in systems of different dimension and the strongest light-matter coupling we can reasonably expect to achieve in materials.

[1] Niclas S. Mueller, Yu Okamura, Bruno G. M. Vieira, Sabrina Juergensen, Holger Lange, Eduardo B. Barros, Florian Schulz, and Stephanie Reich, Deep strong light-matter coupling in plasmonic nanoparticle crystals. Nature 583, 780 (2020).
[2] F. Schulz, O. Pavelka, F. Lehmk�uhler, F. Westermeier, Y. Okamura, N.S. Mueller, S. Reich, and H. Lange. Structural order in plasmonic superlattices. Nat. Comm. 11, 3821 (2020).
[3] Niclas S. Mueller, Emanuel P tzner, Yu Okamura, Georgy Gordeev, Patryk Kusch, Holger Lange, Joachim Heberle, Florian Schulz, and Stephanie Reich. Surface-enhanced raman scattering and surface-enhanced infrared absorption by plasmon polaritons in three-dimensional
nanoparticle supercrystals. ACS Nano 15, 5523 (2021).
[4] Eduardo B. Barros, Niclas S. Mueller, and Stephanie Reich, Plasmon-Polaritons in Nanoparticle Supercrystals: Microscopic Quantum Theory Beyond the Dipole Approximation. Phys Rev B, in print (2021).

Pseudoelasticity in metal nanocrystals allows for shape recovery from strains much larger than their bulk counterparts. This fascinating property could be used to engineer self-healing or reconfigurable materials, but to take full advantage of its possibilities a deeper understanding of its mechanism and limitations is needed. For instance, it is unknown whether room-temperature pseudoelasticity can occur in all metal nanocrystals without the introduction of plastic damage. Here we report the use of non-hydrostatic compression of gold nanocrystals in a diamond anvil cell to a range of maximum pressures, up to 11.4 GPa. Optical absorbance spectroscopy of the localized surface plasmon resonance is used to non-invasively monitor changes in particle shape and crystallinity, as indicated by the plasmon resonance peak position and intensity, respectively. We find that while complete shape recovery occurs following compression to all pressures tested, irreversible crystalline defects are only introduced above a threshold of approximately 2.5 GPa. In this way, we establish the capacity of gold nanocrystals to undergo complete pseudoelastic shape recovery following compression under moderate loads, as well as the onset of limited pseudoelastic behavior at higher loads. This work lays a foundation for future all-optical investigations of the limits of pseudoelastic deformation in a wide variety of metal nanocrystals.

The favorable optical and catalytic properties of bimetallic noble metal nanocrystals have been directly attributed to the atomic-scale arrangement of metal atoms. In AgAu nanocrystals, for example, CO oxidation efficacy has been directly attributed to the presence of Ag-Au bonds. Despite the mechanistic hypotheses that atomic scale structure affects observed phenomena, atomic distributions within bimetallic nanocrystals are rarely investigated in tandem with synthetic advances, and often alloy structure is assumed to be homogeneous based on species miscibility alone. While thermodynamic models have suggested that Ag/Au phase segregation is favored under certain nanoscale conditions, the effects of kinetics on atomic distributions within nanoparticle alloys have not been systematically explored. Based on differences in redox behavior between noble metal species, we hypothesize that altering parameters that affect atomic reduction within bimetallic systems will lead to differences in atomic incorporation and distribution within bimetallic nanocrystals.

Herein, a systematic investigation of the effects of reduction parameters on AgAu alloy atomic-scale distributions will be discussed. In particular, the Ag:Au solution ratio, concentration of reducing agent to metal atoms and volume of reductant added have been varied, and the resulting morphology, composition and atomic coordination environments mapped accordingly using X-ray focused approaches. Electron microscopy and small angle X-ray scattering trace differences in particle morphology, X-ray fluorescence determines composition, UV-vis spectroscopy shows changes in the position of the local surface plasmon resonance peak and X-ray absorption spectroscopy reveals differences in atomic coordination environment. Together, these techniques reveal that indeed the parameters that affect reduction kinetics can alter alloy distributions within nanoparticles, even when nanoparticle composition remains constant. Other notable results include trends in Ag underincorporation and the proposal of synthetic methodology that can be employed to encourage the formation of homogeneous alloys. Results have substantial impact on the use of bimetallic noble metal nanoparticles for plasmonic sensors and as substrates for heterogeneous catalysis.

The growth of highly metallic transition metal nitride films has received considerable attention using methods such as RF sputtering, atomic layer deposition, pulsed laser deposition, and molecular-beam epitaxy. Especially, niobium nitride (NbN) is known for its superconducting properties with relatively high superconducting T_c which is crucial for superconducting nanowire single-photon detectors. However, the wavelength sensitive detection with high detectivity remains unexplored. In this work, we demonstrate a plasmon enhanced photodetection in NbN superconducting photodetectors. We deposited niobium nitride (NbN) thin film by radio-frequency (RF) magnetron sputtering on MgO (100) substrate at 800 °C with high crystalline quality. Since the superconductivity of the NbN film can be varied with the argon/nitrogen flow rate, target, RF power, growth temperature, and growth substrate during sputtering. Thus, we optimize the metallicity and superconductivity (with T_c ~16 K) of NbN films by adjusting the argon/nitrogen flow rate. To increase the photodetectivity, we design an Ag nanocube nanoresonator with a strong gap-plasmon resonance in the visible range to further enhance the photoresponsivity by engineering the optical response for the superconducting devices. This is due to the superconducting states are broken down by a localizing strong electromagnetic field. To design the plasmonic nanostructures, we calculated the electromagnetic field distribution of Ag nanocube/Al₂O₃/NbN structure by finite-difference time-domain (FDTD). We observed a strong plasmonic resonance field which tightly confined in the Al₂O₃ layer between the Ag nanocube and NbN film at a resonant wavelength of 532 nm. We will discuss the detailed working mechanism and the potential application of plasmon-enhanced photodetection in NbN superconducting photodetectors.

In this talk, we will present an overview of our ongoing work on exploring the theoretical foundations of networks of photonic structures that can perform mathematical operations and analog computation with waves/signals. These structures may include metadevices such as couplers, which can be waveguide-based or material-based couplers, and reconfigurable multipliers that provide phase shifting and amplification/attenuation of signals. We will discuss some of the mathematical operations such networks of metadevices can perform, including matrix-vector multiplication, equation solving and matrix inversion and related capabilities. Moreover, we will discuss the possibility of generalized inversion (also known as pseudoinversion or Moore-Penrose inversion) of matrices that can be singular and rectangular, leading to possible applications of such metadevices for gradient descent computation. We will discuss our theoretical findings and their physical insights, and we will also mention possible future research directions and applications.

A swift electron beam can interact with localized surface plasmon resonance (LSPR) near-fields to excite an LSPR, which may then decay and emit a photon into the far-field [1,2]. The initial excitation process is detected in electron energy loss spectroscopy (EELS); the decay and photon emission is detected using cathodoluminescence (CL). The two techniques together provide a powerful combination for studying both the near- and far-field properties of LSPRs.
We use EELS and CL to study a vertical split ring resonator (VSRR), a popular nanostructure for left-handed metamaterial design and an interesting option for dichroic spectroscopy platforms [3]. The VSRR supports an LSPR mode with an electric and magnetic dipole, both in-plane and relatively strongly excited using light [4]. The ~300 nm long gold VSRRs were fabricated using a double-step electron beam lithography process, then characterized using EELS in a monochromated FEI Titan scanning transmission electron microscope, with an energy resolution of ~60 meV. We acquired polarization-resolved CL spectroscopy data in an FEI XL-30 field emission scanning electron microscope equipped with a parabolic mirror, polarization filters, and IR and visible spectrometers. Using six acquisitions with different polarization filter settings, we derive the full polarization state of the emitted light [5,6] from the VSRR LSPRs. Our experimental data is supported by simulations using the MNPBEM toolbox [7] and custom Python data processing code [8,9].
In EELS we identify the magnetic dipolar mode at a free-space wavelength of approximately 1800 nm, and a series of higher order modes in the visible region of the spectrum [10]. We isolate this dipolar mode in our CL acquisition and derive the emission polarization as a function of electron beam location. The CL signal is strongest when the electron beam is located near the tips of the VSRR, overlapping closely with EELS intensity hotspots for this mode. When the electron beam is located far from either of the symmetry axes in the VSRR, we detect a non-negligible amount of circular polarization in the emission.
At higher energies, there are a series of coupled electric dipoles on the rims of the VSRR pillars. These coupled dipoles form bonding and anti-bonding longitudinal and transverse pairs, with corresponding different linear polarization states. Although these modes are heavily intermixed in a small spectral region, they respond to different polarization states and will produce different near-field distributions under photonic excitation, as we determined from the spatially unique EELS and CL maps at different energies. The dependence of the polarization state on the electron beam location reveals that, reciprocally, the near-fields in different locations may respond strongly to different incident polarizations. With regards to assessing the viability of a nanostructure for dichroic spectroscopies, this relationship could be very fruitful.

Acknowledgements
Experimental EELS work was performed in the Canadian Centre for Electron Microscopy, supported by the Natural Sciences and Engineering Research Council of Canada (ICB, EPB, GAB). Experimental CL work was performed at AMOLF, part of the research program of the Netherlands Organization for Scientific Research and funded by the European Research Council (ICB, SM, TC, AP).

References
[1] Nelayah et al., Nat. Phys., 3, 5, 2007.
[2] García de Abajo, Rev. Mod. Phys., 82, 1, 2010.
[3] García-Etxarri & Dionne, Phys. Rev. B, 87, 23, 2013.
[4] Gay-Balmaz & Martin, J. Appl. Phys., 92, 5, 2002.
[5] Coenen & Polman, Opt. Express, 20, 7, 2012.
[6] C. I. Osorio et al., ACS Photonics, 3, 1, 2016.
[7] Hohenester, Comput. Phys. Commun., 185, 3, 2014.
[8] Bicket (2020), [Python 3], https://github.com/icbicket/CLFields. DOI: 10.5281/zenodo.3739563.
[9] Bicket (2020), [Python 3], https://github.com/icbicket/SpectrumImageAnalysisPy. DOI: 10.5281/zenodo.807763.
[10] Bicket et al., Microscopy, 67, suppl_1, 2018.

Disordered nanostructures are ubiquitous in Nature, usually generating white or black colours due to their broadband optical response and robustness to perturbations. By manipulating the optical environment, we are able to tune the disordered plasmonic networks from broadband absorption to band-limited reflection through deterministic control of the coupling to an external cavity, producing rainbow-liked colours at will [1]. Counterintuitively, fabrication imperfections (intrinsic disorder) can be harnessed as a beneficial factor to improve the purity of structural colours. More surprisingly, the disorder can disentangle optical responses and material prosperities (refractive indices), in sharp contrast with the traditional belief that optical responses vary with the material. With the assist of random scattering inside disordered structures, the same colour is achieved by metallic nanostructures composed of different noble metals including gold, silver, palladium and platinum [2]. Not limited to fundamental physics, the proposed platform may inspire novel applications ranging from Raman spectroscopy, nonlinear optics, photocatalysis.


[1] P. Mao, C. Liu, F. Song, M. Han, S.A. Maier, S. Zhang, “Manipulating disordered plasmonic systems by external cavity with transition from broadband absorption to reconfigurable reflection", Nature Communications 11, 1538 (2020).

[2] P. Mao, C. Liu, Y. Niu, F. Song, R. E. Palmer, M. Han, S. A. Maier, S. Zhang, Disorder-Induced Material-Insensitive Optical Response in Plasmonic Nanostructures: Vibrant Structural Colors from Noble Metals", Advanced Materials, 202007623 (2021).

Light can hybridize with low-dimensional materials, forming half-light-half-matter quasiparticles known as polaritons, featuring deeply subwavelength field confinement and broadband responses from terahertz to visible and infrared range. Among them, hyperbolic phonon polaritons (HP²) in natural materials, especially anisotropic van der Waals (vdW) crystals, are uniquely interesting. Their hyperbolicity originates from the drastic difference in atom interaction strength between in-plane covalent bondings and out-of-plane vdW bondings, and it can support large momentum for enhanced light-matter interactions. Based on these features, directional propagation and dispersion engineering of HP² have been demonstrated in hexagonal boron nitride (hBN) and α-MoO₃, facilitating super-resolution focusing and imaging, canalization, molecular sensing, reconfigurability, and other polariton applications. However, current state-of-the-art discoveries in HP² have been mostly based on studying the amplitude of polaritons, while other degrees of freedom, such as nontrivial geometric phase, remain largely unexplored. Surface plasmon polaritons (SPPs), a counterpart of phonon polaritons in the visible and near-infrared range, have gained more controllability over their phases, as additional degrees of freedom. In particular, a plasmonic vortex featuring orbital angular momentum (OAM) with spiral phases has been shown to support a large number of states in Hilbert space, thus offering the potential to increase the capacity of optical communication networks. Plasmonic OAMs can also couple to the spin of excitation, inducing spin-orbit interactions, enabling spin-controlled plasmonic applications such as unidirectional propagation, spin-locked optical vortex generation, and chiral information detection. However, plasmonic materials act as conductors in the mid-infrared (mid-IR) range, and graphene, which supports infrared plasmons, has a limited light-matter interaction efficiency due to its atomic thickness. Herein, we report mid-IR deep-subwavelength HP²Vs with high tunability in pristine hBN flakes. Our system is composed of a substrate gold disk with Archimedean spiral shapes to launch HP² in hBN. Via scattering-type scanning near-field microscopy (s-SNOM), HP²Vs are observed in real space, and the spin-orbit interaction and high tunability of HP²V are accordingly demonstrated. Our results facilitate effective optical spin-orbit interaction, super-resolution sensing and imaging, enhanced light-matter interaction, and particle manipulation in the mid-IR regime_.

Gap plasmons allow strong optical field confinement, and has many promising applications in plasmonic circuitry, nonlinear optics, sensing etc. In this work, we study both flat and profile modulated three layers sandwich-like structures made of external plasmonic metal layers and a dielectric layer. The excitation of gap plasmons at specific thicknesses of the intermediate layer is predicted with COMSOL simulations. Introducing Eu³⁺ emitters into the dielectric layer, we show that such structures can provide strong control on magnetic and electric dipole emissions and radiation patterns. The experimental structure is fabricated using thermal evaporation of metal and spin coating of dielectric layer. A clear conformity between experimental observations and theoretical predictions is observed.

Since the development of diffractive optical elements in the 1970s, major research efforts have focused on replacing bulky optical components by thinner, planar counterparts. More recently, dynamic flat optics has been gaining importance and new opportunities are emerging for dynamic wavefront manipulation. I will discuss efforts to realize electrically-tunable flat optics based on metasurfaces that employ nanomechanics, electrochemistry, free-carrier dispersion, phase change materials, and atomically-thin semiconductors.

Molybdenum sulfide (MoS₂) shows strong optical absorptions in UV-vis electromagnetic spectrum and high photo-corrosion tolerance, being an excellent photovoltaic and photocatalytic material. Here MoS₂ nanosheets were prepared and defect engineered to tune optical properties. Their microstructures were characterized by diffractions, electron microscopies, and optical methods. The defected nanosheets were then fabricated as electrode catalysts in I^3-/I^- -based dye-sensitized solar cells (DSSCs). The defects in MoS₂ nanosheets enhanced the photoelectric conversion efficiencies of the DSSCs. The work is partially supported by DOE grant DE-FE0031906.

Alloy quantum dots (aQD) represent a new class of nano-materials whose electron confinement is determined not exclusively by size, but also by composition field. While homogeneous quantum dots have been more widely explored for applications in photonics, optoelectronics, and photovoltaics, aQDs remain less investigated. The differences in both size and composition field across differently shaped aQDs lead to differences in material properties. These variations in material properties present compelling potential applications, specifically in the field of energy generation and photovoltaics. This is primarily because of the complexity in characterizing nonuniform fields and establishing the correlation between inhomogeneity and confinement. In order to effectively analyze the varied electron confinement of these materials, we employ a combination of density functional theory based quantum simulations, and finite element analysis based k.p calculations to determine the electron energy levels for a number of uniform and nonuniform physical variables. As a part of the analysis, composition maps were constructed for each of the four different aQD shapes, namely, the steep cone, normal cone, hut, and dome. Each of these composition’s maps was fitted with an analytical function that was then used to analyze the quantum confinement and general electronic properties of the aQDs. Results show that a generic expression for all four of the aQDs exists such that modification of this equation allows each of the disparate aQDs to be modeled.

The steep cone shaped aQD with a 15° sidewall angle corresponds to a composition map where spatial decay occurs from the tip of the cone to the bottom. The normal cone shaped aQD exhibits a similar behaviour, but the sidewall angle is instead 30°, and the fitting function is modified to a product of a quadratic and a linear function. For the hut shaped aQD, the composition map takes the shape of a “hut” where decay occurs from the top edges towards the bottom edges in a non-linear fashion. For the final shape, the dome, spatial decay emanates from the top tip of the dome. The dome is fitted by two sidewalls per side that have angles of 11° and 30° respectively.

For each of the quantum dot shapes above, composition maps obtained are independent of size to determine the singular effects of the compositional variation. Our results suggest that inhomogeneity in composition field has a consequential role in controlling the confinement energy levels, with the highest ground state confinement in the steep cone shape, and the lowest in the hut shape. Thus, the shape of aQDs can play a critical role in tailoring effective electronic behavior. Furthermore, a larger alloy quantum dot can behave electronically like a smaller quantum dot, if the composition field is inhomogeneous. By engineering quantum dots with certain composition maps, the photon absorption properties of the respective quantum dot can thus be tailored substantially. In particular, the most apparent application of this is to solar cells and spintronics; engineering the properties of the quantum dot allows us to increase the absorption efficiency of the photovoltaic cell, which directly leads to greater potential for green energy generation.

A novel Solid State Incandescent Light Emitting Device (SSI-LED) that was fabricated from the MOS capacitor with the high-k gate dielectric was invented and investigated by Kuo’s group (1). Upon the application of a gate voltage to the device, light emission is observed. The principle behind the light emission is the thermal excitation of nano-sized conductive paths, i.e., nanoresistors, formed after the breakdown of the dielectric layer (2). Previously, the light emission from nanoresistors across the gate electrode area was assumed to be uniform (3). However, some experimental results (4,5) showed that the density of nanoresistors near the edge of the gate was larger than that away from the edge.
In this paper, authors utilize the Finite Element method to explore the reason behind the formation of the denser nanoresistors at the circumferential edge of the gate electrode. The current density distribution, which is the foundation behind the generation of the large number of nanoresistors from the dielectric breakdown process, has been simulated using the COMSOL Multiphysics software package and the results are depicted using MATLAB and Python codes. The area circumferential to the gate electrode has a larger density of nanoresistors than the rest of the area due to the accumulation of larger amount of charge carriers before the dielectric breakdown. Possible factors contributing to the uneven distribution of the nanoresistors, such as the dielectric and substrate properties, have been investigated. The substrate-to-gate electrode area ratio has been identified as the key factor in determining the width of the periphery area with the high density of nanoresistors.
1. Y. Kuo and C.-C. Lin, Appl. Phys. Letts., 102(3), 031117 (2013).
2. Y. Kuo and C.-C. Lin, Solid State Electronics, 89, 120 (2013).
3. A. Shukla and Y. Kuo, ECS Trans., 97, 69-77 (2020)
4. C.-C. Lin and Y. Kuo, ECS J. Solid State Sci. Technol., 3, Q182-Q189 (2014).
5. Y. Kuo and C. C. Lin, ECS Solid State Lett., 2, 59-62 (2013).

Gold nanoparticles (Au NPs) have unique surface plasmonic resonances (SPR) and promise a broad variety of optical applications in sensors and imaging, and combining them with organic components can give electronically functional hybrid materials [1]. In general, protecting surface ligands are required to stabilize the Au NPs and prevent their irreversible aggregation. The ligands carry one or more functional anchor groups with a high affinity for gold. Thiols are the most ubiquitously used anchor group, yet the thermal instability and comparably low conductance of the S−Au contact limit the applications of the resulting Au NPs. N-heterocyclic carbenes (NHCs) have been receiving much interest in surface chemistry as an high-performing alternative to sulfur-based ligands: The NHC−Au linkage shows excellent stability under conditions that destroy the S−Au bond (e.g. high temperature, variable pH, and electrochemical redox), and is furthermore highly conductive.

We use NHC anchors as robust and conductive anchors to bind charge-conducting conjugated polymers (CPs) to Au NPs to obtain electronically functional hybrid materials.

Three different Br-NHC-Au-X (X=Br, Cl) complexes with were synthesized in multiple steps. The complexation saturates and stabilizes the NHC, the bromo function on the benzimidazole ring can be used to further functionalize the complexes. 2-Bromo-3-hexyl-5-iodo-thiophene was converted to a Grignard monomer with isopropyl-magnesium chloride, and used for a chain-growth Kumada polymerization starting from the active aryl bromide on the NHC complexes. Using a Ni−bipyridyl complex as intermediate catalyst, P3HTs functionalized with the NHC-Au-X were prepared. The Mn values are in the range of 10k, the head-to-tail regioregularity was visible in ¹H NMR. ¹H MNR and MALDI MS were used to confirm the terminal NHC units, but also showed unmodified chains due to the tendency of the Ni catalyst to dissociate from the initiators into the reaction solution and initiate the polymerization without functional group.

Direct reduction with NaBH4 directly gave Au NPs with P3HT as surface ligands. For these P3HT-NHC@Au NPs, TEM images showed spherical particles with average diameters of 3.2, 2.5, and 6.5 nm, respectively. The presence of the NHC on the surface was confirmed by XPS.The P3HT-NHC@Au NPs have a remarkable thermal stability up to 24 h in 100 °C toluene solutions, while Au NPs stabilized by sulfur-based PEG fully degrade under the same conditions. Also, continuous CV scans revealed only a slight decay of peak current after 50 cycles, indicating the good electrochemical stability of P3HT-NHC@Au NPs.

To investigate the conductive linker, electrochromism (EC) experiments were carried out. P3HT is electrochromically active, and one can assume that binding the polymer to Au NPs via a conductive NHC anchor group will increase the electrochromic response speed, allowing to estimate the organic/inorganic electron transfer of P3HT-NHC@Au NPs. We found that blending 10% of P3HT-NHC@Au NP into an P3HT-NHC-Au films improves the responsive speed signiffcantly, from 1.21/1.05 s for neat P3HT-NHC-Au to 0.79/0.52 s for the blend.

This is consistent with DFT calculations (B3LYP/6-31g(d,p) for H, C and S; B3LYP/lanl2dz for Au) on a simple model compound (NHC linking tetrathiophene to a single Au0 atom): The electron cloud was found to be delocalized over the gold atom and the NHC in the HOMO orbit, suggesting an efficient electron transfer between Au NP and the conjugated P3HT polymer.


[1] Steiner, Lissel, Fery, Lauth, Scheele: Angew. Chem. Int. Ed. 2021, 60 (3), 1152.
[2] Sun, Zhang, Simon, Steiner, Schubert, Du, Qiao, Fery, Lissel: Angew. Chem. Int. Ed. 2021, 60 (8), 3912.

Surface- and graphene-enhanced Raman spectroscopy (SERS and G-SERS) are attractive analytical techniques for the detection of chemical and biological molecules at a single-molecule level registering their chemical fingerprints. Graphene-mediated or G-SERS boosted this phenomenon producing signal enhancement from both electromagnetic (EM) and chemical (CE) mechanisms. Graphene oxide (GO) nanosheets coated with silver; Ag (30 nm) and gold; Au (40 nm) nanoparticles were developed smart array of platforms for direct detection and differentiation of DNA bases: Adenine (A), Cytosine (C), Guanine (G), Thymine (T) as well as environmental relevant β-carotene (β-C) and malachite green (MG) biomolecules. The size and interparticle gap of metal nanoparticles were controlled via dispersion and loading on GO to achieve optimal enhancement factors. These substrates were characterized by electron microscopy, atomic force microscopy, and Raman spectroscopy. The Raman spectra consisted of discrete bands characteristic of (A: 732.8 cm^-1; C: 792 cm^-1; G: 658 cm^-1; T: 796 cm^-1; β-C: 1416 cm^-1; MG: 1174 cm^-1), which represent molecular normal modes of vibration and serve as a fingerprint of three-dimensional structure, intramolecular interaction and steady-state dynamics of biomolecule ensemble. The GO-decorated nanoparticles are capable of sensitive biomolecular detection over a broad concentration range of 100nM-100mM with the limit of detection (LOD) noted at <1ppm for A, C, G, T, β-C, and MG. Moreover, the experimental results illustrated five to six-order magnitude signal enhancement in the following order: GO/Ag30>GO/Au40>Ag30>Au40>GO. Moreover, ssDNA probe and complementary strand target DNA were monitored simultaneously during hybridization on the same substrates assigned confidently to base, local structure, and global conformation of dsDNA. The experimental findings suggest (1) a strong GO-nanoparticle coupling leads to interfacial hybridization (charge transfer), polarization, and molecular orientation, and (2) it will help to design ultrasensitive G-SERS platforms covering a range of inorganic nanoparticles and nanostructured organic surfaces for a wealth of biomedicine and environmental applications.

We Investigated electronic structure and optical properties of transition metal nitrites i.e. TiN, HfN, ZrN, and their composition, TiZrN and TiHfN by using density functional theory and experimental techniques. These metallic materials are envisaged as alternative plasmonic materials in preference with comparison to conventional nobel metals from optical to the infrared wavelength range. Band structures and density of states are calculated in addition with bulk plasma frequency, absorption-extinction coefficients using ab initio procedure. Evaluation of calculation along experimental results for frequency dependent dielectric functions contributed by their interband and intraband transitions depicted optical losses in these refractory materials. Aforementioned materials with optimized thin films were deposited on suitable substrates of MgO and Si. These stoichiometric thin films were characterized to measure real and imaginary part of dielectric function by spectroscopic ellipsometry (SE). From quantitative analysis of measured and calculated quality factors for localized surface plasmon resonance (Q_LSPR)and surface plasmon polaritons (Q_SPP) it is confirmed that HfN, TiN, TiZrN, TiHfN and ZrN possess excellent mechanical, chemical, and optical properties to be used as an alternative plasmonic material of gold in the visible and near-infrared spectral regions respectively. The photonic energy absorbed by these material is far less than gold (Au) at optical frequency, calculated and demonstrated as concrete evidence in absorption spectrum.

The necessity of increasing the optical radiation absorption in two-dimensional semiconductors significantly slows down the possibility of their technological implementation in modern photovoltaic and photosensor applications. In an unmodified transition metal dichalcogenides monolayer, the optical absorption in the visible range does not exceed 7%, which is extremely low for generating a sufficiently high photoelectric effect. There are several approaches, among which the most effective is the use of low-dimensional plasmonic metallic elements. Such structures make it possible to generate local amplification of optical radiation near their surface, leading to a significant increase in absorption in adjacent semiconductor structures.
Here we demonstrate a successful approach for increasing absorption in two-dimensional transition metal dichalcogenides films using ordered matrices of plasmonic elements of various shapes. The monatomic layer WS₂ was used as an active element. To achieve the highest efficiency of photodetectors, theoretical modeling of the plasmonic structures was made. Using this simulation, the optimal parameters of silver nanostructures were determined. Based on the obtained results, a series of experimental photodetectors was made. It was found that the created plasmon detectors have an almost tenfold increase in photosensitivity in comparison with conventional structures. It is shown that such an approach makes it possible to develop polarization-sensitive and spectrally selective optical devices. Moreover, their performance characteristics can be changed in a wide range by varying the geometric parameters of the plasmonic structures.
This work was supported by the Russian Science Foundation No. 19-72-10165. E.D. M. acknowledges the support of the Ministry of Education and Science of the Russian Federation (state task No. FSFZ-0706-2020-0022).

The coupling physics of localised surface plasmons of metal nanoparticles and phonons of graphene has attracted a great interest in the last few years. Surface-Enhanced Raman Scattering (SERS) is an effective method to probe this effect, where the intensity of the graphene Raman peaks are enhanced by the near field plasmonic effect originating from metal nanoparticles. In this work, we have examined the plasmonic property of the interface of a twisted multilayer graphene system and Ag nanoparticles (~ 100 nm). The twisted graphene produced in the laboratory has been established to possess decoupled layers leading to strong 2D bands [1]. The presence of plasmonic interface is found to cause relative enhancement of the 2D band intensity, the extent of enhancement varying with the nature of the substrate. Further, we have studied the particle size and laser energy dependence on this differential enhancement.

[1] Mogera, U., Dhanya, R., Pujar, R., Narayana, C., & Kulkarni, G. U. (2015). Highly decoupled graphene multilayers: turbostraticity at its best. The journal of physical chemistry letters, 6(21), 4437-4443.

Localized surface plasmon resonance (LSPR) brings about some interaction between metal nanostructures and the surrounding materials. Generation of hot electrons through LSPR allows a lot of applications such as photothermal manipulation of materials surrounding the metal nanostructures as well as promoting photovoltaics and photocatalysis.[1-5] Fabric is very familiar with us in our daily life. We focus on it as an attractive substrate to densely modify metal nanostructures with the viewpoint of the large surface area, chemical resistant property, and flexible workability. In this study, we prepared a gold nanostructure-modified fabric in such a way of dyeing and investigated the photothermal effect on the fabric.
Immersion of tetrachloroauric ions-adsorbed cotton fabric in a heated aqueous solution of sodium citrate gave dark purple color. The coloration is due to strong plasmonic absorption through densely formation of isolated gold nanostructures on the cotton fabric. The X-ray diffraction pattern of the gold nanostructure-modified cotton fabric (Au-cotton) shows peaks assigned as Au(111) and (200) as well as cellulose. The Au-cotton has an absorption band in visible light region with a peak around 550 nm. Irradiation of visible light to the Au-cotton causes the temperature rise at the surface. The largest temperature rise per unit given photon number occurs by irradiation at 550 nm. The rising rate to the light power at 550 nm was estimated to be ca. 0.5 K/mW between 293 and 343 K. The results of the dependence of the temperature rise on the irradiation wavelength suggest that the temperature rise comes from generation of hot electrons based on LSPR.
We demonstrate an interaction between the gold nanocrystals and the surrounding material as an example of the photothermal effect. The Au-cotton soaked with paraffin was placed on a glass plate at room temperature and irradiated with visible light. The paraffin in the Au-cotton melted and seeped out onto the glass surface attributed to the temperature rise of the Au-cotton based on LSPR. When the light irradiation was stopped, the paraffin solidified. Consequently, the Au-cotton adhered to the glass surface.
[1] A. Furube, S. Hashimoto, NPG Asia Mater. 2017, 9, e454.
[2] G. Liu, K. Du, J. Xu, G. Chen, M. Gu, C. Yang, K. Wnag, H. Jakobsen, J. Mater. Chem. A 2017, 5, 4233-4253.
[3] J. Li, S. K. Cushing, F. Meng, T. R. Senty, A. D. Bristow, N. Wu, Nat. Photonics 2015, 9, 601-608.
[4] H. Tang, C.-J. Chen, Z. Huang, J. Bright, G. Meng, R.-S. Liu, N. Wu, J. Chem. Phys. 2020, 152, 220901.
[5] T. E. Tesema, B. Kafle, T. G. Habteyes, J. Phys. Chem. C 2019, 123, 8469-8483.

One of the key requirements in solar harvesting is to have strong absorptions in the NIR region as the solar spectra provide nearly 50% of the infrared irradiation. In some particular photonic material applications, transparent thin films are desirable especially in dye-sensitized solar cells, therefore requiring high average visible transmittance (AVT). We have developed and synthesized transparent photonic thin films based on a bio-inspired porphyrin compound chlorophyllin that typically exhibits two peaks respectively near 400 nm (blue-violet) and 700 nm (NIR) with a high AVT above 90%. To further enhance NIR absorptions, a unique porphyrin hybrid is developed with systematic additions of another porphyrin compound phthalocyanine with much stronger NIR absorptions. Phthalocyanine has more pronounced UV and NIR absorptions, but a narrower “optical window” as compared to that of chlorophyllin. We have found that congruent mixtures between chlorophyllin and phthalocyanine can result in broader NIR absorptions while maintaining the optical window between 400 and 650 nm. This will provide an ideal spectrum for a highly transparent but strongly UV/NIR absorbing characteristic. We report the optical properties of the porphyrin hybrids based on UV-vis and Raman spectroscopy. Also discussed is the operating mechanism underlying the thermal-photonic behaviors associated with the hybrid structures.

Noble metallic nanoparticles (NPs) become a great scientific and industrial interest due to their unique physicochemical properties, and play important roles for a number of catalytic applications, such as oxidation and reduction reactions, environmental catalyzes, fuel cells and energy storage. Among these, platinum NPs (Pt NPs) have been receiving large attention because they are the most efficient and common catalyst for both anodes and cathodes in fuel cells. Multi-metallic Pt-based NPs were reported for enhancement of catalytic properties in comparison to monometallic Pt NPs, while reducing the Pt consumption [1]. Although possessing such potentials, the synthetic techniques for multi-metallic NPs have not yet achieved the controllability over the NPs morphology and structure, unless going through complex reaction conditions. In this study, Au@Ag@Pt core@multi-shell NPs were synthesized in one-pot by facile co-reduction method, a combination of galvanic replacement reaction (GRR) and a co-reducing agent, to obtain a well-defined nanostructure with enhanced surface area of Pt towards catalytic applications. The as-synthesized NPs were found to have Au core covered by Ag-Pt double-shell, which contained a degree of hollows with the inner surface composed of both Ag and Pt, while the outer surface composed of granular Pt. The shell-formation mechanism will also be discussed in aspect of competition role between GRR and co-reduction reactions, under the influence of unique charge transfer effect from Au core to Ag middle shell [2]. The NPs were characterized with UV-Vis, DLS, and especially high-end electron microscopic techniques, including ADF-STEM and EDS elemental mapping. Through understanding the complexity of the reaction mechanism, one can expect to design and fabricate various types of trimetallic nanoparticles with intricate structures, thus further expanding their nanotechnology and applications.
[Acknowledgements] This work was performed under the Cooperative Research Program of “Network Joint Research Center for Materials and Devices”, and partially supported by JSPS KAKENHI JP19K15388 to A.T.N.D and JP18K18953 to K.K.
[References] [1] Y. Li et al. J. Mater. Chem. A 3 (2015) 368; [2] Anh T.N. Dao et al. Appl. Phys. Lett. 99 (2011) 073107.

Most common plasmonic materials such as silver, gold, and aluminum have been proven to have excellent localized surface plasmon resonance (LSPR). However, the research for hafnium nitride (HfN) plasmonic nanostructures, one of the alternative plasmonic materials with high chemical stability, high melting point (T~3583 K), still remains unexplored. In this work, we experimentally demonstrated a full-color plasmonic color filter based on HfN metasurfaces with a high melting point. By tuning the LSPR the optical response can be controlled. Thus, we demonstrated full-color imprinted color pixel including red, green, blue, and black in the subwavelength scale. By using finite-difference time-domain (FDTD) simulation, the LSPR can be tuned by the designed HfN nanodisk structure with a diameter varied from 80 nm to 640 nm. The key reason to achieve full-color plasmonic resonance is the HfN has a high bulk plasmon frequency around 3.1 eV (λ_p = 400 nm). This result provides evidence that refractory plasmonic HfN metasurface is applicable in the full-visible spectrum, thus can pave the way in the thermostable optoelectronic applications.

Recent advances by my group on structuring the wavefront of light and on new optical singularities are presented. Metasurfaces suffer from limited performance and efficiency when multiple functions with large deflection angles are required because the non-local interactions due to optical coupling between nanostructures are not fully considered. Supercell metasurfaces can achieve multiple independent optical functions at arbitrary large deflection angles with high efficiency. In one implementation the incident laser was simultaneously diffracted into Gaussian, helical and Bessel beams over a large angular range. A compact wavelength-tunable external cavity laser with arbitrary beam control capabilities – including beam shaping operations and the generation of freeform holograms has also been demonstrated utilizing supercell metasurfaces.
The majority of structured light work has been concerned with shaping the wavefront in the plane transverse to the propagation direction. Multiple degrees of freedom, such as spin and optical angular momentum, can be controlled simultaneously along the optical path with a single metasurface by sculpting the incident waveform into an ensemble of co-propagating modes with different longitudinal wavevectors which beat along the direction of propagation, thereby modulating the resulting envelope at-will via multimode interference. Note that replicating this behavior with bulk polarization optics or SLMs is inherently cumbersome (if not impossible) as the former can only modify incident polarization globally whereas the latter operates on one incident polarization at a time. Two classes of metasurfaces have been realized for structuring light along the propgation direction: namely: polarization-switchable plates that couple any pair of orthogonal polarizations to two vortices in which the magnitude and/or sense of vorticity vary with propagation, and multifunctional plates that can structure both , spin and orbital angular momentum, independently, along the optical path while operating on incident light of any polarization. Compact and integrated devices of this kind can be exploited in light-matter interaction and imaging and may facilitate new applications that are not accessible with other wavefront shaping tools.
Optical phase singularities are zeros of a scalar light field. The most systematically studied class of singular fields is vortices: beams with helical wavefronts and a linear (1D) singularity along the optical axis. Beyond these common and stable 1D topologies, a broader family of zero-dimensional (point) and two-dimensional (sheet) singularities can be engineered. Sheet singularities are realized by maximizing the field phase gradient at the desired positions. Furthermore, by applying an analogous procedure to the full vectorial electric field, one can engineer paraxial transverse polarization singularity sheets. As validation, the experimental realization of phase and polarization singularity sheets with heart-shaped cross-sections using metasurfaces has been realized. Singularity engineering of the “dark” enables new degrees of freedom for light-matter interaction and can inspire similar field topologies beyond optics, from electron beams to acoustics.
Contributions by Christina Spägele, Michele Tamagnone, Ahmed H. Dorrah, Soon Wei Daniel Lim, Joon-Suh Park, Maryna L. Meretska, Dmitry Kazakov, Marcus Ossiander, Marco Piccardo, Noah A. Rubin, and Aun Zaidi are gratefully acknowledged.

Metasurfaces have attracted a lot of attention because of their great capability to manipulate the attributes of an electromagnetic wave in demand. Thus, nanophotonic metasurfaces provide great flexibility in light management that highly benefits the development of low-profile optical systems. However, metasurfaces severely suffer from high optical losses that highly limit their optical performance. In addition, optical modulation in metasurface unit elements relies on strong resonances at a sub-wavelength resolution. It leads to a significant amplitude variation when a broad phase shift range is obtained. Such amplitude-phase covarying response can further degrade the optical performance of metasurfaces. In this talk, I will present two strategies to realize high-performance metasurfaces. Firstly, I will present a design method that enables a significant enhancement of the optical efficiency in a hybrid plasmonic metasurface. By integrating a solid nano-structure with its inverse complementary, we realized a plasmonic metasurface with a circular cross-polarization conversion efficiency higher than 50% in transmission at near-infrared wavelengths. Such high optical performance metasurface is achieved by simultaneously exciting the electric, magnetic, and toroidal multipolar modes, which satisfies the generalized Kerker condition and improves the transmission efficiency. We further demonstrate a couple of metasurface-based components such as beam deflector and flat focusing lens with record operating efficiency based on the proposed metasurface. Secondly, I will report an unsupervised genetic-type tree search algorithm in a virtual space wherein an automatic inverse design of metasurfaces is achieved with low computational complexity. With the developed method, we design and characterize beam deflection/steering metasurfaces, which are capable of deflecting normally incident light into target directions with minimal sidelobes; other functions of metasurfaces could also be achieved by revising the learning criterion in the virtual space. Due to the co-optimization of amplitude and phase shift, the inversely-designed metasurface possesses higher optical directivity than the forward-designed metasurface.

Spin Hall effect of light (SHEL) refers to a transverse and spin-dependent displacement of light at a planar optical interface. The SHEL is a universal phenomenon that can be readily observed in any interface, but has been neglected mostly due to the subwavelength scale of the shift. Therefore, enhancement of the shift up to wavelength scale and even beyond has been the main trend. However, the enhancement of the shift generally entails degradation of efficiency below 1%. To overcome the limitation, the simultaneous enhancement of the shift and efficiency under horizontal polarization was proposed and demonstrated in the microwave regime by using anisotropic impedance mismatching [1].
Here, we present our second proposal to achieve the large SHEL with high efficiency [2]. In contrast to the first approach, both the shift and efficiency is enhanced under arbitrarily polarized source due to the degenerate reflection coefficients [3]. The principle is a total reflection, in which both s- and p-polarized light is reflected at a denser-to-sparser interface with unity amplitudes. The simultaneous enhancement of the shift and efficiency is demonstrated experimentally using a near-zero-index metamaterial in the microwave regime. The large SHEL with near-unity efficiency under an arbitrarily polarized source will enable highly efficient and versatile devices with spin-selective functionalities.
Furthermore, we extend the recent attempts to achieve the large and efficient SHEL, which have been limited only to the microwave, to the optical regime [4]. The large SHEL with high efficiency can be also realized in the near-infrared wavelength by using an all-dielectric metasurface. A capability of metasurfaces to modulate complex transmission coefficients can be used to design a system that supports the large SHEL with high efficiency. Our recent progress in demonstrating the SHEL using Stokes polarimetry measurement will be introduced.

References
[1] M. Kim et al. Spin Hall effect of light with near-unity efficiency in the microwave. Laser & Photonics Reviews 15, 2000393 (2021)
[2] M. Kim et al. Total reflection-induced efficiency enhancement of spin Hall effect of light. ACS Photonics (In review)
[3] M. Kim et al. Spin Hall effect under arbitrarily polarized or unpolarized light. Laser & Photonics Reviews 15, 2100138 (2021)
[4] M. Kim et al. Spin Hall effect of light with high efficiency in optical wavelengths using an all-dielectric metasurface (In preparation)

High index dielectric nanostructures have unique optical responses driven by the Mie resonance, offering a variety of functionalities for directional nanoantennas, metalenses, etc. Recently, in addition to the electric and magnetic multipole resonances, toroidal multipole resonances have attracted much attention as a third multipole family. The lowest-order toroidal dipole (TD) mode is composed of a current loop along a meridian of a torus, which leads to strong confinement of an electromagnetic field in a dielectric nanostructure [1]. The enhanced electromagnetic field associated with the TD resonance has been exploited for the enhancement of nonlinear optical responses, [2] photocatalytic activities, [3] etc.
We have studied a low aspect ratio circular silicon (Si) nanodisk, e. g., ~500 nm in diameter and ~50 nm in thickness, as a nanoantenna supporting the TD mode, and showed that a metasuraface composed of the two-dimensional hexagonal array supports high quality (Q) factor TD resonances arising from coupling of the TD modes of individual nanodisks. [4] The high-Q coupled TD resonance results in narrow-band absorption enhancement in the wavelength range, where Si is almost transparent. This suggests that the Si metasurface supporting the coupled TD resonance enhances responsivity of a Si photodetector in a specific narrow wavelength range depending on the structural parameters.
In this work, we produce a Si nanodisk array metasurface having a coupled TD resonances and demonstrate that the resonant absorption results in the enhancement of the photocurrent in the near infrared range, especially in the wavelength range below the bulk Si band gap. First, we explore the structural parameters of the metasurface by the FDTD simulations, and show that at maximum ~120-fold enhancement of the absorption compared to that of a flat Si film with the same thickness is possible. We reveal from the multipole decomposition that the strong absorption enhancement is due to TD modes having an enhanced electric field within a Si nanodisk (intra-nanodisk TD mode) and between Si nanodisks (inter-nanodisk TD mode). Finally, we fabricate Si nanodisk metasurfaces and demonstrate narrow-band enhancement of the absorption and photocurrent due to coupled TD resonances.

[1] A. Miroshnichenko et al., Nature Communications, 6, 8069 (2015)
[2] G. Grinblat et al., Nano Letter, 16, 7, 4635 (2016)
[3] L. Hüttenhofer et al., ACS Nano, 14, 2, 2456 (2020)
[4] H. Hasebe et al., Advanced Optical Materials, 20, 2001148(2020)

Solid-state nanopore technologies have shown a great progress for developing single-molecule DNA sequencing, in which single-stranded DNA molecules passing through a nanoscale ion channel are observed by detecting current blocking signals. At present, nanopore ionic current detection enables discrimination of four bases (adenine, thymine, cytosine, and guanine) included in DNA, but it is much more challenging to identify chemically modified bases for epigenetic sequencing. Optical DNA sequencing based on surface-enhanced Raman scattering (SERS) is promising as an alternative method for spectroscopic identification of DNA bases. SERS utilizes extremely high field enhancement in a sub-nanometer volume (hot spot) of metal nanostructures to provide single-base spatial resolution and sensitivity. In this study, we develop a method to spontaneously form Au nanoparticle (AuNP) dimers, which sandwiches a single DNA oligonucleotide within a gap of less than 1 nm. SERS measurement of single AuNP dimers in aqueous suspension demonstrates single-base resolution obtained within a 10-ms measurement period.
We formed oligonucleotide sandwich AuNP dimers in aqueous suspension by using colloidal AuNPs with a diameter of 60 nm and oligonucleotides containing adenine (A) and cytosine (C) bases with a sequence of CCCCCCCCCCCCACCCCCCCCCCCC (12C1A12C; 25 mers length). The adsorption of oligonucleotides on AuNP surface and the formation of AuNP dimer was promoted by protonation of oligonucleotides, addition of salt, and incubation at elevated temperature. The conditions are carefully examined and optimized for enhancing the dimerization efficiency and for maximizing the probability to find the oligonucleotide in the dimer gap, which should be minimized to obtain pronounced SERS activity and single-base resolution. The absorption spectrum of AuNP dimers, which were reproduced from the second band (dimer band) of gel electrophoresis separation after incubation and addition of BSPP into the AuNP dimer suspension, exhibits a large red shift of the longitudinal mode of AuNP dimer, demonstrating a strong coupling of AuNPs due to an interparticle gap smaller than 1 nm.
We performed micro-Raman measurements for oligonucleotide sandwich AuNP dimers under Brownian motion in aqueous suspension. The dimer suspension was diluted such that single dimer spectroscopy could be conducted. The measurement time was set to 100 ms while the dwell time of dimer in the observation volume was approximately 40 ms. By considering the fact that the AuNP dimer experiences rotational diffusion, the effective measurement time should be less than 10 ms. Among a number of Raman spectra obtained from single dimers, some exhibited a characteristic peak purely from adenine (734 cm^-1) while others purely from cytosine (800 cm^-1). Because we observed nucleotides containing both adenine and cytosine, the SERS signal should in principle always be a mixture of adenine and cytosine even at a single nucleotide observation. The fact that we obtained the SERS signal of purely adenine is clear evidence that the measurement achieved single-base (sub-nanometer) resolution. A similar result was obtained for oligonucleotides containing guanine (G) and cytosine (C) bases.

Implantable sensors continuously monitor biomarkers in bodily fluids, facilitating chronic disease observation, supporting personalized drug therapy, and enhancing the patient’s comfort. Nevertheless, there are two major issues limiting long-term application: Firstly, the sensor must be well integrated into the host tissue to prevent encapsulation, and thus impediment of analyte transport [1]. Secondly, the sensor-element for external signal transduction must be robust against degradation and exhibit a long signal lifetime [2].
To overcome both issues, we have developed an implant based on plasmonic gold nanoparticle sensors embedded in a macroporous polymeric hydrogel [3]. While the hydrogel’s macropores allow tissue and capillary ingrowth, the nanoparticles’ plasmonic signal exhibits a potentially unlimited lifetime. The nano-sensors are coated with aptamer and show a change of their resonance wavelength upon binding the antibiotic drug kanamycin. We implant our sensor-system subcutaneously in the abdominal skin of live hairless rats and measure the plasmon spectrum with a self-built spectral imaging setup non-invasively through the skin.
Upon administering the drug kanamycin in the rat’s tail vein, we can observe a concentration-dependent change of the plasmon signal verifying the drug’s presence in the tissue surrounding the implant. Control experiments, where we inject analyte-free saline solution, show no such signal change. The shift of the resonance wavelength is consistent with values we expected from theoretical calculations. Finally, histological sections of the working implants confirm that they are well integrated into the host tissue.
Our sensor design is not restricted to a specific analyte and allows many possible applications due to the versatile functionalization options of gold nanoparticles. The potentially unlimited lifetime of the plasmon signal enables long-term application and is advantageous over e.g. fluorescence-based systems, which are subject to photobleaching. Additionally, the macroporous polymeric hydrogel matrix is well-integratable into the host tissue, preventing encapsulation, facilitating analyte transport, and hence further boosting the service life of the implant.
Taking all these points into account, we believe that our implant paves a realistic way towards a universal sensor-platform for continuous monitoring of biomarkers.


[1] Anderson, J. M.; Rodriguez, A.; Chang, D. T. Foreign body reaction to biomaterials. Semin. Immunol. 2008, 20 (2), 86−100.
[2] Vaddiraju, S.; Burgess, D. J.; Tomazos, I.; Jain, F. C.; Papadimitrakopoulos, F. Technologies for continuous glucose monitoring: Current problems and future promises. J. Diabetes Sci. Technol. 2010, 4 (6), 1540−1562.
[3] Kaefer, K.; Krüger, K.; Schlapp, F.; Uzun, H.; Celiksoy, S.; Flietel, B.; Heimann, A.; Schroeder, T.; Kempski, O.; Sönnichsen, C. Implantable sensors based on gold nanoparticles for continuous long-term concentration monitoring in the body. Nano Lett. 2021, 21 (7), 3325–3330.

Plasmonics and metamaterials have a common thread of enhanced phenomena and shape/composition independent dielectric and light behavior. A common application of plasmonic materials in sensing where the "enhanced light" can be used to further the use in detecting low levels of analyte and combined with a waveguide or guided layer detection. This is especially useful when combined with spectroscopic and imaging methods for characterizing the potential for artificial intelligence and machine learning (AI/ML) paradigm. This work will report on the use of plasmon, localized surface plasmon resonance LSPR and SPR waveguiding methods to demonstrate high sensitivity and selectivity for the detection of chemical and biological analytes at a very low level of concentrations. Using electrochemically molecularly imprinted polymers enabling tuning of the dielectric parameters based on the oxidation state of the polymer to facilitate the MIP process and evaluate the figures of merit of a sensor device. The main sensitivity is achieved by the plasmonic enhancement and relationship with multilayer films and dielectric behavior closer to a guided wave platform. Thus there is a high possibility of combining in-situ and in-operando measurements methods that can be relevant for future AI/ML applications.

Surface-enhanced Raman scattering (SERS) spectroscopy is a very powerful analytical technique for the detection of various analytes in various mediums. For concentrating the electromagnetic waves (laser light), plasmon resonance is a minimum requirement for realizing the SERS effect. Therefore, various types of SERS substrates based on plasmonic metallic were developed since its discovery four decades ago. It is also required that SERS substrates are equipped with a roughened metallic surface with some nanoscale gaps and junctions that create hotspots. SERS techniques enabled the detection at the ultimate limit, i.e., detection of a single molecule. Though solid substrates have potential, the mobile substrate (solution dispersion of nanoparticles) is very promising because of its small size, movability, high surface area, and ability to dealing with light efficiently. In this regard, smaller-sized metal nanoparticles with hotspots are very useful. However, smaller-sized metal nanoparticles are good for one-time use as it is difficult to wash and regenerate the nanoparticles because of their very small size and the possibility of uncontrolled aggregation and oxidation. To avoid such concern and also for regeneration and sequential SERS sensing purpose, polymer-based metal composite particles are very promising. Polymer-supported sensor particles (as substrate) are versatile due to the fascinating characteristics of the polymers such as swellability, porosity, transparency, metal loading ability, and high surface area. Microfluidics is a promising technique for the fabrication of polymer-supported sensor particles of nanoscale and microscale with uniformity. For nanoscale sensor particles, the metal nanoparticles can be incorporated and anchored at the surface of polymer nanoparticles electrostatically, and a ligand-free metal layer can be deposited by a metal-catalyzed metal enforcement process. Here, the development of the polymer-metal composite particles at nanometer and micrometer length scales, and their applications as sensor particles for SERS sensing are presented. Microfluidic techniques were utilized for the generation of uniform sensor particles with the precise distribution of metal nanoparticles in the interior and at the surface. Furthermore, metal-catalyzed metal enforcement approaches initiated for achieving a ligand-free metal surface for direct analyte interactions, and experimental demonstration of sequential SERS analysis of multiple analytes through microfluidics has been established.

Many applications of high economic value within the energy sector operate at high temperature or under chemically harsh environments, posing a challenge for in-situ operation with traditional sensor technology. Oxide thin films as sensing layers for conductive sensors or on the optical fiber platform provide one solution to this problem, as these thin films have the ability to operate at high temperature under a wide range of chemical conditions. The optical fiber platform in particular offers scalable, spatially distributed sensing, with off-the-shelf silica fiber providing stability up to 800 °C under oxidizing and reducing conditions. The incorporation of tailored plasmonic nanoparticles offers an additional response in the visible spectrum which may be utilized to provide enhanced cross-sensitivity discrimination under multi-variate sensing conditions. Gold nanoparticles, for example, have been shown to be stable at high temperature when incorporated within an oxide matrix. However, the plasmonic response can be impacted by the complex interaction between free carriers and atomic vacancies (which may be mobile at elevated temperature).

Computational modeling and experimental results demonstrate the impact of defect chemistry on the high temperature sensing response of the following two model oxides on the optical fiber platform: 1) La-doped SrTiO₃, an n-type doped system for sensing reducing gas streams; and 2) Fe-doped SrTiO₃, a p-type doped system for sensing oxidizing gas streams. On optical fiber and planar substrates, the interaction between the free-carrier (Drude) response of the oxide film in the NIR and the plasmonic response in the visible spectrum is demonstrated. The dynamics of the combined electronic-ionic-plasmonic physics are also modeled and used as a tool to guide the development of sensing layers with improved sensing response time and reduced drift at extreme temperatures.

The integration of multiple devices in a single functional unit is boosting the advent of a series of compact optical sensors for rapid and on-site analysis. In this context, the huge potential of plasmonic-based sensors has been affected by the strict constraints of the detection scheme. The resulting need for laboratory equipment, such as laser sources and expensive prism-based optics, therefore ends up in not-portable systems.
Here, an ultra-compact plasmonic sensor is demonstrated through the smart integration of an organic light-emitting transistor (OLET), an organic photodiode (OPD), and a nanostructured plasmonic grating (NPG). The direct integration of the OPD onto the planar structure of the OLET enabled unprecedented proximity of the light source and light detecting areas, which allowed the exploitation of the angle-dependent sensing characteristics of the NPG.
The most effective 3D layout of integration, including the optimal size and relative positioning of the three elements (i.e. OLET, OPD, and NPG), was unraveled by an advanced simulation tool, which also predicted the signal variation of the sensor under different conditions. Accordingly, the effectiveness of the new plasmonic-based detection scheme was demonstrated by the dependence of the OPD photocurrent on the surrounding environment of the NPG. In particular, a variation of the OPD photocurrent of about 10^-9 A was recorded when exposing the NPG from water to alcoholic solutions at different concentrations.
A miniaturized plasmonic sensor with a total size of 0.1 cm³ was therefore obtained through the smart integration of cost-effective, versatile, and nanometer-thick optoelectronic and plasmonic components.

In this talk, we discuss our recent progress in the area of polaritonics, and in particular on the role of polaritons in molding the optical wavefront over engineered ultrathin surfaces. We discuss the opportunities of these concepts to manipulate the propagation of light strongly coupled with matter. We will discuss the role that polaritons can have in inducing giant nonlinear responses in metasurfaces, and we unveil the emergence of topological transitions at photonic magic angles enabled by the strong coupling between two polaritonic bilayers with a twist angle between them. We will also discuss other exotic opportunities enabled by lower symmetry materials, such as monoclinic crystals or tailored metasurfaces. During the talk, we discuss the unusual phenomena supported by these structures, and their opportunities for nano-imaging and information transport.

Metasurfaces allow the manipulation of the amplitude, phase, and polarization of light on an ultrathin platform, and have started to be explored also in the context of digitizing optical holograms. To increase the bandwidth of a metasurface hologram, essential for high-capacity holographic memory devices, different properties of light including polarisation, wavelength, and incident angles have been exploited for holographic multiplexing; however, the bandwidth of a metasurface hologram has remained too low for any practical use.

We present the design of a complex-amplitude metasurface hologram for ultrahigh-dimensional OAM-multiplexing holography in momentum space. To realise a complex-amplitude Fourier hologram, we have introduced an OAM diffuser array with a random phase function, capable of scaling down the amplitude variation in a typical Fourier image as well as eliminating the coherence of holographic image channels. We will demonstrate first realizations of holograms based on this concept.
The second part of the talk will focus on demonstration of metasurfaces on fiber end facets for optical trapping and achromatic focusing with record bandwidth over the whole telecom range.

Biosensors play an essential role in numerous healthcare applications ranging from diagnosis of life-threatening conditions to the management of infectious diseases for preventing pandemics. Unfortunately, current biosensors are lacking precision, bulky, and costly, as well as they require long detection times, sophisticated infrastructure and trained personnel, which limit their use. In our laboratory, we address these challenges by developing next-generation nanophotonic biosensors, bioimaging, and spectroscopy technologies. We exploit nanophotonics using plasmonic and dielectric metasurfaces for achieving high sensitivity, integration and multiplexing. We integrate our sensors with microfluidics for efficient sample manipulation and automation. We use smart data science tools to improve sensor performance. In this talk, I will present some of our recent effort towards developing ultra-compact, portable, rapid, and low-cost nanophotonic biosensor and their use for early disease diagnostics [1-4].

Our lab has been developing various point-of-care (PoC) biosensors with plasmonic nanohole arrays (NHAs) supporting extraordinary optical transmission (EOT) [5-7]. Recently, we showed that gold nanoparticle (Au-NP) bindings on NHAs can dramatically enhanced the sensitivity and enable digital counting of even individual binding events [1]. The detection principle is based on local suppression of the EOT signal. Although the sizes of NPs are subwavelength, their plasmonic interaction with NHAs allows for high-contrast imaging of surface-bound NPs in a bright field optical reader over large field-of-views and eliminates the need for bulky and expensive spectrometry instrumentation. In a follow-up work, we used this new principle to develop a portable PoC device to assist sepsis diagnosis [2]. The optical is made of off-the-shelf optical components and comprises an LED, a simple sample holder, and a CMOS camera. We tested the portable reader in a hospital setting with patient samples and showed detection of two inflammatory biomarkers related to sepsis diagnosis, procalcitonin and C-reactive protein with limit-of-detection down to few tens of pg/mL directly in blood serum. Our sensor provided performance equivalent to gold-standard immunoassay based laboratory tests. Moreover, our results revealed that our sensor can perform rapid tests in less than 15 minutes.

Concurrently, we use low-loss dielectric metasurfaces and exploit their high-Q resonance modes with accessible near-field enhancements for sensing. In one work, we combined silicon-based metasurfaces supporting BIC modes in NIR spectrum with hyperspectral imaging and demonstrated detection limits at the levels of ~3 IgG molecules/μm2 [3]. Recently, we introduced a single-wavelength imaging-based dielectric biosensor for achieving superior sensitivity based on reconstructed spectral shift without requiring wavelength scanning and spectrometers [4]. Here, we employed diatomic metasurfaces and combined them with advanced data processing techniques for aided imaging. Our method acquires large-area intensity images of the metasurface in real-time by using a single wavelength illumination and processes them optimally by a linear estimation algorithm to reconstruct spectral shift data at high accuracy. We integrated the metasurfaces with microfluidics in a microarray format and performed in-flow detection of breast cancer extracellular vesicles (EV), which are important biomarkers for diagnostics. Our optofluidic sensor has enabled real-time quantification of EV bindings from solution at concentrations as low as ~200 femtomolar.

References:
[1] Belushkin et al. ACS Nano (2019)
[2] Belushkin et al. Small (2020)
[3] Yesilkoy et al. Nature Photonics (2019)
[4] Jahani et al. Nature Communications (2021)
[5] Cetin et al. Light Science and Applications (2014)
[6] Coskun et al. Scientific Reports (2014)
[7] Yesilkoy et al. Light Science and Applications (2018)

Plasmonic wire has drawn immense scientific interests since the inception of artificial chiral media [1], which supports dominant electric mode upon applied electric polarization parallel to its axis and does not usually induce non-axial circulating current (i.e., dominant transverse magnetic modes) in it [2 – 6]. To induce transverse magnetic moment (circulating current distribution) in conducting systems, resonator geometry must be suitably tuned to manipulate its charge distribution along a loop (e.g., split- or closed ring, spherical particles, solenoid, toroid, etc.) [2 – 5]. Moreover, plasmonic systems possess inherent ohmic and radiative losses which eventually degrades system performance in large extent. As a result, plasmonic systems like metal wires, nonmagnetic plasmonic gratings were barely used to tailor its magnetic characteristics. Motivated from this viewpoint to excite magnetic resonances in a sole plasmonic wire, a mechanism was proposed recently in periodically arranged electromagnetically coupled plasmonic cut wires with applied polarization along its length [7]. By tuning length of one cut-wire (implying asymmetry), one can effectively transform physical functionality of other wire from being ‘electric’ to ‘magnetic’ in nature at terahertz (THz) regime by interplaying with near-field coupling and lattice coupling. We may highlight, as of earlier literature [1 – 5], realization of artificial transverse magnetism had only been possible by combining or coupling of at least two coupled wires; whereas nature of a single wire (be it isolated or coupled) always remains electric in nature, unlike our case where we break this convention. Moreover, the behavior of continuous wire (constituting grating) would be significantly different than periodically oriented subwavelength cut wires. Thus, we investigated the coupling between a continuous wire (grating) and a cut-wire (dipole) to tune the nature of grating element from electric to magnetic. Hybridization between grating and metasurface responses leads to generation of magnetic Fano and nonradiative anapole modes (i.e., low loss characteristics), makes this hybrid (combination of subwavelength grating (guided mode) and cut-wire dipole (diffractive, metamaterial mode)) plasmonic device useful for prospective THz magnetic applications like realization of femtosecond opto-magnetism, magnetic cloaking, magnetic storage, spin rectification, magnetic imaging, etc.

References
[1] J. C. Bose, “On the rotation of plane of polarisation of electric wave by a twisted structure,” Proc. R. Soc. Lond. 63146–152 (1898).
[2] D. R. Smith, W. J. Padilla, D. C. Vier, S. C. Nemat-Nasser, and S. Schultz, “Composite Medium with Simultaneously Negative Permeability and Permittivity,” Phys. Rev. Lett. 84, 4184 (2000).
[3] S. Karmakar, D. Kumar, R. K. Varshney, and D. Roy Chowdhury, “Lattice induced plasmon hybridization in metamaterials,” Opt. Lett. 45 (13), 3386-3389 (2020).
[4] S. Karmakar, S. Banerjee, D. Kumar, G. Kamble, R. K. Varshney, and D. Roy Chowdhury, “Deep Subwavelength Coupling Induced Fano Resonances in Symmetric Terahertz Metamaterials,” Phys. Status solidi RRL 13, 1900310 (2019).
[5] G. T. Papadakis, D. Fleischman, A. Davoyan, P. Yeh, and H. A. Atwater, “Optical magnetism in planar metamaterial heterostructures,” Nat. Commun. 9, 296 (2018).
[6] S. Karmakar, D. Kumar, R. K. Varshney, and D. Roy Chowdhury, “Strong terahertz matter interaction induced ultrasensitive sensing in Fano cavity based stacked metamaterials,” J. Phys. D, 53 (41), 415101 (2020).
[7] S. Karmakar, D. Kumar, B. P. Pal, R. K. Varshney, and D. Roy Chowdhury, “Magnetic wire: transverse magnetism in a one-dimensional plasmonic system,” Opt. Lett. 46 (6), 1365-1368 (2021).

Plasmonic resonances can concentrate light into exceptionally small volumes, approaching the molecular scale. Such extreme light confinement provides an advantageous pathway to probe molecules at the surface of plasmonic nanostructures with highly sensitive spectroscopies. Beyond the chemical information available through surface-enhanced Raman scattering (SERS), unavoidable losses associated with metals, which are usually seen as a nuisance, encode invaluable information on energy transfer to the adsorbed molecules through the resonance linewidth. We target the sharp gap resonances of gold nanocubes deposited on a gold mirror coated with a thin dielectric layer (NCoMs) and measure a thousand single nanocavities with resonances spanning the orange to red spectral range. We closely monitor changes in linewidth and peak energy of the nanocavities and combine these observations with SERS spectra to monitor, in an energy-resolved manner, energy transfer and plasmon-driven chemical reactions at their surface. Using methylene blue (MB) as a model system, we show how this approach can be used to measure the absorption spectrum of molecules following their adsorption onto a metal surface and reveal a rich plasmon-driven reactivity landscape. We show that within different energy windows distinct reaction pathways dominant: near-field driven desorption at ~1.7 eV and direct charge-transfer driven N-demethylation at ~1.9 eV.

Quantum technologies harness quantum effects usually in small and pure arrangements such as through cold atoms or superconducting systems. From the start, photonic quantum effects have played a central role, too, but most photonic solutions and systems based on light-matter interaction also require cryogenic environments. Strong coupling of light and matter at the single emitter level is a fundamental quantum resource offering deterministic energy exchange between single photons and a two-level system, and the possibility to achieve single-photon nonlinearities via the anharmonicity of the Jaynes-Cummings ladder. Until recently, however, the conditions for achieving strong-coupling were most commonly met at cryogenic temperatures such that de-coherence processes are suppressed. As a major step forward, we have recently demonstrated room-temperature strong coupling of single molecules [1] and single quantum dots [2] to ultra-confined light fields in plasmonic resonators at ambient conditions. The fact that strong-coupling conditions may be reached at room temperature is of immense interest because it represents a clear route to a practical implementation and use of quantum behaviour in nanophotonic systems and its application in bio-sensing [3].
Here we discuss the principles of room-temperature single-emitter strong coupling in nanoplasmonics and illuminate perspectives for quantum nanophotonics [4]. We will highlight the physics associated with recently demonstrated room-temperature strong coupling of single molecules in a plasmonic nano-cavity [1] and near-field generated strong coupling of single quantum dots [2] and single quantum emitter Dicke enhancement [5] paving the road towards single-photon quantum nonlinearities. We will also explain near-field enhanced single-photon emission in near-zero index materials and multipartite dynamic quantum entanglement [6].

ACKNOWLEDGEMENTS
This work was supported by the Science Foundation Ireland (SFI) through grant 18/RP/6236.

REFERENCES
[1] R. Chikkaraddy, B. de Nijs, F. Benz, S. J. Barrow, O. A. Scherman, E. Rosta, A. Demetriadou, P. Fox, O. Hess, and J. J. Baumberg, Single-Molecule Strong Coupling at Room Temperature in Plasmonic Nanocavities, Nature 535, 127 (2016).
[2] H. Groß, J. M. Hamm, T. Tufarelli, O. Hess, and B. Hecht, Near-Field Strong Coupling of Single Quantum Dots, Science Advances 4, eaar4906 (2018).
[3] N. Kongsuwan, X. Xiong, P. Bai, J.-B. You, C. E. Png, L. Wu, and O. Hess, Quantum Plasmonic Immunoassay Sensing, Nano Lett. 19, 5853 (2019).
[4] X. Xiong, N. Kongsuwan, Y. Lai, C. E. Png, L. Wu, and O. Hess, Room-Temperature Plexcitonic Strong Coupling: Ultrafast Dynamics for Quantum Applications, Appl. Phys. Lett. 118, 130501 (2021).
[5] T. Tufarelli, D. Friedrich, H. Groß, J. Hamm, O. Hess, and B. Hecht, Single Quantum Emitter Dicke Enhancement, ArXiv:2010.12585 [Cond-Mat, Physics:Quant-Ph] (2021).
[6] F. Bello, N. Kongsuwan, J. F. Donegan, and O. Hess, Controlled Cavity-Free, Single-Photon Emission and Bipartite Entanglement of Near-Field-Excited Quantum Emitters, Nano Lett. 20, 5830 (2020).

Controlling a state of material between its crystalline and glassy phase has fostered many real-world applications including photonic switches and phase change materials. Nevertheless, design rules for crystallization and vitrification kinetics as well as the contrast of the corresponding optical properties still lack predictive power. Here, we identify stoichiometry trends for these processes in phase change materials, i.e. along the GeTe-GeSe, GeTe-SnTe, and GeTe-Sb2Te3 pseudo-binary lines employing a pump-probe laser setup and calorimetry. We discover a clear stoichiometry dependence of optical properties and crystallization speed along a line connecting regions characterized by two fundamental bonding types, metallic and covalent bonding. Increasing covalency slows down crystallization by six orders of magnitude and promotes vitrification. The stoichiometry dependence is correlated with material properties, such as the optical properties of the crystalline phase and a bond indicator, the number of electrons shared between adjacent atoms. A quantum-chemical map explains these trends and provides a blueprint to design crystallization kinetics and property contrast.

We present a fabrication technology to produce highly ordered metal nanoparticle arrays on large surface areas of a chosen substrate. The nanoparticles were formed on a nano-dimpled aluminum template which allows for highly ordered nanoparticle distributions and also allows for their direct transfer to a substrate of choice using a polymer lift-off. The influence of several experimental parameters on the nanoparticle ordering was studied. By controlling the dimple size and the thickness of the deposited metal film, it was possible to influence the size of the nanoparticles and its spacing within the array. Post-deposition, the metal film was annealed and the annealing temperature as well as the rate of rise in temperature during the annealing process were factors that influenced the ordering of the metal nanoparticles.
The nano-dimpled alumina substrate was prepared by a two-step anodization method. Three different deposition voltages (40 V, 60 V, and 70 V) were used to study the effects of voltage on pore diameter and the spacing between the metal nanoparticles. It was observed that the pore diameter scales approximately linearly with voltage in agreement with results reported in previous works. By controlling the pore diameters, it was possible to tune the diameters and interpore distance of the metal NP’s. Large pore sizes yielded metal nanoparticles with large diameters and smaller interpore distance. Constant DC voltages of 40, 60, and 70 volts resulted in pore diameters of 60 ± 10nm, 100 ± 10 nm, and 120 ± 10 nm respectively.
Following fabrication of an ordered nano-dimpled surface, different metals (gold, silver and indium) were evaporated onto the dimpled surface to form a thin film. The thickness of the metal was varied from 8 nm to 12 nm and its effect on nanoparticle size and morphology was studied. Subsequent annealing of the metal film at different temperatures with varying rates of temperature rise enabled a study of the metal film thickness and annealing parameters on the morphology and ordering of the metal nanoparticles. The inter-particle distance was found to be inversely proportional to the thickness of the metal film. Moreover, rapid thermal annealing resulted in less uniform nanoparticles whereas slow anneal provided much more spherical nanoparticles with hexagonal close-packed ordering. The annealing temperature did not strongly influence the size and morphology of the nanoparticles, but the duration of the annealing process had a greater effect on the result. For gold nanoparticles, the optimum annealing process was a slow anneal at a temperature of about 300 C. Results of fabricating ordered arrays of indium and silver nanoparticles will be presented in this talk.

Photoelectrochemical (PEC) solar energy conversion applications rely on chemical reactions driven by photogenerated minority carriers (electrons or holes) at a semiconductor-liquid junction. The optical, electronic, and chemical transport processes characteristic of these PEC reactions occur on independent and generally disparate length scales. Fabricating electrodes with hierarchical structure can optimize the performance for each of these processes simultaneously. Disordered materials with dielectric contrast on the length scales of the wavelength of light can trap light in localized modes. The simplicity of the fabrication alone makes this approach a particularly attractive one for engineering light trapping into scalable photoelectrode structures. One significant issue is that disordered materials can only be defined by ensemble or statistical parameters (pore diameter, scatterer diameter, relative volume fractions) rather than as precise structures, which results in a real, physical variance intrinsic to the ensemble structure. Simulations of the properties of a given ensemble (local light absorption, for example) require a large number of examples for generating statistically accurate measurements for those properties. In this talk, we will describe our recent efforts to use neural network emulation to explore the absorption properties of defects in a simplified, disordered photonic glass. This system provides an apt model for developing mathematical representations for electrode structure (input) and finite element simulation data (output) to interface with machine learning algorithms. It also provides an opportunity to develop appropriate error metrics for quantifying the accuracy of prediction and improve the training of a neural network emulator. We will outline the practical application of these models and show how algorithmic predictions can be used to identify the most efficient ensemble configuration for a given semiconductor photoelectrode.

Photon management techniques are widely applied in solar cells and photodetectors to improve device efficiencies by light absorption enhancement. Conventional photon management strategies like dielectric nanophotonic structures in thin film photonics are limited by complicated fabrication process, incompatibility with Si foundry, high cost of materials, and narrow resonant bandwidth. Surface-textured transparent conducting oxide (TCO) thin films, typically composed of heavily doped wide bandgap semiconductor oxides like indium tin oxide (ITO) [1], are widely developed to overcome the above challenges. However, their sub-micron surface textures tend to weaken the photon management effect on thin absorber with thickness <1 μm. These facts leave the development of photon management strategies for two-dimensional (2D) absorbers such as graphene and molybdenum disulfide (MoS2) an open challenge for the current optics and materials research societies. In this paper, we report two semimetal composite nanostructures designed for 2D MoS2: pseudo-periodic Sn nanodots and SnOx (x<1) core-shell nanoneedle structures. 2D MoS2 shows up to 15x enhancement in absorption at λ=700-1000 nm under pseudo periodic Sn nanodots and 20-30x enhancement with 11% absorption in MoS₂ at λ=700-900 nm under SnOx (x<1) nanoneedles. Both cover broad optical bands from visible to near infrared (NIR) light regimes and are self-assembled from physical vapor deposition without sophisticated nanolithography. The enhanced absorption has been verified by notable increase in Raman scattering and Photo luminance intensities as a result of field enhancement under photon management. One mechanism of light absorption increase is the ultrahigh permittivity of Sn [2]. Same as Si which is the key element for almost all the current electronic and photonic devices, Sn is a group IV semimetal in the chemical periodic table. Compared with other conventional high-index dielectric materials (typically ε=10-15), it has an ultra large dielectric constant of ε > 50 at λ=500-3000 nm to induce strong near field enhancement in the nearby lower index regions according to the boundary condition of Maxwell's equations. Thus, Sn as a type of semimetal itself, could serve as a novel material system beyond plasmonic metals and high index dielectrics for optical interactions. The other unique feature of increased light absorption is that it covers a broadband which even achieve long wavelengths beyond intrinsic inter-band transition in 2D MoS2. This interesting behavior could be explained by local strain induced band gap engineering. Band gap engineering techniques are developed for photoelectronic devices operated at long wavelengths. These devices require especially small bandgap semiconductors. Conventional technique of tuning the stoichiometry of different elements inside a material system would need advanced material processing method like molecule beam epitaxy (MBE) and could only boost the absorption at a specific wavelength. Our self-assembled nanostructures effectively decrease the band gap of 2D MoS2 by introducing local tensile strain to the material which enable the broadband long wavelength light absorption improvement. Thus, our semimetal composite nanostructures synergistically address the photon management and band gap engineering challenges of 2D MoS2 photonic devices at long wavelength. Besides, the preparation of self-assembled Sn composite nanostructures would not require sophisticated nanolithography which would potentially hinder large-scale manufacture. The compatibility with Si also makes it favorable for current semiconductor foundries. Therefore, our semimetal composite nanostructures would potentially be a good feature on future 2D photonic devices.
[1] Tak, Yoon-Heung, et al. Thin Solid Films 411.1 (2002): 12-16.
[2] Fu, Sidan, et al. ACS Photonics 6.1 (2018): 50-58.

The employment of metal thin films and nanostructures in photonic devices has widespread applications in displays, lasers, sensors, electro-optical modulation and photovoltaics. By far, intense research has focused on conventional metals (e.g., Au, Ag and Cu) because of their low optical losses, paramount to achieving excellent plasmonic responses [1]. Yet, these metals are neither CMOS-compatible nor earth-abundant, which constrains their usage in advanced optical devices. Therefore, research for other potential candidates such as non-conventional metals/alloys is called for. Magnesium (Mg), which dissolves in water, is the eighth most abundant material on the earth's crust with confirmed biocompatibility, and has not been explored to its full potential in photonics. By investigating its etching properties with the control of thin film deposition method (radio frequency sputtering, RFS, and electron-beam evaporation, EBE), temperature (10 – 60 ^oC) and structural dimension, we demonstrate the promise of utilizing Mg for achieving optical tuning [2]. We fabricate scalable, transmissive color filters formed by triple-stack Mg/MgO/Mg thin films based on Fabry-Perot resonance, which feature tunable optical transmission to any color within the sRGB gamut [3]. Additionally, we fabricate tunable reflective color filters consisting of two stacked metal-insulator-metal layers (top: Mg-MgO-Mg, and bottom: Ag/Al₂O₃/Ag). After immersion in water, dynamic color tuning is successfully achieved (e.g., vivid yellow to pink), conditioned on etching time and temperature. Therefore, the transient properties of Mg-based photonics hold great promise for use in biodegradable color display, data encryption, and anticounterfeiting. Furthermore, we also exploit the modulation of optical properties of other unconventional metals such as Nickel (Ni), Cobalt (Co) and Platinum (Pt) and their alloy systems. Specifically, we implement the dewetting process of thin films to obtain large-area arrays of nanostructures. We characterize their optical behavior by in-situ spectroscopic ellipsometry. We establish a platform for engineering these metallic materials with on-demand optical responses. Research into these unconventional metals (e.g., Mg, Ni, Co and Pt) and their nanostructures opens up a new opportunity for game-changing nanophotonic components.


[1] T. Gong, P. Lyu et al, Advanced Optical Materials 2020, 8(23), 2001082
[2] T. G. Farinha et al, ACS Photonics 2019 6 (2), 272-278
[3] T. G. Farinha, T. Gong, P. Lyu et al, Optical Materials Express 2021, 11(5), 1555-1565

Understanding the ultrafast temporal dynamics of free carriers in epsilon-near-zero (ENZ) materials is important for zero-index nonlinear photonic applications ^[1]. Unique and fascinating features of ultrathin ENZ materials such as strong field intensity enhancement (FIE) along with high inherent nonlinearity and sub-picosecond temporal response have enabled enhanced high harmonic generation ^[2] and sub-picosecond optical switching ^[3]. However, the origin of such giant optical nonlinearity in epsilon-near-zero (ENZ) materials is still not well understood. Additionally, there is a lack of theoretical models that can accurately describe the temporal dynamics of hot electrons, especially in the strong nonlinear optical regime. In this work, we observe abnormal temporal dynamic of hot electrons in ENZ thin films under different pump fluences and excitation angles using a degenerate pump-probe spectroscopy technique. An strong intensity- and angle-dependence (for a TM polarized pump) on the relaxation time of the hot electrons are experimentally observed for the first time. Furthermore, a novel hydrodynamic model ^[4] has been used to validate the experimental results theoretically.

Two different ENZ materials (ITO with a thickness of 305 nm and AZO with a thickness of 205 nm) are fabricated using magnetron sputtering and atomic layer deposition respectively. The optical properties of the thin films are measured with spectroscopic ellipsometry, showing that the ENZ wavelength is 1190 nm for ITO and 1480 nm for AZO. The ultrafast temporal response of free carriers is measured with linearly polarized femtosecond laser pulses (pulse duration ~70 fs, repetition rate 1 kHz, peak wavelength (λ_p) tunable between 1100-1600 nm). Experiments are performed for normal and oblique incidences of TE/ TM polarized light pulses with different pump intensities and peak wavelengths (λ_p= λ_ENZ, λ_p> λ_ENZ, and λ_p< λ_ENZ). The temporal dynamics of the free carriers are evaluated by the relaxation time of carriers upon fitting the normalized change in probe transmission using the two-temperature model ^[5] and compared with the theoretical value predicted with the hydrodynamic model. Our results suggest that free carriers’ relaxation time are highly dependent on the pump intensity as well as the excitation angles and wavelengths. We believe that our findings on understanding the tunability of the ultrafast free carrier dynamics of ENZ materials will be significant for efficient nonlinear zero-index photonic applications in the strong nonlinear optical regime.

Gold and silver are the most commonly used plasmonic materials because of their excellent optical properties in the visible and near-infrared spectral ranges. However, realization of wide-spread plasmonic applications requires additional material advantages, such as low cost, material stability, device scalability, spectral tunability, as well as device integration with existing semiconductor technologies. In this work, we demonstrate growth of atomically smooth aluminum (Al), copper (Cu), gold (Au), and silver (Ag) epitaxial films on two-dimensional (2D) layered muscovite (mica) substrates via van der Waals (vdW) heteroepitaxy with controllable film thickness from a few to hundreds of nanometers. In our approach, the mica thin sheet act as a flexible and transparent substrate with an atomically flat surface for vdW heteroepitaxy, which allows for large-area formation of atomically smooth, single-crystalline, and ultrathin plasmonic materials without the conventional issue of film dewetting. The metal thin films grown on mica allow us to design and fabricate well-controlled Al and Cu plasmonic nanostructures (the focus of this work) with tunable localized surface plasmon resonance (LSPR) and propagating surface plasmon polariton (SPP) properties from visible to the near-infrared spectral region. Furthermore, we show that the plasmonic responses of Al and Cu nanostructures fabricated on mica substrates remain unchanged under large substrate bending conditions. Based on these results, we expect that the metal-on-mica vdW epitaxy platform can lead to transformative plasmonic device applications (“flexible plasmonics”) based on novel bendable functionalities.

Deep eutectic solvents (DESs) as a new generation of low-melting-point solvents have been used for syntheses of noble metal nanoparticles owing to their unique properties, such as structure-directing abilities, strong solvating power, wide reaction temperature range, non-volatility, and high thermal stability. Despite these advantages, to the best of our knowledge, the high-throughput and large-scale productions of noble metal nanoparticles using DESs have been rarely reported to date. In this work, we present the gram-scale synthesis of various noble metal nanoparticles with controlled shapes and sizes using a polymerizable DES. Acrylamide (AAm) is used to play a role as a hydrogen bond donor in DES and monomer. Importantly, the synthesis of noble metal nanoparticles and the polymerization of AAm take place simultaneously, resulting in the stabilization of highly concentrated nanoparticles in a polyacrylamide (pAAm) solid matrix. In addition, the overall synthesis of the nanoparticles in the polymerizable DES takes only 20 min in one-pot, and can be easily extended to carry out multiple reactions in a high-throughput manner. The intense optical properties of the nanoparticles are investigated, demonstrating the localized surface plasmon resonance of noble metal nanostructures. The molecular weights of the pAAms are investigated using gel permeability chromatography, indicating the roles of the reagents participating in the synthesis reactions. Moreover, alloy nanoparticles are also synthesized in the polymerizable DES. Importantly, we demonstrate that the production of the noble metal nanoparticles using the polymerizable DES could be as large as a few grams, demonstrating that our method could be truly practical for various applications, such as molecular diagnostics and therapeutics. As an example of applications that utilize a large number of nanoparticles, we demonstrate a concept of transparent displays using the synthesized nanoparticles on a round glass plate with a diameter of 10 cm.

Strong plasmon-phonon (pl-ph) coupling can modify the energy landscape of mid-infrared antennas, thus providing opportunities to alter the energy states of infrared (IR) polaritons beyond the standard tunability approaches. Optical probes have dominated strong-coupling studies, but they offer limited spatial resolution. Nowadays, atom-wide monochromatic electron beams formed in transmission electron microscopes allow us probing and mapping IR excitations (e.g. plasmon, phonon polaritons) with better spatial sensitivity [1,2]. Here, we present a spectroscopical study of strong pl-ph coupling in mid-IR antennas using spatially-resolved electron energy loss spectroscopy (EELS). We imaged the spatial distribution of strongly-coupled ph-pl modes, revealing unique spatial behavior of polaritonic hybrid modes in mid-IR antennas [3].
We fabricated mid-IR double-antennas with extreme aspect ratio (up to 40) by joining a single rod-like Al plasmonic antenna and a rod-like a-SiO₂ phononic antenna, side by side. We adjusted the length of the double-antenna in such a way that the plasmon response of the Al antenna lies within the upper Reststrahlen band (RB) of the SiO₂. Optimal coupling conditions are achieved for antennas of 3 μm in length. The spatially-resolved EELS work was conducted using a transmission electron microscope equipped with an aberration corrector and monochromator that produces a ~ 2 Å electron probe with an energy resolution of ~ 9 meV.
The EELS spectra of the Al/SiO₂ antennas show double-peak resonances as a result of the pl-ph coupling. Each peak in the double-peak resonance is associated with the formation of one hybrid pl-ph mode. We determined the dispersion relationship associated with the polaritonic modes and found Rabi splittings as large as 26 meV within the RB. Our simulations indicate the appearance of only two coupled pl-ph modes, despite the large variety phonon polariton modes involved in the coupling. We measured the coupling constant of the pl-ph interaction (g = 18 meV), indicating that the coupling is in the strong-regime. Despite the modest quality-factor antennas built using low-cost materials, the fabricated Al/SiO₂ antennas equals performance of platforms of different geometries that capitalize on novel materials [4]. Our work suggests that geometry plays a relevant role in the development of strong-coupling hybrid platforms, likely due to the large variety of phonon polariton modes that participate in the coupling.
We mapped out the scattering associated with the strongly-coupled pl-ph modes and found that its spatial distribution is akin to the distribution of dipole optical modes in rod-like antennas. The maps of both coupled modes share several similarities along the double-antenna main axis, however minor variations can be noticed in the transverse direction, likely due to their unique charge configurations. Spatially-resolved EELS measurements across SiO₂/Al interfaces revealed a drastic scattering variation within sub 10 nm distances due to the charge symmetry of each pl-ph mode.
In summary, we have built and probed a double-antenna composed by one plasmonic Al antenna attached to a phononic SiO₂ antenna. We found evidence of strong coupling between plasmon and phonon polaritons, as revealed by the energy split in the dispersion curve. We have imaged strongly-coupled pl-ph modes bringing important physical insights into the local density of electromagnetic states. Our results suggest that geometry plays an important role in the design of hybrid strong-coupling platforms.
Acknowledgement:
For financial support, M.J.L. acknowledges NSERC under a discovery grant, and P.E.B. acknowledges support of U.S. DOE (# DE-SC0005132).

References:
[1] O. Krivanek, et al, Nature 514 (2014) 209.
[2] M. J. Lagos, et al, Nature 543 (2017) 529.
[3] M. J. Lagos, et al, ACS Photonics 8 (2021) 1293.
[4] L.Tizei, et al, Nano Letters 20 (2020) 2973.

Conventional laser cavities require discontinuity of material property or disorder to localize a light field for feedback. Recently, an emerging class of materials, i.e. twisted van der Waals materials have been explored for the applications in electronics and photonics. Here, we propose and develop magic-angle lasers where the localization is realized in a periodic twisted photonic graphene superlattices [1]. We reveal that the confinement mechanism of magic angle lasers does not reply on a full band gap but on the mode coupling between twisted two layers of photonic graphene lattice. Without any fine tuning in structure parameters, a simple twist can result in nanocavities with strong field confinement and high quality factor. Furthermore, the emissions of magic-angle lasers allow direct imaging of the wavefunctions of magic-angle states. Our work provides a robust platform to construct high quality nanocavities for nanolasers, nanoLEDs, nonlinear optics and cavity quantum electrodynamics at the nanoscale.
Reference:
[1] X. R. Mao, Z. K. Shao, H. Y. Luan, S. L. Wang & R. M. Ma. Magic-angle lasers in nanostructured moiré superlattice. Nature Nanotechnology, https://doi.org/10.1038/s41565-021-00956-7, 2021.

Lead halide perovskites have become promising materials for future lasers and optoelectronic devices. However, the conventional solution-based fabrication process greatly limits the possibility to include precision nanostructures that can further enhance the performance. In this study, we implement thermal evaporation to adapt conventional nanolithography technique, which allow us to construct MAPbI₃ perovskite nanodisk arrays. Optical pumped single-mode lasing centered at 785 nm is observed at room temperature. The Q-factors is estimated to be larger than 1500. The lasing peaks corresponds to the lattice mode and can be tuned for at least 20 nm by varying the periodicity of the nanodisk arrays. Results in this study can be extended to obtain perovskite nanolasers with tunable lasing wavelength.

In recent year, high responsivity and low-cost phototransistors have become prospective for various applications, such as optical communication, imaging, and wearable electronics. While the semiconductor materials will dominate the transistor performances, there have been several studies to develop thin film transistors using zinc-tin oxide (ZTO) due to its high electron mobility and on/off ratio. In addition, ZTO have shown the potential for optoelectronic applications owing to their significant photocurrent arising from generation of electron-hole pairs under UV light illumination. When using the visible light sources, photoexcited electrons can be generated in ZTO via the ionization of neutral oxygen vacancies (V_o) to positively charged oxygen vacancies (V_o²⁺) and the photoexcited electrons under illumination of visible light of wavelength less than 450 nm. However, it will be difficult to get significant light response of ZTO at wavelengths of greater than 500 nm because the neutral oxygen vacancies are mostly locating at deep levels (>2.5 eV) within the bandgap.
In order to expand the light responding spectrum of ZTO, in this work, we report a hybrid device by embedding gold nanoparticles (Au NPs) in the zinc-tin oxide (ZTO) thin film transistor. Integrating with plasmonic metals provides a promising approach for visible light photodetectors and photocatalysis since the photocurrent can be enhanced by hot electrons generated by local surface plasmon resonance (LSPR). Au NPs may act as the charge trapping/detrapping media to further control the electron concentration in ZTO via gate bias modulation. As a result, the responsivity and sensitivity of ZTO/Au NPs phototransistor depend on the gate bias under illumination and the wavelength of the light. Compared to the same lighting condition without applying negative gate bias, the responsivity and the sensitivity of ZTO/Au NPs hybrid device can be greatly increased by 5 and 200 folds, respectively, with the application of negative gate bias under 405 nm light illumination. Similar enhancement effects are also observed under illumination using 520 nm and 635 nm light, which is partially attributed to LSPR effect of Au NPs. The key factor to generate the higher photocurrent with applying negative gate bias is that the charged oxygen vacancies (V_o²⁺) will accumulate at the ZTO/Au NPs interface, and thus form a build-in electric field to induce the electrons to tunnel from Au NPs into ZTO.
A superb memory retention performance upon light/bias operation can be achieved in ZTO/Au NPs hybrid phototransistor since the electrons tunneling from Au NPs are excessive as compared to the charged oxygen vacancies (V_o²⁺) in the ZTO channel. As a result, the recombination process is minimized, and the current is retained. Multiple memory state can be achieved by repeated programming operation and the memory states can be erased by applying positive gate bias which trigger the recombination/trapping of photocarriers. Therefore, this novel ZTO/Au NPs hybrid phototransistor exhibits broad spectrum photoresponse and multi-level photo-memory performance that highlights the promises for optical communication and computation technologies.

Light-matter interactions with conventional metal nanostructures can be limiting as it is unduly challenging to modify their static properties after fabrication. To circumvent this issue and open up avenues to dynamically control light at the nanoscale, research is shifting towards dynamic plasmonic systems with switchable states. Our recent study showed for the first time that nanodisks of the highly conducting polymer, poly(3,4-ethylenedioxythiophene:sulfate) (PEDOT:Sulf), can function as dynamic nano-optical antennas with plasmonic resonances that are tunable via gas- or liquid-induced chemical redox reactions. Here, we take the next crucial step and present the first electrically tunable conducting polymer nano-optical antennas. In addition to switching the antennas on and off, we demonstrate gradual tuning of the plasmonic resonances by careful electrical modulation of the mobile polaronic charge carrier density. The concept can facilitate many future applications such as smart windows with tunable plasmon-induced heating or ultrathin optical components with tunable focal lengths and beam steering. Moreover, tailoring of the polymer antenna geometry enabled a significant blue-shift of the resonance peak from a previously reported lower limit of ~1800 nm to ~1270 nm, opening up the possibility to use such nano-optical antennas for applications in the visible to near-infrared ranges.

Owing to the tremendous demands for high-resolution thin lenses, ultrathin flat lenses, which are different from conventional lenses, have been extensively investigated with various applications in telescopes, aerospace technology, holograms, and glasses-free 3D displays. However, such ultrathin flat lenses need some characteristics such as thickness and tunable focal length to satisfy the requirement of various applications. So, realize these characteristics of ultra-thin flat lenses, diffractive optical elements or metasurfaces are being used. In particular, Fresnel zone plates which is comprised of a set of concentric rings using diffraction to focus light can be designed as thin, low-volume for applications in various fields. In this study, ultrathin flat lenses based on Fresnel zone plates using graphene have demonstrated high focusing efficiency and broadband performance. Unlike in most other 2D material, graphene which has a very thin thickness of per layer is intrinsically an ideal conductor with zero energy bandgap. How the transmittance change when a DC voltage is applied to ultrathin flat lens by experiments and COMSOL simulations. The result indicates that the transmittance is the lowest at 0 V and increases as the magnitude of the voltage bias is increased in either direction. Furthermore, as the voltage bias increases, the carrier density increases at the edge of the pattern leading to the shift in Fermi level which results in the difference in absorption of the incident light and thus the reduction of the Fresnel zone plate ring width. Our ultrathin flat lens is able to modify the focal length while achieving high transmission efficiency and nanoscale thickness. This study will be the cornerstone of the development of ultra-thin variable focus displays in the future, and it will be able to develop into a hologram display, reducing weight and increasing efficiency of glasses-free 3D displays and multi-focusing display for vehicles.

Lead halide perovskite materials have been proven to be promising candidates for laser application owing to their extraordinary optical properties. However, the environmental pollution risk of lead-based devices has become a challenge. Thus, searching for non-toxic, lead-free perovskite materials is essential. Here, we report a plasmonic upconversion laser from a lead-free halide perovskite CsSnBr₃ nanowire which synthesized at room temperature. The length of CsSnBr₃ nanowires ranging from 5 to 10 mm, and the width is from 100 nm to 1 mm. We characterized the spontaneous emission rate using time-resolved photoluminescence (PL) and PL measurements and observed an emission centered at 675 nm from the CsSnBr₃ nanowire. However, we only observed spontaneous emission from CsSnBr₃ nanowires due to their low PL quantum yield. Hence, we designed a plasmonic nanocavity to enhance the two-photon absorption rate and photon emission rate to reach the lasing action. The plasmonic upconversion nanolaser consists of a nanowire on a sputtered plasmonic hafnium nitride film with a thin Al₂O₃ capping layer by using atomic-layer-deposition. We will discuss the potential applications of lead-free halide perovskite CsSnBr₃ nanowire lasing such as nonlinear optics, optical communication, information processing, and sensing.

2D semiconductors in the transition metal dichalcogenide family (MoS₂, MoSe₂, MoTe₂, WS₂, WSe₂) have attracted much recent attention because of their direct-gap character at monolayer thickness and the pronounced role of excitons in these systems. In addition, the possibility of combining them in vertical stacks creates a wide range of new properties, such as the emergence of interlayer excitons. In this paper, we will highlight some the distinctive ways in which these robust building blocks for optoelectronic devices can be tuned, often in ways not available or practical in bulk materials. In particular, large and spatially localized strain fields can be used to modify the gaps and to localize excitation. Also, the external dielectric environment can modify the electrical and optical gaps of these materials at monolayer thickness. Finally, in monolayers and especially in heterostructures, electrical gating can be used to tune the material properties both through doping and the Stark effect. We will give an overview of these distinctive properties and how they might be harnessed in a device context.

In exciton-polariton coupling, the understanding of electronic energy exchange in Rabi splitting is crucial. The favorite structures that have been adopted to study such coupling are “nanoparticle-on-mirror”. Integrating with transition metal dichalcogenide (TMDC) monolayers can realize large Rabi splitting. However, such a hybrid system has a significant localized electric field in out-of-plane that cannot satisfy the couple with in-plane exciton dipole of TMDC. Here, we propose one-dimensional plasmonic nanogrooves integrated with MoS₂ monolayers. The strongly localized fields are parallel with MoS₂ exciton orientation can address the aforementioned issue. As a result, a significant Rabi splitting (65 meV) in the intermediate coupling regime is demonstrated. In addition, Kelvin probe force microscopy is used to image charge transportation in the hybrid system. We directly observed the surface potential change on the proposed system via photo-excitation caused by the charge rearrangement. Furthermore, the giant photoluminescence enhancement can be attributed to the parallel exciton-polariton interactions. Our findings can provide great potential for improving the performance of plasmonic-TMD heterodevices in the field of photoconversion.

Polaritonic modes in van der Waals crystals provide an unexcelled degree of field confinement and a superior capability for light manipulation at nanoscale. Very recently, a new class of polaritonic modes has been spotlighted as a superior platform for low-dimensional nanophotonics: the image polaritons, supported by a polaritonic material in proximity to a metal when the polaritonic mode couples with its mirror image.
We use scanning scattering-type near-field optical microscope (s-SNOM) to study the hyperbolic image phonon-polaritons (HIP) in hexagonal boron nitride (hBN) on gold. In our experiments, atomically flat gold crystals are used as a substrate, eliminating any roughness-mediated scattering of HIP. In such system, HIP dispersion properties can be accurately measured by the near-field mapping of their interference fringes. We use the polariton propagation length in optical cycles as a figure of merit (FOM).
We observe the first-order HIP mode with FOM of 6.5, independently of hBN thickness, while its wavelength varies from 192 nm in a 30 nm-thick hBN slab to 565 nm in an 88 nm-thick slab, all measured at 1480 cm^–1 in the upper reststrahlen band. In agreement with theory, measured FOM is 1.8 times larger than that of hyperbolic phonon-polaritons in the same hBN on a typically used SiO₂ substrate. At the same time, HIP possess an impressively 3.9 times shorter wavelength. While the hBN used in our experiments has isotope composition of ¹⁰B/¹¹B ≈ 82/18, we note that FOM of the HIP in the isotopically pure hBN with ¹⁰B = 98.7% is expected to be as high as 19.
Our results are even more interesting in a context of the recent work by Menabde et al. reporting on the experimental observation of significantly improved FOM of the mid-infrared image (also called acoustic) graphene plasmons. Near-field probing has revealed image plasmon mode that is twice as compressed and has 1.4 times higher FOM compared to graphene surface plasmons under similar conditions. Importantly, these results have been obtained with a large-area, chemically grown graphene, suggesting that image graphene plasmons are less sensitive to loss in graphene. This can be explained by the field distribution of the image plasmons which is mostly confined within the dielectric spacer, in contrast to the surface graphene plasmons localized at the graphene plane.
Summarizing, the direct near-field probing of image polaritons in van der Waals materials clearly demonstrates their superiority over pristine polaritonic modes in terms of both FOM and field confinement, promising the ultra-compact photonic devices where the wave phenomena and the exceptionally strong light-matter interaction can be put to work together.

The exciton dynamics in layered transition metal dichalcogenides (TMDCs) have spurred immense interest for next generation optoelectronics. Many works regarding this topic have been reported on either mechanically exfoliated TMDCs or monolayer flakes made via the chemical vapor deposition (CVD). Meanwhile, the exciton dynamics in Janus TMDCs monolayer with structural symmetry-breaking remains unexplored in any scalable fashion. Here, we report the ultrafast exciton dynamics in millimeter-scale Janus WSeS monolayer grown by chemical vapor deposition with H₂ plasma strip method. We performed PL measurement, Raman mapping, and steady-state absorption measurement to study the optical properties of the Janus WSeS monolayer. The ultrafast carrier dynamics of A/ B /C excitons were studied by using pump-probe transient absorption spectroscopy with a broadband white-light probe beam. The transient absorption pump-probe spectra of Janus WSeS varying with pump fluences and time delay will be discussed. We find that the relaxation of A exciton in Janus WSeS follows a triple-exponential decay process of approximately 1.1 ps, 14.4 ps, and 52.4 ps, respectively. We compare the carrier dynamics in Janus WSeS monolayer to the symmetric WSe₂ and WS₂ monolayers. We conclude that the short exciton lifetime in the Janus structure is due to the built-in dipole moment. The findings provide fundamental insight into the optical properties of 2D materials. We will discuss the practical applications in the field of 2D optoelectronics and ultrafast 2D photonics.

Metasurfaces based on plasmonic materials have revolutionized nanooptics [1]. The ability to shape beams with surfaces that are only a few hundred nanometers high has added tremendous design freedoms and functionalities to optics. In principle, plasmonic metasurfaces are the ultimate spatial light modulators that include amplitude and phase control on a deep subwavelength scale. The combination of propagation phase with geometrical phase allows for a full 2π phaseshift for each individual spatial element.
The forefront of metasurface research is geared towards active metasurfaces. These devices allow for changing their functional properties. Examples include beam steering with varying angles, bifocal and zoom lensing, and, ultimately, switchable spatial light modulation.
Several approaches have been brought forward: Liquid crystals or polymers have been combined with plasmonic nanoantennas or high-index dielectric resonators, and an electric field can switch the refractive index next to the nanoantennas, hence shifting the resonance wavelength. Phase-change materials such as GST have been used in hybrid or high-index Mie resonator fashion to create metasurfaces in the infrared. Typically, materials of the GST family are switched by heat, as they can shift their refractive index drastically, depending on whether they are in the amorphous or crystalline state, which can be switched depending on heating and cooling parameters [2]. Another approach uses materials such as ITO which can shift its refractive index in the infrared depending on its charge carrier densitiy. In combination with a perfect absorber geometry, this results in electrical switchable functionalities such as beam steering for LIDAR applications. Other possibilities include metal-to-insulator transition materials such as Mg or VO₂, which can be switched by proton incorporation or heating [3].
All of the previous approaches have their individual benefits and drawbacks. Hybrid approaches often suffer from the fact that the plasmonic resonances are shifted only spectrally, hence influencing other wavelengths. This holds true also for several of the GST approaches. Perfect absorber geometries that can in principle give the full 2π phaseshift and can be electrically switched commonly work in reflection geometries with very low reflectances, hence making applications such as LIDAR difficult. Nanoantennas that can switch plasmonic resonances on or off have switching speed issues, as incorporation of protons from the hydrogen gas phase often requires palladium as molecule splitting catalyst and hence results in switching times of seconds [4].
Here, we present a completely different and novel approach to the above problem: We present electrically switchable nanoantennas whose plasmon resonance can be switched on and off electrically and individually. We demonstrate video-rate switching times, hence overcoming the previous speed issues.
In order to validate our concept, we combine our electrically switchable plasmonic nanoantennas in functional metasurfaces. In one implementation, we demonstrate beam steering. In another example, we demonstrate a flat metalens whose focusing ability can be switched on and off electrically. Our system can be operated in transmission as well as in reflection geometry.
Our approach represents the breakthrough that will ultimately enable spatial light modulators with densities higher than 1000 lp/mm. This is the crucial element to make possible high-angle variable beam steering, electrically adressable zoom metalenses, and, ultimately, wide-angle holographic videos. These devices are key to enable compact augmented and virtual reality devices.

[1] W.T. Chen, A.Y. Zhu, and F. Capasso, Nature Reviews Materials 5, 604 (2020).
[2] Y. Wang, M.L. Brongersma, et al., Nature Nanotechnology 16, 667 (2021).
[3] F. Neubrech, X.Y. Duan, and N. Liu, Science Advances 6, eabc2709 (2020).
[4] F. Sterl, H. Giessen et al., Nano Lett. 15, 7949 (2015).

Highly-confined or large-momentum polaritons are usually associated with transverse-magnetic (TM) surface modes residing at the interface between two bulk materials. Commonly, the condition for the existence of such a mode is that one of the materials exhibits a permittivity whos real part is negative, while the other is a dielectric. For example, electron in metals give rise to the existence surface-plasmon-polariton (SPP) at a metal/dielectric interface, and optical phonons give rise to the surface-phonon-polariton in polar dielectrics. Both exhibit a negative real part of the permittivity in a certain spectral range, and the response holds also for 2D monolayers with the same properties [1, 2]. In that aspect, up until recently, monolayer TMDs have been known to exhibit a positive real part of their permittivities and thus in principle, should not support such a polaritonic mode.

In this work [3], we predict the existence of a highly-confined exciton-polariton mode, which propagates in-plane and confined to the monolayer in the out-of-plane direction. It belongs to the same family of polaritons mentioned above, only here the oscillations of the impinging TM electromagnetic light field couples with collective oscillations of the TMD excitons, and indeed exhibits the known signature of negative real part of its permittivity. The latter is attainable by encapsulating the monolayer TMD with hBN and cooling down to cryogenic temperatures. We show that owing to the resonant nature of the excitonic optical response in TMDs, the formation of the 2DEP and its detection strongly depend on achieving extremely narrow excitonic linewidths.

From experimental data, we extract the temperature-dependent real part of the permittivity, for an hBN-encapsulated WS2 monolayer, obtained from reflection measurements, and find that it can sustain increasingly negative values with decreasing temperature. We analyze the 2DEP properties under these conditions at the lowest temperature and compare them to those of SPPs at the same spectral range. We find that the 2DEP exhibits over two orders of-magnitude larger wavelength confinement.

Such highly-confined polaritons in the visible spectrum can pave the way to new light-matter interaction possibilities, based on 2D-materials, and compete with current SPP-based applications.

References

[1] Basov D.N. et al, Science 354-95(2016).
[2] Low T. et al, Nat. Mater. 16 182–94(2017).
[3] I. Epstein et al, "Highly Confined In-plane Propagating Exciton-Polaritons on Monolayer Semiconductors", 2D Materials 7, 035031 (2020).

Optical nonlinearity in photonic materials enables a large number of applications such as frequency conversion, all-optical signal processing, and non-classical sources of light. The speed of solid-state electronic devices, determined by the temporal dynamics of charge carriers, could potentially reach unprecedented petahertz frequencies through direct manipulation by optical fields, consisting in a million-fold increase from state-of-the-art technology. In graphene, charge carrier manipulation is facilitated by exceptionally strong coupling to optical fields, from which stems an important back-action of photoexcited carriers. Here we report the instantaneous response of graphene to ultrafast optical fields, elucidating the role of hot carriers on sub-100 fs timescales. The observed nonlinear response and its dependence on interaction time and field polarization reveal the back-action of hot carriers over timescales commensurate with the optical field. An intuitive picture is given for the carrier trajectories in response to the optical-field polarization state. We note that the peculiar interplay between optical fields and charge carriers in graphene may also apply to surface states in topological insulators with similar Dirac cone dispersion relations. Furthermore, we report harmonic generation and saturable absorption in graphene, discussing free-carrier generation and their ultrafast temporal dynamics in the atomically thin material. We further discuss the nonlinear optical properties of transition metal dichalcogenides, illustrating cavity-enhanced second-harmonic generation and parametric down-conversion and demonstrating that phase-matching free operation can be achieved in photonic micro-cavities embedding two-dimensional semiconductors as nonlinear optical media. Our results are promising for the development of integrated optical parametric oscillators and micro-sources of entangled photons.

Arrays of plasmonic gold nanoparticles, embedded in fused silica plates, were prepared by implanting 2 MeV Au⁺ ions through a silica particle lithographic mask. The nonlinear optical response of these periodic arrays was experimentally investigated using the z-scan technique at 532 nm with 26 ps pulses from a Nd:YAG laser. The open-aperture z-scan measurements were performed following different linear polarization angles for the incident light beam. A strong saturable absorption and a very clear nonlinear dichroic behavior were determined, the latter indicated by a sinusoidal modulation of the nonlinear absorption coefficient β (up to about 20% of its absolute value) as a function of the polarization angle. This result gives clear evidence of the formation of ordered arrays of embedded Au nanoparticles, corresponding to a honeycomb lattice symmetry with a modulation period of 30°.

Simulating diffuse, glossy and specular albedo materials for enhanced bifacial solar module output.

An effective way of improving photovoltaic (PV) yield while keeping the costs low is by using bifacial modules. As opposed to conventional monofacial modules, bifacial modules produce electricity by accepting solar irradiance from the front and the rear side. A crucial element in enhancing the bifacial output is the ground reflectance, i.e., albedo. The module output strongly depends on the angle and wavelength of the incident irradiance and many nanophotonic light management strategies are particularly sensitive to these parameters [1, 2, 3, 4, 5]. Thus, understanding the spectral and angular albedo of the ground-reflector becomes important for the accurate understanding of albedo-dependent module output. Various sophisticated models [6, 7, 8, 9] help in calculating the bifacial output due to albedo but often use wavelength and/or angle averaged albedo.
To overcome this, we developed an albedo-centric reverse ray-tracing algorithm that can simulate short-circuit current density of a bifacial module under the influence of albedo materials with complex spectral and angular reflectance. The software can model a wide range of reflectors, i.e, diffuse, glossy [10], specular, and a combination of the three. It can also accommodate custom reflectors like metamaterials or free-space luminescent solar concentrators [11]. The algorithm also accepts any spectral and angular irradiance and tailored PV modules, while accounting for self-shading.
Our study shows that under AM1.5G illumination at 85° relative to the horizontal, a diffuse reflector outperforms a specular reflector for any value of module height and tilt. A diffuse reflector also proves to be more robust to the changes in module orientation and consequently, self-shading.
Such an approach can help in the fundamental understanding of the influence surface properties of a reflector on a solar module. It can facilitate in assessing the performance of currently existing natural and artificial reflectors and inspire ideas to devise novel albedo materials for higher bifacial output. Furthermore, when combined with the open-circuit voltage and local spectro-angular irradiance [12] measurements, the above methodology can assist in selecting the best module-albedo pair and their optimal configuration.

References
[1] R. Saive, T. C. R. Russell, and H. A. Atwater, IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC), Waikoloa Village, HI, USA, 2018.
[2] M. Brennan, A. Abramase, R. Andrews and J. Pearce, Solar Energy Materials & Solar Cells, vol. 124, pp. 111-116, 2014.
[3] T. C. R. Russell, R. Saive and H. A. Atwater, 44th IEEE Photovoltaic Specialist Conference (PVSC), Washington, D.C., USA, 2017.
[4] N. Tavakoli and Esther Alarcon-Llado, Optics Express, vol. 27, pp. A909-A923, 2019.
[5] R. Saive, Progress in Photovoltaics: Research and Applications, pp. 1-13, 2021.
[6] A. Asgharzadeh, M. A. Anoma, A. Hoffman, C. Chaudhari, S. Bapat, R. Perkins, D. Cohen, G. M. Kimball, D. Riley and F. Toor, IEEE 46th Photovoltaic Specialists Conference (PVSC), Chicago, IL, USA, 2019.
[7] C. E. Valdivia, C. T. Li, A. Russell, J. E. Haysom, R. Li, D. Lekx, M. M. Sepeher, D. Henes, K. Hinzer and H. P. Schriemer, IEEE 44th Photovoltaic Specialist Conference (PVSC), Washington, DC, 2017.
[8] M. R. Vogt, T. Gewohn, C. Schinke and R. Brendel, EUPVSEC 2018, 2018.
[9] T. O. Fartaria and Manuel Collares Pereira, Solar Energy, vol. 91, pp. 93-101, 2013.
[10] B. Walter, S. Marschner, H. Li and K. Torrance, Proceedings of the Eurographics Symposium on Rendering Techniques, Grenoble, France, 2007.
[11] G. Heres, L. Einhaus and R. Saive, IEEE Photovoltaic Specialists Conference (PVSC), online, 2021.
[12] S. Pal, A. Reinders and R. Saive, IEEE Journal of Photovoltaics, vol. 10, no. 6, pp. 1803-1815, 2020.

We report a novel approach to analyze bandgap and defect states within semiconductors by fluorescence spectroscopy using a plate reader conventionally used for the study of biological samples in solution. We use two-dimensional fluorescence mapping (2DFM) to deconvolve spectral signatures that would be superimposed in 1D-spectral space; for example, the 570nm emission peak in ZnO whose emissive state is of a different physical nature depending upon the excitation wavelength used. The broad applicability of the technique is shown for a library of widely studied inorganic semiconductors CdS, CdSe, ZnS, ZnSe, ZnO (analytical standard and nanorods) & TiO₂. Anthracene is also included as a representative example of an organic semiconductor. The 2DFM approach is able to identify spectral features from the bandgap, defects and trace impurities. The technique has applicability in both academic and industrial settings for semiconductor studies and quality control (QC) applications.

Composites of ZnO-Au are promising materials for use in optical security applications due to multiple aspects of the materials fabrication process impacting its spectral fingerprint. We report a one-pot strategy to synthesize ZnO-Au hetero-structures using a peptide chimera with sequence GLHVMHKVAYSSGAPPMPPF (ZA2). The novel ZA2 chimera combines most of a ZnO binding sequence, G-12 (GLHVMHKVAPPR) and a gold binding sequence, A3 (AYSSGAPPMPPF) that has been extensively studied in the literature. The one-pot peptide mediated synthesis was performed in the presence and absence of HAuCl₄ to separately determine the impact of the peptide and gold inclusions on the structural and electronic properties of the ZnO component. Using a combination of optical spectroscopy and electron microscopy we show that both spherical and non-spherical gold nano-inclusions were successfully embedded within the ZnO forming Ohmic contacts. Simulation of the plasmon resonance of nanoscale gold inclusions in ZnO using Mie theory and finite difference time-domain (FDTD) numerical approaches report on collective plasmon phenomena from embedded inclusions suggesting an inter-inclusion spacing of 10 nm or less and informs on the tuneable spectral nature of these materials. Spectra calculated using the FDTD approach show the synthetic approach described here has the potential to generate a library of materials with varying spectral features that could be used for identification purposes.

Materials whose dielectric functions approach zero have many unique properties ranging from enhanced non-linear behavior to suppression of quantum electromagnetic fluctuations. Beyond these interesting physical phenomena, epsilon-near-zero (ENZ) concepts can be applied to devices to exploit these effects. In this talk, I will discuss our recent work on the development of devices that take advantage of these effects to create super-absorbing optical films for novel photodetectors and hydrogen sensors, modified thermal emission, and quantum effects that can be exploited using ENZ materials. Two examples of the later are the suppression of spontaneous emission and modifications to the Casimir force, a force that it purely quantum in nature and is related to the electromagnetic boundary conditions placed on vacuum fluctuations. We will conclude with an outlook for this emerging area of research.

The spontaneous emission rates of emitters can be manipulated by tailoring the optical density of states of its local environment. Traditionally, this has been achieved through interaction of emitters with different types of photonic cavities. Using the same principles, metasurfaces and other types of nanostructured media have been used to control spontaneous emission of single and ensembles of quantum emitters. In this talk I will give an overview of the control and manipulation of spontaneous emission from different types of emitters (dopants in carbon nanotubes, excitons in two dimensional materials and epitaxial/colloidal quantum dots) using dielectric metasurfaces. Finally, I will describe how semiconductor metasurfaces can be used to control the directionality of spontaneous emission at ultrafast time-scales.

The achievement of thresholdless lasing is a long-sought goal for many applications. Yet, the inherent working principle behind lasers hinders its realization, due to the necessity to reach population inversion. An interesting strategy to overcome this limitation is based on exciton-
polaritons. These hybrid light-matter excitations can undergo a phase transition to a coherent state, known as Bose-Einstein condensate, eliminating the requirement for population inversion.
In order to undergo this phase transition that leads to the emission of laser-like light, exciton-polaritons need to live long enough to macroscopically occupy a single quantum state. Up to now they have been mainly studied in vertical microcavities but their lifetime is usually limited by the quality of the mirrors and deep etching processes.
In this work we show exciton-polariton lasing from a bound state in the continuum (BIC), obtained via shallow patterning of a GaAs/AlGaAs slab. The extremely long lifetime of the BIC, alongside the high-quality post-processed cavity, allowed us to achieve an ultra-low lasing threshold in state-of-the-art polariton lasers. We focus on the impact of the nanofabrication processing over the polariton lasing from a BIC, leading to low threshold polariton condensation, and we characterize its emission in both real and reciprocal space.
In conclusion, this work unveils new possibilities of energy efficient coherent light sources to be used in future integrated photonic devices. Our new approach to achieve exciton-polariton lasing shows several advantages over the more classical vertical microcavities, since no mirrors are required and the etching needed to manipulate the photonic mode is only a few tens of nanometers. Aside from the interesting technological advantage, bosonic condensation in symmetry-protected photonic eigenmodes could also bring new topological properties onto macroscopic quantum states, which could give new insights on the spatial properties of BIC states.

Epsilon-near-zero (ENZ) materials have drawn significant research interest in recent years, owing to their unique electromagnetic/optical properties at the frequencies of vanishing real part of permittivity. Effects include negligible phase advances, strong spatial coherence, and enormous field enhancement at the interfaces with dielectric materials. They have found extensive applications in supercoupling, nanocircuit boards, ultrafast optical nonlinearity, perfect optical absorption, etc. In addition, ENZ materials have also been successfully exploited to modulate the spontaneous emission intensity, including the spectrum and directionality, of quantum emitters (QEs). Here we propose the concept of electromagnetic “bandgaps” for nanoparticles comprising an ENZ material. Depending on the frequency, the spontaneous emission of a QE in close proximity to a nanoparticle can be either substantially enhanced (as a result of the coupling into surface plasmon or dielectric resonances) or significantly suppressed (due to the lack of modes available for excitation around the ENZ frequency). Our theoretical results show an enhancement factor of ~38000 for the total emitted power at the surface plasmon resonance frequency and of ~0.002 for the radiatively emitted power around the ENZ frequency from a QE placed in close proximity of a low-loss ENZ sphere with a diameter of 600 nm, compared with that placed in free space. Even for realistic ENZ materials (e.g. ITO), enhancements of ~1610 and suppressions of ~0.13 are predicted. The effective bandgap of the nanoparticles can therefore be probed by tracking the radiated power from the emitter. Our research provides an alternative perspective to explore the correlation between plasmonic resonances, ENZ behavior, and spontaneous emission dynamics of quantum emitters.

Organic light-emitting diodes (OLED) are widely used light-emitting diodes that utilize the light emission from an electron-hole annihilation inside an organic material. In an OLED device, the multiple reflections and the light interference inside a layered structure cause its optical characteristics to depend heavily on the device shape. Also, it leads to an angle dependence of the emission spectra while narrowing down the emission spectra. In this manner, an optimization process is required for any OLED device for the maximization of light extraction efficiency and color gamut. Some classical approaches have been made to optimize the structure but the results are hardly global optima since the global optimization requires heavy computation.
In the meantime, it has been reported that the neural network works as an excellent platform for calculating optical response of photonic devices, and tackling photonic inverse design problems^1,2. In the field of OLED, machine learning techniques have also been performed to facilitate material screening processes³. However, no approach has been made to address the link between the device design parameters and the light extraction efficiency (LEE) of a given OLED device for the purpose of the inverse design and optical performance optimization.

In this work, we present a neural network that receives the layer thicknesses and refractive indices as the inputs and predicts the LEE response of a given OLED structure. The network is trained with a dataset containing 260,000 structure parameter-LEE pairs which were obtained from the in-house Chance-Prock-Silbey (CPS) model⁴. The neural network was able to complete the spectrum predictions with 4×10⁶ faster rate than the rigorous electromagnetic simulation based on CPS formulation depending on the degree of parallelization with RMSE of 1.86×10^-3. Two different routes were taken for the inverse design of LEE responses. One was the inverse neural network built by joining additional layers to the pre-trained forward neural network and the other was the neural network assisted genetic algorithm (NNGA) which uses the forward network as the platform for LEE prediction. NNGA showed superior performances in the inverse design of both existing and non-existing LEEs whereas the inverse neural network was unmatched in terms of the computation speed. The NNGA also successfully solves two representative OLED optimization problems: maximizing LEE and minimizing angle-dependent white color variation. Considering that both problems possess large design spaces that are difficult to be addressed by conventional numerical approaches employing rigorous electromagnetic simulations, the neural network-based methodology presented in this work provides a promising platform for tackling computation-heavy optimization tasks with one-time computation cost. Furthermore, different aspects of the neural network including its one-to-many mapping behaviour, extrapolability, and comparison between the inverse design methods were investigated extensively.
Acknowledgements
This work was supported by LG Display
References
[1] Kiarashinejad, Y, et al., Deep learning approach based on dimensionality reduction for designing electromagnetic nanostructures. Npj Comput Mater 2020, 6 (1).
[2] Ma, W., et al, Deep-Learning-Enabled On-Demand Design of Chiral Metamaterials. ACS Nano 2018, 12 (6), 6326-6334.
[3] Gomez-Bombarelli, R., et al, A., Design of efficient molecular organic light-emitting diodes by a high-throughput virtual screening and experimental approach. Nat Mater 2016, 15 (10).
[4] Chance, R. R., et al., Molecular Fluorescence and Energy Transfer Near Interfaces. In Advances in Chemical Physics, 1978; pp 1-65.

Organic light-emitting diodes (OLED) have become one of the major light sources for display use due to high color accuracy and luminosity. The external light efficiency of conventional OLEDs ranges around 20% while the internal quantum efficiency approaches ~100%, which is sufficiently low. The discrepancy arises from the total internal reflection and a relatively large refraction index of emission layers. It has been reported that the external light efficiency of an OLED can be improved by introducing a periodically corrugated layer boundary [1, 2], but the effect has not been analyzed quantitatively. Electromagnetic analysis of a dipole layer in a periodically corrugated LED has been done previously [3], but the approach used in that work cannot deal with the emission from a single dipole, which is crucial for analyzing a periodically corrugated dipole layer. In this work, we demonstrate how the emission from a single dipole in a periodically corrugated LED structure can be calculated by diffraction tracking. We decomposed the light emission of a point dipole source into plane waves, and tracked each wave to calculate light emission. We tracked the diffraction and polarization conversion at a periodically corrugated layer boundary. Diffraction efficiency was calculated by rigorous coupled-wave analysis method (RCWA) [4]. We have confirmed that the results from our calculation model agree well with the simulation values from finite-difference time-domain method (FDTD) in terms of far-field image and light extraction efficiency (LEE) while alleviating the computational load by a significant amount. We even showed that the results from our calculation model agree well with FDTD methods at two-dimensional periodically corrugated layer boundary. We also implemented direct calculation of incoherent dipole layer in a periodically corrugated OLED, which makes our method more time efficient.
The advantages of our method become clear during the OLED structure optimization process. The diffraction coefficients obtained from the RCWA calculation depends only on the shape of the corrugation, hence the calculation for different dipole vertical/horizontal position, dipole orientation and different emission layer thicknesses can be completed with minimal computation. Also, the analysis involving multiple dipoles can be calculated in parallel, facilitating the optimization task even more.
References
1. Zhang, Xu-Lin, et al, Optics Letters, Vol. 36, Issue 19, pp. 3915-3917 (2011)
2. Jeon, Sohee, et al, Nanoscale 6.5 (2014): 2642-2648.
3. Delbeke, Danaë, et al, Journal of the Optical Society of America A Vol. 19, Issue 5, pp. 871-880 (2002)
4. J.P.Hugonin, et al, arXiv:2101.00901