We discuss our contributions to describe the optical response of photonic materials made from periodically arranged scatterers. These scatterers can be molecules or macroscopic objects. A unifying description is possible by representing the scatterers with a T-matrix. While considering the renormalization of the object’s T-matrix upon interaction with all scatterers in the lattice, any optical quantity of interest can be expressed on numerical grounds. We also derive analytical expressions for many of those quantities while considering scatterers up to octupolar order at normal and oblique incidence for subwavelength and diffracting metasurfaces. Exemplarily, design challenges using these methods are presented.

Subwavelength grating (SWG) metamaterial structures are excellent platforms for guided-wave nonlinear optics, but their design and optimization are challenging due to the large number of geometric degrees of freedom and the need for compute-intensive 3D simulations. Here, we demonstrate inverse design of χ(2) SWG waveguides using an efficient and accurate differentiable plane-wave expansion (PWE) eigensolver. Our solver, which incorporates sparse iterative algorithms and subpixel smoothing, enables efficient eigensolution and end-to-end differentiation from geometric parameters to the SWG figure of merit, which depends both on the eigenvalues (first-order perturbation theory) and the eigenvectors and group indices (second-order perturbation theory), both in forward- and reverse-mode. We apply this solver to the design and optimization of metamaterial waveguides for two types of backward SHG: idler-reversed and pump-reversed. This approach may find use in designing periodic structures more generally, including nanobeam cavities, slow-light modulators, and vertically coupled resonators.

Since the development of diffractive optical elements in the 1970s, major research efforts have focused on replacing bulky optical components by thinner, planar counterparts. The more recent advent of metasurfaces, i.e. nanostructured optical coatings, has further accelerated the development of flat optics through the realization that nanoscale antenna elements can be utilized to facilitate local and nonlocal control over the light scattering amplitude and phase. In this presentation, I will start by showing how passive and active metasurfaces can start to impact Mixed Reality applications. I will discuss the creation of high-efficiency, metasurfaces for OLED displays, and eye tracking systems. I will also highlight recent efforts in our group to realize electrically-tunable metasurfaces employing nanomechanics, tunable transparent oxides, electrochemistry, microfluidics, phase change materials, and atomically-thin semiconductors. Such elements are capable of dynamic wavefront manipulation for optical beam steering, wavefront manipulation, and future displays. The proposed optical elements can be fabricated by scalable fabrication technologies, opening the door to a wide range of commercial applications.

By combining the piezoelectric MEMS mirror with plasmonic OMSs, we successfully demonstrated an electrically driven MEMS-OMS platform for dynamic 2D wavefront shaping with high modulation efficiencies, broadband operation, and fast speed [2]. One functionality of particular interest to the commercial applications (e.g., LiDAR) is the possibility of switching between multiple diffraction orders to allow for quasi-continuous beam steering. To realize this suggested functionality, we randomly interleaved two meta-gratings optimized at two different air gaps with distinct grating periods of Λsc and 2 Λsc. In both simulations and experiments, polarization-independent reflection power redistributions between 0th, +1st, +2nd diffraction orders are visualized, corresponding to 0⁰, 7.6⁰, and 15.4⁰ reflection directions, under normal incidence. As a future perspective, on this MEMS-OMS platform, we foresee more versatile functionalities available, thanks to the plenty space of design freedom in both MEMS mirror and OMS sides. Currently, we are also working on other designs to explore other possibilities on this platform. [1] Meng, C., Thrane, P., Ding, F., Gjessing, J., Thomaschewski, M., Wu, C., Dirdal, C. & Bozhevolnyi, S. I. (2021). Dynamic piezoelectric MEMS-based optical metasurfaces. Science Advances, 7(26), eabg5639.

We explore the differences between gap surface plasmon (GSP) and Fabry-Perot resonances for use in dynamic optical metasurfaces enabled by combining plasmonic nanostructures with piezoelectric micro-electro-mechanical mirrors. This work builds upon research presented in [1] where the dynamic metasurface platform was demonstrated using GSPs. By extending the working range into the Fabry-Perot regime it is possible to design dynamic metasurfaces with new functionalities in addition to combining the two domains. [1] Meng, C., Thrane, P., Ding, F., Gjessing, J., Thomaschewski, M., Wu, C., Dirdal, C. & Bozhevolnyi, S. I. (2021). Dynamic piezoelectric MEMS-based optical metasurfaces. Science Advances, 7(26), eabg5639.

Optical nanostructures enable molding the flow of light on the subwavelength scale rendering possible the realization of compact, cost-effective, flat and highly efficient optical components for imaging, spectroscopy, sensing, and communications. From the fundamental science viewpoint, light-matter interactions at the nanoscale reveal themselves in new and often unexpected ways. In fact, the usual linear and nonlinear macroscopic theories that describe the behavior of the electromagnetic field merit revision. From the applications viewpoint, miniaturized, tunable and reconfigurable functionalities implemented on a chip are of paramount importance. Here, we reassess dielectric materials such as chalcogenides as promising candidates for science and applications in wavelength ranges once thought to be hostile to the propagation of light. We show that, in fact, these materials constitute new, excellent contenders for developing new, multi-functional nanophotonic devices due to their high linear refractive indices, strong nonlinear response, and fast switching speeds. To date, applications of these glasses have been limited to wavelength ranges where absorption meticulously avoided, for example, above 500nm for As2S3: high absorption at visible and ultraviolet wavelengths manifests itself through the large imaginary parts of the dielectric constants. However, we demonstrate that, as counterintuitive as it may appear, highly nonlinear, properly nanostructured chalcogenide glass, transparent in the mid- and near-infrared, can also be used to generate third harmonic frequencies in the visible and ultraviolet part of the spectrum, despite the presence of strong material absorption in this range. Such third-harmonic generation can be tuned and further enhanced using stacked metasurfaces and various mechanisms of local field enhancement, including Mie and quasi-bound states in continuum resonances.

Metasurfaces hold promise to miniaturize many bulk (~mm) optical elements and concurrently enable the implementation of multiple optical functionalities in individual integrated optical devices. Nonlinear optics is a fast-rising area of application, albeit the sub-wavelength thickness of metasurfaces means that their nonlinear conversion efficiency is often very low. In this framework, lithium niobate (LN) is a ferroelectric material widely employed for optoelectronics and nonlinear optics thanks to its high refractive index, broad transparency window, and large electro-optical response and second-order nonlinearity. Over the last few years, much effort has thus been directed towards the realization of LN nanoparticles and nanostructures [1,2]. Here, we report highly directional second-harmonic (SH) generation of visible light by a monolithic LN metasurface [3]. The metasurface was milled on a commercially available z-cut LiNbO3 substrate via dual-beam focused ion beam lithography. Using an 80 MHz train of 140 fs laser pulses with 500 MW/cm2 peak intensity as a pump, we measured SH conversion efficiencies (SH power /exciting power) up to 2.5E-8. The numerically-assisted design of the metasurface [4] exploits the resonant modes of the single pillar: a magnetic dipole at the exciting wavelength of 820 nm, along with an electric dipole and a magnetic quadrupole at the SH wavelength. The interplay between the single-pillar emission and the grating mode of the metasurface brings about an efficient (~20 dB) rerouting of the SH emission from the zeroth into the first diffraction orders, which exhibit a unconventional polarization behavior. The studied system exemplifies a design concept of NL diffractive metasurface and is suitable for applications such as NL holography with polarization encoding. [1] A. Fedotova, et al., Nano Letters 20, 8608–8614 (2020) [2] T. Santiago-Cruz, et al. arXiv 2103.08524 (2021) [3] L. Carletti, et al. ACS Photonics 8, 731–737 (2021) [4] L. Carletti, et al. Optics Express 27, 33391–33398 (2019)

Light-matter strong coupling provides a powerful way to modify the nonlinear optical properties of materials. The coupling strength of the state-of-the-art strongly coupled systems are restricted by weak field confinement of the cavity, which limits the enhancement of the optical nonlinearity. Here, we investigate a new type of strong-coupling system, where the Mie resonant modes of the dielectric nanocavities are strongly coupled to an epsilon-near-zero mode, which results in an Rabi splitting of 220 meV. Static and dynamic nonlinear optical measurements reveal a very large enhancement in the intensity-dependent refractive index change at the coupled resonance. In addition, we observe a transient response of hundreds of femtoseconds for the coupled system. The ultrafast and large optical nonlinear coefficients in this work offer a new route towards strong coupling-assisted high-speed photonics.

Symmetries and broken symmetries play important roles in physics. In particular, they constrain the electromagnetic response of materials and the allowed light-matter interactions. In such a context, symmetry breaking can lead to unique and peculiar physical phenomena. The Hall effects, for example, result from broken symmetries. In the usual linear Hall effects, the time-reversal symmetry is broken by a magnetic bias. Alternatively, nonlinear Hall effects can occur in systems with a broken inversion symmetry rather than broken time-reversal symmetry. Here we explore several physical platforms that can enable nonreciprocal and non-Hermitian responses based on the nonlinear Hall effect and 2D material layers biased with a static electric field. It will be shown that the electric field bias may create unique physical responses, including regimes of loss and gain controlled by the wave-polarization, asymmetric responses, and others. In this talk, we will present an overview of our on-going work on this topic.

I identify a fundamentally new type of Bragg phenomenon, one that occurs in a uniform medium that relies on resonant tuning of the medium parameters, rather than coherent superposition of reflections of monochromatic light from a periodic structure. Since the medium is uniform, reflection occurs over a potentially wide range of wavelengths, much like a metal. Unlike a metal, however, the reflection is polarization dependant: one circular polarization is transmitted, whilst the opposite polarization is reflected. The basis of this remarkable phenomenon is a uniform medium that is simultaneously birefringent and optically active. Solving for the eignmodes for light propagating in the medium, it is found that a resonance occurs when the optical activity is matched to the average transverse refractive index. When this matching occurs, the index associated with one circular polarization is purely imaginary, resulting in it being completely reflected when incident from vacuum. The opposite polarization is, by contrast, transmitted. The ‘bandwidth’ of the resonance is set by the birefringence. Heuristically, one circular polarization progressively, and synchronously, samples the alternating transverse refractive indices as it propagates. It is this alternation that makes the light ‘feel’ a Bragg grating, while the medium remains uniform, an interpretation that has been confirmed by a coupled wave analysis. The resonance condition requires a huge optical activity: a full polarization rotation over a wavelength. This is precisely the condition that triggers the onset of negative refraction in a purely optically active medium, and one that has been observed recently in meta-media at THz frequencies. The medium can provide a low-loss white-light polarization divider. Furthermore, modulation of the medium parameters results in an efficient wide-band polarization switch. We believe that the identified mechanism has wide application in optics and optoelectronics. The proposed medium is hereby christened ‘bire-active’.

This Conference Presentation, “Nanophotonic chirality transfer to dielectric Mie resonators,” was recorded for Photonics Europe 2022, in Strasbourg, France.

We show that combined loss-gain media, including parity-time symmetric media, can offer novel prospects in molecular chirality sensing and enantiomer discriminaion, as they can greatly enhance the circular dichroism response and the disymmetry factor characterizing molecular chiral systems.

The resolution of conventional microscopy is limited to a half of the wavelength of light. We report on recent advances in applications of deep learning and topologically structured light to far-field non-destructive imaging with deep subwavelength resolution and to picometric metrology.

Plasmonic resonators featuring nanoscale can exhibit strongly enhanced optical near-fields that have been extensively used in surface enhanced spectroscopy (Raman and Fluorescence) and in biosensing. However, deterministic nanostructures do not provide numerous degrees of freedom to control optically these local field enhancements. By comparison, wavefront shaping techniques in disordered scattering media provide numerous degrees of freedom to control light focusing in space and time. To associate local field enhancements and far-field wavefront control, we use disordered plasmonic metasurfaces close to the percolation threshold that feature both hotspots and delocalized plasmonic modes that can be controlled using a spatial light modulator. By controlling the phase of an incoming femtosecond pulsed laser on a disordered gold metasurface, we optimize the two-photon induced luminescence (TPL) at a chosen position with a typical two-order of magnitude enhancement, indicating a far-field optimization of the optical near-field. The optimization process is performed using a random genetic algorithm and provides the highest enhancement when the metasurface features the highest degree of morphological complexity, close to the percolation threshold (Nano Lett. 20, 3291 (2020)). Far-field wavefront shaping also provides new degrees of freedom to provide statistical imaging schemes that offset the structural complexity of disordered nanophotonic systems. For instance, using nonlinear luminescence images measured with randomly wavefront-shaped femtosecond excitations, we can independently map the localized and delocalized plasmonic modes on a disordered gold metasurface (ACS Photonics 8, 1973 (2021)).

Dielectric and plasmonic metasurfaces provide excellent control over the shaping of optical wavefronts via the manipulation of polarisation, phase and amplitude of the light. Taking advantage of their subwavelength thicknesses, metasurfaces are a very promising technology in a variety of applications including beam steering and focusing, polarisation and angular momentum control. Recently, holographic information encoding for 3D displays using metasurfaces has also been of interest, creating effective techniques for the 2D visualisation of 3D objects. Normal mapping, for example, is widely used in computer graphics to create shading effects and recreate 3D-like features of surface textures. Here, we report on the development of the concept of surface normal mapping for the representation of 3D objects and shading effects with optical metasurfaces. In this work, the metasurface is designed to implement diffuse reflection and uses the concept of normal mapping to control its scattering properties. As a proof of principle, a flat diffuse metasurface imitating lighting and shading effects of a 3D cube was characterised under incoherent illumination. The “3D image” is displayed directly on the illuminated metasurface and its shading varies in response to the change in illumination angle. The metasurface performs in a broad range of the visible spectrum, including the three RGB wavelengths. The 3D images created via normal mapping based on optical metasurfaces provide an effective technology for 3D security features and anti-counterfeiting. This type of metasurfaces can also be useful in the design of efficient optical diffusers for display technology and etalons for metrology.

Optical metasurfaces (OMSs) represent subwavelength-dense planar arrays of nanostructured elements designed to control local phases and amplitudes of scattered optical fields, thus being able to manipulate radiation wavefronts at a subwavelength scale. In the past decade, many configurations involving different physical mechanisms have been developed, and numerous diverse applications have been demonstrated, including free-space wavefront shaping, versatile polarization transformations, optical vortex generation, optical holography and many others [1]. However, to date, most reported OMSs are static, featuring well-defined optical responses determined by OMS configurations that are set during fabrication, although, for more intelligent and adaptive systems, dynamic OMSs with externally controlled functionalities are in high demand. Realization of dynamic OMSs is, however, very challenging because of the high density of array nm-sized elements that are also arranged in nm-thin planar configurations, circumstances that limit severely the interaction volumes. In this talk, I present recent results that we obtained with two different configurations for realizing electrically controlled OMSs operating in reflection: piezoelectric micro-electro-mechanical systems (MEMS) integrated with gap surface-plasmon (GSP) based OMSs [2] and FabryPerot resonators based on thin electro-optic crystal layers sandwiched between nanostructured electrodes [3]. Their operational principles are described, and the main experimental results obtained are presented. Furthermore, new developments and functionalities demonstrated will be presented at the conference. References: 1. F. Ding, A. Pors, and S. I. Bozhevolnyi, “Gradient metasurfaces: a review of fundamentals and applications,” Rep. Prog. Phys. 81, 026401(2017). 2. C. Meng, P.C.V. Thrane, F. Ding, J. Gjessing, M. Thomaschewski, C. Wu, C. Dirdal, and S.I. Bozhevolnyi, “Dynamic MEMS-based optical metasurfaces,” Science Advances 7, eabg5639 (2017). 3. C. Damgaard-Carstensen, M. Thomaschewski, F. Ding, and S.I. Bozhevolnyi, “Electrical tuning of Fresnel lens in reflection,” ACS Photonics 8, 1576 (2021).

The capability to manipulate the polarization state of light at the nanoscale is of paramount importance in many emerging research areas ranging from optical communication to quantum information processing. Gap-surface plasmon (GSP) metasurfaces, which provide unprecedented advantages and abilities of molding reflected fields, have been demonstrated excellently suited for integration of multifunctional polarization optics into a single device. Here, we establish a versatile GSP metasurface platform based on nanoscale quarter-wave plates (nano-QWPs) that enable efficient circular-to-linear polarization conversion along with the complete phase control over reflected fields. Capitalizing on the nano-QWP design, we demonstrate, both theoretically and experimentally, how resonance and geometric phases can be used in concert to achieve independent and simultaneous phase modulation of both co- and cross-polarized circularly polarized (CP) waves by realizing arbitrary beam steering of co- and cross-polarized CP channels in a broadband near-infrared range. The GSP metasurface platform established in our work provides versatile and flexible solutions to enrich multiple functionalities for diversified metasurface-based polarization optics exploited in modern integrated photonic devices and systems.

For miniaturized active photonic components, resonance tuning of nanoantennas is essential. Phase-change materials (PCMs) have been established as prime candidates for non-volatile resonance tuning based on a change in refractive index [1]. Currently, a novel material class of switchable infrared plasmonic PCMs, like In3SbTe2 (IST), is emerging. Since IST can be locally optically switched between dielectric (amorphous phase) and metallic (crystalline phase) states in the whole infrared range, it becomes possible to directly change the geometry and size of nanoantennas to tune their infrared resonances by more than 4 µm. In particular, resonant nanostructures on sub-meta-atom level can be directly written, erased and modified in the thin IST film without cumbersome nanofabrication techniques. Additionally, prepatterned nanoantennas can be screened by a thin IST film resulting in an on/off functionality. With an IST patch two nanoantennas can be soldered together to shift the resonance [2]. Here, crystalline IST split-ring resonators (SRRs) are directly optically written and reconfigured in their arm size to continuously tune their magnetic dipole resonances over a range of 2.4 µm without changing their electric dipole resonances. The SRRs are further modified into crescents and J-antennas, which feature more complex resonance modes dependent on the polarization of the incident light. The ability of erasing and modifying the structures enables reversible and fast adaptions of the fabricated antenna geometries. In addition, the experimental results and the corresponding mode assignments are confirmed with full-wave simulations [3]. Our concepts are well-suited for rapid prototyping, speeding up workflows for engineering ultrathin, tunable, plasmonic devices for infrared nanophotonics, telecommunications or (bio)sensing. [1] Wuttig et al., Nature Photonics 11, 465 (2017) [2] Heßler et al., Nature Communications 12, 924 (2021) [3] Heßler, Conrads et al. Nano Letters (submitted) (2021)

Recently in nanophotonics, a rigorous evolution from passive to active metasurfaces has been witnessed. This advancement not only brings forward new physical phenomena but also elicits opportunities for practical applications. However, active metasurfaces operating at visible frequencies often exhibit low performance due to design and fabrication restrictions at the nanoscale. In this work, we demonstrate electrochemically-controlled metasurfaces with high intensity contrast, fast switching rate, and excellent reversibility at visible frequencies. We employ a conducting polymer, polyaniline (PANI) that can be locally conjugated on preselected gold nanorods to actively control the phase profiles of the metasurfaces. The optical responses of the metasurfaces can be in situ monitored and optimized by controlling the PANI growth of subwavelength dimension during the electrochemical process. In particular, we showcase electrochemically-controlled anomalous transmission and holography with good switching performance. Such electrochemically powered optical metasurfaces lay a solid basis to develop metasurface devices for real-world optical applications.

Structured light finds many interesting applications in numerous fields, such as information encoding, wavefront manipulation and imaging. The opportunity to engineer the field distributions of optical beams opens up many exciting possibilities for achieving new optical interactions with materials and molecules. In this work we theoretically and experimentally investigate the interaction of cylindrical vortex beams (CVBs) with a strongly anisotropic plasmonic metamaterial, focusing on the case of radially and azimuthally polarised beams. We developed a semi-analytical model to describe the propagation of CVBs through an anisotropic slab, describing the metamaterial by means of an effective medium theory. In the tight focusing regime, the extinction properties of the metamaterial show the sample sensitivity to different symmetries of the electric field distributions, as well as the important role of the longitudinal field components of the beam on the extinction. Strong dichroism of the anisotropic metamaterial results in variations of the beam modal structure by differently influencing the three components of the field. Moreover, decomposing the beam intensity profiles in the Laguerre-Gauss mode basis reveals a non-negligible variation of the modal content of the beam, induced by the nanorod metamaterial anisotropy. Linear, radial and azimuthal polarisation states have been tested for metamaterials with different anisotropy parameters. Experimental results show a good agreement with the theoretical predictions, proving the promising potential of anisotropic metamaterials for manipulation of complex vector beams.

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.

In this invited talk, I will review our recent theoretical, and experimental results in the field of artificial intelligence (AI) assisted inverse design of a new class of flexible ultra-flat optics (thickness < 100nm) for high efficiency (close to unitary) vectorial control of light. These devices are supported by a layer of “physical” neural network units in suitably engineered optical nanoresonators. These systems act as universal approximators of arbitrary defined input-output responses, processing information at the speed of light. I will discuss basic optical components and new integrated systems for processing high-dimensional visual information in real-time and a new concept of universal integrated, ultra-flat equipment for metrology applications based on data-driven science. Finally, I will also discuss some recent challenges in manufacturing high-resolution, large-scale samples up to 3-inches in size.

I will review the recent trends in metamaterial research driven by the emergence of a new field of metaphotonics that is driven by the physics of Mie resonances excited in high-index dielectric nanoparticles and also voids created in a dielectric media. This provides a novel platform for localization of light in subwavelength photonic structures and opens new horizons for metamaterial-enabled photonics. I aim to review the recent advances in Mie-resonant metaphotonics (also called "Mie-tronics") for isolated high-index dielectric nanoparticles and nanoparticle structures such as dielectric metasurfaces, and also discuss its applications to different problems, including nonlinear harmonic generation, non-plasmonic nanolasers, and topological photonics

Dynamic tuning of metamaterials is a critical step towards achieving advanced functionality and improved operational bandwidth. Approaches to modulation can be separated into two categories. The first is mechanical reconfiguration of the scatterer geometry or scatterer arrangement. The second is manipulation of the optical properties of the scatterer, or its environment. Although there are a variety of paths to achieve modulation, electrochemical actuation provides a method that can belong to both categories by providing for both volume expansion through lattice rearrangement as well as modulation of free electron density for permittivity control. In this talk we will outline several approaches towards electrochemical modulation including modulation of popular metasurface materials such as silicon and titanium oxide. The use of nanostructured metasurfaces allow for new freedom in controlling the spectral response of the material while also minimizing diffusion lengths for higher speed and lower power operation compared to traditional electrochromic materials.

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. Our recent discovery of single-photon emitters in technologically important SiN platform will be also discussed.

Optical metasurfaces, two-dimensional arrangements of designed nanoresonators, offer unique opportunities for controlling light fields and for tailoring the interaction of light with nanoscale matter. Due to their flat nature, their integration with two-dimensional materials consisting of only a single molecular layer is particularly interesting. This talk reviews our recent and ongoing activities in hybridizing optical metasurfaces composed of resonant metallic or dielectric building blocks with different types of two-dimensional materials, including monolayer transition metal dichalcogenides (2D-TMDs) and carbon nanomembranes (CNMs). On the one hand, we will show that CNMs can serve as mechanically stable substrates for free-standing metasurface architectures of nanoscale thickness. On the other hand, we will demonstrate that the ability of the nanoresonators to concentrate light into nanoscale volumes can be utilized to carefully control the properties, such as pattern and polarization, of light emitted by 2D-TMDs via photoluminescence or nonlinear processes. Here, the ability of tailored nanostructures to interact selectively with exciton populations located at inequivalent conduction band minima at the corners of the 2D-TMD's Brillouin zone is of particular interest. Such a selective interaction is an important prerequisite for the realization of future miniaturized valleytronic devices.

Recycling low-temperature waste heat is a key issue to realize de-carbonization in our society. Nevertheless, conventional technologies including thermoelectric generators are incapable of gathering thermal energies from the surrounding environments. Here, we report metamaterial thermoelectric conversion driven by the thermal energies extracted from the surrounding environment, which can be available for energy harvesting. The metamaterial thermoelectric generator can extract thermal energies from the surrounding environment and convert them into electricity. The metamaterial thermoelectric generator consists of a bismuth telluride thermoelectric element loaded with metamaterial-fabricated copper electrode at one edge. The metamaterial arrays fabricated on the copper electrode absorb thermal radiation emitted from the surrounding environment, leading to local heat generation as a consequence of plasmon loss. The local heat propagates to the bismuth telluride thermoelectric element via the copper electrode, resulting in an additional thermal gradient across the thermoelectric element and generation of electricity. Previously, we numerically investigated the metamaterial thermoelectric conversion. In this report, we will show the experimental demonstration of metamaterial thermoelectric conversion.

In this work we make use of an inverse design methodology for the design of high efficiency deformation robust flat optics. Our approach leverages neural network predictors trained to quickly estimate the results of finite difference time domain (FDTD) simulations. By rapidly exploring the solution space, we find geometries that exhibit an optical response tolerant to dimensional errors. We validate our approach by fabricating and characterizing flat optics polarizers on top of polyamide tape. The devices exhibit a polarization efficiency of 85% over a 200 nm bandwidth and retain high performance when subjected to large deformations, in contrast to a control non-robust design.

Optical metasurfaces have been extensively investigated, but most are, however, static. We suggest an approach to realize electrically tunable optical metasurfaces, based on tuning the light reflectivity near a high-fidelity Fabry-Perot resonance. This concept is implemented in dynamic Fresnel lens focusing. The active Fresnel lens (AFL) exploits the electro-optic Pockels effect in a 300 nm thick lithium niobate layer sandwiched between a continuous thick and a nanostructured gold film serving as electrodes. We fabricate and characterize the AFL, focusing 800-900 nm radiation at a distance of 40 µm with the focusing efficiency of 15%, and demonstrating the modulation depth of 1.5%, with the driving voltage of ±10 V within the bandwidth of ~6.4 MHz.

The laser beam shaping is a practical problem in both information transmission, detection and material processing. Many applications focus on beam intensity and phase control in the so-called near-field, but far-field phenomena in microsystems should be no less interesting. One new way to control far-field distributions is by applying what is called “supercollimation”. It can be observed in systems consisting of periodically arranged phase gratings made from concentric features that repeat along the optical axis. Supercollimation is defined as the transformation of a broad-angular-intensity profile beam into a very thin well-defined high-peak-intensity and low-divergence beam. Such a beam can be compared to a Bessel Beam, however, a sharp central peak does not appear in the near field, but on the contrary: in the far field. There is no simple way to understand how a supercollimated beams are formed because it does not occur with 1D or 2D periodic systems. What is clear is that it is a cascading process that requires multiple diffraction. In sufficiently, long structures, plane waves diffracted from 0-order to higher diffraction orders are diffracted back resonantly with a broadened angular distribution that can overlap with the zero-angle component in the spatial spectrum. Thus, the aim of this paper is to reveal how such a phenomenon can occur in periodic systems with rotational (axial) symmetry, such as photonic crystals and resonators with concentric gratings. Finally, I will discuss how to use the “broken” symmetry of asymmetric photonic crystals in interpreting this phenomenon in practical contexts.

Non-Hermitian physics is an emerging field that for the unconventional management and control of wave fields. PT-symmetric metamaterials or dipoles have already been proposed as a flexible platform to redirect electromagnetic fields. Acoustic PT-dipoles have also been recently studied for sound guidance, showing an asymmetric scattering response. Therefore, we experimentally propose pairs of identical Helmholtz resonators with different losses distributed in the plane to create such dipoles (as electromagnetic analogues) and either concentrate the field, redirect it or even silence predefined areas. The experimental results confirm that the two-dimensional sound pressure field can be successfully modified on the bases of non-Hermitian physics.