Response Of Metasurfaces Research Articles

Toroidal dipole resonances enhanced second-harmonic generation with shallow etching of lithium niobate metasurface.

Lithium niobate (LiNbO3) has shown great potential for applications in nonlinear metasurfaces, thanks to its large second-order nonlinear coefficients and high integration capabilities. Optical resonances play a crucial role in further enhancing the nonlinear optical responses of LiNbO3 metasurfaces (LNMS). In this study, both numerically and experimentally, we designed and fabricated a metasurface structure that supports toroidal dipole (TD) resonance to enhance second-harmonic generation (SHG). This structure, which consists of an array of shallow-etched square columns on a continuous thin film, intensifies the SHG signal at 400 nm within the LiNbO3 film by means of strong local field confinement. Experimental results indicate that this signal is ten times stronger compared to that of lithium niobate on insulator (LNOI). These findings emphasize the potential of TD resonance in enhancing the performance of LiNbO3 in integrated nonlinear nanophotonic applications.

Optimize Fusion Skeleton for Multimodal Neural Network by Sequential Sampling to Efficiently Estimate Electromagnetic Response of Metasurfaces

Optimize Fusion Skeleton for Multimodal Neural Network by Sequential Sampling to Efficiently Estimate Electromagnetic Response of Metasurfaces

On the Optical response of all-dielectric metasurface in the exceptional point

All-dielectric nanophotonics leverages high-refractive-index dielectric materials to manipulate light at the nanoscale with minimal energy loss, offering strong electric and magnetic resonances. These properties make all-dielectric structures ideal for a range of advanced photonic applications, from ultra-thin lenses to sensors. In this study, we explore the multipole response of all-dielectric metasurfaces when operating near an exceptional point (EP), a singularity in non-Hermitian systems where eigenmodes coalesce. Numerical simulations reveal that at the EP, the metasurface’s resonance splits, with significant contributions from magnetic dipole (MD) and electric quadrupole (EQ) modes. These findings enhance our understanding of EPs in metasurface physics and highlight the potential of this geometry for applications in photonic devices.

Constraints on co- and cross-polarization reflection and transmission of Babinet-complementary metasurfaces

A study of constraints on reflection and transmission coefficients for Babinet-complementary metasurfaces is presented in this work. These coefficients have been found to describe circular paths in the complex plane, whose centers and radii are affected by losses. Additionally, it has been shown that it is possible to fully control the cross-polarization coefficients by rotating the metasurface with respect to the polarization direction. Our findings have been verified through numerical simulations and experiments. The properties discussed in this paper could be useful in limiting the types of possible responses of metasurfaces.

Meta-Attention Deep Learning for Smart Development of Metasurface Sensors.

Optical metasurfaces with pronounced spectral characteristics are promising for sensor applications. Currently, deep learning (DL) offers a rapid manner to design various metasurfaces. However, conventional DL models are usually assumed as black boxes, which is difficult to explain how a DL model learns physical features, and they usually predict optical responses of metasurfaces in a fuzzy way. This makes them incapable of capturing critical spectral features precisely, such as high quality (Q) resonances, and hinders their use in designing metasurface sensors. Here, a transformer-based explainable DL model named Metaformer for the high-intelligence design, which adopts a spectrum-splitting scheme to elevate 99% prediction accuracy through reducing 99% training parameters, is established. Based on the Metaformer, all-dielectric metasurfaces based on quasi-bound states in the continuum (Q-BIC)for high-performance metasensing are designed, and fabrication experiments are guided potently. The explainable learning relies on spectral position encoding and multi-head attention of meta-optics features, which overwhelms traditional black-box models dramatically. The meta-attention mechanism provides deep physics insights on metasurface sensors, and will inspire more powerful DL design applications on other optical devices.

Revealing Mode Formation in Quasi-Bound States in the Continuum Metasurfaces via Near-Field Optical Microscopy.

Photonic metasurfaces offer exceptional control over light at the nanoscale, facilitating applications spanning from biosensing, and nonlinear optics to photocatalysis. Many metasurfaces, especially resonant ones, rely on periodicity for the collective mode to form, which makes them subject to the influences of finite size effects, defects, and edge effects, which have considerable negative impact at the application level. These aspects are especially important for quasi-bound state in the continuum (BIC) metasurfaces, for which the collective mode is highly sensitive to perturbations due to high-quality factors and strong near-field enhancement. Here, the mode formation in quasi-BIC metasurfaces on the individual resonator level using scattering scanning near-field optical microscopy (s-SNOM) in combination with a new image processing technique, is quantitatively investigated. It is found that the quasi-BIC mode is formed at a minimum size of 10×10-unit cells much smaller than expected from far-field measurements. Furthermore, it is shown that the coupling direction of the resonators, defects and edge states have pronounced influence on the quasi-BIC mode. This study serves as a link between the far-field and near-field responses of metasurfaces, offering crucial insights for optimizing spatial footprint and active area, holding promise for augmenting applications such as catalysis and biospectroscopy.

Control of the near-field radiative heat transfer between graphene-coated nanoparticle metasurfaces

The control of near-field radiative heat transfer (NFRHT) between two metasurfaces can be achieved by manipulating the geometric and dielectric parameters of their components. Based on a 2D effective medium approximation, we describe the dielectric response of each metasurface composed of graphene-coated nanoparticles (GCNPs) on a 2D square lattice as a homogeneous uniaxial film. Wrapping Drude-like nanoparticles (NPs) with graphene enhances the effective plasmonic response of metasurfaces by significantly broadening the frequency range in which surface and hyperbolic waves can be excited by thermal photons. Consequently, the NFRHT between GCNP metasurfaces improves that observed between uncoated Drude-like nanoparticle arrays. We found that the heat flux (Q) grows with increasing metasurface packing fraction (PF) and is also sensitive to GCNP size. By tuning the graphene chemical potential (μ)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$(\\mu )$$\\end{document}, Q reaches a maximum improvement of 88%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$88\\%$$\\end{document} for μ≈0.1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mu \\approx 0.1$$\\end{document} eV with cores made of Drude-like material, while using cores made of the polar dielectric SiC, Q increases up to 226%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$226\\%$$\\end{document} for μ≈0.45\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\mu \\approx 0.45$$\\end{document} eV. Our results show that, in addition to the geometric control achieved with uncoated NP arrays, the tunable optical properties of the graphene shell allow dynamic control of the heat flux, expanding the possibilities for NFRHT engineering offered by GCNP metasurfaces.

Topology Optimization Enables High-Q Metasurface for Color Selectivity.

Nonlocal metasurfaces, exemplified by resonant waveguide gratings (RWGs), spatially and angularly configure optical wavefronts through narrow-band resonant modes, unlike the broad-band and broad-angle responses of local metasurfaces. However, forward design techniques for RWGs remain constrained at lower efficiency. Here, we present a topology-optimized metasurface resonant waveguide grating (MRWG) composed of titanium dioxide on a glass substrate capable of operating simultaneously at red, yellow, green, and blue wavelengths. Through adjoint-based topology optimization, while considering nonlocal effects, we significantly enhance its diffraction efficiency, achieving numerical efficiencies up to 78% and Q-factors as high as 1362. Experimentally, we demonstrated efficiencies of up to 59% with a Q-factor of 93. Additionally, we applied our topology-optimized metasurface to color selectivity, producing vivid colors at 4 narrow-band wavelengths. Our investigation represents a significant advancement in metasurface technology, with potential applications in see-through optical combiners and augmented reality platforms.

Stretchable Dual-Axis Terahertz Bifocal Metalens with Flexibly Polarization-Dependent Focal Position and Direction.

Varifocal lenses are essential components in any optical system, while traditional lenses suffer from bulky volume, fixed focal position, and limited working spectra. As well-arranged subwavelength structures, metalenses overcome the abovementioned obstacles and exhibit merits of ultrathin thickness, flexible focal length, and multifocus. The electromagnetic responses of metasurfaces, including metalens, rely on the phase distributions of phase-shifting elements. The steerable focal direction is investigated to obtain the combinations of focusing and anomalous refraction phase distribution. To fully explore the flexibility of focal length and direction, seven designs of double layers of terahertz (THz) bifocal metalenses are proposed and investigated in this study. They exhibit dependent and independent relationships of tunable focal length and direction with flexible tuning mechanisms. Along with polarization multiplexing, two different focuses can be obtained when the incident waves are x-linear and y-linear polarization states, respectively. The simulation results agreed well with the theoretical predictions. These designs provide a new method to modulate the focal position precisely with promising applications in wireless communication, imaging, and on-chip optical integration systems.

Deep learning-enhanced prediction of terahertz response of metasurfaces

Metasurfaces offer an exciting opportunity to manipulate electromagnetic waves, presenting vast potential across diverse applications. In this study, we introduce a novel deep learning approach that integrates an Autoencoder with a Multi-Layer Perceptron to effectively forecast the Terahertz (THz) spectral response of metasurfaces. By harnessing a large dataset of training examples, our model adeptly captures the intricate correlation between metasurface structures and their optical responses, circumventing the traditionally time-consuming analysis of complex patterns. This proposed methodology furnishes a valuable tool for examining the THz transmission response of metasurfaces and has the potential to expedite metasurface design processes.

Chirality tuning and reversing with resonant phase-change metasurfaces.

Dynamic control of circular dichroism in photonic structures is critically important for compact spectrometers, stereoscopic displays, and information processing exploiting multiple degrees of freedom. Metasurfaces can help miniaturize chiral devices but only produce static and limited chiral responses. While external stimuli can tune resonances, their modulations are often weak, and reversing continuously the sign of circular dichroism is extremely challenging. Here, we demonstrate the dynamically tunable chiral response of resonant metasurfaces supporting chiral bound states in the continuum combining them with phase-change materials. Phase transition between amorphous and crystalline phases allows for control of chiral response and varies chirality rapidly from -0.947 to +0.958 backward and forward via the chirality continuum. Our demonstrations underpin the rapid development of chiral photonics and its applications.

Collective response in planar random-flip coherent metasurface by modulating multipole moments

Collective response in planar random-flip coherent metasurface by modulating multipole moments

Optimizing an electromagnetic wave absorber for bi-anisotropic metasurfaces based on toroidal modes

The design and optimization of an electromagnetic wave absorber for far-field wireless power transmission (WPT) is the subject of this research study. The goal of the research is to effectively absorb energy from ambient RF electromagnetic waves without the usage of a ground plane by employing metasurfaces with chiral components.By integrating trioidal moments into the design theory, the objective is to create a metasurface that functions in two frequency bands and produces high-quality resonance. The study also explores the dual non-homogeneity property of structures, polarization tensor coefficients, and the electromagnetic response of non-homogeneous metasurfaces. Based on the relative orientation of induced fields and moments, it delves deeper into the two basic possibilities for dual non-homogeneous elements. The development of chiral metasurfaces and the notion of electromagnetic chirality and its implications for polarization properties are introduced.

A coupled-mode-theory formulation for periodic multi-element metasurfaces in the presence of radiation losses

We derive a coupled-mode theory (CMT) formulation for the fast analysis of periodic multi-element metasurfaces in the presence of radiation losses. Full-wave simulations of periodic multi-element metasurfaces are very time- and memory-consuming, especially as the size and complexity of the metasurface increase. The CMT formulation provides a considerably faster and efficient alternative. It results in a small system of equations with size equal to the number of supported resonator modes in the frequency range of interest, allowing to calculate the resonator mode amplitudes and, consequently, the metasurface response. Subsequently, we systematically derive analytical closed-form expressions for the coupling coefficients between two weakly coupled resonators in the presence of radiation losses and incorporate them into the CMT model, which is found important for the accurate description of the metasurface, while also providing insight into the underlying physics of complex metasurfaces. We validate the proposed formulation on benchmark examples of both metal- and dielectric-based metasurface absorbers (MSAs) by comparing the CMT results to spectral FEM simulations of the composing supercell. To further demonstrate the potential of the proposed formulation, as a proof of concept, we use the CMT to synthesize a larger optimized periodic multi-element MSA. A comprehensive comparison to full-wave FEM simulations of the composing supercell is included in terms of time and computational requirements, which shows that our method provides a valuable and efficient alternative solver for synthesizing complex metasurfaces.

Analytical modeling of terahertz graphene metasurfaces

Control over the optical response of graphene metasurfaces enables many promising applications in terahertz devices, including beam filters, polarizers, and absorbers. In this paper, we provide an analytical theory that enables us to use equivalent circuit models to simulate these responses. Closed-form expressions are provided for their associated scattering spectra, including an infinity of parallel R-L-C circuits for each metasurface of arbitrary morphology. These models’ accuracy is verified by comparing their results with those that are derived from full-wave simulations. Moreover, applications of these models are presented to anticipate both tunable optical response in interacting graphene disks and refractive index sensing via the response of graphene squares to ambient media. These models open up a new way of controlling the structure’s performance and refining it to meet our application-specific requirements.

Chalcophosphate metasurfaces with multipolar resonances and electro-optic tuning.

We computationally analyze the electro-optic response of metasurfaces consisting of interconnected nanoantennas with multipolar resonances using a chalcophosphate, Sn2P2S6, as the active material. Sn2P2S6 has large electro-optic coefficients and relatively low Curie temperature (<70 °C), allowing for strong changes in the refractive index of the material under moderate electric fields if the temperature can be finely controlled in proximity of the Curie point. Through numerical simulations, we show that metasurfaces designed with this nanostructured material demonstrate a significant shift of multipolar resonances upon biasing, despite moderate refractive-index values of the chalcophosphate and reduced mode localization due to this. The magnetic octupolar resonance of a dense array provides the strongest shift of spectral features upon changes in the refractive index, and we attribute it to the high mode localization of this higher-order multipole. We numerically demonstrate that narrow lattice resonances of collective nature do not provide an advantage in shifting the spectral features because of the nonlocal and delocalized nature of the modes, which are spread in the nanoantenna surrounding rather than confined inside nanoantennas. Both in-plane and out-of-plane biasing of the chalcophosphate crystals are similarly efficient with suitable electrode design, choice of electrode material, and crystal orientation within the nanoantennas. These designs exhibit optical properties similar to those of metasurfaces with isolated nanoantennas in the dense array.

Deep Learning-Based Metasurface Design for Smart Cooling of Spacecraft

A reconfigurable metasurface constitutes an important block of future adaptive and smart nanophotonic applications, such as adaptive cooling in spacecraft. In this paper, we introduce a new modeling approach for the fast design of tunable and reconfigurable metasurface structures using a convolutional deep learning network. The metasurface structure is modeled as a multilayer image tensor to model material properties as image maps. We avoid the dimensionality mismatch problem using the operating wavelength as an input to the network. As a case study, we model the response of a reconfigurable absorber that employs the phase transition of vanadium dioxide in the mid-infrared spectrum. The feed-forward model is used as a surrogate model and is subsequently employed within a pattern search optimization process to design a passive adaptive cooling surface leveraging the phase transition of vanadium dioxide. The results indicate that our model delivers an accurate prediction of the metasurface response using a relatively small training dataset. The proposed patterned vanadium dioxide metasurface achieved a 28% saving in coating thickness compared to the literature while maintaining reasonable emissivity contrast at 0.43. Moreover, our design approach was able to overcome the non-uniqueness problem by generating multiple patterns that satisfy the design objectives. The proposed adaptive metasurface can potentially serve as a core block for passive spacecraft cooling applications. We also believe that our design approach can be extended to cover a wider range of applications.

Enhancing Tandem Solar Cell's efficiency through convolutional neural network-based optimization of metasurfaces

Tandem Solar cells are composed of multiple layers with varying bandgap materials to facilitate the absorption of a wider range of solar energy. However, their performance is hampered by interface losses leading to current mismatching. To overcome these limitations, tandem solar cells often require the use of mirrors and lenses to concentrate and trap light to achieve optimal efficiency. Recently, the use of nanoscale 2D meta-materials has emerged as a promising alternative to traditional lenses. However, the optical optimization of these meta-materials can be both time-consuming, resources, and space-intensive, posing a challenge to their implementation. This study explores the use of deep learning to predict the optimal optical design for the top cell in tandem solar cells to maximize power conversion efficiency. Computational techniques are used to analyze the optical responses of metasurfaces. 2D-Convolutional Neural Networks (CNN) are used to train a dataset of 10,578 TiO2/CH3NH3PBr3/ZnO metasurfaces. Nine different CNN models were used with different architectures to identify the best hyperparameters that give the low mean square error. CNN displayed fast high prediction accuracy taking an average of 0.3 ± 0.05 seconds per prediction. The Deep SHapley Additive Explanations (SHAP) algorithm was used to gain insights into CNN's predictions and understand the behavior of complex metasurfaces integrated into a typical reference tandem solar cell architecture. The proposed metasurfaces can significantly enhance the efficiency of tandem solar cells. The active layer comprises of near 90% absorption of the solar spectrum. The average absorption of the top cell increased in the UV-vis region (650-800nm) up to 93.4%.

Spin-polarization control of in-plane scattering in arrays of asymmetric U-shaped nanoantennas.

We study projection-enabled enhancement of asymmetric optical responses of plasmonic metasurfaces for photon-spin control of their far field scattering. Such a process occurs by detecting the light scattered by arrays of asymmetric U-shaped nanoantennas along their planes (in-plane scattering). The nanoantennas are considered to have relatively long bases and two unequal arms. Therefore, as their view angles along the planes of the arrays are changed, they offer an extensive range of shape and size projections, providing a wide control over the contributions of plasmonic near fields and multipolar resonances to the far field scattering of the arrays. We show that this increases the degree of the asymmetric spin-polarization responses of the arrays to circularly polarized light, offering a large amount of chirality. In particular, our results show the in-plane scattering of such metasurfaces can support opposite handedness, offering the possibility of photon spin-dependent directional control of energy routing.

Active control of resonant asymmetric transmission based on topological edge states in paired photonic crystals with a Ge2Sb2Te5 film.

For many high-precision applications such as filtering, sensing, and photodetection, active control of resonant responses of metasurfaces is crucial. Herein, we demonstrate that active control of resonant asymmetric transmission can be realized based on the topological edge state (TES) of an ultra-thin G e 2 S b 2 T e 5 (GST) film in a photonic crystal grating (PCG). The PCG is composed of two pairs of one-dimensional photonic crystals (PCs) separated by a GST film. The phase change of the GST film re-distributes the field distributions of the PCG; thus active control of narrowband asymmetric transmission can be achieved due to the switch of the on-off state of the TES. According to multipole decompositions, the appearance and disappearance of the synergistically reduced dipole modes are responsible for the high-contrast asymmetric transmission of the PCG. In addition, the asymmetric transmission performances are robust to the variation of structural parameters, and good unidirectional transmission performances with a high peak transmission and high contrast ratio can be balanced, as the layer number of the two PCs is set as four. By changing the crystallization fraction of GST, the peak transmission and peak contrast ratio of asymmetric transmission can be flexibly tuned with the resonance locations kept almost the same.