All-dielectric Metasurfaces Research Articles

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.

Controllable dual resonances of Fano and EIT in a graphene-loaded all-dielectric GaAs metasurface and its sensing and slow-light applications

Abstract Active nanophotonic metasurfaces have attracted considerable attention for their promise to develop compact, tunable optical metadevices with advanced functions. In this work, we theoretically demonstrated the dynamically controllable dual resonances of Fano and electromagnetically induced transparency (EIT) using a graphene-loaded all-dielectric metasurface with U-shaped gallium arsenide (GaAs) nanobars operating in the near-infrared region. The destructive interference between a subradiant mode (i.e., a dark mode) supported by two vertical GaAs bars and two radiative modes (i.e., two bright modes) supported by a horizontal GaAs nanobar gives rise to a Fano resonance and an EIT window with high transmission and a large quality factor (Q-factor) in the transmission spectrum. Importantly, the transmission amplitudes can be flexibly modulated by adjusting the graphene Fermi levels without rebuilding the nanostructures. This modulation results from the controllable light absorption by the loaded graphene monolayer due to its interband losses in the near-infrared spectrum. Furthermore, the peak wavelengths of the Fano resonance and EIT window with high Q-factors are highly sensitive to variations in the refractive index (RI) of the surrounding medium, giving the proposed metasurface a relatively good sensitivity of ~700 nm/RIU and a high figure of merit (FOM) of 280, making it an effective RI sensor. Additionally, the metasurface features an adjustable slow light effect, indicated by the adjusted group delay time ranging from 0.12 ps to 0.38 ps. Therefore, the metasurface system proposed in this work offers a viable platform for advanced multi-band optical sensing, low-loss slow light devices, switches, and potential applications in nonlinear optical fields.

Stacking integrated learning-based inverse design of four-nanopore high-Q all-dielectric metasurface

In this paper, we establish a four-nanophole hollowed out all-dielectric metasurface structure as the object of reverse design, which can realize many types of high Q-value Fano resonance effects through dynamic tuning of a single structure, with Q value up to 4401. We used the time domain finite difference method for data collection, and then select appropriate models according to the characteristics of the data from the perspective of the data itself. We select in small sample prediction excellent support vector regression (SVR), gradient promotion decision tree (GBDT), prediction generalization performance and stability of high random forest (RF) as a base learners, with good nonlinear fitting ability BP neural network (BPNN) yuan learners, establish a fusion model based on Stacking integrated learning strategy. Based on this fusion model, the multi-parameter all-dielectric metasurface structure is reverse designed instead. The results show that the proposed method has high prediction accuracy with an absolute average error of only 0.0043, and excellent average accuracy of 82.5%, 77.9% and 14% compared with the combined model GBDT-SVM-BP, GBDT-RF-BP, and RF-SVM-BP. This study provides a new perspective on the reverse design of all-dielectric metasurface structures.

Generation of arbitrary vector orbital angular momentum beams on higher-order Poincaré spheres based on an all-dielectric metasurface with complex amplitude modulation

Vector orbital angular momentum (OAM) beams, described by higher-order Poincaré (HOP) sphere, are generalized forms of waves carrying OAM with an inhomogeneous polarization of wavefronts. We construct all-dielectric metasurfaces with adjustable amplitude, polarization, and phase to generate arbitrary vector OAM beams. The metasurface is composed of two pairs of silicon nanopillars arranged alternately. Using the interference effect of the four meta-atoms related to the circular polarization, combined with the propagation and geometric phases, two OAM beams with controlled amplitude, phase, and equal topological charge but opposite signs can be obtained under the incidence of orthogonally circularly polarized lights. For the x linearly polarized light, arbitrary vector OAM beams on the HOP sphere are generated via the superposition of the above OAM beams. Additionally, the evolution process of the beam on the longitude and latitude of the Poincaré sphere is revealed by changing the amplitude and phase of the two OAM beams. This work provides a simple, effective, and flexible method for realizing vector OAM beams while having potential implications for the generation and manipulation of vectorial light fields at the micro-nano scale.

Optical absorption modulation and image display capabilities of all-dielectric metasurface featuring integrated graphene

Optical absorption modulation and image display capabilities of all-dielectric metasurface featuring integrated graphene

Superchiral Light Emerging from Bound States in the Continuum in Metasurfaces of Si Nanorod Dimers.

Bound states in the continuum (BICs) in all-dielectric metasurfaces enhance light-matter interaction at the nanoscale due to their infinite Q factors and strong field confinement. Among a variety of phenomena already reported, their impact on chiral light has recently attracted great interest. Here we investigate the emergence of intrinsic and extrinsic optical chirality associated with the excitation of BICs in various metasurfaces made of Si nanorod dimers on a quartz substrate, comparing three cases: parallel nanorods (neutral) and shifted and slanted dimers, with/without index-matching superstrate. We analyze both the circular dichroism (CD) of the far-field (FF) interaction and the helicity of the near-field (NF) distribution. We show that the best approach to achieve chiral response in the FF based on extrinsic chirality is to exploit quasi-BICs (q-BICs) appearing in the case of slanted nanorod dimers. By contrast, the helicity density is largely enhanced in the case of shifted dimers, as it presents intrinsic chirality, with values 2 orders of magnitude larger than those of circularly polarized plane waves. These so-called superchiral electromagnetic fields concentrated at the nanoscale within the metasurface hold promise of appealing implications in phenomena such as strong-coupling, photoluminescence emission, or other local light-matter interactions.

Quasi-bound states in the continuum induced by in-plane and out-of-plane asymmetry in all-dielectric metasurfaces

In this paper, we propose a periodic structure consisting of a pair of square holes etched through square silicon substrates to achieve a high-Q quasi-BIC resonance peak at 1524 nm. The simulation results indicate that the Q factor of the excited quasi-BIC can reach 12,660. Multipole decomposition and near-field distribution show that the quasi-BIC of the square etched hole structure is primarily dominated by magnetic dipoles, electric quadrupoles, and magnetic quadrupoles. When structural parameters shift in other directions, or when the square hole is rotated or changed to a circle, this quasi-BIC demonstrates good robustness.

Extreme high-temperature effects on optical response of CsPbBr3 perovskite

The temperature sensitivity of CsPbBr3 perovskite impacts the performance of related devices, with their optoelectronic behavior characterized by the dielectric function (ε = ε1+iε2). Yet, research on optical properties (ε) beyond 200 °C remains limited, hindering the evaluation of their high-temperature stability. This study first grew a high-quality CsPbBr3 single crystal, then examined its dielectric function from −55 to 550 °C using spectroscopic ellipsometry, and explored the optical response of all-dielectric CsPbBr3-based metasurfaces under varying temperatures. High temperature induces dynamic disorder in CsPbBr3, and thermal decomposition initiates above 300 °C, leading to a substantial change of dielectric function. CsPbBr3-based metasurfaces exhibit near-perfect light absorption (average 0.987) with only a meta-atomic height equivalent to one-seventh of the wavelength. They also demonstrate enhanced absorption at longer wavelengths, attributed to high-temperature-enhanced magnetic resonance. This work aims to improve the high-temperature performance of CsPbBr3 perovskites and expand their applications in energy and ultrafast optics.

Multimodal ultra-high Q-factor C2 asymmetric all-dielectric metasurface induced by quasi-BIC states

A high Q-factor resonant cavity indicates minimal energy loss at the resonance frequency. In this paper, we propose a four-leaf clover sensor with air pillars based on the all-dielectric metasurface and discuss the C2 asymmetric induced quasi-bound states in the continuum(quasi-BIC). Firstly, we obtain dual Fano resonance by C2 symmetric configuration. Secondly, by introducing C2 asymmetric parameters, leading to a new Fano resonance peak, and the Q-factor exceeds 104. Furthermore, When the incident laser changes from TE mode to TM mode, the original Fano resonance peaks gradually disappear, and a new peak is generated near the 1273 nm. The calculated polarization sensitivity is up to 0.012 per degree. The dynamic range (DR) of the peak value of transmission spectrum with polarization is 58 dB. By calculation, we have achieved a maximum sensitivity (S) of 270 nm/RIU for an optical refractive index sensor, with a figure of merit (FOM) value of 2250 RIU-1.

Giant ultrafast dichroism and birefringence with active nonlocal metasurfaces

Switching of light polarization on the sub-picosecond timescale is a crucial functionality for applications in a variety of contexts, including telecommunications, biology and chemistry. The ability to control polarization at ultrafast speed would pave the way for the development of unprecedented free-space optical links and of novel techniques for probing dynamical processes in complex systems, as chiral molecules. Such high switching speeds can only be reached with an all-optical paradigm, i.e., engineering active platforms capable of controlling light polarization via ultrashort laser pulses. Here we demonstrate giant modulation of dichroism and birefringence in an all-dielectric metasurface, achieved at low fluences of the optical control beam. This performance, which leverages the many degrees of freedom offered by all-dielectric active metasurfaces, is obtained by combining a high-quality factor nonlocal resonance with the giant third-order optical nonlinearity dictated by photogenerated hot carriers at the semiconductor band edge.

Analogue of High-Q Transparency Band and Sensitivity in All-Dielectric Metasurfaces Supporting Bound States in the Continuum

Bound states in the continuum (BICs), which are characterized by their high-quality factor, have become a focal point in modern optical research. This study investigates BICs within a periodic array of dielectric resonators, specifically composed of a silicon rectangular bar coupled with four silicon rectangular blocks. Through the analysis of mode coupling, we demonstrate that the interaction between the blocks significantly modulates the eigenmodes of the bar, causing a redshift in all modes and enabling the formation of electromagnetically induced transparency based on BICs (EIT-BIC). Unlike typical EIT mechanisms, this EIT-BIC arises from the coupling of “bright” and “dark” modes both from the rectangular bar, offering novel insights for nanophotonic and photonic device design. Further, our systematic exploration of BIC formation mechanisms and their sensing properties by breaking structural symmetries and changing environmental refractive indices has shed light on the underlying physics. This research not only consolidates a robust theoretical framework for understanding BIC behavior but also paves the way for high-quality factor resonator and sensor development, as well as the precise control of photonic states. The findings significantly deepen our understanding of these phenomena and hold substantial promise for future photonic applications.

An all-dielectric chiral metasurface with circular dichroism and asymmetric transmission characteristics

An all-dielectric chiral metasurface with circular dichroism and asymmetric transmission characteristics

Metamaterial-induced-transparency engineering through quasi-bound states in the continuum by using dielectric cross-shaped trimers

This study presents a novel approach to activate a narrowband transparency line within a reflecting broadband window in all-dielectric metasurfaces, in analogy to the electromagnetically-induced transparency effect, by means of a quasi-bound state in the continuum (qBIC). We demonstrate that the resonance overlapping of a bright mode and a qBIC-based nearly-dark mode with distinct Q-factor can be fully governed by a silicon trimer-based unit cell with broken-inversion-symmetry cross shape, thus providing the required response under normal incidence of a linearly-polarized light. Our analysis that is derived from the far-field multipolar decomposition and near-field electromagnetic distributions uncovers the main contributions of different multipoles on the qBIC resonance, with governing magnetic dipole and electric quadrupole terms supplied by distinct parts of the dielectric “molecule”. The findings extracted from this research open up new avenues for the development of polarization-dependent technologies, with particular interest in its capabilities for sensing and biosensing.

High Q-factor Fano resonances based on an all-dielectric metasurface for high-performance refractive index sensing

High Q-factor Fano resonances based on an all-dielectric metasurface for high-performance refractive index sensing

All-dielectric meta-surface transmission-mode ultra-thin GaAs negative electron affinity photocathode

Transmission-mode (t-mode) GaAs negative electron affinity photocathodes (NEA-PCs) can be integrated with the optical focusing lenses and microchannel plates to produce high-quality electron beams and high-sensitive detectors. Quantum efficiency (QE) of ∼40% has been reported for the t-mode thick (>1000 nm) GaAs NEA-PCs. Nevertheless, practical applications of these devices have been seriously restricted by their long response time (tens of picoseconds). In this work, the all-dielectric meta-surfaces (ADMS) were designed as the light managers for the t-mode ultra-thin GaAs NEA-PCs. For the 500–850 nm waveband, high light absorption (>80%) can be obtained through coupling the electromagnetic dipole moments of ADMS into the leaky optical modes in 100 nm ultra-thin GaAs NEA-PC layer, which leads to enhanced QE higher than that of the thick ones, the response time less than 5 ps, and the mean transverse energy less than 60 meV, respectively. Given these properties, ADMS t-model ultra-thin NEA-PCs represent a promising photocathode to provide the high-brightness short-pulse spin-polarized electron beams and high-sensitive fast-response detectors for the electron accelerator and low-light-level photodetection applications, respectively.

Active tuning of near-infrared interaction of single-walled carbon nanotubes with quasibound states in the continuum in all-dielectric metasurfaces

Active tuning of near-infrared interaction of single-walled carbon nanotubes with quasibound states in the continuum in all-dielectric metasurfaces

Double-parameter analysis of an asymmetric herringbone temperature and refractive index sensor based on all-dielectric metasurface.

The all-dielectric metasurface is a tremendously efficacious path to seek out planar optical manipulators. The application of extremely sensitive optical sensors is expected to benefit from the Fano resonances created in all-dielectric metasurface. An optical sensor basaed on the all-dielectric hollow herringbone metasurface is tuned for high-sensitivity temperature sensing and refractive index sensing. In the continuous near-infrared band, two resonance responses activated by magnetic toroidal dipole and magnetic quadrupole can be generated simultaneously. According to the simulation results, a superior properties refractive index sensor holding a Q factor as high as 2.6 × 104 is achieved, its maximum FOM of 3980 RIU-1 is displayed, and its sensitivity is 232 nm/RIU. And sensitivity of the temperature sensor is proved to be 63 pm/K, which shows a prominent improvement in temperature sensing. After analyzing it in the experiment, it is found that the Q factor is 5366 and FOM of 465 RIU-1, with the sensitivity of 178 nm/RIU. This refractive sensor provides a favorable groundwork for developing high-sensitivity sensing devices in many biochemical disciplines, which also increases the extensive application possibilities for biochemical analysis and environmental detection.

Broadband high-efficiency meta-holography from all-dielectric quasi-continuous metasurfaces

The compactness and particular optical design make metasurface a competitive candidate for holographic display and storage. Recently, the selection and optimization for the used metasurface structures and types have become research spots. Now the most researched and demonstrated meta-holograms are often based on discrete structures, which can achieve high efficiency but comparatively narrow working bandwidths or a wide wavelength range but low power efficiency. Therefore, contemporary meta-holograms struggle for realizing simultaneous broadband and high efficiency. In this paper, all-dielectric quasi-continuous metasurfaces composed of nanostrips are introduced to expand the operating bandwidth for high efficiency meta-holography. Benefiting from the associated Pancharatnam–Berry phase, the nanostrips with spatially orientation angles continuous changes can realize arbitrary phase modulation. For the first time, the average power efficiency of a meta-hologram is experimentally measured to be 56.63% over a broad wavelength band ranging from 500 to 1000 nm. In addition, based on this kind of all-dielectric quasi-continuous nanostrips, we also design and experimentally achieve multicolor three-dimensional (3D) holographic images. Actually, such all-dielectric quasi-continuous methodology proposed here can be used to design other functional meta-devices, including optical metalens, nanoprinting, and information encryption.

Optical switching with high-Q Fano resonance of all-dielectric metasurface governed by bound states in the continuum.

The discovery of bound states in the continuum (BIC) of optical nanostructures has garnered significant research interest and found widespread application in the field of optics, leading to an attractive approach to achieve high-Q (Quality factor) Fano resonance. Herein, an all-dielectric metasurface consisting of four gallium phosphide (Gap) cylinders on the MgF2 substrate is designed and analyzed by the finite element method (FEM). By breaking the symmetry of the plane, specifically by moving the two cylinders to one side, it is possible to achieve a transition from the symmetry-protected BIC to quasi-BIC. This transition enables the excitation of sharp dual-band Fano resonance at wavelengths of 1,045.4 nm and 1,139.6 nm, with the maximum Q factors reaching 1.47 × 104 and 1.28 × 104, respectively. The multipole decomposition and near-field distributions show that these two QBICs are dominated by the electric quadrupole (EQ) and magnetic quadrupole (MQ). Furthermore, bidirectional optical switching can be accomplished by changing the polarization direction of the incident light. As a result, the maximum sensitivity and figure of merit (FOM) are 488.9 nm/RIU and 2.51 × 105 RIU-1, respectively. The results enrich our knowledge about BIC and reveal a platform for the development of high-performance photonics devices such as optical switches and sensors.

Generation of dual vortices with controlled topological charges based on spin-decoupled moiré metalens.

By their powerful talent in manipulating optical parameters, metasurfaces demonstrate great ability in the generation of the vortex beams. Until now, vortex beam generators constructed by metasurfaces mostly lack tunability, which reduces the scope of their applications. Here, spin-decoupled moiré metalenses composed of two cascaded all-dielectric metasurfaces are designed. Utilizing mathematical derivation and numerical simulation, dual vortices with variable topological charge can be generated under the incidence of orthogonal circularly polarized light by tuning the mutual rotation between the two cascaded metasurfaces. Meanwhile, vector vortex beams can be produced by superposition of dual focused vortices under the linearly polarized light illumination and whose vector polarized states can also be manipulated by mutual rotation. This work provides a flexible design strategy for continuous manipulation of singular beams, which have potential applications in optical communication, microparticle manipulation, and super-resolution imaging.