Dielectric Metasurfaces Research Articles

Global rigorous coupled wave analysis for design of multilayer metasurface absorbers.

In this work, a novel Global NV-ETM RCWA method is proposed to accelerate the optimization of the periodic stepped radar absorbing structure. This method is based on the rigorous coupled-wave analysis (RCWA) utilizing the normal vector field (NV) and enhanced transmittance matrix (ETM) approach. The NV field dramatically improves the convergence rate for both dielectric and magnetic metasurfaces. The Global NV-ETM RCWA algorithm is developed to further accelerate the complete search calculations. Using the proposed method, the periodic stepped radar absorbing structures are efficiently optimized to realize the entire band absorption in 2-18 GHz. The optimization results demonstrate the Global NV-ETM RCWA method significantly increase the computational efficiency, with a 38-fold improvement over direct NV-ETM RCWA calculations when the truncation order N=3. This method provides a powerful tool for designing metasurface absorbers with various desired functionalities.

Dielectric Tetramer Nanoresonators Supporting Strong Superchiral Fields for Vibrational Circular Dichroism Spectroscopy.

Chirality (C) is a fundamental property of objects, in terms of symmetry. It is extremely important to sense and distinguish chiral molecules in the fields of biochemistry, science, and medicine. Vibrational circular dichroism (VCD) spectroscopy, obtained from the differential absorption of left- and right- circularly polarized light (CPL) in the infrared range, is a promising technique for enantiomeric detection and separation. However, VCD signals are typically very weak for most small molecules. Dielectric metasurfaces are an emerging platform to enhance the sensitivity of VCD spectroscopy of chiral molecules via superchiral field manipulation. Here, we demonstrate a dielectric metasurface consisting of achiral germanium (Ge) tetramer nanoresonators that provide a proper and accessible high C enhancement (CE). We realize a maximum C enhancement (CE_max) with respect to the incident CPL (CE_max = Cmax/CRCP) of more than 750. The volume-averaged C enhancement (CE_ave = Cave/CRCP) is 148 in the 50 nm thick region above the sample surface and 215 in the central region of the structure. Especially, the corresponding CE_ave values are more than 89 and 183 even when a 50 nm thick chiral lossy molecular layer is coated on the metasurface. The metasurface benefits from geometrically achiral nanostructure design to eliminate intrinsic background chiral-optical signal from the substrate, which is useful in chiral sensing, enantioselectivity, and VCD spectroscopy applications in the mid-infrared range.

Rotational photonic spin Hall effect

Abstract Multidimensional manipulation of photonic spin Hall effect (PSHE) has attracted considerable interest due to its potential in a wide variety of spin-based applications. Plenty of research efforts have been devoted to transverse or longitudinal spin-dependent splitting; however, the splitting pattern that can self-rotate in a three-dimensional (3-D) space appears to be missing in literature. In this paper, we introduce a novel 3-D rotational PSHE, which can be realized and tuned using well-designed Pancharatnam–Berry phase metasurfaces. To demonstrate this phenomenon, we first show that when a single dielectric metasurface is used, the lobe-structured spin-splitting patterns on the transverse planes rotate and evolve along the propagation path. Then, we present that under two cascaded metasurfaces, the rotation angle of the splitting patterns are tunable by adjusting the relative rotation angle between the two metasurfaces. Finally, we manifest that the lobe number of the two spin-dependent splitting patterns can be independently controlled once we introduce a dynamic phase, which produces an asymmetrical rotational PSHE. The demonstrated phenomena can be used to achieve active manipulation of spin photons in multiple dimensions, and the developed device might find potential applications in various areas, e.g., optical microscopy.

Ionic terahertz metasurface in ultrathin-layered conductive hydrogel by ultrafast laser tailoring

The ion-interactive semi-conductive hydrogel emerged as a promising optoelectronic material due to its desirable electrical conductivity, optical clarity, and switchable dielectric properties beyond incumbent inorganic materials. However, developing the actively modulated terahertz (THz) devices via hydrogel remained barely studied at an early stage. Herein, a scalable high-throughput picosecond laser system rapidly tailored hydrogel into ion-interactive dielectric metasurfaces (IDMs). The IDMs exhibited minus permittivity (ɛ < 0 F/m) not belonging to hydrogel, tunable time/frequency modulation, and high-consistency geometries for specific THz wave manipulations. The interplay between anions and the π-conjugated thiophene ring of hydrogel dynamically changed the dielectric properties and electrical conductivity through mixed electron/hole transportation, making IDMs change their microscale electric fields anomalously. Other as-prepared IDMs (split resonators, interleaved antennas, and their array) further confirmed the native capability of generating orbital angular momentum, spatially manipulating THz waves, or tuning frequencies. This work ushers in a new platform to customize and deploy non-metal THz meta devices in batches.

Electromagnetic heating-assisted metasurface for stably tunable, fast-responding chiroptics.

Herein, a graphene-dielectric metasurface with the function of stably tunable and fast responding on the chiroptics is theoretically investigated and numerically demonstrated. Via utilizing the intrinsic thermo-optical effect of the silicon, the circular dichroism (CD) peak position can be linearly scaled with a spectral sensitivity of up to 0.06 nm/K by artificially adjusting the temperature. Moreover, a perfectly adjusting manipulation with a wavelength shift of full width at half maximum for the resonant spectrum and the simultaneously maintained CD values can be realized by a slight temperature variation of ∼0.8 K. Additionally, we take a graphene layer as the heating source to actually demonstrate the ultra-fast thermal generation. Applying an input voltage of 2 V to the graphene with only 10 µs can rapidly increase the metasurface temperature of up to 550 K. Such performances hold the platform with wide applications in functional chiroptics and optoelectronics.

Multifunctional Metasurface Tuning by Liquid Crystals in Three Dimensions.

Optical metasurfaces present remarkable opportunities for manipulating wave propagation in unconventional ways, surpassing the capabilities of traditional optical devices. In this work, we introduce and demonstrate a multifunctional dynamic tuning of dielectric metasurfaces containing liquid crystals (LCs) through an effective three-dimensional (3D) control of the molecular orientation. We theoretically and experimentally study the spectral tuning of the electric and magnetic resonances of dielectric metasurfaces, which was enabled by rotating an external magnetic field in 3D. Our approach allows for the independent control of the electric and magnetic resonances of a metasurface, enabling multifunctional operation. The magnetic field tuning approach eliminates the need for the pre-alignment of LCs and is not limited by a finite set of directions in which the LC molecules can be oriented. Our results open new pathways for realizing dynamically reconfigurable metadevices and observing novel physical effects without the usual limitations imposed by the boundary conditions of LC cells and the external voltage.

Dynamic Nonlocal Dielectric Metasurfaces: Tuning Collective Lattice Resonances via Substrate–Superstrate Permittivity Contrast

Contrary to local resonances of single nanostructures, collective (or nonlocal) resonances in periodic metasurfaces, such as surface lattice resonances (SLRs), can significantly enhance light–matter interaction, leading to higher spectral selectivity. The dynamic control of such nonlocal response represents an emerging field of research. While tuning of SLRs has been demonstrated in plasmonic metasurfaces, the use of dielectric metasurfaces provides additional conditions to control both reflectance and transmittance, with minimum absorption effects. A close‐to‐homogeneous environment is usually required to guarantee the excitation of SLRs. Here, we propose theoretically and demonstrate experimentally a practical strategy for the tuning of SLRs in dielectric metasurfaces when an arbitrary index mismatch is considered between substrate and superstrate. The approach is based on a generalized lattice sum theory that accounts for the presence of a substrate. Dynamic tuning of the SLRs in silicon metasurfaces placed on a substrate is achieved with a changeable superstrate via an optofluidic process. Two tuning mechanisms are revealed corresponding to shifting and damping of the SLR, depending on the superstrate–substrate refractive index contrast. The demonstrated dynamic manipulation of transmission and reflection may be exploited in dielectric metasurfaces for tunable spectral selectivity, sensing, or novel display technologies.

Ultra-high NA optical image differentiator based on dielectric metasurfaces

Metasurface-based optical differentiators have recently attracted attention for their potential advantages in real-time and high-throughput image processing technology. However, the existing differentiators suffer from limitations in numerical aperture (NA). Here, we propose a differential metasurface based on silicon clusters covered with polydimethylsiloxane, achieving a remarkable NA of 0.7. We numerically demonstrate the proposed differentiator can perform second-order differentiation on a 1D spatial function and high NA can accurately compute smaller-scale variations. We further demonstrate its ability to achieve edge detection on 2D images. Our results advance the development of ultra-high numerical aperture optical differentiator and provide new opportunities for image processing.

Lattice constant and polarization-independent high transmission in tellurium-based dielectric metasurfaces

High-refractive-index dielectric metasurfaces give rise to unprecedented control of light manipulation, such as control of phase, polarization and amplitude, giving rise to interesting properties, such as directional beam steering, polarization detectors and sensors. Dielectric metasurfaces of sub-wavelength dimensions have tremendous applications in the field of optics, such as negative refractive index, cloaking, perfect absorbers and reflectors. The study of light–matter interactions in such materials has gained impetus due to the formation of novel states, such as anapoles, and transparent states obtained by interference between resonant electric, magnetic and higher-order modes. In this article, we investigate the light–matter interaction of an array of periodic dielectric metasurfaces made from high-refractive-index tellurium in cubic geometries and study its electromagnetic response as a function of the lattice constant, angle of incidence and angle of polarization. More specifically, we observe a non-resonant transparent state at 60.69 THz, which is independent of both the lattice constant and polarization of the input radiation. Moreover, this state shows high transmission for a broad range of incident angles with potential applications as optical filters. It also depends on the incident polarization, thus acting as a perfect polarization detector. Detailed investigations of scattering parameters, the spatial distribution of electric and magnetic fields in the near- and far-field regions and detailed multipole analysis are carried out to analyze the electromagnetic response of the metasurface.

Tunable full-space light field manipulation based on double-layer Ge2Sb2Te5 metasurfaces

Metasurfaces have the strong ability to reshape wave front and are attractive for a wide range of applications. However, metasurface based light field manipulation can be realized usually in either transmission or reflection space individually, which will limit its application scenario of metasuface. In this paper, we design a phase change material Ge2Sb2Te5 (GST) based double-layer dielectric metasurface in mid-infrared bands, which can achieve the light field manipulation in full space and dynamically control the focal intensity of reflected and transmitted light. As proof-of-concepts, two functional metasurfaces are designed and investigated: the first metasurface realizes the dynamically adjustable focus intensity in full-space under left circularly polarized (LCP) light incidence, the second metasurface is designed to provide spin-dependent bifocal in full space, which can generate custom designed two focal points under LCP or right circularly polarized (RCP) light incidence. Our proposed tunable full-space dielectric metasurfaces can be potentially applied for energy-harvesting, optical communication and optical imaging.

An All‐Silicon Metalens Integrated with a Mid‐Wave Infrared Black Phosphorus Photodiode

AbstractBlack phosphorus (BP) is a promising material for mid‐wave infrared (MWIR) photodetection because of its direct tunable bandgap and compatibility with a wide range of substrates, particularly silicon. However, optical signal collection in BP devices can be limited by its absorber thickness or active area. Dielectric metasurface lenses, which can be directly integrated into a device's substrate, are a scalable approach to increase the signal collected by BP photodetectors. This work presents the design, fabrication, and characterization of an all‐silicon metasurface lens for focusing MWIR light through its substrate. Then, a BP‐MoS2 heterojunction photodetector is integrated with the metalens, and MWIR photodetection increases at room temperature. These results demonstrate that integrated metasurface lenses are an excellent approach for boosting the performance of MWIR BP photodetectors.

Delocalized Electric Field Enhancement through Near-Infrared Quasi-BIC Modes in a Hollow Cuboid Metasurface.

The two main problems of dielectric metasurfaces for sensing and spectroscopy based on electromagnetic field enhancement are that resonances are mainly localized inside the resonator volume and that experimental Q-factors are very limited. To address these issues, a novel dielectric metasurface supporting delocalized modes based on quasi-bound states in the continuum (quasi-BICs) is proposed and theoretically demonstrated. The metasurface comprises a periodic array of silicon hollow nanocuboids patterned on a glass substrate. The resonances stem from the excitation of symmetry-protected quasi-BIC modes, which are accessed by perturbing the arrangement of the nanocuboid holes. Thanks to the variation of the unit cell with a cluster of four hollow nanocuboids, polarization-insensitive, delocalized modes with ultra-high Q-factor are produced. In addition, the demonstrated electric field enhancements are very high (103-104). This work opens new research avenues in optical sensing and advanced spectroscopy, e.g., surface-enhanced Raman spectroscopy.

Multi-Bound States in the Continuum and Lineshape Tailoring of the Multi-Layers Dielectric Metasurface

Bound states in the continuum (BICs) with high quality factors (Q-factors) by confining resonances embedded in a continuous spectrum by eliminating radiation loss can boost substantially light-matter interaction for various applications in ultrathin, active, and nonlinear metadevices. Regulating and improving the modulating freedom of the BICs are important in tailoring the spectra lineshape. Here, we suggest a way to manipulate the number of BICs in the spectra by controlling the layers and the shape or arrangement of the rectangular nanoholes in each layer of the metasurface, illuminated by a p-polarized plane wave. It is convenient to achieve symmetry protected multiple at-Γ BICs and multiple off-Γ BICs associated with the decoupling of the radiation waves and improve their regulation depth only by adjusting the layers and one single nanohole arrangement and shape in one lattice in each layer, without introducing extra geometric complexity of the metasurface. Additionally, we demonstrate the multiple BICs are robust and have a good tolerance to fabrication imperfections. Beyond the multiple BICs of both types, we also explore the huge sensitivity around the multi-BICs of the metasurface on the surrounding medium refractive index. Multiple BICs with high Q-factor realized in this multi-layer metasurface promise various practical applications including ultra-high sensitivity sensors and filters.

Chirality-selective electromagnetically induced transparency in a dielectric metasurface based on chirality transfer between bright and dark modes

Chiral metasurfaces have wide applications in chiral sensing and functional devices, such as ultrathin circular polarizers. By analytical coupled mode theory and finite-difference time domain simulation, we investigate the chiroptical properties of designed dielectric metasurface with unit cell of corner-stacked nanorods and stacked nanorings, paying attention to the bright-dark-mode coupling effects. With the help of phase modulation and mode hybridization, we can realize chirality transfer from bright modes of chiral nanorods to dark modes of achiral nanorings, which results in chirality-selective transparency due to chirality-selective excitation of binding/antibonding dark modes. Moreover, one can switch between different coupling regimes with a distinct physical effect (Fano effect vs Rabi splitting) by changing only the chirality of the incident field without varying the structure of the metasurface. Based on the mechanisms of chirality transfer and mode hybridization, our designed metasurface has achieved chirality-selective transparent window with tunable central frequency and bandwidth, which provides insight and guidance for the optoelectronic device design.

Electrically switchable anapole dielectric metasurface based on conducting polymer

Anapole state is a non-radiative resonance resulting from the interference between electric dipole and toroidal dipole moments. The active control of the anapole state plays a crucial role in the implementation of tunable photonic devices. In light of this significance, in this work, an electrically switchable anapole dielectric metasurface based on the highly conducting polymer PEDOT:SULF is proposed. The metasurface consists of a periodic array of silicon nanobrick resonant units and is supported by an ITO/SiO2 substrate covered with a PEDOT:SULF layer. PEDOT:SULF enables rapid transition between plasmonic and insulating states under an external bias, thereby enabling dynamic control the contributions of multipole mode in the dielectric metasurface, and then achieve rapid dynamic switching of the anapole state between its "on" and "off" states. This anapole metasurface provides an effective platform for controlling far-field radiation and near-field enhancement, thereby facilitating the potential applications of active micro-/nano-photonic devices in light modulators.

Terahertz narrowband filter metasurfaces based on bound states in the continuum.

The electromagnetically induced transparency (EIT) effect realized by metasurfaces have potential for narrowband filtering due to their narrow bandwidth. In optics, bound states in the continuum (BIC) can produce strong localized resonances, which means that light can be trapped and stored for long periods of time to produce very high Q-factors. This has potential applications in designing highly efficient sensors and narrow bandpass filters. Here, we present two metal-flexible dielectric metasurfaces consisting of copper structures and polyimide substrates. Quasi BICs are obtained by breaking C2 symmetry of the metal structures. Resonance-captured quasi-BICs with ultra-high q-factor resonances satisfy the dark modes required to realize the EIT and couple to the bright modes in the structure to achieve narrowband filtering. The peak transmission rates are around 0.9 at 0.29 THz-0.32 THz and 0.23 THz-0.27 THz, respectively. Our results have practical implications for the realization of low-frequency terahertz communications.

Precisely constructing hybrid nanogap arrays via wet-transfer of dielectric metasurfaces onto a plasmonic mirror.

We propose a new method for fabricating hybrid metasurfaces by combining Mie and plasmonic resonances. Our approach involves obtaining an ultrasmooth gold film and separately structuring monocrystalline silicon (c-Si) nanoantenna arrays, which are then wet-transferred and finally immobilized onto the gold film. The experimental and simulation analysis reveals the importance of the native oxide layer of Si and demonstrates fascinating dispersion curves with nanogap resonances and bound states in the continuum. The localized field enhancements in the nanogap cavities result from the coupling between multipolar Mie resonances and their mirror images in the gold film. This effective method improves our understanding of hybrid modes and offers opportunities for developing active metasurfaces, such as depositing c-Si nanoantenna arrays onto stretchable polydimethylsiloxane substrates or electro-optic and piezoelectric sensitive lithium niobate films for potential applications in MEMS, LiDAR, and beyond.

Efficient Second-Harmonic Generation from Dielectric Inter-subband Polaritonic Metasurfaces Coupled to Lattice Resonance.

Nonlinear optical metasurfaces offer a possibility to perform frequency mixing without the phase-matching constraints of bulk nonlinear crystals and with control of the local nonlinear response at a sub-wavelength scale. Nonlinear inter-subband polaritonic metasurfaces created by combining the semiconductor heterostructures with quantum-engineered inter-subband nonlinear response and electromagnetically engineered metal-clad nanoresonators offer by far the largest second-order nonlinear response of all condensed matter systems reported to date. However, the nonlinear optical response of these metasurfaces is limited by optical intensity saturation in the nanoresonator hot spots that prevented the achievement of power conversion efficiencies over 0.2% in three-wave mixing experiments. In this study, we propose and experimentally demonstrate dielectric inter-subband polaritonic metasurfaces for second-harmonic generation that achieve 0.37% power conversion efficiency. Our structure is created by a new design approach that combines dielectric resonators inducing Mie resonant modes with a lattice resonance to achieve a uniform and high field enhancement throughout the meta-atom volume.

Asymmetric transmission devices empowered by a cascaded structure of a dielectric metasurface-photonic crystal.

In this Letter, we theoretically propose an all-dielectric quasi-three-dimensional subwavelength structure constructed by a dielectric metasurface cascaded with a multilayer photonic crystal (PC) to achieve a high-performance asymmetric optical transmission (AOT). The desired optical control of the AOT is realized by combining the predetermined anomalous beam steering of a phase gradient metasurface with a unique bandgap as well as transmission characteristics of the multilayered stacked PC. The simulated results demonstrate that the proposed AOT device operating at the center wavelength of 633 nm with a circularly polarized state exhibits a high transmission of up to 62.4% with a contrast ratio exceeding 606. The excellent performance of AOT is achieved by making disassembled transverse magnetic and transverse electric polarized light under the same deflection angle concurrently match with respective high-efficient transmission bands in the multilayer PC. Furthermore, dependence of the performance of the proposed device on structural dimensions is also explored. Fortunately, the designed AOT structure is applicable to any linearly polarized light but is accompanied by double diffraction channels as compared to the circularly polarized light case. Owing to its planar configuration, passive operation, and compelling performance under various polarization states, the proposed strategy for achieving AOT paves a new road for realizing high-performance optical metadevices in compact optical systems.

Dual-wavelength dielectric metasurface for full-space light manipulations

Dielectric metasurfaces have been credited for their salient capability in manipulating visible light with high efficiency, yet their operations are usually specified to the transmission space with limited functionality at a specified single wavelength. In this work, we show that dielectric metasurface can also be considered as an effective platform to independently control transmitted and reflected light wavefronts with high efficiency at two distinct wavelengths. In specific, silicon meta-atoms allowing high transmission and reflection of orthogonally linear-polarized light at wavelengths of 690 nm and 750 nm along with independent 2π phase modulations are successfully established. The first demonstrated dual-functional metasurface can transform incident plane wave into focused point beam spot and focused vortex beam in transmission and reflection spaces, respectively. The spatial multiplexing strategy is further considered to enrich the functionality diversity of a single meta-device, on which two off-axis light focusing phenomena and focused vortex beams carrying opposite topological charges in either transmission or reflection spaces with reduced crosstalk are successfully witnessed. Lastly, the metasurface enabling broadband anomalous deflection further demonstrates that the device’s efficiency can be further improved with densely packed meta-atoms. Our study provides a new thought in light control, which can be further considered in the development of multi-wavelength and full-space meta-devices.