Multilayer Metasurfaces Research Articles

Analysis of two-dimensional planar gratings and metasurfaces in the quasi-static approximation

Currently, composite materials based on metal gratings with a period D much smaller than the wavelength l are widely used. With the use of such materials, it is possible to control the amplitude, phase and polarization of an electromagnetic wave, which opens wide possibilities for the creation of new types of antennas, radio-absorbing coatings, etc. This paper considers gratings of metallic strip or patches on the surface of a thin metal-backed dielectric slab to control the phase of the reflected wave. In the quasi-static approximation, the input impedance of such materials can be calculated using simple analytical formulas. This paper investigates the accuracy of the quasi-static approximation at different angles of incidence of the electromagnetic wave on two-dimensional gratings. Using the equivalent circuits method, analytical expressions for the input impedance of metasurfaces are obtained. The dispersion properties of composite materials “inductive or capacitive grating on the surface of a thin metal-backed dielectric slab” are investigated, and a technique for determining the resonant frequency, which corresponds to the maximum of the input impedance of the metasurface, is presented. The obtained results show that at any angle of incidence, it is possible to evaluate the condition of the quasi-static approximation applicability for two-dimensional gratings and for metasurfaces based on such gratings. If this condition is satisfied, that simple analytical expressions can be used to calculate the input impedance. The use of these expressions allows us to determine the frequency at which the input impedance reaches its maximum. It is established that for inductive and capacitive gratings the impedance boundary condition of M.A. Leontovich is equivalent to the averaged boundary condition of M.I. Kontorovich. It is obtained that for grating in free space, quasi-static approximation corresponds to the M.A. Leontovich impedance condition. The dispersion properties of composite materials “inductive or capacitive grating on a thin metal-backed dielectric slab” are investigated. It is shown that the phase transition of the reflection coefficient through zero occurs at maximum values of the input impedance. Using the considered method of equivalent circuits, the presented results can be easily generalized to any type of multilayer metasurfaces.

Experimental Realization of a One‐Directional Broadband Transmissive Cloak in Microwaves

AbstractHiding an isolated object in free space using a transmissive invisibility cloak has become a significant research area, propelled by advancements in metamaterials and transformation optics over the past decade. Despite the availability of various simplified methods for implementing transmissive cloaks, issues such as impedance mismatches and narrow working bandwidths often arise, posing challenges. Achieving a broadband transmissive cloak in free space has proven to be particularly arduous. This study presents a near‐perfect one‐directional broadband transmissive cloak constructed from multilayer metasurfaces of arbitrary shapes, showcasing superior performance across a broadband frequency range. The phase distribution of the metasurfaces and the efficacy of the transmissive cloak are assessed using the generalized Snell's law. An experimental near‐perfect broadband transmissive cloak is successfully demonstrated to operate within the frequency range of 8.5 to 11.2 GHz. This study contributes to reducing the density and mass of cloaks, thereby facilitating the expansion of cloaking capabilities in various directions and across different frequency bands.

Scalable Nanoimprint Manufacturing of Functional Multilayer Metasurface Devices

AbstractOptical metasurfaces, consisting of subwavelength‐scale meta‐atom arrays, hold great promise of overcoming the fundamental limitations of conventional optics. Due to their structural complexity, metasurfaces usually require high‐resolution yet slow and expensive fabrication processes. Here, using a metasurface polarimetric imaging device as an example, the photonic structures and the Nanoimprint lithography (NIL) processes are designed, creating two separate NIL molds over a patterning area of > 20 mm2 with designed Moiré alignment markers by electron‐beam writing, and further subsequently integrate silicon and aluminum metasurface structures on a chip. Uniquely, the silicon and aluminum metasurfaces are fabricated by using the nanolithography and 3D pattern‐transfer capabilities of NIL, respectively, achieving nanometer‐scale linewidth uniformity, sub‐200 nm translational overlay accuracy, and <0.017 rotational alignment error while significantly reducing fabrication complexity and surface roughness. The micro‐sized multilayer metasurfaces have high circular polarization extinction ratios as large as ≈20 and ≈80 in blue and red wavelengths. Further, the metasurface chip‐integrated CMOS imager demonstrates high accuracy in broad‐band, full Stokes parameter analysis in the visible wavelength ranges and single‐shot polarimetric imaging. This novel, NIL‐based, multilayered nanomanufacturing approach is applicable to the scalable production of large‐area functional structures for ultra‐compact optic, electronic, and quantum devices.

Deep-learning-assisted inverse design of polarization-multiplexed structural color filters with ultrahigh saturation based on all-dielectric metasurface

Although great progress has been made for metasurface-based structural color filters, there are still significant challenges in encoding metasurfaces with complex color images, especially in high saturation or multi-channel cases, where traditional iterative optimization calculations demand substantial computational resources. Here, we proposed a dual-channel multilayered all-dielectric metasurface that can generate structural colors accounting for 224% Standard RGB space, 168% Adobe RGB space and 78% of the 1931 CIE chromaticity space under vertically-polarized incidence. A deep learning-based designing approach is introduced to significantly reduce the computing consumption while maintaining high accuracy. Numerical simulations further demonstrated that the proposed design method is effective for encoding both complementary and independent images with high saturation and wide color gamut. Therefore, the proposed color filters may find potential applications in image display, information encryption and optical anti-counterfeiting.

Extension of effective permittivity for metallic elements on the substrates interface

Previously, the influence of substrate interface on the metallic element is always described by the specific effective permittivity. Here, the metallic element on the substrate interface can be converted into that in a homogenized equivalent substrate with almost arbitrary permittivity. Specifically, according to the definition of the permittivity, the substrate interface with a small thickness can be regarded as a homogenized equivalent substrate. Based on the tangential components of the electrical fields, it can be derived that the equivalent permittivity is strongly related to the thickness of the equivalent substrate. Finally, the simulation results demonstrate that the electromagnetic response of the metallic element placed in the homogenized equivalent substrate keeps unchanged as that on the substrate interface. Thus, the proposed method not only provides a generalized means to describe the electromagnetic influence of the substrate interface on the metallic element, but also greatly simplifies the analytical design of multi-layered metasurface.

Nonlocal Acoustic Moiré Hyperbolic Metasurfaces.

The discovery of the topological transition in twisted bilayer (tBL) materials has attracted considerable attention in nano-optics. In the analogue of acoustics, however, no such topological transition has been found due to the inherent nondirectional scalar property of acoustic pressure. In this work, by using a theory-based nonlocal anisotropic design, the in-plane acoustic pressure is transformed into a spatially distributed vector field using twisted multilayer metasurfaces. So-called "acoustic magic angle"-related acoustic phenomena occur, such as nonlocal polariton hybridization and the topological Lifshitz transition. The dispersion becomes flat at the acoustic magic angle, enabling polarized excitations to propagate in a single direction. Moreover, the acoustic topological transition (from hyperbolic to elliptic dispersion) is experimentally observed for the first time as the twist angle continuously changes. This unique characteristic facilitates low-loss tunable polariton hybridization at the subwavelength scale. A twisted trilayer acoustic metasurface is also experimentally demonstrated, and more possibilities for manipulating acoustic waves are found. These discoveries not only enrich the concepts of moiré physics and topological acoustics but also provide a complete framework of theory and methodologies for explaining the phenomena that are observed.

Broadband High-Efficiency Co-Polarized Wavefront Manipulation Based on Multi-Layered Metasurface for Terahertz Wave

Broadband High-Efficiency Co-Polarized Wavefront Manipulation Based on Multi-Layered Metasurface for Terahertz Wave

Constrained tandem neural network assisted inverse design of metasurfaces for microwave absorption.

Designing microwave absorbers with customized spectrums is an attractive topic in both scientific and engineering communities. However, due to the massive number of design parameters involved, the design process is typically time-consuming and computationally expensive. To address this challenge, machine learning has emerged as a powerful tool for optimizing design parameters. In this work, we present an analytical model for an absorber composed of a multi-layered metasurface and propose a novel inverse design method based on a constrained tandem neural network. The network can provide structural and material parameters optimized for a given absorption spectrum, without requiring professional knowledge. Furthermore, additional physical attributes, such as absorber thickness, can be optimized when soft constraints are applied. As an illustrative example, we use the neural network to design broadband microwave absorbers with a thickness close to the causality limit imposed by the Kramers-Kronig relation. Our approach provides new insights into the reverse engineering of physical devices.

Full-space and multi-polarization holograms realized by a non-Hermitian bilayer metasurface.

Multilayer metasurfaces break the mirror symmetry along the path of light propagation, thereby increasing the potential for light manipulation. Herein, a paradigm is proposed that building a non-Hermitian bilayer metasurface, which is composed of two identical, orthogonally oriented, chiral J-shaped Au structures in each layer, allows exceptional points (EPs) to exist in full-space. Specifically, in the reflected half-space that adheres to mirror symmetry, the circularly polarized eigenstates coalesce at the EP, while in the transmission half-space, where mirror symmetry is broken, the linearly polarized eigenstates converge at the EP. By considering the intrinsic property of topologically protected 2π-phase accumulation encircling both EPs, we investigated full-space holography through using circularly polarized light (in reflection half-space) and linearly polarized light (in transmission half-space).

Antireflective Multi‐Dielectric Metasurfaces Operating in the Visible

AbstractAs increasingly more demanding photonics applications are brought on‐chip, more complex design solutions are employed to deliver enhanced performance: e.g., meta‐molecules, inverse‐designed freeform structures, and multilayer metasurfaces. Instead, this study introduces anti‐reflective metasurfaces fabricated in a single nanolithography step that follows deposition of multiple dielectrics onto a substrate. Anti‐reflective metasurfaces offer fundamentally better transmission efficiencies compared to conventional equivalents. Conventional and anti‐reflective metalenses, both fabricated in the Ta2O5/SiO2 material platform for 461 nm operation, show an improvement in focusing efficiencies from ≈ 55% to ≈ 90% at low numerical apertures and from ≈ 35% to ≈ 65% at high numerical apertures. Simulations of ideal and imperfect metalenses indicate that anti‐reflective metasurfaces not only improve transmission at the design library level, but also reduce spurious diffraction and protect against higher‐order scattering.

Metasurface modulator of frequency diverse multi-mode OAM beams with equal divergence angle for coincidence imaging

Traditional multi-mode orbital angular momentum (OAM) beams suffer from low energy efficiency in detection since the divergence angle tends to increase with increasing order of the OAM mode. Thus, OAM beams with equal divergence angle are highly desirable. In this paper, a frequency-diverse multi-mode (FDMM) OAM modulator with approximately equal divergence angle is proposed. The modulator is composed of an OAM modulation metasurface and a spatial filter (SF) with narrow passband. Firstly, the OAM modulation metasurface is designed based on multiple ring-like regions with different radii in the metasurface aperture. Each mode of OAM beam is generated at a corresponding frequency point in one region. Thus, the metasurface aperture can generate different modes of OAM beams at different frequencies (i.e., FDMM OAM beams) simultaneously. Then, the SF with narrow passband is proposed through synthesizing multi-layer frequency selective metasurfaces, aiming to reduce interferences between different modes of OAM beams. Finally, the FDMM OAM modulator is realized by superposing the SF on the OAM modulation metasurface. The FDMM OAM modulator can generate six modes of OAM beams (i.e., , and modes) at 12.5 GHz, 13.0 GHz and 13.6 GHz, respectively. A multi-mode OAM coincidence imaging model based on the FDMM OAM modulator is established. The simulation and experiment results show that the proposed FDMM OAM modulator can be used for target reconstruction and the coincidence imaging performance can be improved assisted by an optimization algorithm.

Multimaterial Additively Manufactured Transmissive Spin‐Decoupled Polarization‐Maintaining Metasurfaces

AbstractTaking advantage of multimaterial additive manufacturing, this work demonstrates a kind of polarization‐maintaining metasurface (MS) for spin‐decoupled beam shaping. The wavefronts of the two spin states, i.e., the left‐hand circular polarization (LHCP) and the right‐hand circular polarization (RHCP), can be independently manipulated while the output spin state remains the same as the input. The meta‐atom is implemented with dual‐polarized antennas at the top and the bottom with phase delay lines connecting them. The dual‐polarized antenna on the receiving side is fixed to receive the LHCP/RHCP incident waves, while the dual‐polarized transmitting antenna is rotated, providing the opposite phase shifts for the LHCP and RHCP. The absolute value of the phase shift is the same as the transmitting antenna's rotation angle. Meanwhile, varying the length of phase delay lines introduces 2π phase shifts for both LHCP and RHCP inputs. Thus, combining the two phase‐shifting degrees of freedom, the LHCP and RHCP wavefronts can be independently controlled while the polarization is not flipped after transmitting through the MS. One MS produces vortex beams with different topological charges for LHCP and RHCP. The other achieves spin‐decoupled focusing. The multilayered MSs are conveniently printed using multimaterial additive manufacturing without postprocessing.

Flexible plasmonic nanoparticle-on-a-mirror metasurface-enabled substrates for high-quality surface-enhanced Raman spectroscopy detection

Surface-enhanced Raman scattering (SERS) technology with the advantages of ultra-high sensitivity, non-destructive analysis, and quick measurement for molecular detection applications is receiving increasing attention. However, traditional rigid SERS substrates face challenges in in-suit conformal detection and weak structure coupling effect for real-life applications. Here we report a flexible polydimethylsiloxane (PDMS) substrate loaded with plasmonic nanoparticle-on-a-mirror (NPOM) metasurface for SERS detection that featured outstanding sensitivity, uniformity, repeatability, and excellent mechanical flexibility. The upper multilayered NPOM metasurface can be fabricated in a single-step process by ion beam sputtering of various targets. This NPOM configuration consists of dense silver nanoparticles over a silver mirror, separated by a SiO2 spacer layer, which can realize near-total power absorption and exhibits superior SERS ability. The bottom PDMS layer for support can provide excellent mechanical properties. In the test, the as-prepared NPOM/PDMS substrates show high SERS performance in detecting crystal violet and chlorpyrifos molecules. This flexible metasurface SERS substrate promises to provide an in-suit and efficient approach for trace substance detection.

Terahertz wideband wave plates designed by stacking multilayered metasurface with different geometries

AbstractThe use of metamaterials in terahertz (THz) polarization wave plates is widespread. Metallic metamaterials, however, typically operate within a constrained bandwidth and have a large ohmic loss. All‐dielectric metamaterials could address these issues. To achieve an ultrawide‐band at THz frequencies, a multilayered silicon metasurface with elliptical or rectangular pillars has been employed. According to simulations, the achromatic relative bandwidth of the dielectric metamaterial can remain flat within a broadband with the appropriate structural parameters. Additionally, a scoring system with error analysis is added to help with choosing the geometry of terahertz polarization wave plates with various layers of metasurface. Due to its flexible polarization control capability, the proposed multilayer metasurface may open the door for arbitrary phase delays of incident waves.

High-efficiency silicon metasurface mirror on a sapphire substrate

For a possible implementation of high-efficiency Si-nanosphere metasurface mirrors functioning at telecom wavelengths in future gravitational wave detectors, exact dimensional and configuration parameters of the total system, including substrate and protective coating, have to be determined a priori. The reflectivity of such multi-layer metasurfaces with embedded Si nanoparticles and their potential limitations need to be investigated. Here we present the results on how the substrate and protective layer influence optical properties and demonstrate how dimensional and material characteristics of the structure alter light reflectivity. Additionally, we consider the impact of manufacturing imperfections, such as fluctuations of Si nanoparticle sizes and their exact placement, on the metasurface reflectivity. Finally, we demonstrate how high reflectivity of the system can be preserved under variations of the protective layer thickness, incident angle of light, and its polarization.

A universal metasurface transfer technique for heterogeneous integration.

Metasurfaces offer a versatile platform for engineering the wavefront of light using nanostructures with subwavelength dimensions and hold great promise for dramatically miniaturizing conventional optical elements due to their small footprint and broad functionality. However, metasurfaces so far have been mainly demonstrated on bulky and planar substrates that are often orders of magnitude thicker than the metasurface itself. Conventional substrates not only nullify the reduced footprint advantage of metasurfaces, but also limit their application scenarios. The bulk substrate also determines the metasurface dielectric environment, with potentially undesired optical effects that undermine the optical performance. Here we develop a universal polymer-assisted transfer technique to tackle this challenge by decoupling the substrate employed on the fabrication of metasurfaces from that used for the target application. As an example, Huygens' metasurfaces with 120 nm thickness in the visible range (532 nm) are demonstrated to be transferred onto a 100 nm thick freestanding SiNx membrane while maintaining excellent structural integrity and optical performance of diffraction-limited focusing. This transfer method not only enables the thinnest dielectric metalens to the best of our knowledge, but also opens up new opportunities in integrating cascaded and multilayer metasurfaces, as well as the heterogeneous integration with nonconventional substrates and various electronic/photonic devices.

Polarization-insensitive electromagnetically induced transparency and its sensing performance based on spoof localized surface plasmons in vanadium dioxide-based terahertz metasurfaces.

The multi-layer terahertz metasurfaces are designed to achieve polarization-insensitive electromagnetically induced transparency (EIT) effect and its sensing performance based on spoof localized surface plasmons (S-LSPs). The unit cell of the proposed metasurfaces is comprised of a metallic spiral (MS) structure, square metal frame (SMF) structure, and vanadium dioxide (VO2) layer. The EIT effect is realized by the bright-bright coupling between spoof electric localized surface plasmons (S-ELSPs) and electric dipole, which can be proved by the multipole scattering theory. The maximum value of transmission amplitude at the transparent window is 0.91, and the modulation depth can reach 51% by adjusting the conductivity of VO2. The theoretical results based on the two-particle model show excellent agreement with the simulated results. Moreover, the change of polarization angle has little effect on the EIT effect and the proposed metasurfaces show polarization-insensitive characteristics. The slow light effect of the proposed metasurfaces can also be dynamically controlled by tuning the conductivity of VO2. Due to the high Q value of the transparent window, the proposed metasurfaces exhibit excellent sensing performance, and the sensitivity is 0.172 THz RIU-1. Our study provides a method for the fabrication of EIT metasurfaces and has a broad application prospect in slow light devices, sensors, and modulators.

Thickness bound for nonlocal wide-field-of-view metalenses

Metalenses—flat lenses made with optical metasurfaces—promise to enable thinner, cheaper, and better imaging systems. Achieving a sufficient angular field of view (FOV) is crucial toward that goal and requires a tailored incident-angle-dependent response. Here, we show that there is an intrinsic trade-off between achieving a desired broad-angle response and reducing the thickness of the device. Like the memory effect in disordered media, this thickness bound originates from the Fourier transform duality between space and angle. One can write down the transmission matrix describing the desired angle-dependent response, convert it to the spatial basis where its degree of nonlocality can be quantified through a lateral spreading, and determine the minimal device thickness based on such a required lateral spreading. This approach is general. When applied to wide-FOV lenses, it predicts the minimal thickness as a function of the FOV, lens diameter, and numerical aperture. The bound is tight, as some inverse-designed multi-layer metasurfaces can approach the minimal thickness we found. This work offers guidance for the design of nonlocal metasurfaces, proposes a new framework for establishing bounds, and reveals the relation between angular diversity and spatial footprint in multi-channel systems.

Broadband stealth devices based on encoded metamaterials.

Based on the generalized Snell's law, the relationship between the phase gradient of the metasurface and the incident frequency is demonstrated, and the principle of the achromatic metasurface is developed. By adjusting the phase gradient and linear dispersion simultaneously, the function of achromatic aberration is realized, and the influence of chromatic aberration on the metasurface is reduced. We propose a metasurface stealth device with achromatic multilayer frame metasurfaces with beam deflection, steering, and collection functions so that the incident electromagnetic beam is transmitted around the stealth object without scattering. In the range of 0.45-0.9THz, the stealth function can be achieved. We have shown that the achromatic principle, design method, and stealth structure provide a guide for achieving transmissive cloaking.

Oblique incident achromatic cloaking based on all-dielectric multilayer frame metasurfaces

Oblique incident achromatic cloaking based on all-dielectric multilayer frame metasurfaces