Efficient Metasurface Research Articles

Wideband Radar-Cross-Section Reduction Using Parabolic Phased Metasurfaces

This article presents a fast and efficient metasurface design approach without using time-consuming optimization algorithms, for wide-angle low scattering applications and wideband radar-cross-section (RCS) reduction. Using ray-tracing theory and by engineering the proposed metasurfaces to exhibit a diffusive parabolic phase distribution across its aperture at frequencies other than the center frequency ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f =</i> 18 GHz), significantly diffused backscattering is guaranteed by redirecting the reflected energies over all directions over a wide range of frequencies. Based on the proposed approach, the scattering behavior of metasurfaces with parabolic phase distributions from 1 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> to 20 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> were carefully investigated. Numerical and experimental results both demonstrated that a metasurface of phase profile at 5 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> is extremely powerful in distributing the scattering energy more uniformly than a metasurface of 1 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> phase profile. The designed metasurfaces using the proposed approach achieved more than 10-dB monostatic/bistatic RCS reduction over a wideband frequency range from 12 to 24 GHz (66.7%) and even for off-normal incidence up to 60°. The proposed approach can overcome the inherent challenges of ensuring wide scattering angle over wideband frequencies of conventional chessboard and coding metasurfaces in the literature.

GaAs Linear Polarizer with a High Extinction Ratio for Extended Short-Wave Infrared Detection

Metasurfaces have shown an unprecedented ability to modulate electromagnetic waves at subwavelength scales, especially polarized optical metasurfaces, applied for imaging, navigation and detection. In this work, a kind of efficient all-dielectric diatomic metasurface for polarization and phase changing, consisting of a pair of GaAs nanopillar and nanocube, is proposed. By adjusting the unit cell structural parameters, the polarization state can be controlled and adjusted at the short-wave infrared (SWIR) band (1~3 μm). At the wavelength of 2125 nm, the maximum transmission efficiency, the extinction ratio and the linear polarization degree can reach 93.76%, 40.99 dB and 0.99, respectively. Overall, this all-dielectric diatomic metasurface has broad application potential in extended SWIR polarization detection.

Efficient digital metasurfaces for full-space manipulation of acoustic waves with low crosstalk between reflection and transmission

Metasurfaces, as a class of emergent platforms, have shown enormous potentials in controlling classical waves. However, achieving simultaneous and dynamic manipulations of reflected and transmitted waves in a low-crosstalk and high-efficiency manner via a single metasurface still remains huge challenges in fact, especially for acoustic waves that lack of available vector degrees of freedom. Here, we propose and experimentally demonstrate an acoustic full-space digital metasurface consisting of arrays of dual-channel elements with variable coding states, and the reflected and transmitted sound waves can be controlled independently and dynamically. Benefitting from the dual-frequency design and the mode suppression effect, the presented element not only attains high efficiency (up to 80% in transmission mode, and nearly 100% in reflection mode), but also enables decoupled modulations of the reflection and transmission coefficients. To illustrate the presented acoustic metasurface, tunable beam deflection and alterable wave focusing are experimentally validated without inter-modal crosstalk on both sides of the platform, respectively. This work opens an avenue to control sound waves in multiple dimensions, which may further promote the development of miniaturized acoustic devices with high integration and versatility.

Uniform Huygens Metasurfaces with Postfabrication Phase Pattern Recording Functionality

With the rapid progress in the field of metasurfaces and their use in miniature integrated devices arises the quest for cheap mass production of efficient metasurfaces. We suggest a novel way to design and fabricate phase-gradient Huygens metasurfaces using laser-annealing of uniform particles made of As2S3 chalcogenide glass. We show that a phase gradient metasurface can be realized by tuning the refractive index of otherwise identical meta-atoms instead of tuning their geometry. We are using an array of identical As2S3 particles with the possibility to locally change their refractive index using a short-wavelength illumination (green laser) in order to tune the phase pattern at the postfabrication stage. Metasurfaces fabricated with this method can be used for operation in the red or IR spectral range. We fabricate uniform As2S3 Huygens metasurfaces using electron beam lithography and demonstrate their postfabrication tuning with exposure of comparatively low intensity. Sample characterizations with transmittance measurements and quantitative phase microscopy provide results in good correspondence with numerical predictions confirming postfabrication spectral tuning. Using such tuning, we demonstrate the possibility of transferring the intensity pattern produced by modifying a writing beam with a spatial light modulator to a phase pattern recorded on a uniform As2S3 metasurface. Our method has potential advantages for the low-cost production of large-scale metasurfaces because uniform geometries are better adjusted for mass manufacturing.

Design and experiment of terahertz unidirectional transmission structure based on neural network

In metasurface design, massive meta-atoms have to be optimized to produce the desired properties, which are time-consuming and sometimes prohibitive. In this paper, neural network model composed forward modeling and inverse design network is proposed to realize rapid, efficient, and automatic metasurface design of THz unidirectional transmission. Specifically, massive metasurface samples are generated by Python-CST co-simulation to train the neural network model and make it robust to predict the THz responses and vice versa. The consistency of simulation and prediction results imply that the model can accurately capture the nonintuitive complex relationship between structural parameters and electromagnetic spectra. To further verify the model, retrieved structural parameters for unidirectional transmission are deduced due to inverse design network and double L-shape arrays metasurface is experimental fabricated. It is shown that the experimental asymmetric transmission responses by retrieved structural parameters are identicalwith the simulations by design parameter. Such results effectively verify the design and model, providing a paradigm for functional metasurface design and the development of applications in optoelectrical fields.

Highly angle-sensitive and efficient optical metasurfaces with broken mirror symmetry

Abstract Optical metasurfaces have great potential to overcome the functional limitations of conventional optical devices. In addition to polarization- or wavelength-multiplexed metasurfaces, angle-multiplexed metasurfaces can provide new degrees of freedom, enabling previously unrealized complex functionality in diverse applications such as LiDAR, augmented reality glasses, and imaging. However, there have been fundamental trade-offs in transmission efficiency and angular sensitivity for practically important paraxial rays. In this paper, we overcome this limitation by breaking mirror symmetries of single-layer metasurface structures. Based on an effective medium theory, we intuitively explain which material parameters affect the sensitivity and efficiency and prove that high sensitivity and high efficiency can be achieved simultaneously by breaking the mirror symmetry. Based on this, we propose optimized metasurfaces for two applications: an angle-multiplexed beam-steering device with up to 93% relative efficiency and an angle-multiplexed metalens array that can break the fundamental resolution–density trade-off of microlens arrays with high efficiency. The proposed angle-selective designs could pave the way for the development of new classes of compact optical devices with novel functions.

Revisiting Optical Material Platforms for Efficient Linear and Nonlinear Dielectric Metasurfaces in the Ultraviolet, Visible, and Infrared

The high refractive indexes and low optical losses of dielectrics are preferred for designing highly efficient metasurfaces with unprecedented wavefront control such as near-unity numerical aperture metalenses and wide-angle beam spreading. Regardless of such intuitive material selections, the correlation between metasurface performance and material properties has not been clearly defined. Notwithstanding the unclear correlation, the intensity ratio of manipulated light to input beam, often called the efficiency, is an important factor for constructing various photonic applications from augmented reality to sensors. In this context, the efficiency records of the previous metasurfaces should be classified with materials aspects to understand current limitations on their efficiencies. This perspective organizes the efficiency records of metasurfaces depending on optical materials, introducing a way to engineer the optical properties of various dielectric materials to improve metasurface efficiencies. Furthermore, this perspective covers the candidates for nonlinear optical materials that can potentially be used for efficient frequency conversion.

Ultraviolet-Visible Multifunctional Vortex Metaplates by Breaking Conventional Rotational Symmetry.

Metasurfaces have shown remarkable potential to manipulate many of light's intrinsic properties, such as phase, amplitude, and polarization. Recent advancements in nanofabrication technologies and persistent efforts from the research community result in the realization of highly efficient, broadband, and multifunctional metasurfaces. Simultaneous control of these characteristics in a single-layered metasurface will be an apparent technological extension. Here, we demonstrate a broadband multifunctional metasurface platform with the unprecedented ability to independently control the phase profile for two orthogonal polarization states of incident light over dual-wavelength spectra (ultraviolet to visible). In this work, multiple single-layered metasurfaces composed of bandgap-engineered silicon nitride nanoantennas are designed, fabricated, and optically characterized to demonstrate broadband multifunctional light manipulation ability, including structured beam generation and meta-interferometer implementation. We envision the presented metasurface platform opening new avenues for broadband multifunctional applications including ultraviolet-visible spectroscopy, spatially modulated illumination microscopy, optical data storage, and information encoding.

Tunable Metasurface Based on Plasmonic Quasi Bound State in the Continuum Driven by Metallic Quantum Wells

AbstractEffectual free‐space optical metasurface is essential for telecommunication and information processing. However, the lack of efficient optical nonlinearity is still an obstacle to empower its full capability and practical applications. Metallic quantum wells (MQWs) with large nonlinear susceptibility may pave a new way. Here, such MQWs are implemented into a resonance configuration of the quasi bound state in the continuum (BIC) and an efficient tunable plasmonic metasurface is proposed. Such metasurface is composed of MQWs‐based nanostructure, which supports the tunable plasmonic quasi‐BIC. The tunability is controlled through substantial change in refractive index of MQWs induced by Kerr‐type nonlinearity, which leads to around 9 dB extinction ratio and extremely high modulation speed up to terahertz level. The quasi‐BIC with narrow linewidth is obtained through symmetry‐breaking of nanoelliptical elements, further enhancing the modulation depth. This work satisfies the ultrafast‐speed and high‐efficiency requirement of free‐space all‐optical metasurfaces, including the spatial light modulators in optical computing field.

A highly efficient deca-band metasurface absorber for diverse microwave applications

We propose a highly efficient, deca-band, and single-layered metasurface absorber (MA) based on the idea of split-ring resonators (SRRs). The unit cell of the proposed MA is a combination of 2 × 2 symmetrically arranged meta-structured designs, each comprising long and short L-shaped SRRs, two concentric semi-circular rings, and two T-shaped conducting strips. The absorption phenomenon of the proposed structure is successfully demonstrated using the standing wave resonance theory (SWRT), detailed current distribution, and the electric field distribution analysis. The thickness of the proposed MA unit cell is 0.96 mm having 1/56, 1/41, 1/35, 1/21, 1/20, 1/18, 1/17, 1/13, and 1/12 respective free-space wavelengths and has compact unit cell dimensions of 0.018λo×0.018λo, where λo corresponds to the lowest absorption frequency. The proposed structure offers absorption peaks of more than 90% at 10 distinct frequencies to demonstrate its applicability in diverse microwave applications. It is shown in simulations and experimentally that the proposed MA is angularly stable up to 70˚and has polarization insensitivity behavior up to 90˚ for both TE and TM polarized waves. A good agreement between simulation and measurement results demonstrates the efficacy of the proposed MA in a wide range of microwave applications.

High efficient and ultra-wideband polarization converter based on I-shaped metasurface for RCS reduction

In modern communication systems, polarization has always been a major issue. Traditional polarization converters have limited uses because of their enormous size and narrow bandwidth. In this paper, an ultra-wideband and high efficient polarization converting metasurface (PCMS) is designed for radar cross section (RCS) reduction. The designed PCMS consists of I-shape unit cell manifesting linearly polarized (LP) incident wave to its orthogonal equivalent. In a frequency range of 10.5 GHz to 29.5 GHz, the design PCM achieves a polarization conversion ratio (PCR) more than 90% with 95% fractional bandwidth (FBW). The designed PCMS is used in a triangular chessboard configuration to reduce the RCS. To obtain destructive interference, the proposed array consists of two-unit cells, PCMS and its mirror, with a phase difference of 180 ± 37°. To reduce RCS, the proposed PCMS unit cell is rotated at 90°, 180°, and 270° for phase cancellation. The proposed array reduced RCS in the ultra-wide frequency range of 8.2 GHz to 29.4 GHz with 113% FBW. The proposed PCMS is validated experimentally. Simulation and experimental results are found in good agreement.

Topology optimization of the azimuth-rotation-independent polarization conversion metasurface for bandwidth enhancement.

Metasurfaces offer an unprecedented opportunity for flexible manipulation of electromagnetic wave. The azimuth-rotation-independent (ARI) polarization conversion metasurface (PCM) is an ultrathin device, which could convert an arbitrary linearly-polarized incident wave to its cross-polarized state. However, the bandwidth of an ARI PCM with a high cross-polarized transmission is usually limited. Here, a topology optimization method of multi-feature points based on the differential evolution (DE) algorithm is adopted to enhance the bandwidth of the traditional ARI PCM while maintaining a high transmission and polarization conversion ratio. The simulated results of the optimized structure indicate a 2.08 times bandwidth expansion in the cross-polarization conversion compared with the original structure. In addition, the measured results are consistent with the simulated ones and the ARI characteristic is validated. The proposed method provides a promising route for efficient high-performance metasurface designs.

Characteristic Model and Efficient FDTD-SPM Algorithm for Fishnet Metasurfaces Analysis

Intelligent metasurfaces to create smart electromagnetic environments have attracted a surge of interest. The multiscale property of metasurfaces results in a huge computational challenge, further hindering their engineering development. Our previous paper has developed a fast finite difference time domain (FDTD) algorithm for patchlike metasurfaces analysis using the surface susceptibility model (SSM). This study extends that to efficient fishnetlike metasurfaces analysis using the surface porosity model (SPM), a useful complement to SSM. We introduce the SPM to FDTD, replacing brute force simulations, to accelerate fishnetlike metasurfaces analysis. At last, we conduct a series of numerical experiments, demonstrating that the proposed FDTD-SPM algorithm greatly reduces the number of computational meshing lattices and the consumed time.

Efficient metasurface for electromagnetic energy harvesting with high capture efficiency and a wide range of incident angles

This work introduces a miniaturized metasurface (MS) energy harvester operating at 5.54 GHz with a high capture efficiency and a wide incident angle. The proposed MS structure is comprised of an ensemble of electric ring resonator (ERR) array. The MS's impedance is engineered to match free space, so that the incident electromagnetic (EM) power is efficiently captured with minimal reflection and delivered to the optimal resistor load through an optimally positioned metallic-via. According to the simulation results obtained by CST Microwave Studio, the proposed MS harvester achieves higher conversion efficiency of about 91% under normal incidence and more than 78% across a wider incident angle up to 60°. In order to verify the simulation results, a 5×5 cell array of the proposed MS harvester is fabricated and tested. The simulation and experimental results are well match. The proposed MS configuration can be attractive for high-efficiency and compact wireless sensor network harvesting systems.

Highly Efficient Perfect Vortex Beams Generation Based on All-Dielectric Metasurface for Ultraviolet Light.

Featuring shorter wavelengths and high photon energy, ultraviolet (UV) light enables many exciting applications including photolithography, sensing, high-resolution imaging, and optical communication. The conventional methods of UV light manipulation through bulky optical components limit their integration in fast-growing on-chip systems. The advent of metasurfaces promised unprecedented control of electromagnetic waves from microwaves to visible spectrums. However, the availability of suitable and lossless dielectric material for the UV domain hindered the realization of highly efficient UV metasurfaces. Here, a bandgap-engineered silicon nitride (Si3N4) material is used as a best-suited candidate for all-dielectric highly efficient UV metasurfaces. To demonstrate the wavefront manipulation capability of the Si3N4 for the UV spectrum, we design and numerically simulate multiple all-dielectric metasurfaces for the perfect vortex beam generation by combing multiple phase profiles into a single device. For different numerical apertures (), it is concluded that the diffracted light from the metasurfaces with different topological charges results in an annular intensity profile with the same ring radius. It is believed that the presented Si3N4 materials and proposed design methodology for PV beam-generating metasurfaces will be applicable in various integrated optical and nanophotonic applications such as information processing, high-resolution spectroscopy, and on-chip optical communication.

Compact and efficient elastic metasurface based on mass-stiffness relation for manipulation of flexural waves

Metasurfaces are a group of lower dimensional metamaterials. They have shown excellent capacities in precise and efficient manipulation of wavefront at-will. However, most of them suffer from low transmission performances since they only consider the control of phase but ignore the impedance that determines transmission behaviours. To optimize the behaviours of metasurface, we employ the mass-stiffness relation to realize efficient manipulation of flexural waves. We use external mass blocks (pillars) to tune the effective mass and cut grooves in connecting beams to release effective stiffness. The two degrees of freedom are used to control the two design targets: local phase shift and impedance. Based on the theoretical analysis and finite element simulations, a three-unit metasurface with high transmission performances and compact geometry is designed. From the comparisons with currently existed designs, we show that our proposed metasurface is much more efficient and compact. This research may play an important role in guidance of the design of highly efficient and compact metasurfaces for precise engineering of elastic wavefront.

Design for Highly Efficient Nanometagratings and Theory of Extreme Subdiffraction Photon Control

Conventional planar metasurfaces are always much larger than the wavelength level. Previous studies on extreme wavefront control based on superoscillation suggest a fundamental tradeoff between the size and efficiency of diffraction-based metasurfaces. Herein, the theory of extreme subdiffraction photon control is proposed and the abnormal wave propagation phenomenon supporting logical design for highly efficient metasurfaces of subwavelength size is pointed out. A new class of novel metasurfaces named nanometagratings is demostrated, which is smaller than one wavelength, and manages to achieve arbitrary wavefront engineering with high conversion efficiency, enabling extreme photon control within subwavelength scale. The theoretical approach is expected to open the avenue for ultrasmall highly efficient functional metasurfaces and promises novel applications such as highly integrated on-chip quantum optical information computation.

Micro-Optical Line Generator Metalens for a Visible Wavelength Based on Octagonal Nanopillars Made of Single-Crystalline Silicon

Conventionally, silicon is less often selected as the material of dielectric metasurfaces in the visible band than other lossless materials, including titanium dioxide, silicon nitride, and gallium nitride. The reason is its relatively high extinction coefficient and resulting low transmittance. This study demonstrated that accurately designed nanopillars made of single-crystal silicon could be used satisfactorily on a metasurface, even in the visible band. Four line-focusing metasurface lenses were designed to verify the lens performance effectiveness of silicon nanopillars in the visible band. In addition, a combination of the character projection and the variable-shaped beam modes in the electron beam lithography was operated to evaluate the compatibility between mass productivity and accuracy. We successfully obtained a highly efficient line-focusing metasurface lens composed of single crystalline silicon nanopillars. The parameters of the metasurface lens at a wavelength of 532 nm were as follows: lens thickness, 300 nm; focal length, 3.91 mm; square aperture, 2 mm; numerical aperture, 0.25; measured transmittance, 38.4% to 46.8%; and measured beam spot width, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.68\,\, \mu \text{m}$ </tex-math></inline-formula> (full width at half maximum, FWHM) at the focal point. The results obtained in this study show a promising use of silicon metasurface for optical sensor applications in the visible band.

Single-Atom Trapping in a Metasurface-Lens Optical Tweezer

Optical metasurfaces of subwavelength pillars have provided new capabilities for the versatile definition of the amplitude, phase, and polarization of light. In this work, we demonstrate that an efficient dielectric metasurface lens can be used to trap and image single neutral atoms with a long working distance from the lens of 3 mm. We characterize the high-numerical-aperture optical tweezers using the trapped atoms and compare with numerical computations of the metasurface lens performance. We predict that future metasurfaces for atom trapping will be able to leverage multiple ongoing developments in metasurface design and enable multifunctional control in complex quantum information experiments with neutral-atoms arrays.

The Role of Epsilon Near Zero and Hot Electrons in Enhanced Dynamic THz Emission from Nonlinear Metasurfaces.

We study theoretically and experimentally the nonlinear THz emission from plasmonic metasurfaces and show that a thin indium-tin oxide (ITO) film significantly affects the nonlinear dynamics of the system. Specifically, the presence of the ITO film leads to 2 orders of magnitude stronger THz emission compared to a metasurface on glass. It also shows a different power law, signifying different dominant emission mechanisms. In addition, we find that the hot-electron dynamics in the system strongly modify the coupling between the plasmonic metasurface and the free electrons in the ITO at the picosecond time scale. This results in striking dynamic THz emission phenomena that were not observed to date. Specifically, we show that the generated THz pulse can be shortened in time and thus broadened in frequency with twice the bandwidth compared to previous studies and to an uncoupled system. Our findings open the door to design efficient and dynamic metasurface THz emitters.