Gradient Metasurface Research Articles

Wide-field large-angle beam splitters based on polarization-insensitive coding metasurfaces

Metasurfaces have been used to make various optical devices such as beam splitters because of their excellent capability to control light. The most recent work on metasurface beam splitters focused on realizing one-dimensional beam splitting. Based on generalized Snell’s law, we designed the beam splitters using a coding strategy by phase gradient metasurfaces, which can divide vertically incident light into two-dimensional space. Meanwhile, the beam splitters are polarization-insensitive because highly rotationally symmetric nanorods are used as structure units. Using different code groups, especially applying 0 and π binary phases, the proposed beam splitters have various functions such as beam deflection, two-beam splitting, and multi-beam splitting. The flexible design of the coding maps allows the light transmission to cover a full-view field. The maximum splitting angles in two-beam and multi-beam splitters are 35.7° and 28.3°, respectively. All the designed beam splitters have a power efficiency of over 80%. The beam splitters have the advantages of small size, easy integration, large beam splitting angle, wide beam splitting area, and high efficiency. They could be applied to many optical systems, such as multiplexers and interferometers in integrated optical circuits.

High-efficiency terahertz surface plasmon metacoupler empowered by bilayer bright–dark mode coupling

Conversion from free-space waves to surface plasmons has been well studied as a key aspect of plasmonics. In particular, efficient coupling and propagation of surface plasmons via phase gradient metasurfaces are of great current research interest. Hereby, we demonstrate a terahertz metacoupler based on a bilayer bright–dark mode coupling structure attaining near-perfect conversion efficiency (exceeding 95%) without considering absorption loss of the materials and maintaining a high conversion level even when the area of the excitation region changes. To validate our design, a fabricated metacoupler was assessed by scanning near-field terahertz microscopy. Our findings could pave the way for developing high-performance plasmonic devices encompassing ultra-thin and compact functional devices for a diverse range of applications, especially within the realm of high-speed terahertz communications.

High-Efficiency Microwave Wireless Power Transmission via Reflective Phase Gradient Metasurfaces and Surface Wave Aggregation.

Microwave Wireless Power Transfer (MWPT) technology is crucial for emergency power supply during natural disasters and powering off-grid equipment. Traditional antenna arrays, however, suffer from low energy capture efficiency, difficult impedance matching, complex synthetic networks, and intricate manufacturing processes. This paper introduces a microwave energy receiver design utilizing Reflective Phase Gradient Metasurfaces (R-PGMs) and surface wave energy convergence technology. The design leverages the effective plane wave-to-surface wave conversion capability of R-PGMs to transform incident microwave energy into a surface wave mode, which is then efficiently harvested using a circular energy convergence array before being output to a coupling port. By optimizing R-PGM parameters, an ideal 60° phase gradient distribution is achieved, facilitating the focus of surface wave energy via dispersion characteristics. These components are integrated into a hybrid antenna array, complemented by a matched energy output port structure. Numerical simulations show that this array can efficiently convert microwave energy from plane waves to surface waves, achieving a conversion efficiency of 85.32% and a collection efficiency of 68.26%. Experimental results corroborate these findings, with peak energy collection efficiency reaching 64.68% at 5.8 GHz and an RF-DC conversion efficiency of 42%, confirming the design's efficacy. Compared to conventional methods, this design simplifies the system by avoiding complex combining networks and significantly enhances the efficiency of microwave MWPT.

Origami multifunctional metagrating for a mechanically controlled electromagnetic wavefront

Metasurface or artificial electromagnetic (EM) structures offer viable options for manipulating EM waves with compact periodic structures. Tunable metasurfaces are ideal for many engineering and scientific applications because they can manipulate EM wavefronts. However, it is challenging to implement tunable metasurfaces that can achieve multiple functions for integration. In addition, integrated multifunctional metasurfaces are often structurally complicated and bulky. This paper proposes a simple spatial modulation mechanism that uses Miura origami reconfigurable one-dimensional metagrating to achieve multiple control of EM wavefront. Theoretical predictions, numerical simulations, and experiments confirm and validate the basic concepts. In particular, the continuous geometric deformation of the Miura-ori lattice is a promising method for compensating the dispersion of characters in gradient metasurfaces. Considering origami structures’ outstanding mechanical properties and strong deformation abilities, this discovery opens up another avenue for lightweight and deployable meta-devices.

Gradient metasurface covered with water-based channels for reconfigurable RCS reduction

A gradient metasurface covered with water-based channels (GMWC) is proposed for RCS (radar cross section) reduction reconfiguration. The amplitude and phase response of the metasurface unit covered by the water-based channel (WC) is altered, which results in a change in the monostatic scattering of the GMWC. Under the normal incidence of X-polarized plane waves, the RCS reduction of GMWC shows different levels when different water channels are activated. The simulated results show that the average RCS reduction of GMWC increases from 5 dB to 20 dB in 5 dB steps in the 6 GHz to 8.2 GHz frequency band under four different reconstruction states (10000;01000;00100;00001). Besides, GMWC has an average RCS reduction of 10 dB within UWB (4–18 GHz) under state 10000. Finally, GMWC is fabricated and measured, and the measured results are in agreement with the simulated results. The RCS reduction reconfiguration characteristic of GMWC has potential application in target camouflage.

Rectangular Slot based Beam Focusing Dual-Band Phase Gradient Metasurface

This study proposes method for enhancing the performance of previously developed or produced weapon systems without developing new detection equipment by using Phase Gradient Metasurfaces(PGMS). Metasurface has been studied and utilized a lot to improve the detection performance of inorganic systems, but its utilization is low due to its narrow bandwidth and complex structure. To address some of these limitations, dual-band power focusing PGMS is proposed. Its frequency independent unit cell characteristic enables creating metasurface that concentrates signals in dual-band. The designed PGMS was validated through near-field and far-field measurement, confirming signal concentration at the specified focal length and improved gain in the dual bands.

Phase gradient metasurface for arbitrary three-dimensional shaped near-field

This paper proposes a methodology for generating arbitrary three-dimensional (3D) shaped near-field using phase gradient metasurface (PGMS) based on phase conjugation (PC) backtracking and shaped diffraction-free Bessel beam array techniques. The PGMS is designed by combining MATLAB and electromagnetic (EM) simulation software (CST). To the knowledge of the authors, for the first time, a 3D-shaped near-field is generated without increasing cost, structural and calculating complexity, and a methodology for generating arbitrarily shaped diffraction-free beam array is developed by MATLAB. To verify the technique, a planar PGMS is designed for dual x-polarized 3D-shaped near-fields: qq image “” in direction (θ1 = 5°, φ1 = 0°) and a flower image “” in direction (θ2 = 10°, φ2 = 180°). The advantages of the 3D-shaped near-fields are as follows: high conversion efficiency 30.5% (measured), wideband 60.5%, and low sidelobe -15 dB. The proposed technique is confirmed by the calculated, simulated, and measured results.

Parallel Beam Splitting Based on Gradient Metasurface: from Classical to Quantum

Parallel Beam Splitting Based on Gradient Metasurface: from Classical to Quantum

Excitonic van der Waals Metasurfaces for Resonant Wavefront Shaping at Deep Subwavelength Thickness Scale.

Dielectric phase gradient metasurfaces have emerged as promising candidates to shrink bulky optical elements to subwavelength thickness scale based on dielectric meta-atoms. These meta-atoms strongly interact with light, thus offering excellent phase manipulation of incident light. However, to fulfill 2π phase control using meta-atoms, the metasurface thickness, to date, is limited to the order of 102 nm. Here, we present the thickness scaling down of phase gradient metasurfaces to <λ/20 by using excitonic van der Waals metasurfaces. High-refractive-index enabled by exciton resonances and symmetry-breaking nanostructures in the patterned layered tungsten disulfide (WS2) corporately enable quasibound states in the continuum in WS2 metasurfaces, which consequently yield complete phase regulation of 2π with the thickness down to 35 nm. To illustrate the concept, we have experimentally demonstrated beam steering, focusing, and holographic display using WS2 metasurfaces. We envision our results unveiling new venues for ultimate thin phase gradient metasurfaces.

A high-efficiency hybrid microwave power receiving metasurface array with dual matching of surface impedance and phase gradient

This paper proposes a hybrid microwave power receiving (MPR) metasurface array with efficient dual matching of surface impedance and phase gradient. The hybrid array comprises three components: a reflective phase gradient metasurface (R-PGM) array, a surface wave focusing array, and an energy harvesting port. The R-PGMs efficiently convert incident electromagnetic waves into surface waves. The surface wave focusing array then concentrates the energy onto the integrated harvesting port through dual matching of surface impedance and phase gradient. Connecting a rectifier circuit enables efficient microwave energy reception and RF-DC conversion, avoiding the need for multiple rectifiers or complex feeding networks in traditional MPR designs. Numerical analysis and experimental tests verify the superior performance of this hybrid array design in efficient microwave energy harvesting and RF-DC conversion, achieving 90.84% plane wave-to-surface wave conversion efficiency, 76.67% surface wave energy harvesting efficiency, and 49.28% overall RF-DC conversion efficiency. Across the wideband range of 5.6–6.0 GHz, the energy harvesting efficiency remains consistently high, demonstrating the superior characteristics and promising potential of this design in MPR device development.

Focusing beam splitters based on gradient metasurfaces in the visible

Beam splitters are ubiquitous in optical measurements, communications, and biomedicine. To miniaturize optical systems, the integration of optical functions is essential. Metasurfaces offer a novel method for designing optical elements to control the phase, amplitude, and polarization of light. This study presents a focusing beam splitter that can simultaneously bifurcate and focus a light beam in the visible regime. The focusing beam splitter has high tolerance to various wavelengths from 400 nm to 700 nm. At the design wavelength of 532 nm, the focusing efficiency is 81.2% and the splitting angle is 42.8°. The half-angle between the bifurcated beams can be adjusted from 28.2° to 66.3° by varying the unit cell sizes from 300 nm to 600 nm. Simultaneously, we design three-channel focusing beam splitters and freely adjust the energy ratio of the three beams by changing the phase gradients. And the three-channel beam splitter has high wavelength tolerance over the bandwidth of 100 nm.

Flat optics of nonuniform phase gradient metasurfaces

Flat optics of uniform phase gradient metasurfaces based on the generalized Snell’s law has been extensively studied. The optics of nonuniform phase gradient metasurfaces (NPGMs) is less clear. Here based on Huygens’ principle and finite-difference time-domain (FDTD) simulation, we explore the optical properties of NPGMs made of nanorods (meta-atoms), which can be tuned by modulation of propagation phase and geometric phase. It is found that the nonuniformity of phase gradient may lead to multichannel anomalous reflection/refraction. The multiple beam splitters with arbitrary intensity ratio can be achieved by using the amplitude/phase modulation. Based on our design principle, we can obtain different anomalous reflection patterns (with channels ±1, ±2, both ±1 and ±2, or no reflection at anomalous angle) by different arrangement of just two types of meta-atoms. In addition, we are able to achieve chiral anomalous reflections for designed NPGMs made of nanorods and L-shaped nanoparticles. Our formulism provides general design guidance for NPGMs and can be used to realize the beam splitting function with adjustable angle and intensity ratio.

Ultrasensitive refractometric sensing via centimeter-scale metasurfaces with spatially gradient geometry generated by elastomer mechanical stretching.

Plasmonic metasurfaces with gradient geometry are emerging two-dimensional optical elements with unique capabilities, such as manipulating light by imparting local, space-variant phase changes to an incident electromagnetic wave, eliminating the chromatic aberration. However, the costly and time-consuming fabrication process and the requirement of sophisticated optical characterization instruments restrict practical applications of plasmonic metasurfaces. Herein, we present a novel nanofabrication method to generate centimeter-scale metasurface with spatially gradient geometry over the whole metasurface by directional stretching of a trapezoid elastic carrier patterned with regular metallic nanostructures. This strategy eases the requirement of time-consuming and expensive lithographic techniques in conventional methods. The spatially gradient plasmonic metasurface exhibit variable transmittances under monochromatic and polar light illumination, resulting in grayscale patterns with different transmittance intensity distributions. An ultrahigh imaging-based sensitivity of 1495 pixel/refractive index unit (RIU) and a detection limit of 0.00068 RIU can be achieved based on the spatially gradient plasmonic metasurface, which is superior to the performance of the regular metasurface before stretching. This novel strategy is expected to be promising for fabrication of gradient metasurfaces to be employed in many fields of nanophotonics.

Miura Origami‐Inspired Reconfigurable Phase Gradient Metasurface for Linearly and Circularly Polarized Differential Modulation

Miura origami's reconfigurable characteristic and structural asymmetry, combined with electromagnetic (EM) wave manipulation, crashed a unique spark. However, the complexity of the three‐dimensional (3D) origami structure after folding makes it challenging to study the phase regulation mechanism. Here, we propose a reconfigurable phase gradient metasurface based on Miura origami and derive the underlying mechanism of phase modulation in detail under linearly and circularly polarized (LP and CP) incidence. We adopt the one‐dimensional (1D) gradient design along the x direction to verify the idea. The phase calculation formulas are given under LP and CP incidence through the Jones matrix's derivation. The beam deflector angles corresponding to LP and CP waves are identical in the planar state. As the folding angle increases, the phase evolution rules corresponding to the LP and CP waves are discrepant, leading to differential beam steering. Finally, the origami sample is processed for verification, and the experimental data are consistent with the simulation and theoretically calculated values. We believe this work can help analyze the EM behavior of complex 3D origami structures and lay a foundation for designing a multifunctional EM origami metasurface.

Frequency-doubling perfect negative reflection in phase gradient metasurfaces

Phase gradient metasurfaces (PGMs) have demonstrated powerful capacities for manipulating light waves freely. However, PGMs have limitations of bulky size and narrow bandwidth that hinder further applications. In this work, we present the design and analysis of a reflection-type PGM by utilizing the phase choice freedom in the supercell. It is found that in this well-designed PGM, perfect negative reflection including perfect retro-reflection can be observed not only at the initially designed frequency f1 = f0, but also at its double frequency, i.e., f2 = 2f0, which is named as the frequency-doubling perfect negative reflection. As a proof of concept, we design and fabricate a reflection-type PGM with 4π phase coverage in a supercell, with experimental results in perfect agreement with the theoretical ones. Our work offers an approach to design planar optical devices at multiple operating frequencies, holding potential applications for frequency division multiplexing communication, remote sensor technology, laser tracking, and more.

Broadband asymmetric acoustic vortex generator based on integrative meta-atoms

The significance of asymmetric acoustic vortices primarily derives from their distinctive influence on acoustic vortex applications, such as acoustic communication receiving and sending isolation. However, the existing asymmetric acoustic vortices suffer from strong narrow-band confinement. In this paper, we eliminate this constraint by proposing a novel asymmetric scheme based on integrative meta-atoms (IMs). The IM is assembled by a phase gradient metasurface (PGM) and a mode selection structure (MSS), which serve the mode conversion and selection functions respectively. The PGM is constructed by a coiling-up space, converting the incident plane wave mode into a first-order vortex mode. We introduce three mirror scatterers in a cylindrical waveguide to form the MSS, allowing the first-order vortex mode to pass but blocking the plane wave mode in the mode band gap. The mode selection mechanism of MSS is revealed by employing the dispersion relation of the unit cell and the mode transmission performance of the MSS. By coupling the acoustic modulation effects of PGM and MSS, the asymmetric generation for the first-order vortex mode is realized, and further demonstrated numerically and experimentally. Significantly, the designed IMs exhibit commendable performance in both broadband and mode purity. In detail, a relative bandwidth of 25.3 % and a purity metric of 0.88 can be achieved. Our study may offer an alternative approach to producing asymmetric acoustic vortex devices, paving the way for a myriad of advanced applications.

Mirror Symmetry Broken of Sound Vortex Transmission in a Single Passive Metasurface via Phase Coupling.

Asymmetric transmission in a passive vortex system is highly desirable, as it enables the development of compact vortex-based devices. However, breaking the mirror symmetry of transmission via a single metasurface poses challenges due to the inherent symmetric transmission properties in reciprocity. Here, we theoretically propose and experimentally demonstrate a novel transmission-reflection phase coupling mechanism to achieve the broken mirror symmetry of sound vortex transmission. This mechanism establishes a special coupling link between transmission and reflection waves, superimposing asymmetric reflection phases on the transmission phases. By utilizing a single passive phase gradient metasurface with asymmetric reflection phase twists, distinct transmission phase twists for mirror-symmetric incident vortices can be achieved within a cylindrical waveguide. This is typically difficult to imple-ment in a reciprocal system. Numerical and experimental results both demonstrate the broken mirror symmetry of vortex transmission and reflection. Our findings offer a new strategy for controlling vortex wave propagation, which may inspire new directional applications and extend to the field of photonics.

OAM vortex wave interaction with phase modulated metasurface

Phase modulated metasurface (PMM) can control the transmission state of electromagnetic (EM) wave through phase modulation scheme on the interface. Orbital angular momentum (OAM) vortex wave is denoted by the helical phase distribution of the wave front. The interaction between OAM vortex wave and PMM is investigated in this paper. The mathematical model is firstly established according to the array theory. Whereafter, two typical PMMs of chessboard metasurface (CBM) and phase gradient metasurface (PGM) are exploited as examples to uncover the scattering characteristics under illumination of OAM vortex wave. For CBM, the phase cancellation scheme is found to be broken when the OAM order l equals to ±2 under both normal incidence and oblique incidence. It reveals that the OAM vortex wave is a promising approach for metasurface stealth target detection. For PGM, the scattered wave still keeps the OAM feature but is deflected to the non-specular direction, which reveals that the generalized Snell’s law is also suitable to OAM vortex wave. The discoveries of this paper may find applications in radar detection fields using OAM vortex wave.

A restrictive design strategy of diffuse metasurfaces based on fibonacci sequences for RCS reduction

The rapid and efficient design strategy represents a highly intriguing research domain within the realm of metasurfaces. It necessitates minimizing the data load in coding metasurface design, facilitating the development of a precise approach to designing metasurfaces with specific characteristics. In this paper, we proposed a restrictive design strategy of metasurface for coding phase distributions based on the Fibonacci sequences, which is efficient to optimize designated radar cross section (RCS) reduction according to application requirements. Through the given restrictive strategy, the encoding range of the metasurfaces is reduced from the entire metasurfaces to the initial phase units in the corners, greatly reducing the total amount of data in the optimization database. In order to verify the availability of the strategy, RCS reduction metasurfaces with variable phase sequences in microwave region are analyzed. For broadband and wide-angle RCS reduction, the optimized Fibonacci discrete phase gradient (FDPG) metasurface is obtained by adjusting the phase sequences. Both the simulation and measured results show that the optimized FDPG metasurfaces by the proposed designing strategy have the RCS reduction of −10 dB in the frequency ranges of 6 GHz to 18 GHz under wide incidence angles from 0° to 30°. The simulated and measured results are good consistent over the whole frequency ranges and angle regions. This method combines mathematical sequences with physics organically, making it an important field in metasurfaces research and it is expected to play an important role in future studies.

Broadband low-scattering and high-efficiency transmission radome by combining phase gradient metasurface and FSS

In this paper, a novel method to achieve high-efficiency transmission radome with broadband low radar cross section (RCS) is proposed by combining phase gradient metasurfaces (PGMs) and frequency selective surface (FSS). The PGMs-FSS is an ultra-thin single-layered structure composed of a modified Jerusalem resonator backed with an FSS. The FSS works as a high-pass filter for transmitting the in-band signals and reflecting the out-of-band signals, and the PGMs works for reducing the RCS. The insertion loss is less than −1 dB within the passband of 15.95 GHz–17.7 GHz. Moreover, the particle swarm optimization (PSO) algorithm is used to extend the RCS reduction bandwidth as well as in-band high transmission. A prototype with a thickness of 0.08 λl (λl is the wavelength of the low frequency) is designed, fabricated, and measured. The results demonstrate that an over 10 dB RCS reduction is obtained from 7.95 GHz to 18.12 GHz, i.e., the relative bandwidth reaches 78.02%. The proposed method of combining PGM-FSS can be used in designing radomes of stealth platforms for the requirements of broadband RCS reduction as well as high-efficiency signal transmission.