Phase Gradient Metasurface Research Articles

Anomalous Reflection From a Phase Gradient Metasurface With Arbitrary Incident Angle

A planar phase gradient metasurface (PGM) with phase gradients in two in-plane directions will introduce two additional wave vectors to the reflected wave vector. For small incident angles, close to the normal vector, the magnitude of the reflected wave vector is smaller than that of the incident wave vector and the direction of the anomalous reflected wave can be determined from the incident wave vector components and additional phase gradient components. The expanded generalized Snell’s law, which includes diffraction order modes, is combined with array theory to accurately predict the directions of the reflected waves from a planar PGM for incident angles where the magnitude of the reflected wave vector is larger than that of the incident wave vector. The predicted directions of the reflected waves from a planar PGM are compared with simulated RCS results obtained with CST Studio Suite and measured results obtained in a compact range.

Light-driven transport of microparticles with phase-gradient metasurfaces.

Optical tweezers have opened numerous possibilities for precise control of microscopic particles for applications in life science and soft matter research and technology. However, traditional optical tweezers employ bulky conventional optics that prevents construction of compact optical manipulation systems. As an alternative, we present an ultrathin silicon-based metasurface that enables simultaneous confinement and propulsion of microparticles based on a combination of intensity and phase-gradient optical forces. The metasurface is constructed as a water-immersion line-focusing element that enables trapping and transport of 2μm particles over a wide area within a thin liquid cell. We envisage that the type of multifunctional metasurfaces reported herein will play a central role in miniaturized optical sensing, driving, and sorting of microscopic objects, such as cells or other biological entities.

Metasurface Radome to Achieve a Wideband High Gain, Low RCS, and Low SLL Antenna

A Phase- and amplitude-adjustable metasurface radome (PAMR) is proposed to achieve a wideband high gain and low radar cross-section (RCS) antenna with arbitrary sidelobe levels (AS-PGMA) at a 9.8 GHz operating frequency. The proposed PAMR is a special multilayer phase gradient metasurface lens whose amplitude distribution is modified based on the Dolph-Tschebyscheff technique. The AS-PGMA simulation results indicate that a high gain, low RCS, and low sidelobe level (SLL) antenna can be achieved over a broad impedance frequency bandwidth of 9.25–10.1 GHz with in-band average values of 17 dB, −7.5dBsm, −20 dB/−27 dB for gain, RCS, and E-/H-plane SLL, which are measured at 9.8 GHz as 17.4 dB, −9dBsm, and −23.02 dB/−27.91 dB, respectively. Additionally, a novel analytical method for calculating the RCS of the proposed PAMR and AS-PGMA is described and verified by measurement and simulation findings. Measurement results show that the E-/H-plane SLL of the AS-PGMA is improved by 12.2 dB/5.1 dB at the operating frequency, respectively, while its gain and RCS reduction are reduced by 1.6 and 2.1 dB, respectively. Moreover, RCS of the AS-PGMA is reduced over an ultrawide bandwidth of 1–20 GHz for orthogonal polarizations. The analysis, simulations, and measurements are all in good agreement.

ЦИЛИНДРИЧЕСКИЕ АНИЗОТРОПНЫЕ МЕТАПОВЕРХНОСТИ С БИГРАДИЕНТНЫМ СПИРАЛЬНЫМ КОДИРОВАНИЕМ ФАЗЫ ПАНЧАРАТНАМ-БЭРРИ И АНОМАЛЬНЫМ РАССЕЯНИЕМ

Scattering characteristics of anisotropic cylindrical phase-gradient metasurfaces (MSs) with various helical Pancharatnam-Berry phase (PB-phase) coding are studied. The efficiencies of the scattering field reduction of circularly polarized (CP) waves in the forward half-space is compared for three different MS models with the same PB-phase gradient in the azimuthal direction and different phase gradients in the axial direction. The metasurfaces are conformally located on semi-cylindrical metallic polyhedral prisms. The meta-particles are of the form of triple coupled split-ring resonators. Spiral coding of the PB-phase of MS unit cells in the azimuthal and axial directions is carried out by the rotation of the meta-particles. The effect of constant phase gradients on the frequency characteristics of backscattered fields and bistatic diagrams of scattering of CP-waves in the azimuthal and meridional planes is numerically estimated. It is shown that the axial phase gradient causes more efficient anomalous scattering of co- and cross-polarized CP-waves. It leads to the deviation of intense diffraction lobes from the normal in the meridional plane by a large angle, with a shift of the lobes in the azimuthal plane. Bigradient MSs provide the scattering field reduction of CP-waves by at least 10 dB in the band from 10-12 GHz to 18.5 GHz in almost the entire forward half-space in the azimuthal plane and in the sector of angles up to ±45° in the meridional plane.

A Near-Field Meta-Steering Antenna System With Fully Metallic Metasurfaces

In the near-field meta-steering method, the near electric-field phase distribution is dynamically altered by rotating a pair of phase-gradient metasurfaces that are typically made out of dielectric lattices or metal patterns printed on bonded dielectric substrates. As a lightweight alternate, a new class of fully metallic metasurfaces (MMs) is proposed in this article. The lack of dielectrics not only substantially reduces the cost and weight but also increases its potential in space and high-power applications. More importantly, the new MMs are designed such that they can be made by cutting narrow slots in fully planar thin metal sheets, retaining structural rigidity while reducing the complexity, cost, and weight. The same MM works for any polarization. Each cell in the new MMs has close to ideal phase shift and transmission magnitude greater than −0.9 dB. A prototype was designed and fabricated to validate this MM-based beam-steering concept in the Ku-band. The maximum measured steering range in zenith is ±42°, with a full 360° steering in the azimuth. The measured 3 dB gain bandwidth is 700–800 MHz (5.6%–6.4%), and the measured gain variation (scan loss) when steering over the whole range is 2.2–4.5 dB. The weight density and thickness of each MM are 0.88 g/cm2 and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.8\lambda _{0}$ </tex-math></inline-formula> , respectively. Including the feed, the total antenna height is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4.9\lambda _{0}$ </tex-math></inline-formula> .

Coplanar Discontinuous Transmission Line Feed for Phase Gradient Metasurface Antenna

The aim of this study is to efficiently excite the radiation of phase gradient metasurface (PGM), a novel coplanar discontinuous transmission line feed is proposed for reduction of the profile, where 12 uniform discontinuous rectangular rings form the feeding structure. The unit cells of PGM are symmetrically placed on both sides of this feed structure at the same layer, and they are close enough. The measurements agree well with those of simulations in the range of 10.48–11.18 GHz.

A phase-gradient acoustic metasurface for broadband duct noise attenuation in the presence of flow

A phase-gradient acoustic metasurface is designed based on wavefront manipulation for achieving broadband noise reduction in a flow duct. It is constructed by eight periodically arranged varying-depth units filled with porous materials, forming a linear phase shift of reflected wave from 0 to 2π at 3000Hz. The linear reflected phase-shifting can be roughly maintained in a wide frequency range. By employing the broadband phase-shift feature of the units, the metasurface exhibits an enhanced sound absorption performance in terms of the absorption coefficient and bandwidth when comparing to the individual units, which originates from forcing the higher-order modes of reflection waves to be evanescent. Then, its transmission attenuation characteristics in the presence of background flow are assessed by installing it on the side wall of a flow tube. The effects of flow speed (Mach number is up to 0.3) and sound source position (at the upstream and downstream sides of the metasurface) are experimentally and numerically investigated. Results show that the attenuation performance of the metasurface decreases with flow speed for the upstream sound source while increasing for the downstream sound source. For both upstream and downstream sound sources, a high transmission loss (≥20dB) is achieved by the metasurface in the frequency range of 1500–3000Hz at different flow speeds. Hence, the proposed metasurface is promising for achieving broadband noise attenuation in the presence of flow.

Rectangular Waveguide-Fed Surface-Wave Frequency-Scanning Antenna Utilizing Wavevector Mismatch

In this letter, we propose a design of rectangular waveguide-fed surface-wave frequency-scanning antenna utilizing the principle of wavevector mismatch. First, the transition from a side-fed rectangular waveguide mode to spoof surface plasmon polariton (SSPP) is achieved. Then the SSPP propagating on the metal-dielectric interface is modulated through wavevector mismatching that induces the SSPP to spatial radiating wave conversion. Theoretical deduction proves the frequency scanning nature of the generated radiating beam. Phase gradient metasurface (PGM) is used to realize the wavevector engineering that causes artificial wavevector mismatch. Simulated and experimental results, agreeing well with each other, showed that the antenna can realize beam scanning from −8° to 11.6° across 6.8 to 7 GHz, with realized gain varying from 5 to 5.98 dBi.

Integrated acoustic multilayer metasurfaces for high degree of diffractive functionality

Phase-gradient metasurfaces exhibit diverse extraordinary diffractive behaviors. These range from reflective or transmissive scattering of an incident wave toward any targeted direction to temporal trapping of waves inside structures. The behaviors are determined by the design parameters, phase gradient and number of discrete unit cells within a supercell. In this study, we propose a higher-level design scheme by integrating a stack of multiple metasurfaces. We focus on a case with negligible separation in which the constituent metasurfaces perfectly interact with each other. A significantly higher degree of diffractive behavior compared with that of a conventional single layer is observed for multilayer metasurfaces specifically in the regime above the critical angle. The scattering angular characteristics can be enriched remarkably in multilayers because, unlike in a single layer, the transformation of the incident tangential wavenumber is not restricted by the number of integer multiples of the phase gradient. The wavenumber transform is available for the entire range of incident angles in multilayers, whereas it is available only for half of this range in a single layer. The temporal duration of internal wave trapping can also be enhanced dramatically in multilayers because the supercell periodicity can be effectively lengthened from those of the constituents by combining different phase gradients. To our knowledge, this is the first report of the distinctive diffractive behaviors of multilayer metasurfaces, particularly for an incident angle above the critical angle. To fully describe the underlying physics, we derive a generalized diffraction law that robustly addresses the multilayers by proposing a novel physical concept to straightforwardly connect repetitive phase accumulations inside metasurface unit cells to diffraction. The reciprocal lattice of a unit cell, rather than that of a supercell, is observed to play an important role in determining dominant scattering. The proposed theories are thoroughly verified by analytical and numerical simulations.

High-Efficiency Conversion From Rectangular Waveguide Mode to Spoof SPP Using Metasurface

In this letter, we propose a method to realize rectangular waveguide mode to spoof surface plasmon polaritons (SSPP) mode conversion using phase gradient metasurface (PGM). By applying a PGM with designed wavevector to the narrow wall of the rectangular waveguide, propagation mode and direction of the guided waves can be conversed and controlled due to wavevector matching. Such system can provide an excellent transition by matching the momentum of the rectangular waveguide mode and the SSPP mode. A smooth bridge between the rectangular waveguide system and the SSPP waveguiding system can be realized with potential advantages in conformal design, super-resolution imaging or signal manipulation. Simulated and experimental results, agreeing well with each other, validated the effectiveness of rectangular waveguide mode to SSPP mode conversion using PGM from 6.96-7.55 GHz. Furthermore, a dual channel rectangular waveguide to SSPP splitter is proposed and validated with applications for dual-channel power dividing and signal transmission, achieving an efficiency of 72.4%.

Beam manipulation for quantum dot light-emitting diode with an Ag grating and a phase-gradient metasurface.

The quantum dot (QD) light-emitting diode (LED) is a robust scheme for single photon source. However, the spontaneous emission of a QD LED has arbitrary directions and polarizations, which is disadvantage for photon collection and manipulation. We propose a QD LED integrated with an Ag grating and a phase-gradient metasurface. The circular patterned Ag grating is adopted to collimate the emission beam with right phase and improve its spatial coherence, therefore a phase-gradient metasurface can work for beam manipulation. The 10°, 20°, and 30° angle deflection as well as doughnut-pattern generation are demonstrated by numerical simulation. A small metasurface with the width of 6 µm can provide a collection efficiency of 25.9% at the deflection angle of 10°. Furthermore, only one single QD can be selected from a QD assembly with a low-density.

Planar metal-only omnidirectional retroreflector using transmitarray and blazed grating for TE and TM polarizations.

Retroreflector is a device that reflects light along its incident direction over a continuous range of incident angles. The retroreflectors proposed so far are so bulky and uneven that they cannot be mounted on large structures. Recently, two-layer planar retroreflectors have been proposed in the field of optics and acoustics, but at microwave frequencies only one retroreflector with non-planar overall structure has been presented that doesn't cover omnidirectional incidence and only supports TM polarization. The term omnidirectional means the capability of retroreflectivity in all azimuth angles (φi) and a wide elevation angle (θi) range. In this paper, a novel planar and metal-only retroreflector is proposed at microwave frequencies that efficiently reflects the electromagnetic wave along its incident direction in omnidirectional angle range and supports both TE and TM polarization. The proposed structure consists of two separate cascaded layers. The top layer is a transmitarray structure in which incident waves with different angles are focused at different locations on the bottom layer. The bottom layer reflects back focused waves along its focus direction and two distinct structures are proposed to implement it. The first type is a novel spatially varying blazed grating and the second type is a spatially varying phase gradient metasurface, as used in the optics and aucostics. The first type offers omnidirectional property for both TE and TM polarization while the second type offers higher monostatic RCS for only TE polarization. Simulated and measured results of the proposed retroreflector with the first type bottom layer show an omnidirectional retroreflectivity with half-power (3-dB RCS level) elevation field of view of 60° (-30°to 30°) at frequency of 10 GHz with a normal incidence efficiency of 63%.

Highly efficient wavefront control based on extremely anisotropic materials

In this paper, a new strategy to design phase gradient metasurfaces (PGMs) are proposed. Using this way, we can achieve highly efficient control of the electromagnetic wavefronts. Specifically, we design reflection-type PGMs formed by an extremely anisotropic medium and a perfect electric conductor boundary. Instead of using refractive index distributions of materials to control the phases, we use a uniform anisotropic medium to control the reflection phases by changing the shape of the boundary. Using this strategy, three devices of the reflection focusing lens, the generator of the quasi airy beam and the PGMs have been realized, which show excellent performance in wavefront control. Our method is also suitable for the design of transmission-type metasurfaces and other wavefront transformations, which provides a novel approach for wave manipulation.

High-Q filtering and dynamic modulation in all-dielectric metasurfaces induced by quasi-BIC

The all-dielectric metasurfaces can significantly reduce the volume of optical components while having low loss and high performance, which has become a research hotspot in recent years. However, due to the complexity of metasurface geometric design, it is challenging to realize dynamic modulation on all-dielectric metasurface optical elements. Here, we propose a high quality factor (high-Q) pass-band filter designed by introducing the quasi-bound states in the continuum (quasi-BIC) into the silicon array phase-gradient metasurfaces. Our simulations show that due to the quasi-BIC effect only a high-Q resonance with the linewidth less than 1 nm and the corresponding Q value of ∼37000 could transmit along the zeroth order direction, which could be used for ultra-narrow linewidth filtering. Furthermore, our simulations present that the near-fields of the waveguide modes supported by the silicon arrays are partially distributed inside the indium tin oxide (ITO) substrate, which makes it possible to dynamically tune the central wavelength of our proposed filter by varying the ITO refractive index.

Understanding of Wood Anomalies in Metallic Gratings From Phase Gradient Metasurfaces

Phase-gradient metasurfaces (PGMs) have provided unprecedented opportunities for manipulating light. Here, we reexamine ordinary and well-studied subwavelength metallic gratings (OMGs) from the concept of PGMs to provide more insight into their diffraction properties. We will show that due to the existence of gauge invariance in PGMs, i.e., the diffraction law of PGMs is independent of the choice of initial value of abrupt phase shift that induces the phase gradient, the well-studied OMGs can be regarded as a PGM strictly, with its diffraction properties can be fully predicted by generalized diffraction law with phase gradient. In particular, the generalized diffraction law reveals that the phase gradient plays a significant role in the famous effect of Wood’s anomalies and Rayleigh’s conjecture.

Perfect Control of Diffraction Patterns with Phase-Gradient Metasurfaces.

Phase-gradient metasurfaces (PGMs) constitute an efficient platform for deflection of a beam in a desired direction. According to the generalized Snell's law, the direction of the reflected/refracted wave can be tuned by the spatial phase function provided by the PGMs. However, most studies on PGM focus only on a single diffraction order, that is, the incident wave can be reflected or refracted to a single target direction. Even in the case of multiple beams pointing in different directions, the beams are still in the same order mode, and the energy carried by different beams cannot be controlled. In addition, the energy ratio of multiple beams is generally uncontrollable. Here, we propose a general method to perfectly control diffraction patterns based on a multi-beam PGM. An analytical solution for arbitrarily controlling diffraction beams is derived through which the generation and energy distribution in high-order diffraction beams can be achieved. Three metasurfaces with different diffraction orders and energy ratios are designed and fabricated to demonstrate the proposed method. The efficiencies of diffraction for the desired channels are close to 100%. The simulated and measured far-field patterns are in good agreement with theoretical predictions, validating the proposed method that provides a new way to design multi-beam antennas and that has significance in wireless communication applications.

Deep-subwavelength gap modes in all-dielectric metasurfaces for high-efficiency and large-angle wavefront bending.

All-dielectric, phase-gradient metasurfaces manipulate light via a judiciously designed planar distribution of high and low refractive indices. In the established design approaches, the high-index elements play a dominant role, while the electromagnetic field existing between these elements is routinely viewed as either an incidental by-product or detrimental crosstalk. Here we propose an alternative approach that concentrates on exploring the low-index materials for wavefront shaping. In our Si metasurface, the low-index air gap between adjacent Si fins is judiciously tuned, while the high-index Si fins only have a single size across the whole metasurface. These gap modes provide the full 2π phase coverage, as well as high and relatively uniform transmission, at the deep-subwavelength scale. These characteristics are ideal for mapping a steep phase gradient, consequently suitable for high-efficiency and large-angle wavefront bending. This light manipulation capability is exemplified with numerical simulation in PW-SW (freely propagating wave to surface wave) conversion, where the wavefront is deflected by an angle of 90°. In the gap-mode meta-converters, the average unit size can be only 1/60 of free-space wavelength, an order of magnitude smaller than that of conventional all-dielectric metasurfaces. Their conversion efficiency can reach 68%, the highest value reported for any all-dielectric gradient metasurface THz converter.

Accurate optimization technique for phase-gradient metasurfaces used in compact near-field meta-steering systems

Near-Field Meta-Steering (NFMS) is a constantly evolving and progressively emerging novel antenna beam-steering technology that involves an elegant assembly of a base antenna and a pair of Phase-Gradient Metasurfaces (PGMs) placed in the near-field region of the antenna aperture. The upper PGM in an NFMS system receives an oblique incidence from the lower PGM at all times, a fact that is ignored in the traditional design process of upper metasurfaces. This work proposes an accurate optimization method for metasurfaces in NFMS systems to reduce signal leakage by suppressing the grating lobes and side lobes that are innate artifacts of beam-steering. We detail the design and optimization approach for both upper and lower metasurface. Compared to the conventionally optimized compact 2D steering system, the proposed system exhibits higher directivity and lower side-lobe and grating lobe levels within the entire scanning range. The broadside directivity is 1.4 dB higher, and the side-lobe level is 4 dB lower in comparison. The beam-steering patterns for the proposed 2D compact design are experimentally validated, and the measured and predicted results are in excellent concurrence. The versatile compatibility of truncated PGMs with a low gain antenna makes it a compelling technology for wireless backhaul mesh networks and future antenna hardware.

Design of Acoustic/Elastic Phase Gradient Metasurfaces: Principles, Functional Elements, Tunability, and Coding

AbstractAcoustic/elastic metasurfaces as a kind of two-dimensional metamaterials are of subwavelength thickness and show remarkable ability of acoustic/elastic wave manipulation. They have potential applications in various fields such as acoustic imaging, communications, cloaking, camouflage, vibration/noise control, energy harvesting, and nondestructive testing. In this review, we mainly summarize recent developments in acoustic/elastic phase gradient metasurfaces, including design principles, design of functional elements, wave field manipulation with applications, design of tunable metasurfaces, as well as the emerging digital coding metasurfaces. At last, we outline the future research directions in this field.

A compact and high gain metalens antenna based on impedance matching mechanism

A compact wideband metalens antenna which is composed of a log-periodic dipole antenna (LPDA) and metalens is proposed operating over the band of 1.7–2.7 GHz. The metalens is designed by using anisotropic ABA structures (A: impedance matching layer (IML), B: phase gradient metasurface lens (PGMS lens)) to reduce the back-lobe level and achieve high-performance. The PGMS lens is used to achieve a considerable phase shift range, and two IMLs are introduced to eliminate unnecessary reflection caused by the impedance mismatch and further increase phase coverage. Hence, the metalens features a low profile with a thickness of 0.29 λ 0 , where λ 0 is the free-space wavelength at 2.2 GHz. The measured results demonstrate that the proposed metalens antenna has a wide impedance bandwidth of 45% (1.7–2.7 GHz) and −3 dB gain bandwidth of 40% (1.8–2.7 GHz), respectively. Moreover, the gain is averagely increased by 9.56 dB in the whole working band, and the maximum gain achieves 16.1 dBi at 2.2 GHz. These characters make the proposed metalens antenna suitable for microwave imaging systems and other applications.