Gradient Metasurface Research Articles

Nonreciprocal Flat Optics with Silicon Metasurfaces.

Metasurfaces enable almost complete control of light through ultrathin, subwavelength surfaces by locally and abruptly altering the scattered phase. To date, however, all metasurfaces obey time-reversal symmetry, meaning that forward and backward traveling waves will trace identical paths when being reflected, refracted, or diffracted. Here, we use full-field calculations to design a passive metasurface for nonreciprocal transmission of both direct and anomalously refracted near-infrared light over nanoscale optical path lengths. The metasurface consists of a 100 nm-thick, periodically patterned Si slab. Owing to the high-quality-factor resonances of the metasurface and the inherent Kerr nonlinearities of Si, this structure acts as an optical diode for free-space optical signals. This structure also exhibits nonreciprocal anomalous refraction with appropriate patterning to form a phase gradient metasurface. Compared to existing schemes for breaking time-reversal symmetry, our platform enables subwavelength nonreciprocity for arbitrary free-space optical inputs and provides a straightforward path to experimental realization. The concept is also generalizable to other metasurface functions, providing a foundation for one-way lensing and holography.

Dispersion engineering of metasurfaces for supporting both TM and TE spoof surface plasmon polariton

In this study, an anisotropic plasmonic metasurface is proposed to support both TM and TE mode spoof surface plasmon polaritons (SPPs) with the same wave vector. The unit cell consists of a rectangle metal patch with a grounded dielectric substrate. According to the equivalent circuit model, the dispersion relationships of the TM and TE mode spoof SPPs can be manipulated independently by altering different structure parameters. By proper parameters design, the TM and TE mode spoof SPPs working with the same wave vector at the same frequency is achieved. Besides this, a polarization insensitive phase gradient metasurface (PGM) is designed to excite spoof SPPs under linear polarization incident waves. The PGM is designed to have good wave vector matching with the eigen spoof SPPs on the anisotropic plasmonic metasurface. Both the simulations and experiments show that the TM and TE mode spoof SPPs can be excited and propagate with the same wave vector at 8.5 GHz. This work may provide new way in the study of spoof SPPs and have potential in antennas, microwave devices and other practical applications.

Nanoscale beam splitters based on gradient metasurfaces.

Beam splitters are essential components in various optical and photonic applications, for example, interferometers, multiplexers, and so on. Present beam splitters based on cubes or plates are normally bulky. Realizing beam splitters in nanoscales is useful to reduce the total size of photonic devices. We demonstrate here a beam splitter with nanoscale thickness based on a gradient metasurface comprising lithium niobate cylinder arrays. Since one unit cell of metasurface comprising two cylinder rows shows two opposite phase gradients, the incident light is split into different directions according to the generalized Snell's law. The split ratio is proven to be effectively tunable.

Deflection angle switching with a metasurface based on phase-change nanorods [Invited

We propose the active metasurface using phase-change material Ge2Sb2Te5 (GST), which has two distinct phases so called amorphous and crystalline phases, for an ultrathin light path switching device. By arranging multiple anisotropic GST nanorods, the gradient metasurface, which has opposite directions of phase gradients at the two distinct phases of GST, is demonstrated theoretically and numerically. As a result, in the case of normal incidence of circularly polarized light at the wavelength of 1650 nm, the cross-polarized light deflects to −55.6° at the amorphous phase and +55.6° at the crystalline phase with the signal-to-noise ratio above 10 dB.

Broadband absorption with gradient metasurfaces

A metasurface with appropriately designed transverse spatial inhomogeneities can provide the desired phase redistribution in response to an incident wave with arbitrary incident angle. This property of gradient metasurfaces has been used to modify light propagation in unusual manners, to transform the impinging optical wavefront with large flexibility. In this work, we show how gradient metasurfaces can be tailored to offer high absorption in thin absorptive layers, and how to design realistic metasurfaces for this purpose using dielectric materials.

Ultra-wideband manipulation of electromagnetic waves by bilayer scattering engineered gradient metasurface.

Realizing effective scattering manipulation in broad operation band is a long pursuit. Here, we present an ultra-broadband metasurface to manipulate the scattering of electromagnetic waves based on spin–orbit interaction induced virtual shaping. The spin conversion efficiency is over 90% from 4.9 GHz to 22.8 GHz accompanied by a abrupt phase shift covering 0–2π, which is dependent on the orientation of the subwavelength of building blocks. Both Bessel- and triangular-type virtual profile shapes are developed for electromagnetic illusion and camouflage. Simulation and experimental results demonstrate that the significant backward RCS reduction of 10 dB is obtained from 5.5 to 22.5 GHz. The significant improvement in working bandwidth can be attributed to: the application of the dispersionless phase–shift properties of P–B phase, two-dimensional dispersion management methodology and catenary shaped local field enhancement in the metallic gap. The proposed design may have the potential applications in eletromagnetic manipulation and object detection.

Research of a wide-angle backscattering enhancement metasurface

To enhance backscattering, corner reflector and Luneburg lens are usually used. They can operate effectively in a broad angle range and also in a quite wide band. However, corner reflector as a typical structure of backscattering enhancement device, has obvious disadvantages in practical application. For example, it is usually made of metal material, which causes it to be too heavy and bulky. Luneburg lens is generally made of dielectric with strong loss and high cost, which is unfavorable for applications. Thus, it is necessary to explore a new way to realize wide-angle backscattering enhancement. In this paper, a phase gradient metasurface with wide-angle radar cross section (RCS) enhancement property is proposed and demonstrated, which consists of two phase gradients with equal magnitude but in opposite directions. Through designing a reflective phase profile along the surface, an equivalent wave vector can be generated, with doubled magnitude but in an opposite direction to the parallel component of the wave vector of the incident wave. At the incidence angles =-45 and 45, electromagnetic (EM) waves are reflected to the directions just opposite to the directions of incident waves. And at incidence angle =0, the incident EM wave is coupled into spoof surface wave and then guided to another region to decouple into a free space wave. These guarantee RCS enhancement property in a related angular domain. The polarization independent Jerusalem cross unit is used to design the phase gradient, and a wide-angle RCS enhancement metasurface is designed. The simulated results indicate that at the designed incidence angles, directions of the reflected waves are all opposite to the directions of incidence waves for both x and y polarized wave. In order to evaluate the RCS enhancement performances, the mono-static RCS of the designed wide-angle RCS enhancement metasurface is measured. Both the simulations and experiments are in good agreement with each other, and show that the designed metasurface obtains tremendous RCS enhancement performances in a wide-angle domain (-45-45) for both x and y polarized wave with frequencies ranging from 9 GHz to 12 GHz.

Planar Phase Gradient Metasurface Antenna With Low RCS

A low radar cross section (RCS) planar antenna using phase gradient metasurface is proposed in this paper. The proposed antenna works in the X-band and is fed by a parallel plate waveguide power divider. The designed phase gradient metasurface is coplanar with the feeding structure and couples guided waves in the parallel plate waveguide power divider to radiation waves. According to the generalized reflection law, the proposed metasurface is a frequency scanning antenna and the beam direction can be steered slightly from 16.6° to 13°. The proposed antenna is linearly polarized, and however, the RCS is reduced significantly for arbitrary polarized incident waves. The antenna was simulated, fabricated, and measured. The measurement results confirm well with the simulation results.

Frequency Scanning Radiation by Decoupling Spoof Surface Plasmon Polaritons via Phase Gradient Metasurface

On the basis of generalized Snell’s law of reflection, we propose to achieve frequency scanning radiation by decoupling spoof surface plasmon polaritons (SSPPs) from a corrugated metallic strip (CMS). Phase gradient metasurface (PGM) is utilized to modulate the dispersion behavior of the CMS. Since the phase gradient of the PGM is contrary to the wave vector of SSPPs, the dispersion curve of SSPPs can be translated into the fast wave zone, leading to frequency scanning radiation. As an example, we demonstrate a frequency scanning antenna operating at 8.8–10.7 GHz. A prototype was designed, fabricated, and measured. The measured results show that the prototype can realize continuous beam scanning from 4.8° to 37.2°, with realized gain varying from 8 to 13.1 dB. Owing to surface confinement of SSPPs, the proposed method can be readily extended to the design of conformal frequency scanning antennas on curved faces.

End-Fire Surface Wave Antenna With Metasurface Coating

This paper describes a wideband and low-profile end-fire surface wave antenna embedded into a large metallic platform. It consists of a tapered grounded slab fed by a wideband surface wave launcher and coated with a planar gradient metasurface (MS). It is found that by altering the surface reactance of the MS array, the surface wave propagating in the grounded slab can be manipulated to suppress the radiation of TM1 mode region at a specific high frequency. A simple design procedure of MS array is then developed. In addition, a blind-filled cylinder hole is used to further improve the radiation pattern at certain high frequencies. A prototype of the proposed antenna is finally fabricated and tested. Measured results are in good agreement with simulated ones, and they show that the surface wave antenna has a very wide bandwidth from 3.78 to 18.54 GHz for voltage standing wave ratio $0.063~\lambda _{L}$ , $\lambda _{L}$ is the free-space wavelength at the lowest operating frequency). Good radiation pattern and high gain are achieved over a wide frequency band.

Inherent losses induced absorptive acoustic rainbow trapping with a gradient metasurface

Acoustic rainbow trapping represents the phenomenon of strong acoustic dispersion similar to the optical “trapped rainbow,” which allows spatial-spectral modulation and broadband trapping of sound. It can be realized with metamaterials that provide the required strong dispersion absent in natural materials. However, as the group velocity cannot be reduced to exactly zero before the forward mode being coupled to the backward mode, such trapping is temporary and the local sound oscillation ultimately radiates backward. Here, we propose a gradient metasurface, a rigid surface structured with gradient perforation along the wave propagation direction, in which the inherent thermal and viscous losses inside the holes are considered. We show that the gradually diminished group velocity of the structure-induced surface acoustic waves (SSAWs) supported by the metasurface becomes anomalous at the trapping position, induced by the existence of the inherent losses, which implies that the system's absorption reaches its maximum. Together with the progressively increased attenuation of the SSAWs along the gradient direction, reflectionless spatial-spectral modulation and sound enhancement are achieved in simulation. Such phenomenon, which we call as absorptive trapped rainbow, results from the balanced interplay among the local resonance inside individual holes, the mutual coupling of adjacent unit cells, and the inherent losses due to thermal conductivity and viscosity. This study deepens the understanding of the SSAWs propagation at a lossy metasurface and may contribute to the practical design of acoustic devices for high performance sensing and filtering.

Microwave beam steering of planar antennas by hybrid phase gradient metasurface structure under spherical wave illumination

To facilitate the microwave beam steering of planar antennas in both elevation and azimuth planes, a radially gradient quasi-transparent hybrid metasurface (RGHMS) structure is proposed. The circular aperture of RGHMS is comprised of two different phase profiles in the single structure. Half of the circular aperture introduces a gradient phase shift, whereas the other half provides a constant phase shift to the incident spherical wave. Since the obtained wavefront modulation for the beam tilting is realized using the combination of aforementioned phase profiles in a single metasurface (MS), it is considered as a hybrid structure. The proposed circular RGHMS with a radius of 1.2λ0 is placed at a height of 0.43λ0 from the feed antenna by considering the geometrical centers of RGHMS and antenna aperture coinciding with each other. The in-plane translation of the RGHMS modulates the wavefront of the incident wave, which results in 0° to 18° beam steering of planar antenna in the elevation plane. Moreover, in-plane rotation of RGHMS around the antenna axis facilitates the beam steering in the azimuth plane with a full 360° azimuthal coverage. The proposed structure is designed at the center frequency of 10 GHz and introduces uniform beam shapes with the gain of 12.3–14.3 dBi during the beam steering. The strategy of combining two different types of phase profile in a single MS eludes the requirement of the phase correcting lens, and thus can directly be illuminated through the spherical wavefront of antenna in the near field. Moreover, the microwave beam steering in both planes with fairly high gain and compact configuration is revealed.

Metasurface-assisted phase-matching-free second harmonic generation in lithium niobate waveguides

The phase-matching condition is a key aspect in nonlinear wavelength conversion processes, which requires the momenta of the photons involved in the processes to be conserved. Conventionally, nonlinear phase matching is achieved using either birefringent or periodically poled nonlinear crystals, which requires careful dispersion engineering and is usually narrowband. In recent years, metasurfaces consisting of densely packed arrays of optical antennas have been demonstrated to provide an effective optical momentum to bend light in arbitrary ways. Here, we demonstrate that gradient metasurface structures consisting of phased array antennas are able to circumvent the phase-matching requirement in on-chip nonlinear wavelength conversion. We experimentally demonstrate phase-matching-free second harmonic generation over many coherent lengths in thin film lithium niobate waveguides patterned with the gradient metasurfaces. Efficient second harmonic generation in the metasurface-based devices is observed over a wide range of pump wavelengths (λ = 1580–1650 nm).

Random Combinatorial Gradient Metasurface for Broadband, Wide-Angle and Polarization-Independent Diffusion Scattering

This paper proposes an easy, efficient strategy for designing broadband, wide-angle and polarization-independent diffusion metasurface for radar cross section (RCS) reduction. A dual-resonance unit cell, composed of a cross wire and cross loop (CWCL), is employed to enhance the phase bandwidth covering the 2π range. Both oblique-gradient and horizontal-gradient phase supercells are designed for illustration. The numerical results agree well with the theoretical ones. To significantly reduce backward scattering, the random combinatorial gradient metasurface (RCGM) is subsequently constructed by collecting eight supercells with randomly distributed gradient directions. The proposed metasurface features an enhanced specular RCS reduction performance and less design complexity compared to other candidates. Both simulated and measured results show that the proposed RCGM can significantly suppress RCS and exhibits broadband, wide-angle and polarization independence features.

Electromagnetic retroreflection augmented by spherical and conical metasurfaces

The focus of this paper is on phase gradient metasurfaces conformal to spherical and conical bodies of revolution, with an aim of engineering retroreflections and therefore augmenting backscattering cross-sections of those three-dimensional geometries under the illumination of a plane electromagnetic wave. Based on the conducting sphere and cone, the effect of the geometric revolution property on the selection of the unit inclusion of metasurfaces is considered. The procedure for using the selected unit inclusion to implement the proper reflection phase gradient onto the illuminated surfaces of those objects is formulated in detail. Retroreflections resembling conducting plates under normal incidence are observed for both the conducting sphere and cone coated with conformal metasurfaces. As a result, the redirection-induced retroreflection effectively contributes to the backscattering cross-section enhancement. A good agreement between full-wave simulations and measurements demonstrates the validity and effectiveness of backscattering cross-section enhancement using spherical and conical metasurfaces.

Experimental realization of all-angle negative refraction in acoustic gradient metasurface

In this work, we numerically and experimentally demonstrate that all-angle negative refraction can be obtained with the acoustic gradient metasurface of subwavelength thickness. The coiling labyrinthine structures are utilized to build the desired gradient metasurface, and the apparent negative refraction occurring beyond the critical incident angle has been validated by simulations and experimental measurements, which agrees well with the theoretical predictions given by the revised generalized law of refraction while taking the contribution of the Bragg scattering into account. This work provides the solution to manipulate the acoustic waves and shows good promise in building functional diffractive acoustic elements.

All-angle Negative Reflection with An Ultrathin Acoustic Gradient Metasurface: Floquet-Bloch Modes Perspective and Experimental Verification

Metasurface with gradient phase response offers new alternative for steering the propagation of waves. Conventional Snell’s law has been revised by taking the contribution of local phase gradient into account. However, the requirement of momentum matching along the metasurface sets its nontrivial beam manipulation functionality within a limited-angle incidence. In this work, we theoretically and experimentally demonstrate that the acoustic gradient metasurface supports the negative reflection for all-angle incidence. The mode expansion theory is developed to help understand how the gradient metasurface tailors the incident beams, and the all-angle negative reflection occurs when the first negative order Floquet-Bloch mode dominates inside the metasurface slab. The coiling-up space structures are utilized to build desired acoustic gradient metasurface, and the all-angle negative reflections have been perfectly verified by experimental measurements. Our work offers the Floquet-Bloch modes perspective for qualitatively understanding the reflection behaviors of the acoustic gradient metasurface, and the all-angle negative reflection characteristic possessed by acoustic gradient metasurface could enable a new degree of the acoustic wave manipulating and be applied in the functional diffractive acoustic elements, such as the all-angle acoustic back reflector.

Passive acoustic metasurfaces for high-efficiency wavefront transformations

Metasurfaces have offered new degrees of freedom to control the propagation properties of acoustic waves. Gradient metasurfaces, first proposed in optics, and recently extended to acoustics, add an extra transverse momentum to the incident plane wave so that reflected and refracted waves can be rerouted without having to rely on conventional Snell’s Law. However, recent studies have shown that gradient metasurfaces cannot transfer all incident energy into the desired direction without causing unwanted parasitic diffraction and limiting the overall transformation efficiency. More sophisticated metasurface designs have considered impedance matching to address this issue, but this inherently requires the use of active and lossy materials for extreme wave transformations. In this talk, we present our recent efforts in designing passive acoustic metasurfaces with unitary efficiency in beam steering, even when considering extreme steering angles. We consider nonlocal surface impedance profiles and the use of bi...

Flat Engineered Multichannel Reflectors

Recent advances in engineered gradient metasurfaces have enabled unprecedented opportunities for light manipulation using optically thin sheets, such as anomalous refraction, reflection, or focusing of an incident beam. Here we introduce a concept of multi-channel functional metasurfaces, which are able to control incoming and outgoing waves in a number of propagation directions simultaneously. In particular, we reveal a possibility to engineer multi-channel reflectors. Under the assumption of reciprocity and energy conservation, we find that there exist three basic functionalities of such reflectors: Specular, anomalous, and retro reflections. Multi-channel response of a general flat reflector can be described by a combination of these functionalities. To demonstrate the potential of the introduced concept, we design and experimentally test three different multi-channel reflectors: Three- and five-channel retro-reflectors and a three-channel power splitter. Furthermore, by extending the concept to reflectors supporting higher-order Floquet harmonics, we forecast the emergence of other multiple-channel flat devices, such as isolating mirrors, complex splitters, and multi-functional gratings.

Manipulation of acoustic wavefront by gradient metasurface based on Helmholtz Resonators

We designed a gradient acoustic metasurface to manipulate acoustic wavefront freely. The broad bandwidth and high efficiency transmission are achieved by the acoustic metasurface which is constructed with a series of unit cells to provide desired discrete acoustic velocity distribution. Each unit cell is composed of a decorated metal plate with four periodically arrayed Helmholtz resonators (HRs) and a single slit. The design employs a gradient velocity to redirect refracted wave and the impedance matching between the metasurface and the background medium can be realized by adjusting the slit width of unit cell. The theoretical and numerical results show that some excellent wavefront manipulations are demonstrated by anomalous refraction, non-diffracting Bessel beam, sub-wavelength flat focusing, and effective tunable acoustic negative refraction. Our designed structure may offer potential applications for the imaging system, beam steering and acoustic lens.