Efficient beam splitting using zero-load-impedance metagratings: publisher's note.

This publisher's note contains corrections to Opt. Lett.48, 3275 (2023)10.1364/OL.491711.

Efficient beam splitting using zero-load-impedance metagratings.

Metagratings with zero load impedance are proposed to achieve efficient beam splitting. Different from previously proposed metagratings that require specific capacitive and/or inductive structures to achieve load impedance, the metagrating proposed here consists solely of simple microstrip-line structures. Such a structure overcomes the implementation constraints such that low-cost fabrication technology can be applied for metagratings operating at higher frequencies. The detailed theoretical design procedure is presented together with numerical optimizations to achieve the specific design parameters. Finally, several reflection-type beam-splitting devices with different pointing angles are designed, simulated, and experimentally measured. The results show very high performance at 30 GHz, paving the way to simple and low-cost printed circuit board (PCB) metagratings at millimeter-wave and higher frequencies.

Broadband tunable metasurface platform enabled by dynamic phase compensation.

Broadband metasurfaces have attracted significant attention for a variety of applications in imaging and communication systems. Here, a method to alleviate the chromatic aberrations issue is proposed in the microwave region using dynamic phase compensation enabled by a reconfigurable metasurface. The dispersion characteristic of the meta-atom implemented with varactor diodes can be flexibly manipulated electronically, such that the dispersion-induced phase distortions over a wide frequency band can be compensated dynamically to achieve broadband performances. Various aberration-free functionalities can be realized with the proposed active metasurface. Near-field measurements are performed on a fabricated prototype to demonstrate aberration-free beam bending and hologram imaging, showing good agreement with simulation results. Such an active metasurface platform paves the way to efficient devices for wireless power transfer, sensors, and communication and antenna systems at radio or much higher frequencies.

Compact multi-functional frequency-selective absorber based on customizable impedance films.

Multi-functional metamaterial absorbers have attracted considerable attention for applications in the microwave frequency regime. In this paper, we report the design, fabrication, and characterization of frequency-selective absorbers, which exhibit substantial absorption property within a pre-defined frequency band, while at the same time behaving as a highly transparent screen in another targeted frequency band. The proposed designs consist of a symmetrically patterned indium tin oxide film acting as an absorbing layer, two dielectric substrates, and a cross-slot metal sheet frequency selective surface playing the role of a transmitting layer. In order to validate the functionalities of the designed absorbers, equivalent circuit models, full-wave numerical simulations and measurements are presented. The measured results, in good agreement with the numerical ones, show that the proposed designs realize 80% broadband absorption over the desired frequency range and possess a transparent window in a higher or lower frequency band for a wide range of incidence angles up to 60°. These performances suggest that the proposed designs are promising candidates for multi-functional scattering control and communication applications.

Electronically-engineered metasurface for directional beaming of electromagnetic waves through a subwavelength aperture.

Controlling diffracted waves has attracted extensive research interests these last years particularly for the potential application of beaming functionality. In this paper, we propose to realize on-axis beaming of diffracted electromagnetic waves by using a phase- gradient metasurface. The structure is optimally designed in order to transform surface waves to propagating waves and to enhance transmission through a subwavelength aperture. Both numerical simulations and near-field measurements are performed at microwave frequencies to validate the proposed concept. Furthermore, the metasurface is frequency-tunable and can be controlled by an external DC bias voltage. Consequently, by adjusting the electromagnetic response of each unit cell through the bias voltage, different phase gradients can be tailored, enabling broadband operation spanning from 9 GHz to 12 GHz.

Phase-modulation based transmitarray convergence lens for vortex wave carrying orbital angular momentum.

Vortex electromagnetic (EM) waves hold promise for their ability to significantly increase the transmission capacity of wireless communication systems via the torsion resistance defined by different topological charges associated with the orbital angular momentum (OAM). However, the application of vortex waves in remote distance transmission is limited by its characteristic of divergence. In this paper, a lens based on a phase-modulation metasurface (MS) is proposed that enables vortex EM waves to converge, thereby improving their propagation performance at microwave frequencies. A phase-shift distribution on the plane of the MS is obtained based on the concept of the optical converging axicon, which can convert a Laguerre-Gaussian (LG) beam to a Bessel beam based on changing the propagation direction. Simulation results verify the ability of the MS lens to achieve OAM beam focusing, which is advantageous for enhancing the propagation directivity and increasing the gain in the main lobes of vortex waves. This is of particular importance in microwave wireless communication applications.

Realizable design of field taper via coordinate transformation.

Complex electromagnetic structures can be designed by exploiting the concept of spatial coordinate transformations. In this paper, we define a coordinate transformation scheme that enables one to taper the electric field between two waveguides of different cross-sections. The electromagnetic field launched from the wide input waveguide is compressed in the proposed field tapering device and guided into the narrow output waveguide. In closed rectangular waveguide configurations, the taper can further play the role of a mode selector due to the output waveguide's cut-off frequency. Realizable permittivity and permeability values that can be achieved with common existing metamaterials are determined from the transformation equations and simplified by a proposed parameter reduction method. Both a 2D continuous design model and a potential 3D discretized realization model are presented at microwave frequencies and the performances of the tapering devices are verified by full-wave finite element numerical simulations. Finally, near-field distributions are shown to demonstrate the field tapering functionality.

Orbital angular momentum generation method based on transformation electromagnetics.

Orbital angular momentum (OAM) vortex waves generated by conventional spiral phase plates and metasurfaces have been widely discussed. In this work, we propose an innovative OAM generation method based on transformation optics (TO). By solving Laplace's equation with specific boundary conditions, an oblate cylindrical shaped physical domain is designed to imitate a gradient shaped virtual domain which is able to generate a vortex beam upon reflection. As a proof-of-concept demonstration, a broadband all-dielectric microwave lens for vortex beam generation is presented with a topological charge of + 1. The corresponding far-field patterns as well as near-field helical phase and doughnut-shaped amplitude distributions of the lens, obtained from numerical simulations, are reported along with a wide operational bandwidth spanning from 8 to 16 GHz. As a transformation method, the proposed TO technique provides an effective way to realize a conversion from plane waves to vortex waves, which can greatly facilitate the potential implementation of OAM waves in microwave wireless communication systems.

All-dielectric transformation medium mimicking a broadband converging lens.

Radio waves carrying orbital angular momentum (OAM) may potentially increase spectrum efficiency and channel capacity based on their extra rotational degree of freedom. However, due to their divergence characteristics, vortex waves are not suitable to transmit over a long distance in the radio frequency (RF) and microwave domains. In this paper, a transformation optics (TO) based all-dielectric converging lens is proposed. The beam divergence angle of the vortex wave passing through the lens can be decreased from 25° to 9°. The transformed material parameters of the converging lens are determined by solving Laplace's equation subject to specific boundary conditions. Far-field antenna radiation patterns as well as near-field helical phase and electric field amplitude distributions obtained from numerical simulations are reported, demonstrating the broadband characteristics of the proposed microwave lens. Moreover, the all-dielectric compact lens design comprised by a graded permittivity profile can be fabricated by additive manufacturing technology, which greatly facilitates the potential development and application of vortex wave based wireless communications.

Gradient phase partially reflecting surfaces for beam steering in microwave antennas.

A metal-dielectric-metal gradient phase partially reflecting surface based on the combination of a gradient index dielectric substrate with an inductive and a capacitive grids, is designed at microwave frequencies for antenna applications. The gradient index is obtained by realizing air holes of different dimensions in a dielectric host material. A prototype of the gradient index dielectric substrate is fabricated through three-dimensional printing, an additive fabrication technology. It is then associated to two patterned metallic grids to realize a partially reflecting surface with a gradient phase behavior. For experimental validation, the partially reflective surface is used as reflector in a low-profile Fabry-Perot cavity antenna. An angular enhancement of the emitted beam in a desired direction is reported by further engineering the phase introduced by the inductive and the capacitive grids. Far-field measurements are performed on fabricated antenna prototypes to validate the concept. Such gradient phase reflective surface paves the way to low-cost easy-made microwave metal-dielectric surfaces incorporating functionalities such as beam control, forming and collimation.

Conceptual design of a beam steering lens through transformation electromagnetics.

In this paper, based on transformation electromagnetics, the design procedure of a lens antenna, which steers the radiated beam of a patch array, is presented. Laplace's equation is adopted to construct the mapping between the virtual space and the physical space. The two dimensional (2D) design method can be extended to a potential three-dimensional (3D) realization, and with a proper parameter simplification, the lens can be further realized by common metamaterials or isotropic graded refractive index (GRIN) materials. Full wave simulations are performed to validate the proposed concept. It is observed that by placing the lens on a feeding source, we are able to steer the radiation emitted by the latter source.

Additively-Manufactured Broadband Metamaterial-Based Luneburg Lens for Flexible Beam Scanning.

Multi-beam microwave antennas have attracted enormous attention owing to their wide range of applications in communication systems. Here, we propose a broadband metamaterial-based multi-beam Luneburg lens-antenna with low polarization sensitivity. The lens is constructed from additively manufactured spherical layers, where the effective permittivity of the constituting elements is obtained by adjusting the ratio of dielectric material to air. Flexible microstrip patch antennas operating at different frequencies are used as primary feeds illuminating the lens to validate the radiation features of the lens-antenna system. The proposed Luneburg lens-antenna achieves ±72° beam scanning angle over a broad frequency range spanning from 2 GHz to 8 GHz and presents a gain between 15.3 dBi and 22 dBi, suggesting potential applications in microwave- and millimeter-wave mobile communications, radar detection and remote sensing.

Design and Test of Embedded Reconfigurable Mode Converter Based on Spontaneous Deformable Materials.

The mode converter, as a passive mode conversion device in transmission lines, is well-investigated and widely implemented in various electromagnetic systems. However, most traditional mode converters can only realize a single conversion mode. Thus, a mode converter achieving multiple controllable output modes is urgently needed. In this paper, a reconfigurable mode converter operating in the microwave range is achieved by embedding a deformable all-dielectric material with quadrilateral shape into a rectangular waveguide based on coupled-mode theory. It can achieve different target modes with controllable output for the same input by exciting the deformable all-dielectric material. The design principle of the mode converter is expounded concretely and simulation is carried out using HFSS software 2022 R2. Experimental results, consisting of the simulation results, demonstrate that the proposed mode converter can achieve various mode conversions with mode purity higher than 95%. This article innovatively applies deformable materials to waveguide mode conversion, expanding the application of deformable memory materials in electromagnetic devices.

Recent Advances in Reconfigurable Metasurfaces: Principle and Applications.

Metasurfaces have shown their great capability to manipulate electromagnetic waves. As a new concept, reconfigurable metasurfaces attract researchers' attention. There are many kinds of reconfigurable components, devices and materials that can be loaded on metasurfaces. When cooperating with reconfigurable structures, dynamic control of the responses of metasurfaces are realized under external excitations, offering new opportunities to manipulate electromagnetic waves dynamically. This review introduces some common methods to design reconfigurable metasurfaces classified by the techniques they use, such as special materials, semiconductor components and mechanical devices. Specifically, this review provides a comparison among all the methods mentioned and discusses their pros and cons. Finally, based on the unsolved problems in the designs and applications, the challenges and possible developments in the future are discussed.

Suspended Metasurface for Broadband High-Efficiency Vortex Beam Generation.

Electromagnetic (EM) waves carrying orbital angular momentum (OAM) exhibit phase vortex and amplitude singularity. Broadband OAM generation with high efficiency is highly desired with suggested applications such as broadband imaging and communications. In this paper, suspended metasurface structure achieving low-Q factor is proposed to realize broadband phase control and excellent reflection efficiency. Broadband vortex beam generation with OAM order of 1 and 2 are realized using the proposed suspended structure. Furthermore, by analyzing different metasurface aperture phase distribution schemes, the efficiency of the OAM generator is maximally achieved. The designs are validated by simulation and measurement. The proposed OAM generators work across 4-10 GHz with efficiency higher than 82%. This design provides a route to broadband metasurface realization and high efficiency OAM generation.

A Time-Modulated Transparent Nonlinear Active Metasurface for Spatial Frequency Mixing.

In this article, a time-modulated transparent nonlinear active metasurface loaded with varactor diodes was proposed to realize spatial electromagnetic (EM) wave frequency mixing. The nonlinear transmission characteristic of the active metasurface was designed and measured under time-modulated biasing signals. The transmission phase can be continuously controlled across a full 360° range at 5 GHz when the bias voltage of the varactor diodes changes from 0 V to 25.5 V, while the transmission amplitude is between -2.1 dB to -2.7 dB. By applying the bias voltage in time-modulated sequences, frequency mixing can be achieved. Due to the nonlinearity of the transmission amplitude and transmission phase of the metasurface versus a time-modulated bias voltage, harmonics of the fundamental mode were observed using an upper triangle bias voltage. Furthermore, with a carefully designed bias voltage sequence, unwanted higher order harmonics were suppressed. The proposed theoretical results are validated with the measured results.

Band-Pass Filtering Cross-Polarization Converter Using Transmitarrays.

Microwave devices with polarization conversion and band-pass filtering response have great application prospects on radomes. Here, the concepts of band-pass filters and cross-polarization converters are combined to realize a band-pass filtering cross-polarization converter with an extremely high polarization-conversion ratio. Most importantly, the device has an excellent out-of-band rejection level, above 30 and 40 dB for the lower and upper edges, respectively. In addition, the transmission zeros of the passband can be flexibly tuned independently. The band-pass filtering polarization converter was simulated, fabricated, and measured, and the measured results were found to be in good agreement with the simulation results.

Wideband RCS Reduction Using Coding Diffusion Metasurface.

This paper presents a radar cross-section (RCS) reduction technique by using the coding diffusion metasurface, which is optimised through a random optimization algorithm. The design consists of two unit cells, which are elements '1' and '0'. The reflection phase between the two-unit cells has a 180° ± 37° phase difference. It has a working frequency band from 8.6 GHz to 22.5 GHz, with more than 9 dB RCS reduction. The monostatic RCS reduction has a wider bandwidth of coding diffusion metasurface as compared to the traditional chessboard metasurface. In addition, the bistatic performance of the designed metasurfaces is observed at 15.4 GHz, which shows obvious RCS reduction when compared to a metallic plate of the same size. The simulated and measured result shows the proficiency of the designed metasurface.

3D field-shaping lens using all-dielectric gradient refractive index materials.

A novel three-dimensional (3D) optical lens structure for electromagnetic field shaping based on spatial light transformation method is proposed at microwave frequencies. The lens is capable of transforming cylindrical wavefronts into planar ones, and generating a directive emission. Such manipulation is simulated and analysed by solving Laplace's equation, and the deformation of the medium during the transformation is theoretically described in detail. The two-dimensional (2D) design method producing quasi-isotropic parameters is further extended to a potential 3D realization with all-dielectric gradient refractive index metamaterials. Numerical full-wave simulations are performed on both 2D and 3D models to verify the functionality and broadband characteristics of the calculated lens. Far-field radiation patterns and near-field distributions demonstrate a highly radiated directive beam when the lens is applied to a conical horn antenna.

Electromagnetic field tapering using all-dielectric gradient index materials.

The concept of transformation optics (TO) is applied to control the flow of electromagnetic fields between two sections of different dimensions through a tapering device. The broadband performance of the field taper is numerically and experimentally validated. The taper device presents a graded permittivity profile and is fabricated through three-dimensional (3D) polyjet printing technology using low-cost all-dielectric materials. Calculated and measured near-field mappings are presented in order to validate the proposed taper. A good qualitative agreement is obtained between full-wave simulations and experimental tests. Such all-dielectric taper paves the way to novel types of microwave devices that can be easily fabricated through low-cost additive manufacturing processes.