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.

Complex-Amplitude Programmable Versatile Metasurface Platform Driven by Guided Wave.

Metasurfaces have shown unparalleled controllability of electromagnetic (EM) waves. However, most of the metasurfaces need external spatial feeding sources, which renders practical implementation quite challenging. Here, a low-profile programmable metasurface with 0.05λ0 thickness driven by guided waves is proposed to achieve dynamic control of both amplitude and phase simultaneously. The metasurface is fed by a guided wave traveling in a substrate-integrated waveguide, avoiding external spatial sources and complex power divider networks. By manipulating the state of the p-i-n diodes embedded in each meta-atom, the proposed metasurface enables 1-bit amplitude switching between radiating and nonradiating states, as well as a 1-bit phase switching between 0° and 180°. As a proof of concept, two advanced functionalities, namely, low sidelobe-level beam scanning and Airy beam generation, are experimentally demonstrated with a single platform operating in the far- and near-field respectively. Such complex-amplitude, programmable, and low-profile metasurfaces can overcome integration limitations of traditional metasurfaces, and open up new avenues for more accurate and advanced EM wave control within an unprecedented degree of freedom.

Engineering of the Fano resonance spectral response with non-Hermitian metasurfaces by navigating between exceptional point and bound states in the continuum conditions.

We address the engineering of Fano resonances and metasurfaces, by placing it in the general context of open non-Hermitian systems composed of coupled antenna-type resonators. We show that eigenfrequency solutions obtained for a particular case of scattering matrix are general and valid for arbitrary antenna radiative rates, thanks to an appropriate transformation of parametric space by simple linear expansion and rotation. We provide evidence that Parity-Time symmetry phase transition path and bound states in continuum (BIC) path represent the natural axis of universal scattering matrix solutions in this parametric coupling-detuning plane and determine the main characteristics of Fano resonance. Specifically, we demonstrate the control of asymmetry and sharpness of Fano resonance through navigation between BIC and PT-symmetric phase transition exceptional point. In particular, we demonstrate a fully symmetric Fano resonance in a system of two coupled bright and dark mode resonators. This result goes beyond current wisdom on this topic and demonstrates the universality of scattering matrix eigenfrequency solutions highlighted in our study. The validity of our approach is corroborated through comparison with experimental and full 3D numerical simulations results published in the literature making it thus possible to grasp a large body of experimental work carried out in this field. The detrimental impact of absorption losses on the contrast of the Fano resonance, which must be two orders of magnitude lower than the radiative losses, is also evidenced.

Four-channel meta-hologram enabled by a frequency-multiplexed mono-layered geometric phase metasurface.

In recent years, frequency-multiplexed metasurfaces have received extensive attention due to the increasing demand for multifunction integration and communication capacity. However, multi-channel studies achieved with a mono-layered frequency-multiplexed metasurface are limited. Herein, a universal design strategy for a frequency-multiplexed mono-layered geometric phase metasurface is proposed by utilizing Pancharatnam-Berry (PB) phase modulations. The elementary meta-atom is judiciously designed to transmit the cross-polarized component of a circularly polarized incident wave at four distinct frequencies with independent 360° phase shifts and a constant amplitude of 0.48, close to the theoretical limit of 0.5. As a proof-of-concept demonstration, a four-channel meta-hologram is designed to achieve distinct holographic images of "three foci", "five foci", "J" and "X" at 7.2 GHz, 9.1 GHz, 10.9 GHz, and 15.2 GHz respectively. The images are projected in the desired azimuth planes by exploiting the time-shifting properties of the Fourier transform. The experimental and full-wave simulation results are in good agreement, which indicates that the proposed strategy has great potentials in various applications, such as multi-channel imaging and information encryption technology.

Interleaved coding Janus metasurface with independent transmission and reflection phase modulation.

An interleaved coding Janus metasurface is proposed, which can generate bidirectional functionalities with full phase control of the reflected and transmitted waves. By introducing rotation and geometric parameter changes into the meta-atoms, the reflection and transmission channels with required energy distribution and foci are realized. More remarkably, our approach is based on a single metasurface design that arranges two types of unidirectional propagating unit structures with simultaneous desired reflection and transmission properties into a checkerboard configuration to obtain four different holograms. The results verify the excellent performances of the multifunctional metasurface, laying a foundation for manipulation of EM waves with more degree of freedom, and promoting its applications in the entire frequency spectrum.

Metasurface-Based Optical Logic Operators Driven by Diffractive Neural Networks.

In this paper, a novel optical logic operator based on the multifunctional metasurface driven by all-optical diffractive neural network is reported, which can perform four principal quantum logic operations (Pauli-X, Pauli-Y, Pauli-Z, and Hadamard gates). The two ground states | 0 ⟩ $|0 \rangle $ and | 1 ⟩ $|1 \rangle $  are characterized by two orthogonal linear polarization states. The proposed spatial- and polarization-multiplexed all-optical diffractive neural network only contains a hidden layer physically mapped as a metasurface with simple and compact unit cells, which dramatically reduces the volume and computing resources required for the system. The designed optical quantum operator is proven to achieve high fidelities for all four quantum logical gates, up to 99.96% numerically and 99.88% experimentally. The solution will facilitate the construction of large-scale optical quantum computing systems and scalable optical quantum devices.

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.

Dispersion engineering of spoof plasmonic metamaterials via interdigital capacitance structures.

This work presents an approach to realize the dispersion engineering of spoof plasmonic metamaterials with controllable cutoff frequencies. Interdigital capacitance structures are applied to construct the unit cells. Dispersion properties are firstly analyzed to investigate the effects of interdigital capacitance, and the influence of the geometrical parameters of the proposed unit cell on the cutoff frequencies is studied. Then, a spoof surface plasmon polariton (SSPP) transmission line (TL) is developed based on the proposed unit cell together with a smooth transition. The matching principles of the transition are explained by the dispersion curves and the normalized impedance of the corresponding matching unit cells. Finally, the transmission characteristics of the TL are simulated and measured to validate the feasibility of the proposed strategy. Both the lower and upper cutoff frequencies can be tuned jointly by the extra degrees of freedom provided by the interdigital capacitance structures. In comparison with designs based on a substrate-integrated waveguide (SIW), the proposed strategy can reduce the transversal dimension by a factor of two under the same conditions. This work can greatly accelerate the development of versatile microwave integrated circuits and systems based on spoof plasmonic metamaterials.

High gain/bandwidth off-chip antenna loaded with metamaterial unit-cell impedance matching circuit for sub-terahertz near-field electronic systems.

An innovative off-chip antenna (OCA) is presented that exhibits high gain and efficiency performance at the terahertz (THz) band and has a wide operational bandwidth. The proposed OCA is implemented on stacked silicon layers and consists of an open circuit meandering line. It is shown that by loading the antenna with an array of subwavelength circular dielectric slots and terminating it with a metamaterial unit cell, its impedance bandwidth is enhanced by a factor of two and its gain on average by about 4 dB. Unlike conventional antennas, where the energy is dissipated in a resistive load, the technique proposed here significantly reduces losses. The antenna is excited from underneath the antenna by coupling RF energy from an open-circuited feedline through a slot in the ground-plane of the middle substrate layer. The feedline is shielded with another substrate layer which has a ground-plane on its opposite surface to mitigate the influence of the structure on which the antenna is mounted. The antenna has the dimensions 12.3 × 4.5 × 0.905 mm3 and operates across the 0.137-0.158 THz band corresponding to a fractional bandwidth of 14.23%. Over this frequency range the average measured gain and efficiency are 8.6 dBi and 77%, respectively. These characteristics makes the proposed antenna suitable for integration in sub-terahertz near-field electronic systems such as radio frequency identification (RFID) devices with high spatial resolution.

Metagrating absorber: design and implementation.

The extent to which the introduction of subwavelength spatial modulation of electromagnetic properties improves absorption performances is studied. The proposed absorber represents an evolution from the Salisbury screen, whereby the uniform resistive layer is replaced by a metagrating. A periodic supercell that supports only the specular reflection is first designed, and load impedances are then engineered to suppress this diffraction mode. To experimentally demonstrate the concept, four prototypes are fabricated and tested in the microwave domain around 10 GHz. Furthermore, the performances assessed by a merit factor derived from Rozanov's bound show that the use of metagratings opens up good perspectives for improving the state of the art. Our findings can pave the way toward the development of high performance absorbers for applications across a broad frequency spectrum.

Design of a frequency-multiplexed metasurface with asymmetric transmission.

Metasurfaces presenting diversified functionalities have broadened the prospect of manipulating the phase, amplitude, and polarization from the optical to microwave fields. Although the frequency-multiplexing strategy is one of the intuitive and effective approaches to expand the number of channels, demonstrations reporting on the combination between directional asymmetric transmission and frequency-multiplexing via an ultrathin flat device are limited. In this study, a novel, to the best of our knowledge, strategy is proposed to generate four independent holographic images under opposite illumination directions at two operating frequencies, utilizing a single metasurface composed of two types of metallic resonators and one grating layer. Specifically, each scattering channel with independent information makes full use of the whole metasurface. Simulation and experimental results show good agreement, highlighting the attractive capabilities of the multi-functional metasurface platform, which provides more freedom for the manipulation of electromagnetic waves.

Electronic Beam-Scanning Antenna Based on a Reconfigurable Phase-Modulated Metasurface.

Metasurfaces (MSs) have enabled the emergence of new ideas and solutions in the design of antennas and for the control of electromagnetic waves. In this work, we propose to design a directional high-gain reconfigurable planar antenna based on a phase-modulated metasurface. Reconfigurability is achieved by integrating varactor diodes into the elementary meta-atoms composing the metasurface. As a proof of concept, a metasurface prototype that operates around 5 GHz is designed and fabricated to be tested in an antenna configuration. The metasurface is flexibly controlled by different bias voltages applied to the varactor diodes, thus allowing the user to control its phase characteristics. By assigning judiciously calculated phase profiles to the metasurface illuminated by a feeding primary source, different scenarios of far-field patterns can be considered. Different phase profiles are tested, allowing us to, firstly, achieve a highly directive boresight radiation and, secondly, to steer the main radiated beam towards an off-normal direction. The whole design process is verified by numerical simulations and is validated experimentally by far-field antenna measurements. The proposed metasurface enables the design of directive flat antennas with beam-scanning characteristics without complex feeding systems and power-consuming phase shifters, and thus provides potential interests for next generation antenna hardware.

Single-layer spatial analog meta-processor for imaging processing.

Computational meta-optics brings a twist on the accelerating hardware with the benefits of ultrafast speed, ultra-low power consumption, and parallel information processing in versatile applications. Recent advent of metasurfaces have enabled the full manipulation of electromagnetic waves within subwavelength scales, promising the multifunctional, high-throughput, compact and flat optical processors. In this trend, metasurfaces with nonlocality or multi-layer structures are proposed to perform analog optical computations based on Green's function or Fourier transform, intrinsically constrained by limited operations or large footprints/volume. Here, we showcase a Fourier-based metaprocessor to impart customized highly flexible transfer functions for analog computing upon our single-layer Huygens' metasurface. Basic mathematical operations, including differentiation and cross-correlation, are performed by directly modulating complex wavefronts in spatial Fourier domain, facilitating edge detection and pattern recognition of various image processing. Our work substantiates an ultracompact and powerful kernel processor, which could find important applications for optical analog computing and image processing.

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.

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.

Metasurfaces for Far-Field Radiation Pattern Correction of Antennas under Dielectric Seamed-Radomes.

A high-index dielectric radome seam is camouflaged with respect to a low-index dielectric radome panel by tuning the seam with carefully engineered metasurfaces. A transmission-line approach is used to model the metasurface-tuned seam and analytically retrieve the corresponding surface impedance, from which the unit-cell design is then tailored. Full-wave simulations and microwave antenna measurements performed on a proof-of-concept prototype validate the undesired scattering suppression effect in the case of normally and obliquely incident transverse electric and transverse magnetic wave illuminations. Robustness of the proposed solution to fabrication tolerances is also reported. The study presents metasurface-tuning as an easily implementable, frequency adjustable, and polarization insensitive solution to reduce the scattering of dielectric mechanical seams and improve the overall transparency performance of radome structures.

Meta-hologram enabled by a double-face copper-cladded metasurface based on reflection-transmission amplitude coding.

Here, we propose a double-face copper-cladded meta-hologram that can efficiently manipulate the amplitude of electromagnetic waves in both transmission and reflection spaces, depending on the polarization state of the incident electromagnetic wave. The proposed meta-hologram is validated by encoding the transmission-reflection amplitude information of two independent images into a single metasurface. The holographic images obtained from measurements agree qualitatively with simulation results. The proposed metasurface presents a novel, to the best of our knowledge, scheme for electromagnetic wavefront control in the whole space and overcomes the limitations of narrow frequency band operation.

Field Decorrelation and Isolation Improvement in an MIMO Antenna Using an All-Dielectric Device Based on Transformation Electromagnetics.

This work presents a new technique for enhancing the performance of a multiple-input multiple-output (MIMO) antenna by improving its correlation coefficient ρ. A broadband dielectric structure is designed using the transformation electromagnetics (TE) concept to decorrelate the fields of closely placed radiating elements of an MIMO antenna, thereby decreasing ρ and mutual coupling. The desired properties of the broadband dielectric wave tilting structure (DWTS) are determined by using quasi-conformal transformation electromagnetics (QCTE). Next, the permittivity profile of the DWTS is realized by employing air-hole technology, which is based on the effective medium theory, and the DWTS is fabricated using the additive manufacturing (3D printing) technique. The effectiveness of the proposed technique is verified by designing two-element patch-based MIMO antenna prototypes operating at 3 GHz, 5 GHz, and 7 GHz, respectively. The proposed technique helped to reduce the correlation coefficient ρ in the range of 37% to 99% in the respective operating bandwidth of each MIMO antenna, thereby, in each case, improving the isolation between antenna elements by better than 3 dB, which is an excellent performance.

Generation and deflection control of a 2D Airy beam utilizing metasurfaces.

Self-accelerating optical Airy beams present attractive characteristics such as self-bending and non-diffraction, which have rendered this field a research hotspot in recent years. In this paper, the desired phase changes of the unit cell structure for the transmitted cross-polarized wave can be realized by modifying the rotation angle of the unit cell, while the amplitude can be modulated by changing the inner diameter R of the double layer split-ring resonator (SRR). As such, the amplitude and phase modulations can be performed simultaneously and independently to achieve the desired transmitted wave envelope. Furthermore, a novel, to the best of our knowledge, strategy of 2D Airy beam deflection control is also presented by simultaneously modifying the phase and amplitude of the envelope of the transmitted beam, and its feasibility is theoretically and experimentally demonstrated. Our proposed designs suggest high application potentials in the fields of optical particle manipulation, controllable wireless energy transmission, and complex terrain exploration.