Anomalous Reflector Research Articles

Achieving broadband multifunctional modulation of wavefront via terahertz anisotropic metasurface

Vanadium dioxide (VO2) has become a reliable phase change material (PCM) and is increasingly utilized in multifunctional metasurface design. This study introduces designs of bilayer anisotropic VO2meta-atoms in simulation for multifunctional wavefront modulation in the terahertz band. By leveraging the phase change property of VO2 and incident wave polarization, a reconfigurable metasurface composing anisotropic meta-atoms is proposed with four distinct functions. When VO2 is in its insulating state, the metasurface serves as a hologram generator for the x-polarized wave and a focusing reflector for the y-polarized wave. Upon the transition of VO2 into the metallic state, the metasurface functions as a generator of vortex beam for the x-polarized wave, while as an anomalous reflector for the y-polarized wave. The proposed metasurface showcases an outstanding broadband performance, delivering impressive four-channel wavefront modulation within a frequency range of 2.2–3.0 THz. Depending on the compact structure and integrated multi-functionalities, our design may have immense potential across a wide range of terahertz applications, including communication exchange and multiplexing utilization.

Chirality‐Switching and Reconfigurable Spin‐Selective Wavefront by Origami Deformation Metasurface

AbstractOrigami metasurfaces have emerged as a versatile platform for dynamically manipulating electromagnetic waves due to their numerous advantages, including cost‐effectiveness, lightweight nature, and diverse structural transformations. They are particularly well‐suited for reconfigurable chiroptical responses that can find utility in various applications. However, current research has primarily focused on modulating spectral properties like optical chirality and resonant frequencies, which may limit their practical applications. Herein, an origami metasurface enabled by a simple yet highly effective deformation method is proposed to achieve simultaneous tunable chirality and wavefront manipulation, which expands the modulation freedom of mechanical deformation in chiral metasurfaces. The planar metasurface with spin‐insensitive transmittance is bulked into 3D metasurfaces to break the mirror symmetry, thereby triggering pronounced chiral responses. Additionally, through the implementation of propagation phase design, two types of meta‐atoms with distinct phase responses are interleaved to create a chiral anomalous reflector that spin‐selectively modulates the reflection beam. The experimental results agree well with the simulation results and theoretical predictions, verifying the high quality of the proposed design that may offer untapped potential for applications such as reconfigurable photonics devices.

A Novel Geometry-Based 3-D Wideband Channel Model and Capacity Analysis for IRS-Assisted UAV Communication Systems

Intelligent reflecting surface (IRS) composed of a large number of low-cost passive reflecting elements has attracted significant attention from communication communities because of its ability to substantially improve the communication performance. In this paper, the aperture area and radiation pattern of the IRS reflecting elements are considered, and a novel geometry-based three-dimensional (3-D) wideband channel model is proposed for IRS-assisted unmanned aerial vehicle (UAV) communication systems. In the proposed model, the reflection phase is designed by jointly considering the aperture area of the IRS reflecting element and the propagation phases among UAV, IRS, and receiver (Rx), each IRS reflecting element is modeled as an anomalous reflector instead of a specular reflector, and large-scale IRS reflecting elements can jointly beamform the signal in a desired direction. Based on the proposed model, the effects of arbitrary trajectory of UAV and the number and the size of passive reflecting elements on channel statistical characteristics are considered, and the average received signal power and the ergodic sum capacity, which consider the impacts of the number and the size of passive reflecting elements, are also investigated. Furthermore, the path loss of the IRS-assisted link, which is in inverse proportion to the square of aperture area of IRS reflecting elements, is derived, and it coincides with the measured results in real outdoor scenarios. Analysis shows that the communication performance can be enhanced by increasing the number and the size of IRS reflecting elements, and it is validated by numerical results and Monte-Carlo simulation results.

Efficient Anomalous Reflector Design Using Array Antenna Scattering Synthesis

A perfect anomalous reflector is designed by employing the receiving and scattering array antenna theory to optimize the scattering characteristics of a planar reflecting surface. For periodic reflectors with a supercell consisting of a relatively few reactively loaded radiating elements, reflection amplitudes into propagating Floquet modes are controlled in an algebraic optimization of the load reactances, avoiding brute-force optimization via electromagnetic simulations. Wide-angle reflectors are numerically designed and compared with the conventional reflectarray designs, showing that the proposed design approach achieves reflection efficiencies higher than the linear reflection phase gradient method in a computationally efficient manner.

Fully Analytical Design of Dual-Wire PCB Metagratings for Beam Steering and Splitting

The printed circuit board (PCB) type metagrating (MG), a periodic microstrip planar structure with discrete distribution of polarization currents, can realize high-angle wavefront shaping with near-unity efficiency for potential application in high beam-steering antennas or reflectors. For large angle deflection, only a small number of meta-atoms are required in the metagrating’s period. As such, here we focus on a reflective PCB MG in the presence of only two meta-atoms, whose complex load impedance densities is explicitly derived from fully analytical expressions. The passive and lossless conditional equations as well as the equations for the power ratio control of the diffraction modes are given, which allows avoiding numerical optimization procedure. Three practical prototypes, including anomalous reflector and beam splitters are designed, simulated and measured. The measurement results agree well with the simulation results and are consistent with the theoretical predictions.

Vanadium dioxide-based terahertz metasurfaces for manipulating wavefronts with switchable polarization

As a unique phase transition material, vanadium dioxide (VO2) has been widely applied in switchable and modulating metasurfaces. In this work, VO2-based reflective meta-atoms are designed to achieve wavefront reconfiguration in terahertz frequency. When VO2 is metal, four optimized meta-atoms with 90o phase difference provide a full phase coverage, and they achieve high polarization conversion efficiency. After phase transition of VO2, the capability in the metallic state is lost. Thus, the corresponding phase coverage is switched from 360° to 0°, and the phenomenon of polarization conversion almost vanishes. Assembled by these meta-atoms, three metasurfaces are numerically presented. When VO2 is in the state of metal, they are anomalous reflector, metalens, and vortex beam generator. The presented metasurfaces own the abilities of wavefront manipulation and polarization conversion. When VO2 is in the insulating state, three metasurfaces are switched to specular reflector. Simulations are conducted to verify their abilities of wavefront modulation and polarization control. The proposed metasurfaces may have potential applications in the fields of terahertz switching, communication, and focusing.

AVA Analysis of BSR in Fractured filled Gas-hydrates Reservoir in Krishna-Godavari Basin, India

Abstract Gas-hydrates, a major unconventional energy resources of the future, are delineated by seismic experiment through the identification of an anomalous reflector, known as the bottom simulating reflector or BSR on seismic data. The amplitude variation with angle (AVA) from the BSR can be used for quantitative assessment of gas-hydrates and free-gas across the BSR. We have performed the AVA analysis of BSR in the Krishna Godavari (KG) basin of India, where gas-hydrates have been recovered in fractured shale by Exp-01 of Indian National Gas Hydrates Program. The study shows negative intercept and positive gradient along two perpendicular seismic lines, one passing through the well at site-10 of Exp-01. The intercept of -0.01 to -0.15 and gradient of 0.01 to 0.4 are interpreted as class-IV anomaly, which is due to anisotropy caused by fractures filled with gas-hydrates. We have modelled the observed AVA pattern using anisotropic equation for horizontally symmetric transversally isotropic (HTI) media in which gas-hydrates occur in the fractures, roughly aligned at angle of 45° above the BSR and patchy gas distribution below the BSR. The results show 0-31% variable gas-hydrates in fractured morphology and 0-11% free gas below the BSR respectively.

Empirical mode decomposition approach for delineating gas-hydrates and free gas in Mahanadi offshore, eastern Indian margin

Empirical mode decomposition (EMD) is an effective tool for signal analysis that splits the data into individual modes, called the intrinsic mode functions, which are associated with symmetric and narrow-band waveform ensuring that the instantaneous frequencies are smooth and positive. However, some negative features encumber its direct application namely the mode mixing and splitting, aliasing and endpoint artefacts. Two variants, ensemble EMD (EEMD) and complete ensemble EMD (CEEMD) have been recently introduced to overcome these problems. We intend to show the application of the EMD for demarcating the zones of gas-hydrates and free-gas bearing sediments. Gas-hydrates are ice-like crystalline substances that occur in shallow sediments along the outer continental margins and in the permafrost regions, and are considered as viable major future energy resources of the world. Gas-hydrates in marine environment are generally identified by an anomalous reflector, known as the bottom simulating reflector, on seismic section. The present study demonstrates that the EMD can be effectively utilised in demarcating the zones of gas-hydrates and free-gas bearing sediments with a field example in the Mahanadi basin of the eastern Indian margin.

A Novel Physics-Based Channel Model for Reconfigurable Intelligent Surface-Assisted Multi-User Communication Systems

The reconfigurable intelligent surface (RIS) is one of the promising technologies contributing to the next generation smart radio environment. A novel physics-based RIS channel model is proposed. In the model, the signal reflected through the scatters and the RIS elements are jointly studied as multipath components of the overall received envelope. This novel strategy simplifies the mathematical structure of the channel gain and is able to compactly derive the distribution of the overall channel. For the case of continuous phase shifts, the distribution depends on the number of elements of the RIS and the observing direction of the receiver. For the case of discrete phase shifts, the distribution further depends on the number of phase quantization levels. The scaling law of the average received power is obtained from the scale factor of the distribution. For the application scenarios where RIS functions as an anomalous reflector, we investigate the performance of single RIS-assisted multiple access networks for time-division multiple access (TDMA), frequency-division multiple access (FDMA), and non-orthogonal multiple access (NOMA). Closed-form expressions for the outage probability of the proposed channel model are derived. It is proved that a constant diversity order exists, which is independent of the number of RIS elements. Simulation results are presented to confirm that the proposed model applies effectively to the element-based RISs.

Multifunctional phase modulated metasurface based on a thermally tunable InSb-based terahertz meta-atom

Terahertz (THz) metasurfaces composed of Pancharatnam-Berry (PB) meta-atoms have great potential for applications in THz imaging, biological sensing, and optical communication. However, traditional THz PB metasurfaces suffer from inflexible electromagnetic responses and complicated structures. Here, we propose a thermally tunable reflection-type InSb-based THz PB meta-atom, which can not only convert the incident circularly-polarized (CP) wave into cross-polarized components but also adjust the reflection efficiency by increasing the temperature of InSb from 220 K to 360 K. Moreover, various functional devices, including anomalous reflector, reflection-type metalens, and reflection-type OAM beam generators, are investigated with the finite difference time-domain (FDTD) method by using the proposed meta-atom. The working states of these devices can be switched from “ON” to “OFF” at the frequency of 1 THz successfully by changing the temperature of InSb from 220 K to 360 K. This work not only paves a way for the study of tunable multifunctional THz PB devices, but also promotes the practical applications of THz metasurfaces.

Physics-Based Modeling and Scalable Optimization of Large Intelligent Reflecting Surfaces

Intelligent reflecting surfaces (IRSs) have the potential to transform wireless communication channels into smart reconfigurable propagation environments. To realize this new paradigm, the passive IRSs have to be large, especially for communication in far-field scenarios, so that they can compensate for the large end-to-end path-loss, which is caused by the multiplication of the individual path-losses of the transmitter-to-IRS and IRS-to-receiver channels. However, optimizing a large number of sub-wavelength IRS elements imposes a significant challenge for online transmission. To address this issue, in this article, we develop a physics-based model and a scalable optimization framework for large IRSs. The basic idea is to partition the IRS unit cells into several subsets, referred to as tiles, model the impact of each tile on the wireless channel, and then optimize each tile in two stages, namely an offline design stage and an online optimization stage. For physics-based modeling, we borrow concepts from the radar literature, model each tile as an anomalous reflector, and derive its impact on the wireless channel for a given phase shift by solving the corresponding integral equations for the electric and magnetic vector fields. In the offline design stage, the IRS unit cells of each tile are jointly designed for the support of different transmission modes, where each transmission mode effectively corresponds to a given configuration of the phase shifts that the unit cells of the tile apply to an impinging electromagnetic wave. In the online optimization stage, the best transmission mode of each tile is selected such that a desired quality-of-service (QoS) criterion is maximized. We consider an exemplary downlink system and study the minimization of the base station (BS) transmit power subject to QoS constraints for the users. Since the resulting mixed-integer programming problem for joint optimization of the BS beamforming vectors and the tile transmission modes is non-convex, we derive two efficient suboptimal solutions, which are based on alternating optimization and a greedy approach, respectively. We show that the proposed modeling and optimization framework can be used to efficiently optimize large IRSs comprising thousands of unit cells.

MIMO-NOMA Networks Relying on Reconfigurable Intelligent Surface: A Signal Cancellation-Based Design

Reconfigurable intelligent surface (RIS) technique stands as a promising signal enhancement or signal cancellation technique for next generation networks. We design a novel passive beamforming weight at RISs in a multiple-input multiple-output (MIMO) non-orthogonal multiple access (NOMA) network for simultaneously serving paired users, where a signal cancellation based (SCB) design is employed. In order to implement the proposed SCB design, we first evaluate the minimal required number of RISs in both the diffuse scattering and anomalous reflector scenarios. Then, new channel statistics are derived for characterizing the effective channel gains. In order to evaluate the network’s performance, we derive the closed-form expressions both for the outage probability (OP) and for the ergodic rate (ER). The diversity orders as well as the high-signal-to-noise-ratio (SNR) slopes are derived for engineering insights. Moreover, the network’s performance of a finite resolution design has been evaluated. Our analytical results demonstrate that: i) the inter-cluster interference can be eliminated with the aid of large number of RIS elements; ii) the line-of-sight of the BS-RIS and RIS-user links are required for the diffuse scattering scenario, whereas the LoS links are not required for the anomalous reflector scenario.

Multifunctional space–time phase modulated graphene metasurface

Metasurfaces provide special features for manipulating electromagnetic wavefronts that are not possible with conventional optical devices. A common approach in designing metasurfaces has been the use of spatially varying metallic and/or dielectric nanoantennas separated with a subwavelength distance to obtain the required local phase change yielding the desired optical performance. In this paper, we propose a space–time phase modulation technique based on a graphene metasurface with the possibility of actively manipulating the electromagnetic wavefront. In this technique, we utilize graphene microribbon arrays that exhibit resonant behavior at terahertz (THz) frequencies. By applying an alternating voltage with a particular modulation frequency and phase, the time-dependent changes in the complex refractive indices of the graphene ribbons can be induced. This phenomenon results in the active control of the reflection amplitude and phase and the generation of the harmonic frequencies in the output reflection spectra. Theoretically, by using the Floquet analysis, it is shown that the reflected wave has harmonic frequencies, and the phase of the reflection wave at each harmonic component changes through changing the modulation phase of each graphene ribbon. The performance of the wavefront manipulation technique is evaluated using the finite difference time domain method and the circuit model. The results of the proposed circuit model are in good agreement with those of the full-wave simulation. Additionally, the applications of the proposed space–time phase modulated graphene metasurface for realizing an anomalous reflector and a lens with a tunable focal length are explained in detail.

Angularly Dispersionless Scattering Patterns for Impenetrable Surfaces: A straightforward design based on transformation optics

Utilizing transformation optics (TO) methodology, we propose a feasible and straightforward approach to control the scattering pattern of impenetrable surfaces in the desired manner. As a great advantage not reported in previous studies, the presented coating layer can scatter multiple arbitrarily oriented beams whose directions are not affected by the frequency variation, namely as angularly dispersionless scattering. We benefit from the simplicity of classic geometrical optics (GO) as a design tool to avoid the complex procedure of the material derivation resulting from TO. Inspired by irregularities in rough surfaces, we propose a coating layer mimicking the scattering behavior of rough surfaces with high abilities to generate different far-field patterns with arbitrary beam numbers along the desired directions. Moreover, two interesting functionalities, i.e., anomalous reflection and retroreflection, are designed using the proposed coating layer. The material parameters of the proposed coating layer are numerically derived by solving the inverse Laplace equation. Our idea is further extended to design a scattering diffusion device with the quasi-isotropic pattern, which is applicable in radar cross-section reduction. As a demonstration, an anomalous reflector is implemented with an all-dielectric, lossless, and broadband coating layer. The experimental results are in good agreement with the numerical simulations.

Device-quality, reconfigurable metamaterials from shape-directed nanocrystal assembly

Anchoring nanoscale building blocks, regardless of their shape, into specific arrangements on surfaces presents a significant challenge for the fabrication of next-generation chip-based nanophotonic devices. Current methods to prepare nanocrystal arrays lack the precision, generalizability, and postsynthetic robustness required for the fabrication of device-quality, nanocrystal-based metamaterials [Q. Y. Lin et al. Nano Lett. 15, 4699-4703 (2015); V. Flauraud et al., Nat. Nanotechnol. 12, 73-80 (2017)]. To address this challenge, we have developed a synthetic strategy to precisely arrange any anisotropic colloidal nanoparticle onto a substrate using a shallow-template-assisted, DNA-mediated assembly approach. We show that anisotropic nanoparticles of virtually any shape can be anchored onto surfaces in any desired arrangement, with precise positional and orientational control. Importantly, the technique allows nanoparticles to be patterned over a large surface area, with interparticle distances as small as 4 nm, providing the opportunity to exploit light-matter interactions in an unprecedented manner. As a proof-of-concept, we have synthesized a nanocrystal-based, dynamically tunable metasurface (an anomalous reflector), demonstrating the potential of this nanoparticle-based metamaterial synthesis platform.

Perfect anomalous reflection using a compound metallic metagrating.

Metagrating is a new concept for wavefront manipulation that, unlike phase gradient metasurfaces, does not suffer from low efficiency and also has a less complicated fabrication process. In this paper, a compound metallic grating (a periodic metallic structure with more than one slit in each period) is proposed for anomalous reflection. We propose an analytical method for analyzing the electromagnetic response of this grating. Closed-form and analytical expressions are presented for the reflection coefficients of zeroth diffracted order and also higher diffracted orders. The proposed method is verified against full-wave simulations and the results are in excellent agreement. Thanks to the geometrical asymmetry of compound metallic grating, it can be used for designing anomalous reflection at the normal incidence. Given analytical expressions for reflection coefficients, we design a perfect anomalous reflector for a TM polarized plane wave via transferring all the incident power to ( - 1) diffraction order . The structure designed in this study has an unprecedented near-to-unitary efficiency of 99.9%. Finally, a multi-element compound metallic grating is proposed for reflecting the normal incidence to angles of below 30°, which is a challenging accomplishment. This excellent performance of compound metallic grating shows its high potential for microwave and terahertz wavefront manipulation applications.

Low-index second-order metagratings for large-angle anomalous reflection.

A large-angle anomalous reflector based on a low-index polylactic acid metagrating is designed around 140GHz. By breaking the limit of fixed bar-to-bar distance and directing the beam into the second diffraction order through the optimization of the 4π phase supercell, the efficiencies for 70° and 80° reflection under normal excitation both reach 0.82 with a wide bandwidth. In contrast, the efficiency is less than 0.2 in conventional designs. The success of the second-order low-index metagrating for large-angle deflection originates from the appropriate number of Bloch modes with well-engineered mode interactions and couplings. The design can be potentially fabricated by three-dimensional printing, with promising applications in designing flat lenses of high numerical aperture and extending the material system of metadevices.

Ultra-High Efficiency and Broad Band Operation of Infrared Metasurface Anomalous Reflector based on Graphene Plasmonics

Infrared metasurface anomalous reflector with ultra-high efficiency and broad band operation is designed via multi-sheet graphene layer with triangular holes. The anomalous reflection angle covers the range of 10° to 90° with the efficiency higher than 80%, over a broad spectral range from 7 μm–40 μm of infrared spectrum. It reaches above 92% at the center wavelength in the spectral response. By increasing the periodicity of phase gradient, we can expand this frequency band even further without losing efficiency. The compact design of metasurface affords the adjustability of the electrochemical potential level of graphene by means of gating. Additionally, the impact of the number of graphene sheets for the optimum efficiency of the proposed structure is investigated. By adding the secondary graphene metasurface with opposite direction of phase gradient, we demonstrated the tunability of the reflection angle from θr to −θr with bias voltage.

Acoustic anomalous reflectors based on diffraction grating engineering

We present an efficient method for the design of anomalous reflectors for acoustic waves. The approach is based on the fact that the anomalous reflector is actually a diffraction grating in which the amplitude of all the modes is negligible except the one traveling towards the desired direction. A supercell of drilled holes in an acoustically rigid surface is proposed as the basic unit cell, and analytical expressions for an inverse diffraction problem are derived. It is found that the the number of holes required for the realization of an anomalous reflector is equal to the number of diffracted modes to cancel, and this number depends on the relationship between the incident and reflected angles. Then, the "retrorreflection" effect is obtained by just one hole per unit cell, also with only two holes it is possible to change the reflection angle of a normally incident wave and five holes are enough to design a general retroreflector changing the incident and reflected angles at oblique incidence. Finally, the concept of Snell's law violation is extended not only to the incident and reflected angles, but also to the plane in which it happens, and a device based on a single hole in a square lattice is designed in such a way that the reflection plane is rotated $\pi/4$ with respect to the plane of incidence. Numerical simulations are performed to support the predictions of the analytical expressions, and an excellent agreement is found.

Perfect reflection control for impenetrable surfaces using surface waves of orthogonal polarization

For impenetrable electromagnetic surfaces, a metasurface design approach for perfect control of the reflection phenomena using gradient anisotropic tensor surface impedance is presented. It utilizes a set of orthogonally polarized auxiliary surface waves to create pointwise reactive impedance characteristics by channeling power along the tangential direction of the surface in the near zone in a carefully designed manner. The propagating incident and reflected fields do not interfere with the surface waves due to the polarization orthogonality. Design examples of an anomalous reflector and a power splitter for an incident plane wave are presented and numerically verified. Realization possibilities using an array of rotated metallic resonators on a thin grounded dielectric substrate are discussed.