Anomalous Reflection Research Articles

Anomalous reflection from a two-layered marine sediment.

This paper concerns the theory of acoustic reflection from a two-layered marine sediment, the upper layer of which consists of a fine-grained material (mud). The seawater above and basement below the layer are treated as homogeneous half-spaces. Within the mud layer, the density is taken to be constant, and three sound speed profiles are considered: uniform, linear, and inverse-square. The reflection coefficient exhibits a background component that is similar in all three cases, exhibiting only a weak sensitivity to the gradient of the profile, the frequency, and the depth of the layer. Additionally, the two profiles with a non-zero gradient, linear and inverse-square, exhibit a sequence across grazing angle of narrow spikes of total reflection. The angular distribution of this acoustic glint is highly sensitive to the frequency and depth of the layer, and mildly so to the gradient. As the gradient approaches zero, the glint vanishes and the reflection coefficient reduces identically to the form of a uniform sound speed profile. If it were detectable, the angular distribution of the glint, observed at several frequencies, could constitute a unique, sensitive set of "fingerprints," allowing the depth and sound speed gradient of the mud layer to be inferred.

Kissing-loop nano-kirigami structures with asymmetric transmission and anomalous reflection

Kissing-loop nano-kirigami structures with asymmetric transmission and anomalous reflection

All-Angle Scanning Perfect Anomalous Reflection by Using Passive Aperiodic Gratings

All-Angle Scanning Perfect Anomalous Reflection by Using Passive Aperiodic Gratings

Broadband polarization conversion metasurface for beam deflection with switchable characteristics

A broadband polarization conversion metasurface (PCM) for anomalous reflection and beam deflection with switchable characteristics.

Multifunctional metasurface for Terahertz wavefront manipulation based on the combination of vanadium dioxide and graphene

In this paper, we propose a multifunctional wavefront control metasurface based on vanadium dioxide (VO2) and graphene. The unit cell structure contains a VO2 substrate, three rectangular metal strips as the bottom layer, orthogonal graphene strips as the top layer, and an intermediate layer consisting of a split ring resonator and circular metal patches. The desired performances can be achieved by changing the parameters of the split ring resonator in the middle layer, as well as controlling the phase transition state of VO2 and the Fermi level of graphene. When VO2 is in the insulating phase and the Fermi level of graphene is 1 eV, a transmissive metasurface is established, which realizes cross-polarization conversion, anomalous refraction, focusing lens, and orbital angular momentum generation at the frequency of 0.63 THz by changing the geometric sizes of the unit cell. When VO2 is in the metal phase and the Fermi level of graphene is 0.01 eV, a reflective metasurface can be achieved, which realizes terahertz beam splitting, radar cross section, and anomalous reflection of a single beam at the frequency of 0.63 THz. This multifunctional wavefront control metasurface treating the combination of VO2 and graphene exhibits significant application prospects in terahertz transmission, imaging, and communication.

Reflected wave manipulation by aeroacoustic metasurfaces in sheared grazing flows

Acoustic metasurfaces (AMs) are artificial structures possessing exotic acoustic properties, such as negative refraction and perfect absorption, etc., in sub-wavelength thickness scales. Most previous studies on AMs were conducted in laboratory conditions with a static medium, limiting their applications in practical conditions, e.g., the aircraft engines where an aerodynamic background flow is present. To fill this gap, we study the effects of a grazing shear flow on the reflection performance of periodic AMs in this work. An AMs design method is proposed to realize effective wavefront manipulation with the consideration of the shear flow. The strategy is to form a linear reflected phase-shifting covering 0 to 2π above the shear layer. The reflection characteristics of two acoustic porous metasurfaces (APMs), i.e., APM1 and APM2 designed for the uniform and the sheared flows, respectively, are numerically investigated. Results show that the presence of the shear layer reduces the effectiveness of APM1 under oblique incident waves due to the considerable refraction effect. In contrast, under the same flow conditions, APM2 realizes desired wave manipulation behaviors, such as anomalous reflection and surface wave conversion. This study provides guidelines for AMs design in flow conditions, which offers potential for noise control in aeroacoustic problems.

Acoustic Evidence of Shallow Gas Occurrences in the Offshore Sinú Fold Belt, Colombian Caribbean Sea

High-resolution seismic analysis and bathymetry data, used in the Offshore Sinú Fold Belt (OSFB), have revealed seabed and sub-surface anomalies, which were probably caused by the presence of shallow gas within the sedimentary records. Shallow gas is widely detected by the frequent presence of anomalous acoustic reflections including acoustic blanking, enhanced reflections, acoustic plumes, pockmarks, and dome structures. More than 30 anomalies that occur within a subsurface depth of ~65 m were acoustically detected within an area of 1000 km2 on the continental shelf and upper continental slope, in water depths ranging from −20 to −1300 m. Moreover, a map with the spatial distribution of the gas occurrences is shown. A close relationship between the locally elevated seabed (dome structures), pockmarks, and acoustic blanking was found. Most of the active pockmarks may be closely related to the submarine path of the Uramita Fault, indicating that the gas occurrences are controlled by active faulting. The shallow gas occurrence was confirmed by the generation of authigenic carbonate and the occurrence of chemosymbiotic biological communities sampled in the area. Although there is an admixture of biogenic gas, it is believed that many of the features observed relate to thermogenic gas. The identification of these anomalies represents a useful basis for an assessment of marine geohazards and can serve as a hydrocarbon exploration tool.

Accurate P-wave reflection and transmission coefficients for non-welded interface incorporating elasto-plastic deformation

P-wave reflection and transmission coefficients for non-welded interface play crucial roles in broad practical engineering productions, involving fracture properties prediction and seismic inversion. However, the existing reflection coefficient equations for non-welded interface in elasto-plastic media are seldom studied, although the elasto-plastic deformation is frequently encountered in the Earth’s subsurface due to artificial and tectonic activities. In this study, we proposed the accurate reflection and transmission coefficients equation for a non-welded interface embedded in an elasto-plastic deformed medium based on the elasto-plastic acoustoelastic and linear-slip theory. In detail, this paper uses elasto-plastic acoustoelastic theory to derive the reflection and transmission coefficients equation. The reflection and transmission coefficients matrix are solved using the linear-slip theory as the boundary condition. Moreover, we use the hardening parameter and plastic deformation to represent the plastic properties of the rock, which is a function of stress and plastic deformation. Through Numerical analysis, the deformation caused by static stress has significantly changed the amplitude and the slope of the reflection and transmission coefficients amplitude. As the stress increases, the rock’s velocity becomes higher, and all reflection and transmission coefficients (i.e., RPP, RPS, TPP, TPS) abruptly change at the critical angle. Furthermore, with the increase in plastic deformation, the critical angle of the incident P-wave and the hardening parameter becomes larger than the unstressed state. The non-welded interface exhibits a low-pass frequency filter for reflected SV-waves and a high-pass frequency filter for reflected P-waves and transmitted P and SV waves. In addition, we can observe that static vertical stress can weaken the anomalous reflections caused by non-welded formations, but the effect is insignificant. On the other hand, the effect of fracture normal compliance to reflection and transmission is detailly investigated. When N<2.5*10-10(MPa-1), The non-welded interface is close to the welded interface, while N>2.5*10-5(MPa-1), the non-welded interface is close to the solid-air interface.

Synthesis of Polarization-Independent Perfect Anomalous Reflectors via Modulated Metasurfaces and an Analytical Design Model

A systematic approach to the synthesis of polarization-independent metasurfaces for anomalous reflection is presented. A fast analytical procedure is laid out to analyze the electromagnetic (EM) behavior of an impedance-modulated metasurface for arbitrary angles of incidence and reflection, subsequently serving as a means to provide the necessary impedance profile for reflection toward a specific direction, while capturing all propagation mechanisms, including surface waves (SWs) and nonlocal effects. We apply this procedure to design a metasurface for reflecting a normal incident wave to a different reflection angle, where the synthesized impedance is approximated by a discrete design, based on orthogonal metallic patches. This is done to provide control of both cases of incident wave polarization, while the unit cell analysis and extraction of equivalent impedance is easily performed by a full-wave computational eigenmode analysis of the unit cell. The unit cell design is performed by a simple fine-tuning process with minimal geometric modifications, and the actual discrete metasurface design is proven to agree well with the continuous model. The validity of the synthesis approach is perfectly demonstrated by the fabrication and experimental verification of a model design.

Multifunctional and tunable terahertz coding metasurfaces based on vanadium dioxide

Metasurfaces play a pivotal role in terahertz wave modulation and control. However, persistent challenges in metasurface development include non-reusability and the complexity of structural adjustments. In this paper, we present a solution: a multifunctional tunable coding metasurface combined with a phase change material (PCM) utilizing vanadium dioxide (VO2). Initially, we design 1-bit coding metasurfaces based on phase gradients, enabling dual-beam and four-beam reflection. Building on this, we develop 2-bit coding metasurfaces, enabling anomalous reflection of a single beam in four distinct directions. Subsequent implementation of binary addition further expands the azimuthal angle of the reflected wave to encompass eight directions. Notably, by modulating the conductivity of vanadium dioxide, the single-state coding metasurface transition from phase regulation to amplitude regulation. This transition leads to absorption resonance frequencies at 0.35 THz, 0.39 THz, 0.47 THz, and 0.66 THz, achieving absorption rates exceeding 90%. As a result, these multifunctional coding metasurfaces seamlessly integrate electromagnetic wave amplitude and phase control techniques. They boast a straightforward yet powerful structure, demonstrating exceptional reusability and adjustable configurability for the metasurfaces.

Near-perfect retroreflection of flexural waves via optimized elastic metagratings

The potential of elastic wave retroreflection is substantial for various applications such as structural identification and detection. However, realizing accurate retroreflection presents significant challenges owing to the need for high-efficiency anomalous reflection from boundaries. This study delves into the relatively less-explored field of flexural wave retroreflection in thin plates and proposes the use of metagratings for its realization. The present metagratings, which are built from unit cells created by possibly-overlapping circular holes, offer a simple fabrication process owing to their straightforward geometry. They hold an advantage over metasurfaces, which often struggle with wave steering because of their unmanageable higher-order diffraction modes. However, no systematic approach has been proposed so far to design metagratings for flexural wave retroreflection. To address this gap, we have established an optimization problem that identifies the optimal geometries and locations of the circular holes built into the proposed retroreflecting metagratings using a gradient-based optimizer. The design process involves diffraction grating theory and spatial fast Fourier transform. Both single and multiple retroreflections at various angles were considered and the numerical findings were substantiated by experimental results. The findings of this study imply that the investigated metagratings and design approaches have promising potential for more sophisticated wave manipulations.

Manipulation of reflected acoustic wavefront by a comb-like metasurface

This work proposes a design strategy for an acoustic metasurface with the goal of flexibly modulating reflected waveforms. The metasurface unit is composed of a rigid base and a column of rectangular partitions arranged in a comb-like configuration. A full 2π phase shift of the reflected waves can be achieved by adjusting the height of these rectangular partitions. The utilization of a particle swarm optimization algorithm determines the specific phase shift of the units along the metasurface. As a pragmatic example, it is shown that the comb-like metasurface offers a superior ability to direct the propagation of reflected sound waves in the desired directions, and experiments are performed to verify this anomalous reflection phenomenon. Furthermore, numerical calculations effectively demonstrate the application of the acoustic ground illusion. This work presents an effective and feasible design scheme for acoustic wave manipulation.

Design of Anomalous Reflectors by Phase Gradient Unit Cell-Based Digitally Coded Metasurface

In this letter, we propose the designs of 1-bit and 2-bit digitally coded metasurfaces, which achieve anomalous reflection-based beam-steering. First, we demonstrate the systematic design flow of coded metasurface using phase gradient digital unit cells to achieve anomalous reflection in the preferred direction for a normally incident plane wave. Initially, the design procedure is analytically implemented to get the phase profiles for 1-bit and 2-bit digitally coded metasurfaces having equal sizes of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$10\lambda _{0} \times 10\lambda _{0}$</tex-math></inline-formula> (where <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\lambda _{0}$</tex-math></inline-formula> is the free space wavelength at 5.9 GHz) for a specific reflection angle, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$(\theta _{r}=30^{0},\phi _{r}=0^{0})$</tex-math></inline-formula> , and subsequently, the far-field plots are generated. Furthermore, these phase profiles are used to generate 3D-CAD models of the 1-bit and 2-bit metasurfaces using CST, and the desired far-field patterns are obtained having half-power beamwidth (HPBW) of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$5^{0}$</tex-math></inline-formula> . It is observed that the 1-bit coded metasurface produces additional side-lobe levels, which are minimized by use of 2-bit coding. Finally, the proposed 1-bit and 2-bit coded metasurfaces are fabricated, and the anomalously reflected far-field beam is detected by the received power at desired reflection angle and comparing the results with a perfect electric conductor (PEC). The proposed design is suitable for V2X communications and can be scaled to other frequencies.

An anisotropic broadband coding metasurface based on ultralight graphene‐assembled film

AbstractAs theoretical research further develops, coding metasurfaces have been widely studied and applied due to their uniqueness in digital characterization and sequence arrangement. In this work, based on highly conductive graphene‐assembled film (GAF), an ultralight anisotropic broadband coding metasurface (ABCM) is proposed. The GAF ABCM consists of 16 units with specific phase responses and enables complete metal substitution. By arranging the unit cells in a specific order, the GAF ABCM realizes the functions of anomalous reflection, polarization conversion, and radar cross‐section (RCS) reduction. Results validate that the GAF ABCM has a measured 10 dB normal incidence backward RCS reduction within the working frequency band of 7–18 GHz, which fully covers the X‐band and Ku‐band for radar application. Furthermore, due to the lightweight property of GAF, the density of the GAF ABCM is only 0.06 g/cm3 (surface mass density of 0.038 g/cm2). All results indicate that the lightweight GAF metasurface is promising in radar stealth and communication fields.

Phase-Shift Design and Channel Modeling for Focused Beams in IRS-Assisted FSO Systems

As a potential solution for the backhaul of next-generation mobile or low Earth orbit (LEO) satellites, free-space optical (FSO), has received much attention recently. Various techniques have been suggested for employing an intelligent reflecting surface (IRS) in FSO systems, such as anomalous reflection, power amplification, and beam splitting. It is also possible to deliver more power to the receiver (Rx) by collimating or focusing the reflected beam at the Rx lens. In this paper, we propose a phase-shift design of an IRS for beam focusing. In addition, we propose a new pointing error model and an outage performance analysis for the case when the beam width is comparable to or less than the aperture size of the Rx. The analytical results are validated by Monte Carlo simulations. This paper provides essential preliminary results for future research that assumes a focused beam in FSO systems.

The Anomalous Reflection and Refraction of Metasurface in the View of Solar Concentrators

Solar thermal power generation integrates photothermal conversion power generation and low-cost, large-scale heat storage, which is the primary support for constructing a green power system with new energy as the main body. The concentrating system is a critical component of solar thermal power plants. However, traditional concentrators are bulky and require complex solar tracking systems, limiting the reduction of solar thermal power costs. The metasurface can freely regulate electromagnetic waves and can be used in the field of solar concentrators, making it suitable for developing lightweight, low-cost, tracking-free solar concentrators and being a transformative technological solution in the energy field. Considering that the solar position varies with respect to a fixed planar metasurface concentrator, in this paper, we investigate the anomalous reflection and refraction phenomenon on the metasurface when the incident plane is not parallel to the direction of the static phase gradient of the metasurface. Utilizing vector equation derivation, Matlab calculation, and full-wave simulation based on the Finite difference-time domain (FDTD) method, all found the new property of the metasurface: its refracted ray, reflected ray, and normal are in the same plane, which is called the exit plane, and is at a certain angle to the incident plane.

Equivalent Circuit Models of a Bifunctional Optical Metasurface for Beam Splitting and Refractive Index Sensing

Metasurfaces have attracted extensive attention in the micro/nano-optics field depending on their significant ability to modulate optical parameters. However, the current numerical simulation technology cannot meet the demand of the design and analysis of metasurfaces efficiently due to consuming substantial calculating time and memory. Besides, they cannot illustrate the physical mechanism straightforwardly behind the optical responses. Herein, we propose and demonstrate two equivalent circuit models systematically for a bifunctional metasurface with metal–dielectric–metal structural meta-atoms based on polarization multiplexing in the visible band. In the y-polarization state, the equivalent circuit model is established to interpret the phase shift exactly with a high goodness of fit of 0.931, resulting in the beam splitting function from the anomalous reflection phenomenon. The polychromatic light splits from 34 to 69° to produce the grating-type high-saturation structural colors with a large gamut of about 170.07% of the DCI-P3 standard. In the x-polarization state, the metasurface produces the surface lattice resonance that is suitable for the refractive index sensing function due to the high sensitivity to environmental changes. The second equivalent circuit model simulates the linewidth narrowing and red-shift phenomenon with a sensitivity relative error of 7.00%. We introduce a branch with a narrow-band-pass filter and a capacitor in series into the model to mimic the characteristic of Rayleigh anomalies. Both models extend the application of equivalent circuit theory in optics further and provide a crucial approach to enhance the insights into the mechanism of metasurfaces.

An ultra-wideband phase gradient metasurface for anomalous reflection and RCS reduction

In this work, an ultra-wideband phase gradient metasurface (PGM) based on a novel polarization conversion metasurface (PCM) is proposed for anomalous reflection and radar cross section (RCS) reduction. Because the PCM can achieve ultra-wideband circular-polarization(CP)-maintaining reflection in the frequency band from 8.9 to 34.9 GHz, through the rotation of its unit cell structure, the reflection phase in the ultra-wide frequency band can be adjusted at will by the Pancharatnam–Berry (P-B) phase only generated in its co-polarized reflection coefficient under CP incidence. Thus, a PGM is proposed by gradually rotating each unit-cell structure of the PCM, and the proposed PGM can realize ultra-wideband anomalous reflection to reduce the specular RCS. In addition, when the PGM is divided into multiple square elements with different phase gradients, it can work as a diffusion metasurface to reduce the specular RCS and maximum bistatic RCS simultaneously. The simulated results show that the PGM can achieve more than 10 dB specular RCS reduction in the frequency band from 10.0 to 33.5 GHz, but its maximum bistatic RCS is very large; however, when it is changed as a diffusion metasurface, it can reduce not only the specular RCS but also the maximum bistatic RCS in the ultra-wide frequency band of 10.1–34.3 GHz with a relative band-width of 109.5%; in addition, when the incident angle is increased to 45°, the RCS reduction performance of the diffusion metasurface can still be kept well. Finally, an effective experimental verification is carried out for the diffusion metasurface.

Full-space spin-decoupled versatile wavefront manipulations using non-interleaved metasurface

Abstract Achieving multifunctional wavefront manipulations of waves with a flat and thin plate is pivotal for high-capacity communications, which however is also challenging. A multi-layer metasurface with suppressed mode crosstalk provides an efficient recipe primarily for circular polarization, but all multiple functionalities still are confined to locked spin states and modes. Here, a multifunctional metasurface with spin-decoupled full-space wavefront control is reported by multiplexing both linear momentum and frequency degree of freedom. We employed vertically cascaded quadrangular patches and crossbars to integrate both geometric and dynamic phases and realized four channels between two spin states and two frequencies in distinct scattering modes (transmission and reflection). For verification, a proof-of-concept metadevice with four-port wavefront manipulations is experimentally demonstrated, exhibiting distinct functionalities including spin- and frequency-dependent focusing, quad-beam radiation, anomalous reflections, and Bessel beam generation. Our finding of full-space spin-decoupled metasurfaces would be important for high-capacity communications, multifunctional radar detections, and other applications.

A systematic approach for reconfigurable reflecting metasurface synthesis: From periodic analysis to far-field scattering

We present a consistent approach for a systematic design of an intelligent reflecting surface (IRS), based on a new unit cell with efficient reconfigurability functionalities and a three-step electromagnetic methodology for a rigorous IRS synthesis. First, we design and simulate a reconfigurable unit cell, in which the reconfigurability is accomplished by varactor diodes. Next, we model the IRS as a periodic structure of infinite size, according to Floquet theory. By utilizing the theory of anomalous reflection, we arrive at approximate analytical solutions to calculate the set of necessary varactor diodes’ capacitances, which is subsequently fine-tuned by a fast optimization process to achieve enhanced operation. Finally, to arrive at a more realistic design, we properly analyze the IRS as a finite-sized structure. To this end, we employ an efficient finite element method (FEM) analysis, with a properly developed simulation tool, in which the IRS is modeled as an equivalent impedance sheet of zero-thickness. Overall, we extract the far-field scattered pattern of the IRS and evaluate the angular and frequency response of the design for normal and oblique incidence. Our results show that the proposed design exhibits very promising characteristics, both in terms of reconfigurability and frequency response that can be exploited in contemporary and future wireless communication systems.