Anomalous Reflection Research Articles

Influence of viscous shear boundary layers on the sound performance of acoustic metasurfaces

Acoustic metasurfaces are mostly designed in a static medium, ignoring the influence of flow characteristics. However, in actual aeroacoustic noise reduction, e.g., aircraft engine liner design, the background flow can have effects on the sound performance of acoustic metasurfaces, especially for a viscous shear flow. The effect of a viscous shear flow is often neglected in previous studies on the design and sound field prediction of acoustic metasurfaces. For considering the viscous and thermal dissipation effects, an analytical model is developed to predict the sound field of a periodic metasurface in a viscous shear boundary layer. In this model, the effective impedance based on the high-frequency limits is utilized to consider both the actual impedance of the acoustic metasurface and the effect of a finite-thickness viscous shear boundary layer. An acoustic metasurface designed in the static medium or even redesigned with only the effect of an inviscid shear flow is not suitable for wave manipulation when the Reynolds number (Re) changes significantly, since the viscosity is an important and non-negligible factor affecting the sound performance. For the cases in this work, the sound performance gradually deteriorates with the decrease in Re when Re≥5×106. When Re≤1×106, especially at Re=1×105, the existence of viscous shear flows could result in the destruction of expected anomalous reflection and significant intensity change of the reflected waves. This research provides a method for the design of acoustic metasurfaces under viscous shear flow conditions, which is significant for future aeroacoustic applications.

A Low-Profile and Ultra-Wideband Pancharatnam-Berry Coding Metasurface for High-Efficiency and Wide-Angle Circular Polarization Anomalous Reflection.

The manipulation of electromagnetic waves using metasurfaces is important in areas such as stealth and communication. In this paper, we reported on the use of an element-based polarizer for the first step, which enables the incident electromagnetic waves to integrate into the cross-polarized waves with a relative bandwidth of 88% within 15-37.1 GHz. Then, an eight-element coding metasurface based on the Pancharatnam-Berry phase is presented for circular polarization anomalous reflection. The simulated values show that our work can achieve a high-efficiency (94%) and wide-angle (70°) anomalous reflection under normal incidence. The simulated values present good agreement with the experimental values. Our work reveals the ability to manipulate the waves and electromagnetic stealth.

Beam-steering by a phase-transition metal-based 1-bit programmable metasurface

Abstract Here the beam-steering mechanism is demonstrated using a 1 bit coded metasurface at 2.5 THz. Each unit cell of the metasurface is loaded with vanadium dioxide (VO2) which undergoes phase transition upon excitation by an external stimulus. The electrical and magnetic properties of VO2 change as the transition takes place from insulator to metal resulting in a reflection phase change of 180°. When the metasurface is excited by a microstrip patch antenna placed at a focal distance above it, the reflected beam is directed at an angle of 22°, as a result of anomalous reflection. By changing the coding pattern of the metasurface, the reflected beam is redirected at −22°. Four different coding patterns have been proposed to depict beam-steering action by the metasurface. The maximum gain registered by the reflect-array integrated antenna is 21 dBi at 2.5 THz. THz beam steering assists in forming directional beams which can be deployed in wireless transmission of THz waves.

A tunable chiral metasurface useable in terahertz imaging and wavefront shaping

We proposed a tunable chiral metasurface comprising a reflective bottom layer of gold, a dielectric layer of polyimide, and a structural top layer of gold-graphene. Its main properties were studied via numerical simulations conducted using CST Studio Suite. The results indicate that, based on the chiral metasurface, we achieved dual-band circular dichroism of −0.5 and 0.77 at 0.9 THz and 1.06 THz, respectively, and complementary near-field imaging applications were realized by tuning the Fermi level (E f ) of graphene. Subsequently, exploiting the exceptional selective characteristics of circularly polarized waves using a chiral metasurface, eight chiral phase-gradient metasurfaces were constructed by rotating the chiral structure. Moreover, based on the Pancharatnam-Berry phase principle, tunable wavefront shaping applications were further realized, including anomalous reflection, vortex beams, and focusing. In anomalous reflection, the reflection angles for left-circularly polarized (LCP) and right-circularly polarized (RCP) incidences are opposite when adjusting the E f of graphene. For example, when the graphene E f is 0 eV and the LCP wave is incident at 0°, the reflection angle is −18°. Conversely, when the graphene E f is 1 eV and the RCP wave is incident at 0°, the reflection angle is 18°. In the application of vortex beams, by adjusting the E f of graphene, we achieved vortex beams with opposite topological charges under different circularly polarized incidences. In the focusing application, the incident LCP and RCP can achieve focusing and defocusing, respectively. And the graphene E f can dynamically control the focusing efficiency at the incident LCP, increasing it from 13.63% to 44.84%.

Anomalous reflection for highly efficient subwavelength light concentration and extraction with photonic funnels

Abstract Photonic funnels, microscale conical waveguides that have been recently realized in the mid-IR spectral range with the help of an all-semiconductor designer metal material platform, are promising devices for efficient coupling of light between the nanoscales and macroscales. Previous analyses of photonic funnels have focused on structures with highly conductive claddings. Here, we analyze the performance of funnels with and without cladding, as a function of material properties, operating wavelength, and geometry. We demonstrate that bare (cladding-free) funnels enable orders-of-magnitude higher enhancement of local intensity than their clad counterparts, with virtually no loss of confinement, and relate this phenomenon to anomalous reflection of light at the anisotropic material–air interface. Intensity enhancement of the order of 25, with confinement of light to wavelength/20 scale, is demonstrated. Efficient extraction of light from nanoscale areas is predicted.

Phase and amplitude simultaneously coding metasurface with multi-frequency and multifunctional electromagnetic modulations

The integration of multiple functionalities into a single, planar, ultra-compact metasurface has presented significant opportunities for enhancing capacity and performance within compact 5G/6G communication systems. Recent advances in multifunctional metasurfaces have unveiled comprehensive wavefront manipulations utilizing phase, polarization transmission/reflection, and coding apertures. Despite these developments, there remains a critical need for multifunctional metasurfaces with expanded channel capabilities, including multiple operational frequencies, minimal crosstalk, and high-efficiency computable array factors. This study introduces a multifunctional metasurface that integrates phase- and amplitude simultaneous coding meta-atoms at dual frequencies. By altering the polarization of electromagnetic (EM) waves, it is possible to reshape the wave-fronts of reflected waves at these frequencies. The coding metasurface proficiently manipulates both x and y linearly polarized waves through phase and amplitude coding at dual frequencies, thereby enabling distinct functionalities such as anomalous reflection, reflection imaging, and vortex wave beam generation. Both theoretical analysis and full-wave simulation confirm the anticipated functionalities of the designed devices, paving the way for advancements in integrated communication systems with diverse functionalities.

Response to "Comment on 'Anomalous reflection from a two-layered marine sediment' " [J. Acoust. Soc. Am. 156, 1524-1527 (2024)

In their Comment, the authors conclude that acoustic glint is not present in the reflection coefficient of a two-layer sediment in which the top layer is an Airy medium. They conclude, not that the original, inverse-square analysis of the glint is incorrect, but rather that the presence of glint is very sensitive to the detailed shape of the sound speed profile in the top layer.

Design of composite stealth cloak based on anomalous reflections and vortex singularities

In this paper, an ultra-thin (0.072λ) composite stealth cloak is proposed to cover a cloak with a center height of 98.88 mm and an inclination angle of 25° to achieve the dual stealth effect of scattering and gain. Firstly, based on the Generalized Snell theorem, the designed anomalous reflective metasurface has been shown to have a good invisibility in the 14–30 GHz region in simulations as well as in experiments. Secondly, the 3-bit metasurface unit is utilized to generate vortex beams with different number of modes, whose electric field strength at the vortex singularity is 0, which achieves energetically stealth. Finally, the phases of the above two metasurfaces are superimposed using the convolution theorem, and the designed composite stealth cloak significantly reduces the scattering amplitude and reconstructs the wavefront over a wide frequency range when a circularly polarized wave is incident vertically. Based on the above study, the stealth cloak provides a feasible solution for application to radar and microwave antenna stealth systems.

Switchable Terahertz Metasurfaces for Spin-Selective Absorption and Anomalous Reflection Based on Vanadium Dioxide.

Conventional chiral metasurfaces are constrained by predetermined functionalities and have limited versatility. To address these constraints, we propose a novel chirality-switchable terahertz (THz) metasurface with integrated heating control circuits tailored for spin-selective anomalous reflection, leveraging the phase-change material vanadium dioxide (VO2). The reversible and abrupt insulator-to-metal phase transition feature of VO2 is exploited to facilitate a chiral meta-atom with spin-selectivity capabilities. By employing the Pancharatnam-Berry phase principle, complete 2π reflection phase coverage is achieved by adjusting the orientation of the chiral structure. At the resonant frequency of 0.137 THz, the designed metasurface achieves selective absorption of a circularly polarized wave corresponding to the state of the VO2 patches. Concurrently, it reflects the circularly polarized wave of the opposite chirality anomalously at an angle of 28.4° while maintaining its handedness. This chirality-switchable THz metasurface exhibits promising potential across various applications, including wireless communication data capacity enlargement, polarization modulation, and chirality detection.

Terahertz vortex beam generation based on reflective and transmissive graphene metasurfaces

Graphene metasurfaces have substantial potential for generating vortex electromagnetic waves carrying orbital angular momentum (OAM) at terahertz frequencies. In this paper, a deformed H-shaped graphene unit cell is utilized to design a half-space reflective metasurface (with a metal backplane, operating in the reflection mode) and a full-space metasurface (operating in both the reflection and transmission modes) to generate OAM beams. The two structures are intertransformable at the level of physical composition, where one configuration enables efficient anomalous reflection of electromagnetic waves within the 4.6–8.9 THz band, while the other enables uniform modulation efficiency of reflective and transmissive waves within the 5.5–7.0 THz band. Equivalent circuit models have been provided to establish a convenient closed-form mathematical description of the metasurface’s electromagnetic behavior. By performing the near- and far-field simulations, a series of THz vortex generators based on the proposed graphene metasurface are demonstrated. In particular, high-purity reflective and transmissive OAM waves with distinct topological charges are observed. Our study provides a path towards the practical realization of active THz OAM metadevices.

Sound propagation in the presence of non-isothermal sheared flow refraction and metasurface impedance reflection

Sheared flow can profoundly influence the aeroacoustic performance of near-wall structures like metasurfaces on moving vehicles. In this work, a refractive generalized Snell’s law is developed by combining Snell’s law with the generalized Snell’s law to manipulate anomalous reflections in an idealized sheared flow, comprising a uniform flow region and a stationary air region with velocity and temperature gradients. Theoretical analysis and numerical simulations of planar wave reflection by metasurface impedance under non-isothermal sheared flows are conducted, yielding favorable predictions of the zone of silence, as well as anomalous and total reflection behaviors. The total reflection between the shear layer and the metasurface is analyzed with the objective of confining incident wave below the shear layer. Remarkably, a specific configuration of hot-sheared flow and metasurface length can achieve total absorption over a range of incident wave angles from 0° to almost 90°. Moreover, the refractive generalized Snell’s law, extended to account for directional deflection induced by flow convection, is utilized to regulate sound beam reflection, resulting in satisfactory outcomes under sheared conditions. This study offers a theoretical framework for wave control under non-isothermal and sheared flow conditions, which cannot be resolved by the traditional generalized Snell’s law, and can be implemented in the design of metasurfaces for moving applications.

All-dielectric hybrid VIS-NIR dual-function metaoptic

Metasurfaces are a promising technology that can serve as a compact alternative to conventional optics while providing multiple functions depending on the properties of the incident light, such as the wavelength, polarization, and incident angle. Here, we demonstrate a hybrid visible/near-infrared dielectric metaoptic capable of reflecting 940 nm light in a specified direction while allowing transmission of visible light (450–750 nm). This dual functionality is achieved by combining an aperiodic distributed Bragg reflector with dielectric meta-atoms. Experimental demonstration is also reported, showing an anomalous reflection of near-infrared light within a 20° full field-of-view and the transmission of wavelengths from 450 nm to 750 nm.

Flat optics of nonuniform phase gradient metasurfaces

Flat optics of uniform phase gradient metasurfaces based on the generalized Snell’s law has been extensively studied. The optics of nonuniform phase gradient metasurfaces (NPGMs) is less clear. Here based on Huygens’ principle and finite-difference time-domain (FDTD) simulation, we explore the optical properties of NPGMs made of nanorods (meta-atoms), which can be tuned by modulation of propagation phase and geometric phase. It is found that the nonuniformity of phase gradient may lead to multichannel anomalous reflection/refraction. The multiple beam splitters with arbitrary intensity ratio can be achieved by using the amplitude/phase modulation. Based on our design principle, we can obtain different anomalous reflection patterns (with channels ±1, ±2, both ±1 and ±2, or no reflection at anomalous angle) by different arrangement of just two types of meta-atoms. In addition, we are able to achieve chiral anomalous reflections for designed NPGMs made of nanorods and L-shaped nanoparticles. Our formulism provides general design guidance for NPGMs and can be used to realize the beam splitting function with adjustable angle and intensity ratio.

Interlayer coupled dual-layer metagratings for broadband and high-efficiency anomalous reflection

Recent progress in metagratings highlights the promise of high-performance wavefront engineering devices, notably for their exterior capability to steer beams with near-unitary efficiency. However, the narrow operating bandwidth of conventional metagratings remains a significant limitation. Here, we propose and experimentally demonstrate a dual-layer metagrating, incorporating enhanced interlayer couplings to realize high-efficiency and broadband anomalous reflection within the microwave frequency band. The metagrating facilitated by both intralayer and interlayer couplings is designed through the combination of eigenmode expansion (EME) algorithm and particle swarm optimization (PSO) to significantly streamline the computational process. Our metagrating demonstrates the capacity to reroute a normally incident wave to +1 order diffraction direction across a broad spectrum, achieving an average efficiency approximately 90% within the 14.7 to 18 GHz range. This study may pave the way for future applications in sophisticated beam manipulations, including spatial dispersive devices, by harnessing the intricate dynamics of multi-layer metagratings with complex interlayer and intralayer interactions.

All-Oxide Metasurfaces Formed by Synchronized Local Ionic Gating.

Ionic gating of oxide thin films has emerged as a novel way of manipulating the properties of thin films. Most studies are carried out on single devices with a three-terminal configuration,but, by exploring the electrokinetics during the ionic gating, such a configuration with initially insulating films leads to a highly non-uniform gating response of individual devices within large arrays of the devices. It is shown that such an issue can be circumvented by the formation of a uniform charge potential by the use of a thin conducting underlayer. This synchronized local ionic gating allows for the simultaneous manipulation of the electrical, magnetic, and/or optical properties of large arrays of devices. Designer metasurfaces formed in this way from SrCoO2.5 thin films display an anomalous optical reflection of light that relies on the uniform and coherent response of all the devices. Beyond oxides, almost any material whose properties can be controlled by the addition or removal of ions via gating can form novel metasurfaces using this technique. These findings provide insights into the electrokinetics of ionic gating and a wide range of applications using synchronized local ionic gating.

Acoustic Metagrating Holograms.

Metasurface holograms represent a common category of metasurface devices that utilize in-plane phase gradients to shape wavefronts, forming holographic images through the application of the generalized Snell's law (GSL). While conventional metasurfaces focus solely on phase gradients, metagratings, which incorporate higher-order wave diffraction, further expand the GSL's generality. Recent advances in certain acoustic metagratings demonstrate an updated GSL extension capable of reversing anomalous transmission and reflection, whose reversal is characterized by the parity of the number of wave propagation trips through the metagrating. However, the current extension of GSL remains limited to 1D metagratings, unable to access 2D holographic images in 3D spaces. Here, the GSL extension to 2D metagratings for manipulating waves within 3D spaces is investigated. Through this analysis, a series of acoustic metagrating holograms is experimentally demonstrated. These holographic images exhibit the unique ability to switch between transmission and reflection types independently. This study introduces an additional dimension to modern holography design and metasurface wavefront manipulation.

Dynamic PB metasurface for transmissive and reflective wavefront manipulation

Based on the phase transition characteristics of vanadium dioxide (changes in conductivity with temperature), we designed a dynamic metasurface for wavefront manipulation that can switch between transmission and reflection modes at the same frequency. When vanadium dioxide is in the metallic state, the metasurface operates in reflection mode, while it is in the insulating state, it works in transmission mode. The phase distributions of the two operating modes are different and can be easily adjusted by rotating the inner and outer rings. At the frequency of 1.3 THz, we designed a metasurface that can switch between anomalous reflection and refraction with different deflection angles. Additionally, we created a switchable metalens that provides different focal lengths in reflection and transmission modes. The tunable metasurface can provide innovative design possibilities for dynamical wavefront manipulation.

Huygens’ metasurface: From anomalous refraction to reflection

Huygens' metasurfaces (HMS) without any reflection or transmission are widely studied due to the high-efficiency manipulation of electromagnetic waves. In this work, three-layered metallic Huygens' metasurfaces are proposed and the geometry-dependences of anomalous refraction and reflection are investigated. The surface impedance and admittance are independently tailored by changing the length of metallic resonators, which is dictated by the introduction of the intermediate layer. Huygens’ metasurfaces can be geometrically engineered to realize anomalous refraction with an efficiency of 62% and anomalous reflection with an efficiency of 60%, respectively. It is anticipated that the proposed metasurface with tunable materials could achieve reconfigurable terahertz devices.

A tunable terahertz metasurface with function of polarization conversion and circular dichroism

A tunable and multifunctional electromagnetic control metasurface is proposed. The metasurface comprises the graphene sheet and gold open ring, enabling multiband cross-polarization conversion (CPC) and line-to-circular polarization conversion (LTCPC). Simultaneously, it can achieve circular dichroism (CD) with amplitude of 0.99 at 2.28 THz, along with the capability for near-field imaging. The physical mechanism of operation is studied by analyzing surface current distribution and electric field. In addition, because the metasurface exhibits chiral characteristics, appropriate rotation can induce a phase gradient, facilitating a phase coverage of 0–360°. Based on the tunable phase gradient of metasurface, anomalous reflection and vortex beams can be realized.

Vanadium dioxide-based terahertz metasurface device with switchable broadband absorption and beam steering functions

A simple terahertz (THz) metasurface device with dual functionalities of broadband absorption and beam steering is proposed in this paper. The device exploits the phase change properties of vanadium dioxide (VO2), transitioning between its insulating state and metallic states to achieve functional switching. In the metallic state, the device exhibits broadband absorption, surpassing 90% absorption in the 1.03–2.07 THz range. The absorption function maintains exceptional performance across a wide range of incident angles and possesses polarization insensitive properties. In the insulating state of VO2, the metasurface converts into a THz reflective beam steering device. By employing phase coding, 1-bit coding metasurfaces based on phase gradients are designed, enabling dual-beam and four-beam reflection. Building upon this foundation, 2-bit coding metasurfaces facilitate anomalous reflection of a single beam in distinct directions. Consequently, the device not only toggles between broadband absorption and beam steering but also satisfies the design criteria for 3-bit coding metasurfaces. The modulation characteristics of this device serve as a valuable design reference for multifunctional THz devices in diverse scenarios.