Showing papers on "Reflection (physics) published in 2021"

Study on broadband microwave absorbing performance of gradient porous structure

Abstract: In this study, a gradient porous structure was designed for a novel microwave absorbing material, and the effects of various factors on its microwave absorbing characteristics were investigated. The computational and experimental results show that the bandwidth of this composite can reach up to 14.06 GHz with a microwave reflection loss below − 10 dB in the frequency of 1–18 GHz. The appreciable agreement between the simulation and the experiment verified the validity of this structure. The broadband microwave absorbing performance of designed gradient porous structure was significantly enhanced, which was ascribed to the synergistic effect of structural and material characteristics. The square aperture of this structure increased from the bottom to the top, which improved the impedance matching between the surface of gradient porous structure and the air and reduced the reflection of electromagnetic waves. In addition, the transmission path of electromagnetic wave inside the absorbing structure increased, thus facilitating the attenuation of electromagnetic wave. This study could provide a new design strategy for the research of antiradar detection technique and shielding the electromagnetic interference. Summary: 1.The absorbing principle of the gradient porous structure. 2. The simulation and test results of the gradient porous structure.

Wave-Vector-Varying Pancharatnam-Berry Phase Photonic Spin Hall Effect

Abstract: The geometric Pancharatnam-Berry (PB) phase not only is of physical interest but also has wide applications ranging from condensed-matter physics to photonics. Space-varying PB phases based on inhomogeneously anisotropic media have previously been used effectively for spin photon manipulation. Here we demonstrate a novel wave-vector-varying PB phase that arises naturally in the transmission and reflection processes in homogeneous media for paraxial beams with small incident angles. The eigenpolarization states of the transmission and reflection processes are determined by the local wave vectors of the incident beam. The small incident angle breaks the rotational symmetry and induces a PB phase that varies linearly with the transverse wave vector, resulting in the photonic spin Hall effect (PSHE). This new PSHE can address the contradiction between spin separation and energy efficiency in the conventional PSHE associated with the Rytov-Vladimirskii-Berry phase, allowing spin photons to be separated completely with a spin separation up to 2.2 times beam waist and a highest energy efficiency of 86%. The spin separation dynamics is visualized by wave coupling equations in a uniaxial crystal, where the centroid positions of the spin photons can be doubled due to the conservation of the angular momentum. Our findings can greatly deepen the understanding in the geometric phase and spin-orbit coupling, paving the way for practical applications of the PSHE.

Towards Precision Measurements of Accreting Black Holes Using X-Ray Reflection Spectroscopy

Abstract: Relativistic reflection features are commonly observed in the X-ray spectra of accreting black holes. In the presence of high quality data and with the correct astrophysical model, X-ray reflection spectroscopy can be quite a powerful tool to probe the strong gravity region, study the morphology of the accreting matter, measure black hole spins, and possibly test Einstein’s theory of general relativity in the strong field regime. In the last decade, there has been significant progress in the development of the analysis of these features, thanks to more sophisticated astrophysical models and new observational facilities. Here we review the state-of-the-art in relativistic reflection modeling, listing assumptions and simplifications that may affect, at some level, the final measurements and may be investigated better in the future. We review black hole spin measurements and the most recent efforts to use X-ray reflection spectroscopy for testing fundamental physics.

Wavefront-selective Fano resonant metasurfaces

Abstract: Fano resonances are conventionally understood as sharp spectral features with selectivity in the momentum-frequency domain, implying that they can be excited only by plane waves with specific frequencies and incident angles. We demonstrate that Fano resonances can be made generally selective in the space-frequency domain. They can be tailored to resonate only when excited by a frequency, polarization, and wavefront of choice. This generalization reveals that Fano systems are characterized by eigenwaves that scatter to their time-reversed image upon reflection. Although in conventional Fano systems this trivially occurs for normally incident plane waves, we show that, in general, the selected wavefront is locally retroreflected everywhere across the device. These results show that conventional Fano resonances are a subset of a broader dichroic phenomenon with spin, spatial, and spectral selectivity. We demonstrate these concepts with nonlocal metasurfaces whose governing principles are deeply rooted in the symmetry features of quasi-bound states in the continuum. Enhanced light–matter interactions and symmetry-protection make these phenomena uniquely suited for enriching applications in quantum optics, non-linear optics, augmented reality, and secure optical communications, laying the groundwork for a range of novel compact optical sources and devices.

Revisiting the anomalous spin-Hall effect of light near the Brewster angle

Abstract: Optical spin-Hall effect (SHE) exhibits many intriguing features as a linearly polarized (LP) light beam strikes an interface at incident angles around the Brewster angle, but the underlying physics remains obscure. Here, we elucidate the physics through reanalyzing this problem employing rigorous calculations and the Berry phase concept. As a circularly polarized (CP) light beam strikes an optical interface, the reflected light beam contains two components, a spin-flipped abnormal mode acquiring geometric phases (thus exhibiting a spin-Hall shift) and a spin-maintained normal mode without such phases. Strengths of these two modes are determined by the incident angle and the optical properties of the interface. Under the LP incidence, however, a spin component inside the reflected light beam must be the sum of normal and abnormal components of reflected light beams corresponding to CP incidences with different helicity, which thus sensitively depends on the incident angle. In particular, at incident angles near the Brewster one, reflection coefficients for two CP components exhibit opposite signs, leading to significant destructive interferences between normal and abnormal modes, finally generating highly deformed reflected light patterns with anomalously enhanced spin-Hall shifts. These findings can be extended to both reflected and transmitted cases with Brewster-like behaviors. Our analyses reinterpret previously discovered effects, providing an alternative understanding on the SHE of light.

Resonant reflection by microsphere arrays with AR-quenched Mie scattering.

Abstract: Periodic guided-mode resonance structures which provide perfect reflection across sizeable spectral bandwidths have been known for decades and are now often referred to as metasurfaces and metamaterials. Although the underlying physics for these devices is explained by evanescent-wave excitation of leaky Bloch modes, a growing body of literature contends that local particle resonance is causative in perfect reflection. Here, we address differentiation of Mie resonance and guided-mode resonance in mediating resonant reflection by periodic particle assemblies. We treat a classic 2D periodic array consisting of silicon spheres. To disable Mie resonance, we apply an optimal antireflection (AR) coating to the spheres. Reflectance maps for coated and uncoated spheres demonstrate that perfect reflection persists in both cases. It is shown that the Mie scattering efficiency of an AR-coated sphere is greatly diminished. The reflectance properties of AR-coated spherical arrays have not appeared in the literature previously. From this viewpoint, these results illustrate high-efficiency resonance reflection in Mie-resonance-quenched particle arrays and may help dispel misconceptions of the basic operational physics.

Wave trapping by acoustic black hole: Simultaneous reduction of sound reflection and transmission

Abstract: Reduction of vibration and sound energy in the form of traveling waves is of vital importance in many applications. Recent development of acoustic metamaterials opens up unusual ways for sound wave manipulation and control. Among acoustic metamaterials, a much newer concept, Acoustic Black Hole (ABH), has been drawing growing attention in recent years, which shows great potential for acoustic energy trapping and dissipation. In a duct ABH with a properly tailored continuous cross-sectional reduction and impedance variation, it is shown that the sound speed can be progressively reduced, which means that sound waves are eventually trapped in the structure. In this paper, such a wave trapping mechanism is further explored in the context of sound transmission problems, in which an exceptional phenomenon—simultaneous reduction of sound reflection and transmission—is realized. The archived trapping mechanism also ensures that little sound waves will be bounced back to the source to jeopardize the overall performance. Transfer matrix method simulations and impedance tube experiments are performed to characterize the behavior of such a structure and to validate the theory. The promising ABH-specific features arising from the proposed design could overcome many existing limitations of traditional noise control devices.

Selective Mode Conversion and Rainbow Trapping via Graded Elastic Waveguides

Abstract: We experimentally achieve wave-mode conversion and rainbow trapping in an elastic waveguide loaded with an array of resonators. Rainbow trapping is a phenomenon that induces wave confinement as a result of a spatial variation of the wave velocity, here promoted by gently varying the length of consecutive resonators. By breaking the geometrical symmetry of the waveguide, we combine the wave-speed reduction with a reflection mechanism that mode converts flexural waves impinging on the array into torsional waves traveling in opposite directions. The framework presented herein may open opportunities in the context of wave manipulation through the realization of structural components with concurrent wave-conversion and energy-trapping capabilities.

Perfectly-reflecting guided-mode-resonant photonic lattices possessing Mie modal memory.

Abstract: Resonant periodic nanostructures provide perfect reflection across small or large spectral bandwidths depending on the choice of materials and design parameters. This effect has been known for decades, observed theoretically and experimentally via one-dimensional and two-dimensional structures commonly known as resonant gratings, metamaterials, and metasurfaces. The physical cause of this extraordinary phenomenon is guided-mode resonance mediated by lateral Bloch modes excited by evanescent diffraction orders in the subwavelength regime. In recent years, hundreds of papers have declared Fabry-Perot or Mie resonance to be the basis of the perfect reflection possessed by periodic metasurfaces. Treating a simple one-dimensional cylindrical-rod lattice, here we show clearly and unambiguously that Mie resonance does not cause perfect reflection. In fact, the spectral placement of the Bloch-mode-mediated zero-order reflectance is primarily controlled by the lattice period by way of its direct effect on the homogenized effective-medium refractive index of the lattice. In general, perfect reflection appears away from Mie resonance. However, when the lateral leaky-mode field profiles approach the isolated-particle Mie field profiles, the resonance locus tends towards the Mie resonance wavelength. The fact that the lattice fields “remember” the isolated particle fields is referred here as “Mie modal memory.” On erasure of the Mie memory by an index-matched sublayer, we show that perfect reflection survives with the resonance locus approaching the homogenized effective-medium waveguide locus. The results presented here will aid in clarifying the physical basis of general resonant photonic lattices.

Reflection of plane waves from the surface of a piezothermoelastic fiber-reinforced composite half-space

Abstract: This work focuses on the study of plane wave reflection at the surface of a piezothermoelastic fiber-reinforced composite (PTFRC) half-space. The classical dynamical coupled theory, Lord-Shulman th...

Diffuse reflection and reciprocity-protected transmission via a random-flip metasurface.

Abstract: Rough surfaces lead to diffused light in both reflection and transmission, thereby blurring the reflected and transmitted images. Here, we merge the traditionally incompatible diffuse reflection an...

Marchenko redatuming, imaging, and multiple elimination and their mutual relations

Abstract: With the Marchenko method, it is possible to retrieve Green’s functions between virtual sources in the subsurface and receivers at the surface from reflection data at the surface and focusi...

Spectrum Characteristics Preserved Visible and Near-Infrared Image Fusion Algorithm

Abstract: The visible and near-infrared images fusion aims at utilizing their spectrum characteristics to enhance visibility. However, the current visible and near-infrared fusion algorithms cannot well preserve spectrum characteristics, which results in color distortion and halo artifacts. Therefore, this paper proposes a new visible and near infrared images fusion algorithm by fully considering their different reflection and scattering characteristics. According to image degradation model, the reflection weight model and the transmission weight model are established, respectively. The reflection weight model is established by calculating the difference between the visible (red, green, and blue) spectra and the near-infrared spectrum, while maintaining the correlation of the visible spectra. The proposed reflection weight model can preserve the original reflection characteristic of objects in natural scenes. On the other hand, the transmission weight model is explicitly proposed by calculating the gradient ratio of the visible spectra to the near-infrared spectrum. The proposed transmission weight model intends to make full use of the strong transmission performance of the near-infrared spectrum, which can complement the details loss of the visible spectra caused by light scattering. Moreover, the fused image based on two models is further enhanced according to the reflection characteristics of near-infrared spectrum in case of the non-uniform illumination. The experimental results demonstrate that the proposed algorithm can not only well preserve spectrum characteristics, but also avoid color distortion while maintaining the naturalness, which outperforms the state-of-the-art.

Large Deviation Principles of Obstacle Problems for Quasilinear Stochastic PDEs

Abstract: In this paper, we first present a sufficient condition(a variant) for the large deviation criteria of Budhiraja, Dupuis and Maroulas for functionals of Brownian motions. The sufficient condition is particularly more suitable for stochastic differential/partial differential equations with reflection. We then apply the sufficient condition to establish a large deviation principle for obstacle problems of quasi-linear stochastic partial differential equations. It turns out that the backward stochastic differential equations will also play an important role.

Plane wave reflection/transmission in imperfectly bonded initially stressed rotating piezothermoelastic fiber-reinforced composite half-spaces

Abstract: The effects of different types of imperfect interfaces , normal and shear initial stresses and rotation on the reflection and transmission characteristics of plane waves in two dissimilar piezothermoelastic fiber-reinforced composite (PTFRC) half-spaces are analytically studied in this article. The PTFRC structure is modeled employing the Strength of Materials (SM) technique with the Rule of Mixtures (RM) approach. Numerical studies are performed on two distinct PTFRCs comprised of CdSe–epoxy combination and PZT-5A-epoxy combination. Several reasons, like the inevitable presence of interfacial defects because of accumulative damages, have detrimental effects on the life expectancy and efficiency of structures manufactured using smart materials. For the same reasons, the bond between half-spaces is generally not perfect. Thus, the mechanical–electrical-thermally imperfect boundary is classified into seven types, viz. Normal Stiffness Boundary (NSB), Transverse Stiffness Boundary (TSB), Thermal Contact Conductance (TCC), Electrically Imperfect Boundary (EIB), Slip Boundary (SB), Completely Debonded Boundary (CDB) and Welded Contact (WC), which are analyzed individually. Initial stresses and rotation are also considered for making the present model more realistic. The closed-form expressions of amplitude ratios of reflected and transmitted quasi-longitudinal (qP), quasi-transverse (qSV), thermal (T-mode) and electro-acoustic (EA) waves are derived by means of appropriate mechanical–electrical-thermally imperfect boundary conditions. Using these, the energy ratios of all waves along with the interaction energy among various waves are obtained and the Law of Conservation of Energy is validated. Comparative analysis among the Classical dynamical coupled, Lord–Shulman and Green–Lindsay thermoelasticity theories is performed. The influences of the incident angle, imperfect interfaces, rotation, varying magnitudes of normal and shear initial stresses and thermal relaxation parameters on the energy ratios are illustrated graphically. Some special cases exclusive to this study are shown which validate the obtained results with extant literature and possible scientific and engineering applications of the present model are discussed.

Invisible surfaces enabled by the coalescence of anti-reflection and wavefront controllability in ultrathin metasurfaces.

Abstract: Reflection inherently occurs on the interfaces between different media. In order to perfectly manipulate waves on the interfaces, integration of antireflection function in metasurfaces is highly desired. In this work, we demonstrate an approach to realize exceptional metasurfaces that combine the two vital functionalities of antireflection and arbitrary phase manipulation in the deep subwavelength scale. Such ultrathin devices confer reflection-less transmission through impedance-mismatched interfaces with arbitrary wavefront shapes. Theoretically and experimentally, we demonstrate a three-layer antireflection metasurface that achieves an intriguing phenomenon: the simultaneous elimination of the reflection and refraction effects on a dielectric surface. Incident waves transmit straightly through the dielectric surface as if the surface turns invisible. We further demonstrate a wide variety of applications such as invisible curved surfaces, “cloaking” of dielectric objects, reflection-less negative refraction and flat axicons on dielectric-air interfaces, etc. The coalescence of antireflection and wavefront controllability in the deep subwavelength scale brings new opportunities for advanced interface optics with high efficiency and great flexibility. Though existing metamaterial antireflection coatings have been optimized in terms of thickness and functionality, these coatings do not provide phase control in the deep subwavelength scale. Here, the authors report multi-layered metasurfaces that provide both antireflection and phase control.

Joint Beam and Polarization Forming of Intelligent Reflecting Surfaces for Wireless Communications

Abstract: In this paper, we propose a novel intelligent reflecting surface (IRS) structure, which is capable of jointly forming desirable beam and polarization of a reflected wave with the aid of massive reflecting elements. More specifically, the proposed IRS is composed of unit cells, each having multiple variable capacitors. Active tuning of capacitances of the variable capacitors in each unit cell provides the full coverage of $360^\circ$ in the reflection phase. Furthermore, polarization is switched from an incident wave to the reflected wave through the excitation of two orthogonal modes, enabling arbitrary phase distribution over the IRS for the desired polarization. As a result, the proposed IRS allows the increase of the signal-to-noise ratio (SNR) at the receiver in a communication scenario, by adjusting the reflection phase shift of each IRS element. Moreover, the reflection phase shifts of each IRS element are optimized with the aid of the Newton method, such that the received signal power of a receiver is maximized. In our simulations, it is demonstrated that our proposed IRS successfully maximizes the SNR at the receiver, both in the single-user and the broadcast multi-user scenarios, where multi-user interference is absent.

A complex-valued resonance model for axisymmetric screech tones in supersonic jets

Abstract: We model the resonance mechanism underpinning generation of A1 and A2 screech tones in an under-expanded supersonic jet. Starting from the resonance model recently proposed by Mancinelli et al. (Exp. Fluids, vol. 60, 2019, p. 22), where the upstream-travelling wave is a neutrally stable guided-jet mode, we here present a more complete linear-stability-based model for screech prediction. We study temperature and shear-layer thickness effects and show that, in order to accurately describe the experimental data, the effect of the finite thickness of the shear layer must be incorporated in the jet-dynamics model. We then present an improved resonance model for screech-frequency predictions in which both downstream- and upstream-travelling waves may have a complex wavenumber and frequency. This resonance model requires knowledge of the reflection coefficients at the upstream and downstream locations of the resonance loop. We explore the effect of the reflection coefficients on the resonance model and propose an approach for their identification. The complex-mode model identifies limited regions of frequency–flow parameter space for which the resonance loop is amplified in time, a necessary condition for the resonance to be sustained. This model provides an improved description of the experimental measurements.

Decorrelation of the Near-Specular Land Scattering in Bistatic Radar Systems

Abstract: Signal fluctuations at the receiving antenna have been studied from decades by the radar community, especially to understand the decorrelation of the scattering in radar interferometry. This was done assuming uncorrelated point-like scatterers, leading to a simple model for the geometric decorrelation. In this case, the scattering is certainly incoherent. The quasi-specular reflections gathered under the illumination of signals of opportunity can exhibit significant temporal fluctuations. They are related to the statistical features of the surface roughness and can be observed even in almost flat regions, where a predominant coherent reflection could be expected. The presence of gentle undulations, however, i.e., those showed by surfaces having variations of the profiles comparable with the wavelength over the vertical scale, but much longer over the horizontal one, can determine transition regions where the scattering is neither coherent nor completely incoherent. In these conditions, the nature of the fluctuations of the scattering is not well understood and it requires additional studies. A discussion about the dominance of coherent or incoherent reflection in the Global Navigation Satellite System Reflectometry (GNSS-R) community is presently ongoing. To describe the nature of the scattering, and to understand the decorrelation of the near-specular components in GNSS-R, we propose a numerical study of the field collected by a moving airborne receiver based on the Kirchhoff approximation. Our study demonstrates that the near-specular scattering collected over representative natural landscapes by a GNSS-R receiver is partially coherent and essentially incoherent in most cases. Its correlation time is a function of the roughness parameters, namely standard deviation and correlation length, as well as of the system parameters (i.e., spatial resolution and height). The analysis can provide useful information for the interpretation of GNSS data, which present intrinsic variability that can significantly affect the retrieval of the relevant bio-geophysical parameters.

Layered Aluminum for Electromagnetic Wave Absorber with Near-Zero Reflection.

Abstract: Ideal electromagnetic (EM) wave absorbers can absorb all incident EM waves, regardless of the incident direction, polarization, and frequency. Absorptance and reflectance are intrinsic material properties strongly correlated with electrical conductivity; hence, achieving perfect absorptance with zero reflectance is challenging. Herein, we present a design strategy for preparing a nearly ideal EM absorber based on a layered metal that maximizes absorption by utilizing multiple internal reflections and minimizes reflection using a monotonic gradient of intrinsic impedance. This approach was experimentally verified using aluminum nanoflakes prepared via topochemical etching of lithium from Li9Al4, and the impedance-graded structure was obtained through the size-based sorting behavior of aluminum nanoflakes sinking in dispersion. Unlike in traditional shielding materials, strong absorption (26.76 dB) and negligible reflectivity (0.04 dB) with a ratio of >103 can be achieved in a 120 μm thick film. Overall, our findings exhibit potential for developing a novel class of antireflective shielding materials.

Transverse shifts and time delays of spatiotemporal vortex pulses reflected and refracted at a planar interface

Abstract: Transverse (Hall-effect) and Goos--Hanchen shifts of light beams reflected/refracted at planar interfaces are important wave phenomena, which can be significantly modified and enhanced by the presence of intrinsic orbital angular momentum (OAM) in the beam. Recently, optical spatiotemporal vortex pulses (STVPs) carrying a purely transverse intrinsic OAM were predicted theoretically and generated experimentally. Here we consider the reflection and refraction of such pulses at a planar isotropic interface. We find theoretically and confirm numerically novel types of the OAM-dependent transverse and longitudinal pulse shifts. Remarkably, the longitudinal shifts can be regarded as time delays, which appear, in contrast to the well-known Wigner time delay, without temporal dispersion of the reflection/refraction coefficients. These results can have important implications in various problems involving scattering of localized vortex states carrying transverse OAM.

Reflection of plane harmonic wave in rotating media with fractional order heat transfer and two temperature

Abstract: The aim of the present investigation is to examine the propagation of plane harmonic waves in transversely isotropic homogeneous magneto thermoelastic rotating medium with fractional order heat transfer and two temperature. It is found that, for two dimensional assumed model, there exist three types of coupled longitudinal waves (quasi-longitudinal, quasi-transverse and quasi-thermal) in frequency domain. The amplitude ratios, energy ratios, phase velocities, specific loss, penetration depth, attenuation coefficients of various reflected waves are computed and depicted graphically. The conservation of energy at the free surface is verified. The effects of two temperature and fractional order parameter by varying different values are represented graphically