Showing papers on "Reflection (physics) published in 2019"

Intelligent Reflecting Surface-Enhanced OFDM: Channel Estimation and Reflection Optimization

Abstract: In the intelligent reflecting surface (IRS)-enhanced wireless communication system, channel state information (CSI) is of paramount importance for achieving the passive beamforming gain of IRS, which, however, is a practically challenging task due to its massive number of passive elements without transmitting/receiving capabilities. In this letter, we propose a practical transmission protocol to execute channel estimation and reflection optimization successively for an IRS-enhanced orthogonal frequency division multiplexing (OFDM) system. Under the unit-modulus constraint, a novel reflection pattern at the IRS is designed to aid the channel estimation at the access point (AP) based on the received pilot signals from the user, for which the channel estimation error is derived in closed-form. With the estimated CSI, the reflection coefficients are then optimized by a low-complexity algorithm based on the resolved strongest signal path in the time domain. Simulation results corroborate the effectiveness of the proposed channel estimation and reflection optimization methods.

Multi-Beam Forming and Controls by Metasurface With Phase and Amplitude Modulations

Abstract: Owing to the capability of providing a certain phase gradient on the interface between two media, metasurfaces have shown great promise for altering the directions of outgoing electromagnetic (EM) waves arbitrarily. With the suitable arrangement of particles on metasurfaces, anomalous reflection and refraction have been observed in wide frequency ranges. To completely control the propagation of EM waves, both phase and amplitude profiles are required in some applications. Herein, we propose a new type of metasurface with both phase and amplitude modulations, which is composed of C-shaped particles and can generate and control multiple beams using amplitude and phase responses simultaneously. An addition theorem of complex reflection coefficients is presented to acquire various states of multiple beams reflected from designed metasurfaces. Meanwhile, the intensities of multiple beams can be separately modulated as desired benefitting from the independent controls of phase and amplitude profiles. All the experimental results have good agreements with the numerical simulations. The presented method opens a new way to form and manipulate multiple beams using metasurfaces, which can find potential applications in beam shaping, radar detection systems, and high-quality holography.

Wave propagation through a piezoelectric semiconductor slab sandwiched by two piezoelectric half-spaces

Abstract: The paper analyses the wave propagation through a piezoelectric semiconductor slab sandwiched by two piezoelectric half-spaces. The two piezoelectric half-spaces (left and right) are both AlN materials, the middle piezoelectric semiconductor slab is ZnO material and is assumed to be transversely isotropic. The semiconductor effect is emphasized by considering the coupling mechanical displacement, electric potential and the carrier in the slab. The state transition differential equation is derived based on the reduction of order of the governing equations and the transfer matrix of state is obtained by solving the state transition equation. The present method can deal with not only the homogeneous slab but also the heterogeneous slab. Two cases (incident QP wave and incident QSV wave) are considered, and the energy reflection and transmission coefficients varying with the incident angle are calculated. The results show that the steady carrier density and the exterior biasing electric field have obvious influences on the reflection and transmission coefficients. This investigation provides a new thought for the adjustment and controlling of elastic wave propagation in the laminated structures.

Scattering Mechanisms and Modeling for Terahertz Wireless Communications

Abstract: This paper provides an analysis of radio wave scattering for frequencies ranging from the microwave to the Terahertz band (e.g., 1 GHz–1 THz), by studying the scattering power reradiated from various types of materials with different surface roughnesses. First, fundamentals of scattering and reflection are developed and explained for use in wireless mobile radio, and the effect of scattering on the reflection coefficient for rough surfaces is investigated. Received power is derived using two popular scattering models — the directive scattering (DS) model and the radar cross section (RCS) model through simulations over a wide range of frequencies, materials, and orientations for the two models, and measurements confirm the accuracy of the DS model at 140 GHz. This paper shows that scattering can become a prominent propagation mechanism as frequencies extend to millimeter-wave (mmWave) and beyond, but at other times can be treated like simple reflection. Knowledge of scattering effects is critical for appropriate and realistic channel models, which further support the development of massive multiple input-multiple output (MIMO) techniques, localization, ray tracing tool design, and imaging for future 5G and 6G wireless systems.

Low-RCS Reflectarray With Phase Controllable Absorptive Frequency-Selective Reflector

Abstract: This paper presents a methodology to control the reflection phase response from a 3-D bandstop structure backed with an absorbing material. An absorptive frequency-selective reflection structure using the cascaded arc-shaped strip resonators and absorbers is utilized to obtain an absorption–reflection–absorption response. The reflection phase response of the structure within the reflection band is then flexibly controlled by top-loaded metallic patches with variable size. An application of this methodology is demonstrated by designing a high-gain and low-radar cross section (RCS) reflectarray. The in-band phase profile of the reflecting surface is constructively designed to collimate the beam in the far field. Meanwhile, the out-of-band scattering from the planar reflector is significantly reduced. Compared with a conventional reflectarray, the radiation performance of the proposed antenna is maintained with an aperture efficiency of 50.1% and directivity of 23.5 dBi. Significant RCS reduction has been achieved with a fractional bandwidth of 76.9% and 17.1% for the lower and upper bands with reduction levels of 10 and 8 dB, respectively.

Triple modality transmission-reflection optoacoustic ultrasound (TROPUS) computed tomography of small animals

Abstract: Rapid progress in the development of multispectral optoacoustic tomography techniques has enabled unprecedented insights into biological dynamics and molecular processes in vivo and noninvasively at penetration and spatiotemporal scales not covered by modern optical microscopy methods. Ultrasound imaging provides highly complementary information on elastic and functional tissue properties and further aids in enhancing optoacoustic image quality. We devised the first hybrid transmission–reflection optoacoustic ultrasound (TROPUS) small animal imaging platform that combines optoacoustic tomography with both reflection- and transmission-mode ultrasound computed tomography. The system features full-view cross-sectional tomographic imaging geometry for concomitant noninvasive mapping of the absorbed optical energy, acoustic reflectivity, speed of sound, and acoustic attenuation in whole live mice with submillimeter resolution and unrivaled image quality. Graphics-processing unit (GPU)-based algorithms employing spatial compounding and bent-ray-tracing iterative reconstruction were further developed to attain real-time rendering of ultrasound tomography images in the full-ring acquisition geometry. In vivo mouse imaging experiments revealed fine details on the organ parenchyma, vascularization, tissue reflectivity, density, and stiffness. We further used the speed of sound maps retrieved by the transmission ultrasound tomography to improve optoacoustic reconstructions via two-compartment modeling. The newly developed synergistic multimodal combination offers unmatched capabilities for imaging multiple tissue properties and biomarkers with high resolution, penetration, and contrast.Hybridized optoacoustic ultrasound computed tomographyA three-in-one imaging platform combines the advantages of each individual technique to provide whole body tomographic imaging of small animals. Developed by the group of Daniel Razansky from the University of Zurich and ETH Zurich in Switzerland and collaborators in Germany and Spain, the hybrid platform combines optoacoustic tomography with reflection and transmission mode ultrasonography. By launching ultrasound and laser pulses into tissues, the technique allows the construction of cross-sectional tomographic images that reveal fine details on organ function, tissue vascularization, reflectivity, stiffness and density. As an added value of the hybrid combination, images retrieved by one modality are also used to enhance the reconstruction quality of the other two modalities. The platform could thus be used for probing and quantifying multiple anatomical, functional and molecular properties of tissues in health and disease.

Three-dimensional Simulation of the Fast Solar Wind Driven by Compressible Magnetohydrodynamic Turbulence

Abstract: Using a three-dimensional compressible magnetohydrodynamic (MHD) simulation, we have reproduced the fast solar wind in a direct and self-consistent manner, based on the wave/turbulence driven scenario. As a natural consequence of Alfvenic perturbations at its base, highly compressional and turbulent fluctuations are generated, leading to heating and acceleration of the solar wind. The analysis of power spectra and structure functions reveals that the turbulence is characterized by its imbalanced (in the sense of outward Alfvenic fluctuations) and anisotropic nature. The density fluctuation originates from the parametric decay instability of outwardly propagating Alfven waves and plays a significant role in the Alfven wave reflection that triggers turbulence. Our conclusion is that the fast solar wind is heated and accelerated by compressible MHD turbulence driven by parametric decay instability and resultant Alfven wave reflection.

Multifunctional reflection in acoustic metagratings with simplified design

Abstract: In this work, we propose and demonstrate a simple acoustic metagrating with binary phase modulation that can be employed to achieve multifunctional reflection with high efficiency, including three-channel retroreflection, quasi-retroreflection, and specular reflection. Although only two sub-elements are designed for the acoustic metagratings, the efficiency of multifunctional reflection is well preserved. By changing the desired retroreflection angle, the incident range of quasi-retroreflection and specular reflection can be effectively tuned. Our work provides an alternative way for planar acoustic devices with versatility and enables the development of acoustic metasurfaces with a simplified design.

Terahertz Wireless Links Using Diffuse Scattering From Rough Surfaces

Abstract: We describe measurements of the diffuse bistatic scattering of a modulated terahertz beam incident on five metallic rough surfaces, to investigate the implications of surface roughness for non-line-of-sight (NLOS) wireless data links at frequencies at and above 100 GHz. The measurements were performed using transmitter and receiver modules, operating at several frequencies from 100 to 400 GHz. We investigate the dependence of the scattering patterns on surface roughness parameters, including rms height and correlation length. The results are consistent with numerical models for scattering from a rough surface. They support the design of bistatic methods for future multiple antenna systems, remote sensing, imaging, and localization in the terahertz range. We demonstrate for the first time that data links which incorporate an NLOS reflection in a nonspecular direction can be established at frequencies above 100 GHz, with low bit error rates.

Waveform Selective Surfaces

Abstract: The role of frequency is very important in electromagnetics since it may significantly change how a material interacts with an incident wave if the frequency spectrum varies. Here, we demonstrate a new kind of microwave window that has the unique property of controlling transmission and reflection based not only on the frequency of an incoming wave but also on the waveform or pulse width. This surface can preferentially pass or reject different kinds of signals, such as short pulses or continuous waves, even if they occur at the same frequency. Such a structure can be used, for example, to allow long communication signals to pass through, while rejecting short radar pulses in the same frequency band. It is related to the classic frequency selective surface, but adds the new dimension of waveform selectivity, which is only possible by introducing nonlinear electronics into the surface. Thus, our study is expected to provide new solutions to both fundamental and applied electromagnetic issues ranging from traditional antenna design and wireless communications to emerging areas such as cloaking, perfect lenses, and wavefront shaping.

Coherent virtual absorption of elastodynamic waves

Abstract: Absorbers suppress reflection and scattering of an incident wave by dissipating its energy into heat. As material absorption goes to zero, the energy impinging on an object is necessarily transmitted or scattered away. Specific forms of temporal modulation of the impinging signal can suppress wave scattering and transmission in the transient regime, mimicking the response of a perfect absorber without relying on material loss. This virtual absorption can store energy with large efficiency in a lossless material and then release it on demand. Here, we extend this concept to elastodynamics and experimentally show that longitudinal motion can be perfectly absorbed using a lossless elastic cavity. This energy is then released symmetrically or asymmetrically by controlling the relative phase of the impinging signals. Our work opens previously unexplored pathways for elastodynamic wave control and energy storage, which may be translated to other phononic and photonic systems of technological relevance.

Indoor Wireless Channel Properties at Millimeter Wave and Sub-Terahertz Frequencies.

Abstract: This paper provides indoor reflection, scattering, transmission, and large-scale path loss measurements and models, which describe the main propagation mechanisms at millimeter wave and Terahertz frequencies. Channel properties for common building materials (drywall and clear glass) are carefully studied at 28, 73, and 140 GHz using a wideband sliding correlation based channel sounder system with rotatable narrow-beam horn antennas. Reflection coefficient is shown to linearly increase as the incident angle increases, and lower reflection loss (e.g., stronger reflections) are observed as frequencies increase for a given incident angle. Although backscatter from drywall is present at 28, 73, and 140 GHz, smooth surfaces (like drywall) are shown to be modeled as a simple reflected surface, since the scattered power is 20 dB or more below the reflected power over the measured range of frequency and angles. Partition loss tends to increase with frequency, but the amount of loss is material dependent. Both clear glass and drywall are shown to induce a depolarizing effect, which becomes more prominent as frequency increases. Indoor propagation measurements and large-scale indoor path loss models at 140 GHz are provided, revealing similar path loss exponent and shadow fading as observed at 28 and 73 GHz. The measurements and models in this paper can be used for future wireless system design and other applications within buildings for frequencies above 100 GHz.

Improved spectral models for relativistic reflection

Abstract: We have developed improved spectral models of relativistic reflection in the lamppost and disc-corona geometries. The models calculate photon transfer in the Kerr metric and give the observed photon-energy spectra produced by either thermal Comptonization or an e-folded power law incident on a cold ionized disc. Radiative processes in the primary X-ray source and in the disc are described with the currently most precise available models. Our implementation of the lamppost geometry takes into account the presence of primary sources on both sides of the disc, which is important when the disc is truncated. We thoroughly discuss the differences between our models and the previous ones.

Multi-focus hologram utilizing Pancharatnam-Berry phase elements based metamirror

Abstract: An ultrathin reflection-type metamirror is proposed for multi-focusing with any desired focusing fashion including focal number and location. The metamirror is composed of reflection-type Pancharatnam–Berry (P-B) phase elements, which are able to provide full reflection phase of 2π, together with near-unity reflection efficiency by judiciously engineering the rotation angle of each latter element. A holographic algorithm is utilized to calculate the phase distribution at the interface of the metamirror to achieve the desired multi-focus spots. Experimental demonstrations performed in microwave region show good imaging quality with high reflection efficiency and imaging efficiency. The proposed metamirror provides a high-performance solution for low-cost and lightweight beam-shaping and beam-focusing devices.

Transmission compensated primary reflection retrieval in the data domain and consequences for imaging

Abstract: We have developed a scheme that retrieves primary reflections in the two-way traveltime domain by filtering the data. The data have their own filter that removes internal multiple reflections, whereas the amplitudes of the retrieved primary reflections are compensated for two-way transmission losses. Application of the filter does not require any model information. It consists of convolutions and correlations of the data with itself. A truncation in the time domain is applied after each convolution or correlation. The retrieved data set can be used as the input to construct a better velocity model than the one that would be obtained by working directly with the original data and to construct an enhanced subsurface image. Two 2D numerical examples indicate the effectiveness of the method. We have studied bandwidth limitations by analyzing the effects of a thin layer. The presence of refracted and scattered waves is a known limitation of the method, and we studied it as well. Our analysis indicates that a thin layer is treated as a more complicated reflector, and internal multiple reflections related to the thin layer are properly removed. We found that the presence of refracted and scattered waves generates artifacts in the retrieved data.

Three-dimensional simulation of the fast solar wind driven by compressible magnetohydrodynamic turbulence

Abstract: Using a three-dimensional compressible magnetohydrodynamic (MHD) simulation, we have reproduced the fast solar wind in a direct and self-consistent manner, based on the wave/turbulence driven scenario. As a natural consequence of Alfvenic perturbations at its base, highly compressional and turbulent fluctuations are generated, leading to heating and acceleration of the solar wind. The analysis of power spectra and structure functions reveals that the turbulence is characterized by its imbalanced (in the sense of outward Alfvenic fluctuations) and anisotropic nature. The density fluctuation originates from the parametric decay instability of outwardly propagating Alfven waves and plays a significant role in the Alfven wave reflection that triggers turbulence. Our conclusion is that the fast solar wind is heated and accelerated by compressible MHD turbulence driven by parametric decay instability and resultant Alfven wave reflection.

Calculation model of dense spot pattern multi-pass cells based on a spherical mirror aberration

Abstract: We report a novel calculation model for dense spot pattern multi-pass cells consisting of two common identical spherical mirrors. A modified ABCD matrix without the paraxial approximation was developed to describe the ray propagation between two spherical mirrors and the reflection on the mirror surfaces. The intrinsic aberration from the spherical curvature creates a set of intricate variants with respect to a standard Herriot circle spot pattern. A series of detailed numerical simulations are implemented to verify that the input and output beams remain the same and, hence, retrace the same ray pattern. The set of exotic spot patterns obtained with a high fill factor improves the utilization efficiency of the mirror surfaces and produces a longer total optical path length with a low mirror cost.

Interaction of SH guided waves with wall thinning

Abstract: This paper investigates through experiment and finite element modelling, the interaction and mode conversion phenomenon of SH0 and SH1 guided wave modes on a metal plate with machined wall thinning. Quantitative analysis was performed by calculating the reflection and transmission coefficients at the leading and trailing linearly tapered edges, for incident SH0 and SH1 modes. Several geometries were evaluated by varying the taper length and depth. Experiments were performed with periodic permanent magnet array EMATs as transmitters and receivers, generating a single SH mode, whilst both SH0 and SH1 are received. Experimental and numerical data show good agreement, revealing that the interaction of SH guided waves with such defects is complex when mode conversion arises. The values of the reflection and transmission coefficients are non-monotonic along the thinning depth and edge angle ranges. The quantitative results provide insight into the capabilities and limitations of guided SH wave measurements for simple corrosion type defects, indicating that with current capabilities, inspection of real defects will be limited to screening type measurements rather than detailed quantification of the defect region.

Ultrafast Tracking of Exciton and Charge Carrier Transport in Optoelectronic Materials on the Nanometer Scale.

Abstract: We present a novel optical transient absorption and reflection microscope based on a diffraction-limited pump pulse in combination with a wide-field probe pulse, for the spatiotemporal investigatio...

Plasma bullet propagation and reflection from metallic and dielectric targets

Abstract: In this paper, we discuss the results from the computational investigation of the effect of a metal grounded target, metal target at a floating potential and dielectric targets (conductive and non-conductive) on the plasma bullet propagation and reflection. We show that the intensity of the primarily ionization wave (IW) is the highest for the metal target, while it is significantly lower for the non-conductive dielectric. For the conductive dielectric, the wave intensity is greater than that for the non-conductive dielectrics, but lower than for a metal target. After the primarily forward IW touches the target, the reflected waves are observed for all the targets under investigation. For a metal target the reflected IW changes its direction and transforms into the secondary forward wave. We did not observe secondary forward IWs for dielectric targets. For dielectric targets, the reflected waves gradually decayed without changing their direction. The "stopping" path for the reflected wave is introduced and the increase/decrease of this path is discussed in dependence on the target properties.

Single Image Reflection Removal Using Convolutional Neural Networks

Abstract: When people take a picture through glass, the scene behind the glass is often interfered by specular reflection. Due to relatively easy implementation, most studies have tried to recover the transmitted scene from multiple images rather than single image. However, the use of multiple images is not practical for common users in real situations due to the critical shooting conditions. In this paper, we propose single-image reflection removal using convolutional neural networks. We provide a ghosting model that causes reflection effects in captured images. First, we synthesize multiple-reflection images from the input single one based on ghosting model and relative intensity. Then, we construct an end-to-end network that consists of encoder and decoder. To optimize the network parameters, we use a joint training strategy to learn the layer separation knowledge from the synthesized reflection images. For the loss function, we utilize both internal and external losses in optimization. Finally, we apply the proposed network to single-image reflection removal. Compared with the previous work, the proposed method does not need handcrafted features and specular filters for reflection removal. Experimental results show that the proposed method successfully removes reflection from both synthetic and real images as well as achieves the highest scores in peak signal-to-noise ratio, structural similarity, and feature similarity.

Willis Metamaterial on a Structured Beam

Abstract: Bianisotropy is common in electromagnetism whenever a cross-coupling between electric and magnetic responses exists. However, the analogous concept for elastic waves in solids, termed as Willis coupling, is more challenging to observe. It requires coupling between stress and velocity or momentum and strain fields, which is difficult to induce in non-negligible levels, even when using metamaterial structures. Here, we report the experimental realization of a Willis metamaterial for flexural waves. Based on a cantilever bending resonance, we demonstrate asymmetric reflection amplitudes and phases due to Willis coupling. We also show that, by introducing loss in the metamaterial, the asymmetric amplitudes can be controlled and can be used to approach an exceptional point of the non-Hermitian system, at which unidirectional zero reflection occurs. The present work extends conventional propagation theory in plates and beams to include Willis coupling and provides new avenues to tailor flexural waves using artificial structures.