Direction Of Specular Reflection Research Articles

Numerical modeling of acoustic scattering characteristic with wavenumber domain admittance

A conversion from the wavenumber domain acoustic reflection coefficient to the wavenumber domain acoustic admittance is formulated to deal with the acoustic reflection properties of arbitrary materials in numerical simulation. This method, which can represent arbitrary reflection directional properties, is an extended reactive boundary condition modelling method based on a spatial Fourier transform over the boundary surface. The advantages are that it is computationally inexpensive because it does not need to consider the interior of the material in the analysis, and that it can handle reflection characteristics obtained from actual measurements, etc. From the results of applying the wavenumber domain acoustic admittance to the numerical analysis, it has been confirmed that the proposed method can handle not only the specular reflection component, but also when arbitrary scattering components are included, or when the main reflection direction is changed from the specular reflection direction.

Terahertz reflective imaging systems with improved angle compatibility realized by the designed quasi-scattering screen

Active imaging can provide significantly larger signal margins in the terahertz (THz) spectral region than passive imaging. However, the imaging effect of THz active imaging is severely constrained by the orientation requirement of target reflection, which limits the application scenarios of this technique. In view of the shortcomings of existing THz reflective imaging systems, this paper proposes a new system optimization scheme from the aspect of hardware - a compact quasi-scattering screen (QSS) is proposed and designed as a beam-shaping element to eliminate the need for the best direction of specular reflection to the greatest extent. The corresponding relationship between the divergent Gaussian beam and target light field is directly established by using the equal-energy mapping method in non-imaging optics, which rejects the collimation process of the THz source. By customizing the optimization function in ZEMAX® to automatically optimize the surface of the QSS, the direction of incident light propagation can be adjusted. A non-coaxial reflective THz imaging system using a 0.1 THz continuous wave source demonstrates the ability of the QSS to improve the angle compatibility of the traditional THz reflective imaging system. The results show that the imaging angle compatibility range of the system can be increased to more than 5 times without changing the original imaging optical path and its resolution. In addition, it can also improve the imaging quality of the original imaging optical path for the object with a smooth surface or certain structure.

Generation of Terahertz Radiation by Relativistic Laser Pulses on the Surface of Thick Solid Targets and Thin Foils

Terahertz pulse generation from multiterawatt laser surface plasma near thick solid targets and thin foils has been studied. Pulses with energies up to 7 μJ were detected in the spectral region <3 THz in the direction of specular reflection from the surface of the CaF2 target. The dependence of the terahertz pulse energy \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{W}_{{{\ ext{THz}}}}}$$\\end{document} on the laser pulse intensity \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$I_{L}^{\\alpha }$$\\end{document} can be approximated by the power function \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{W}_{{{\ ext{THz}}}}} \\sim I_{L}^{\\alpha }$$\\end{document}. For a fixed laser pulse duration and variable energy the power index lies in the range \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha \\approx 1.5{-} 2.8$$\\end{document}, while for a fixed energy and variable duration \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha \\approx 1$$\\end{document}.

Modeling and analysis of spectral polarization BRDF based on Microfacet theory

The polarization bidirectional reflectance distribution function (p-BRDF) model derived from the microfacet theory is utilized for ascertaining wavelength dependent terms in the model. The spectral p-BRDF model of a given object surface is established according to the spectral models of the complex refractive index and the width parameter. Each parameter in the spectral p-BRDF model has a clear physical meaning, making it more physical than Cook-Torrance BRDF model. The particle swarm optimization algorithm combined with in-plane BRDF experimental data is used for analyzing the spectral p-BRDF of a brass surface in the visible light band. The results show that the BRDF calculated by the model is in good agreement with the experimental data, which is validated by the adjusted R2, the optimal value is 0.9872. The peak value of BRDF is found to increase with increase in wavelength for different polarization states. There exists a certain incident angle at which the BRDF initially increases and then decreases as the reflection angle is increased. The BRDF peak appears in the direction of specular reflection, and all the BRDF peaks increase with the increase in the incident angle. The polarization state, wavelength, surface roughness, and permittivity are found to be the important factors affecting the distribution of BRDF on the object surface. A combination of the spectral p-BRDF model and limited experimental data related to the object surface under different testing conditions can be used to predict the spatial reflection characteristics of the surface of a given object.

Deflection of a reflected intense spatiotemporal optical vortex beam.

In this Letter, we reveal a new deflection effect in the reflection of an intense spatiotemporal optical vortex (STOV) beam. When a STOV beam with relativistic intensities (>1018 W cm-2) impacts on an overdense plasma target, the reflected beam deviates from the specular reflection direction in the incident plane. Using two-dimensional (2D) particle-in-cell simulations, we demonstrated that the typical deflection angle is of a few milliradians and can be enhanced by using a stronger STOV beam with tightly focused size and higher topological charge. Though similar to the angular Goos-Hänchen effect, however, it is worth emphasizing that the deviation induced by a STOV beam exists, even in normal incidence, revealing an essentially nonlinear effect. This novel effect is explained from the viewpoint of angular momentum conservation, as well as the Maxwell stress tensor. It is shown that an asymmetrical light pressure of the STOV beam breaks the rotational symmetry of the target surface and leads to nonspecular reflection. Unlike the shear press of an Laguerre-Gaussian beam, which only acts in oblique incidence, the deflection caused by the STOV beam exists more widely, including in normal incidence.

Analysis of target surface polarization characteristics and inversion of complex refractive index based on three-component model optimization

Traditional imaging detection techniques are used to collect intensity information and spectral information of the target to generate high-resolution target images, but the imaging system is susceptible to external environmental factors, resulting in system instability and image quality degradation. Polarization detection techniques use the polarization properties of light to determine the polarization state and spatial profile of a target, thereby improving the contrast of the target in the image. In this study, the reflection process of the target surface is investigated from a microscopic perspective. A polarization characteristics model was developed to illustrate the link between the properties of the target surface and the degree of polarization of the reflected light. The results show that the degree of polarization is strongly related to the detection of zenith and azimuth angles. Experimental data are used to invert the complex refractive index of the target. The polarization of the target is greatest in the direction of specular reflection and smallest in the direction of light source incidence. This suggests that the established Polarimatric Bidirectional Reflectance Distribution Function (pBRDF) model can provide theoretical support for the analysis of the polarization properties of the target surface. Targets of different materials have different polarization properties and the extinction coefficient can effectively classify the targets.

Design of In-Phase and Quadrature Two Paths Space-Time-Modulated Metasurfaces

Space-time-modulated metasurfaces can manipulate electromagnetic waves in space and frequency domains simultaneously. In this article, an analytical design of space-time-modulated metasurfaces with modulation elements composed of two paths, In-phase (I) and Quadrature (Q), is proposed. The model is derived analytically, the space-/frequency-domain manipulations are achieved by designing the dimension and time sequence of I and Q paths. In the specular reflection direction, an objective frequency shift of the reflected first-order harmonic can be obtained. While, in other directions, the opposite first-order harmonic can be easily controlled by changing the dimension of I/Q paths, and the objective first-order harmonic remains unchanged. Furthermore, with a small dimension of I/Q paths, the first-order harmonic can be used for beam scanning by predesigning the start time of the modulation element. To realize the space-time-modulated metasurface with the required periodically time-varying responses, 2 bit unit cells loaded with dynamically switchable pin diodes are used as I/Q modulation. Both the analytical and numerical results demonstrate that space- and frequency-domain manipulations of the reflected fields by the first-order harmonics can be simultaneously obtained. The proposed designs have potential applications in wireless communications, radar camouflaging, and cloaking.

Link Budget Analysis for GNSS-R Sea Surface Return in Arbitrary Acquisition Geometries Using BA-PTSM

In this article, we present a link budget analysis for Global Navigation Satellite System (GNSS) signals scattered off the sea surface in arbitrary acquisition geometries. The aim of our study is to investigate the reliability of the Geometrical Optics (GO) scattering model, which accurately describes sea surface scattering at and near the specular reflection direction, in properly modeling the sea surface return in far-from-specular acquisition geometries, which are of interest for maritime surveillance purposes and where GO is expected to fail. To this end, we adopted the recent Bistatic Anisotropic Polarimetric Two-Scale Model (BA-PTSM), which revealed good agreement with advanced scattering models, such as the second-order Small Slope Approximation (SSA2), regardless of the acquisition geometry, with the advantage of a reduced computational complexity. Numerical results have been derived for both circular polarization channels and for both spaceborne and airborne GNSS-Reflectometry (GNSS-R). It has been shown that, as long as conventional GNSS-R processing is assumed, GO can be safely adopted for simulation and analysis of spaceborne GNSS-R data regardless of the acquisition geometry and sea state, whereas more accurate scattering models, e.g., BA-PTSM, should be used for airborne receivers in far-from-specular configurations.

Demonstration of light reflection concepts for rendering realistic 3D tree images

With advancements in computer graphics, creating natural images has always been the main purpose, image rendering is all based on principles of physics. So, understanding the physics of image rendering will enable us to create the most realistic images. A ray of light hit a surface with different orientation and reflects as per the rules of physics. It is difficult to calculate the light reflection of complex foliage, such as trees, so, the reflection of this natural complexity needs to be adapted to rendering situations. In this research, the researchers provide demonstrations to enable students to understand the light reflection in nature, light calculation in computer graphics and methods to apply them to render realistic tree images. The researchers assign students to render 3D realistic tree images to assess the students’ understanding by applying the diffuse reflection value, specular reflection value and surface normal direction to render realistic tree images. The researchers find that most students understand of diffuse reflection, specular reflection, and surface normal direction causes the rendering results to be most realistic.

Towards isolated attosecond electron bunches using ultrashort-pulse laser-solid interactions

We investigate MeV-level attosecond electron bunches from ultrashort-pulse laser-solid interactions through similarities between experimental and simulated electron energy spectra. We show measurements of the bunch duration and temporal structure from particle-in-cell simulations. The experimental observation of such bunches favors specular reflection direction when focusing the laser pulse onto a subwavelength boundary of thick overdense plasmas at grazing incidence. Particle-in-cell simulation further reveals that the attosecond duration is a result of ultra-thin (sim tenth of a micron) gaps of zero electromagnetic energy density in the modulated reflected radiation, while the bunching (locally peaked electron concentration) comes from the highly-directional electron angular distribution acquired by the electrons in a grazing incidence setup. To isolate a single electron bunch, we perform simulations using 1-cycle laser pulses and analyze the effect of carrier-envelop phase with particle tracking. The duration of the electron bunch can be further decreased by increasing the laser intensity and the focal spot size, while its direction can be changed by tuning the preplasma density gradient.

Physical models of the bidirectional reflectance of solar receiver coatings

We present measurements of the Bidirectional Reflectance Distribution Function (BRDF) of the state-of-the art solar receiver coating, Pyromark® 2500, and show how the data can be accurately described with physical models. In the conducted measurements of the receiver coating, a peak shift of the radiation away from the direction of ideal specular reflection was observed. Both the Cook–Torrance and the He–Torrance models reproduce this behaviour. For the application of the former model, an empirical angular dependence of the parameter describing the surface roughness has to be assumed in order to obtain an adequate fit to the data. The latter model is less exact in a statistical sense, but it is physically based, with six meaningful fit parameters. As we found that the He–Torrance model with its original form of the specular reflection yielded physically incorrect results for incidence angles larger than 80°, a modification is proposed to overcome this deficiency.Overall, the fitted models are able to provide a good description of the BRDF of the receiver coating over three orders of magnitude. Our results can play an important role in facilitating camera-based flux control or optical simulations of solar tower systems. Moreover, we think that our findings can serve as a sound starting point for the physical modelling of other absorber coatings currently being developed.

Multiple Diffuse Coding Metasurface of Independent Polarization for RCS Reduction

In this article, a multiple diffuse coding metasurface (MDCM) of independent polarization is designed to control the propagation direction of diffuse reflections under different polarizations and to improve the monostatic and bistatic RCS (radar cross section) reduction effect. First, a method for controlling the distribution range and propagation direction of the diffuse field is studied, and the diffuse field distribution of the random phase metasurface is optimized by a genetic algorithm to improve the uniformity of the diffuse scattering distribution. Then, the random phase distribution is superimposed on the periodic gradient phase distributions of the linear and hedge types in the orthogonal direction so that the main propagation direction of the diffuse metasurface deviates from the specular reflection region under different polarizations, showing single and two diffuse beams. Finally, the anisotropic unit cell with a rectangle inside and an improved Jerusalem cross on the outside is employed as the basic coding element of the MDCM due to its independent polarization phase response. The numerical and experimental results show that the MDCM features multiple diffuse scattering, independent polarization and angle insensitivity and can efficiently improve the monostatic and bistatic RCS reduction effect simultaneously. Because the scattered energies are redirected away from the specular reflection direction, the specular scattering reduction effect is better than the isotropic diffuse metasurface. The proposed method increases the difficulty of detection by single or netted radar and has the potential for the applications of stealth techniques.

Polarized Remote Inversion of the Refractive Index of Marine Spilled Oil From PARASOL Images Under Sunglint

The ability to detect oil spills remotely is important in marine environmental monitoring. The optical polarization remote sensing has the unique advantage of inversion of refractive index of spilled oils which is the key parameter for calculation of sunglint reflectance. Compared to nonpolarization optical image, the degree of linear polarization (DOLP) of spilled oil’s sunglint depends on the refractive index and viewing angles but not on the surface roughness. Accurate correction of sunglint reflectance can promote optical estimation of spilled oils. In this article, a polarized optical model was used to calculate equivalent refractive index of Deepwater Horizon (DWH) spilled oils using space-borne Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar (PARASOL) images covering Gulf of Mexico (GOM) in 2010. When the angle ( $\theta _{m}$ ) between the direction of the flat surface specular reflection and that of observation is less than 20°, the PARASOL-derived and modeled DOLPs agree well, and the atmospheric polarization effects can be neglected. The equivalent refractive index of the spilled oil area, which implies the relative proportions of seawater and spilled oil in each pixel, could be estimated using polarized remote sensing under sunglint. Furthermore, if the relationship between the equivalent refractive index and remote sensing reflectance ( $R_{rs}$ ) of spilled oils in the remote sensing images can be given, it might be used to correct the sunglint effect on various spilled oils, thereby leading to an improvement for optical quantifying spilled oil volume.

Analysis on spectral radiative properties of micro-scaled rough ligament surface inside open-cell nickel foam

Accurately predicting the spectral radiative properties of ligament surface is vital for exploring the heat transfer inside open-cell foam materials. In this work, the complex morphology of ligament surface in open-cell nickel foam is reconstructed by analyzing the SEM scanning pictures of ligament surfaces. Then, the radiative parameters including the directional-hemispherical reflectivity, directional-directional reflectivity and proportion of specular reflection of rough ligament surface in the waveband from visible to near infrared light are numerically predicted by applying the FDTD method. Furthermore, the effects of incident angle and wavelength on the predicted radiative properties are compared and summarized. Besides, a fifth-degree polynomial is put forward for characterizing the directional-hemispherical reflectivities under different incident angles and wavelengths. The distributions of directional-directional reflectivities of ligament surface are compared and analyzed in order to predict the various value and dominated scope of reflection peak in the specular reflection direction. Additionally, the proportions of specular reflection under different incident conditions are achieved properly and characterized by proposed mathematical formulas. This work is helpful for the multi-scale numerical prediction of radiative transfer inside open-cell nickel foams, which is beneficial to further researches about engineering applications of open-cell foam materials.

Statistical Properties of Marine Surface Slopes Using Glitter Patterns: The 2-D Case

A glitter pattern is a set of bright spots. To have a bright spot on the water, the slope of a point on the sea surface must provide a specular reflection direction of the sunlight to the detector where the image will be created. This interaction is visible for all of us on a clear day with appropriate wind velocities on the sea surface. In this paper, a mathematical methodology was deduced, which is suggested to apply in real images of the sea. First, the suitable 2-D geometry of the reflection situation was determined. Then, the range of the slopes that satisfies the specular reflection condition for each direction was obtained; in addition, the 2-D statistical relationships between the glitter patterns of the sea surface and the sea surface slopes for different wind velocities on the sea surface were deduced using a 2-D Rect and a Gaussian glitter function, considering Gaussian and non-Gaussian probability density functions. Finally, doing parallel computing, the results consistent with the extension of the 1-D case were obtained. This method could be applied on real images, instead of using pressure sensors and others expensive electronic dispositives to model ocean behavior and other various oceanographic studies.

Observation of backward high-harmonic emission from solids

We experimentally demonstrate backward emission of high-harmonics of a near-infrared laser from MgO and Si crystals in the direction of specular reflection. We show that the variation of the high-harmonic power with the angle of incidence can be predicted with nonlinear reflection coefficients derived originally for perturbative nonlinearities. A comparison of transmission and reflection geometries suggests that backward-propagating high-harmonics are an excellent reference to study nonlinear propagation of intense light in solids. Backward emission will enable phase matching of the high-harmonic beam and the integration of the functionalities of extended gas-phase high-harmonic beamlines into a single optical element. The potential to achieve phase matching paves the way to solid-state based high-harmonic sources with higher flux than the best transmission-based sources, where high-harmonics are strongly absorbed by the crystal itself.

Flexible and polarization-controllable diffusion metasurface with optical transparency

In this paper, a novel coding metasurface is proposed to realize polarization-controllable diffusion scattering. The anisotropic Jerusalem-cross unit cell is employed as the basic coding element due to its polarization-dependent phase response. The isotropic random coding sequence is firstly designed to obtain diffusion scattering, and the anisotropic random coding sequence is subsequently realized by adding different periodic coding sequences to the original isotropic one along different directions. For demonstration, we designed and fabricated a flexible polarization-controllable diffusion metasurface (PCDM) with both chessboard diffusion and hedge diffusion under different polarizations. The specular scattering reduction performance of the anisotropic metasurface is better than the isotropic one because the scattered energies are redirected away from the specular reflection direction. For potential applications, the flexible PCDM wrapped around a cylinder structure is investigated and tested for polarization-controllable diffusion scattering. The numerical and experimental results coincide well, indicating anisotropic low scatterings with comparable performances. This paper provides an alternative approach for designing high-performance, flexible, low-scattering platforms.

A reflective-type, quasi-optical metasurface filter

We introduce a new technique for designing quasi-optical, reflective-type spatial filters. The proposed filter is a reflective metasurface with a one dimensional, frequency-dependent phase gradient along the aperture. By careful design of each unit cell of the metasurface, the phase shift gradient provided by the adjacent unit cells can be engineered to steer the beam towards a desired, anomalous reflection direction over the passband region of the filter. Outside of that range, the phase shift gradient required to produce the anomalous reflection is not present and hence, the wave is reflected towards the specular reflection direction. This way, the metasurface acts as a reflective filter in a quasi-optical system where the detector is placed along the direction of anomalous reflection. The spectral selectivity of this filter is determined by the frequency dispersion of the metasurface's phase response. Based on this principle, a prototype of the proposed metasurface filter, which operates at 10 GHz and has a bandwidth of 3%, is designed. The device is modeled using a combination of theoretical analysis using the phased-array theory and full-wave electromagnetic simulations. A prototype of this device is also fabricated and characterized using a free-space measurement system. Experimental results agree well with the simulations.

Using remote sensing to detect the polarized sunglint reflected from oil slicks beyond the critical angle

AbstractThe critical angle at which the brightness of oil slicks and oil‐free seawater is reversed occurs under sunglint and is often shown as an area of uncertainty due to different roughness and surface Fresnel reflection parameters. Consequently, differentiating oil slicks from the seawater in these areas using optical sensors is a challenge. Polarized optical remote sensing techniques provide complementary information for intensity imagery with different physical properties and, thus, possess the ability to resolve this difficult problem. In the polarized reflectance model, the degree of linear polarization (DOLP) of sunglint depends on accurately knowing the Stokes parameter for the reflected light, and varies with the refractive index of the surface layer and viewing angles. For the polarized detection of oil slicks, the highest sensitivity of the DOLP to the refractive index is located within the specular reflection direction where the sum of the solar and sensor zenith angles is 82.6°. The modeled results clearly indicate that the DOLP of oil slicks is weaker in comparison with oil‐free seawater under sunglint. Using measurements from the space‐borne Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar (PARASOL) over the Deepwater Horizon oil spill in the Gulf of Mexico, we illustrate that the PARASOL‐derived DOLP difference between the oil spill and seawater is obvious and is in accordance with the modeled results. These preliminary results suggest that the potential of multiangle measurement and feasibility of deriving refractive index of ocean surface from space‐borne sensors need further researches.

Laser second-harmonic generation from an overdense plasma slab

AbstractA s-polarized short-pulse laser impinged obliquely on an overdense plasma slab is shown to produce very significant second harmonic in the direction of specular reflection and transmission. The laser induces a non-linear current on electrons, which is curl free. However, with sharp plasma boundary, it gives rise to electromagnetic radiation at the second harmonic. Our formalism includes multiple reflections of the incident and second-harmonic waves from both the front and rear surfaces. The present work includes finiteness of the slab. The normalized second-harmonic amplitude acquires a sharp peak at some specific angle of incidence for a particular set of parameters dependent on thickness of the slab and plasma density.