Near-grazing Incidence Research Articles

An unsplit-field viscoelastic complex-frequency-shifted perfectly matched layer for analysis of transient waves in heterogeneous media based on an efficient voxel element method

Wave propagation analysis of heterogeneous unbounded media employing domain discretization methods necessitates truncation of the finite computational domain at its truncation boundaries. The Perfectly Matched Layer (PML) is a robust absorbing boundary condition, which is extensively employed to favorably absorb outgoing scattered waves from the interior computational domain, regardless of frequencies and directions of outgoing waves. Nonetheless, most available PMLs are developed for elastic domains, while their developments in domain discretization methods, which consider damping using the Rayleigh damping formulation, are insufficient. In this study, a viscoelastic PML formulation is developed that accommodates the Rayleigh-type damping. The PMLs are basically developed from a coordinate transformation concept using a complex stretching function. Suitable stretching functions can efficiently control the absorptive properties and the spectral character of the PML. Besides, the stretching functions play a crucial role in the long-time stability of transient wave simulations. The standard stretching function usually results in spurious reflections, especially when propagating waves incident to the truncation boundary at near-grazing incidence. On the other hand, the complex-frequency-shifted (CFS) stretching function has been shown to mitigate long-time instability. Therefore, the proposed viscoelastic PML in the time-domain and frequency-domain are derived using the CFS stretching function for the first time. Comprehensive formulations of the proposed viscoelastic CFS-PML are given, where the displacement field is represented as the sole unknown variable of the problem. Hence, the proposed CFS-PML can be incorporated into any domain discretization method. However, the proposed CFS-PML is further developed in the context of the voxel element method, where element-by-element iterative strategies are implemented to solve the governing equations without storing global matrices in the computer memory. Comprehensive numerical experiments are conducted to demonstrate the validity and efficiency of the proposed viscoelastic CFS-PML formulation in analyzing soil profiles with various damping ratios and properties. Moreover, long-time stability for problems involving waveguides and near-grazing wave incidence is studied, where no instabilities are detected.

Energy, angle, and temperature dependencies of the sticking of D atoms on Li surfaces

Detailed experimental and computational information on the response of lithium surfaces to irradiation by slow hydrogenic particles (ions, atoms, molecules) is sparse and mainly speculative. In this work, we present a computational study of the reflection and retention of deuterium (D) atoms at crystalline and amorphous lithium surfaces at 300 and 500 K, where the D atoms have an impact energy in the range of 0.025–5 eV and incident angles of 0° (perpendicular incidence) or 85° (near-grazing incidence). Classical molecular dynamics simulations are performed with the reactive bond-order force field (ReaxFF) potentials. This study provides quantitative information on the deuterium sticking probability and recycling coefficient for lithium surfaces. Our results support the ongoing work at the Lithium Tokamak eXperiment-β fusion experiment as well as relevant experiments in the laboratory setting.

Folded chaotic whispering-gallery modes in nonconvex, waveguide-coupled planar optical microresonators

Chaotic whispering-gallery modes have significance both for optical applications and for our understanding of the interplay between wave phenomena and the classical ray limit in the presence of chaotic dynamics and openness. In strongly non-convex geometries, a theorem by Mather rules out the existence of invariant curves in phase space corresponding to rays circulating in whispering-gallery patterns, so that no corresponding modes of this type are expected. Here we discuss numerical computations of the electromagnetic fields in planar dielectric cavities that are strongly non-convex becausethey are coupled to waveguides. We find a family of special states which retains many features of the chaotic whispering-gallery modes known from convex shapes: an intensity pattern corresponding to near-grazing incidence along extended parts of the boundary, and comparatively high cavity Q factors. The modes are folded into a figure-eight pattern, so overlap with the boundary is reduced in the region of self-intersection. The modes combine the phenomenology of chaotic whispering-gallery modes with an important technological advantage: the ability to directly attach waveguides without spoiling the Q factor of the folded mode. Using both a boundary-integral method and the FDTD technique, we explore the dependence of the phenomenon on wavelength in relation to cavity size, refractive-index contrast to the surrounding medium, and the degree of shape deformation. A novel feature that distinguishes folded from regular whispering gallery modes is that a given shape will support high-Q folded chaotic whispering gallery modes only in certain wavelength windows.

EMP Coupling to a Straight Conductor Above Ground: Transmission Line Formulation Based on Electromagnetic Reciprocity

A simple model for coupling of an electromagnetic plane wave incident on a conductor above ground has been developed using reciprocity theory, providing some advantages as compared to the conventional transmission line approach. The model is developed using a semi-infinitely long single conductor above a lossy ground plane and connected by an arbitrary load impedance to a vertical grounding conductor. This configuration corresponds to the worst-case wave coupling, because it leads to a line induced current larger than in the cases of finite and multiconductor lines. A frequency-domain Thevenin equivalent model is developed to relate the incident wave amplitude to the voltage across a generic load, connected at any point on the vertical conductor. The application to the threat analysis of a high-altitude electromagnetic pulse impact on a power transmission line is discussed by considering the time-domain solution (via inverse Fourier transform) for an incident EMP fast-rise transient (E1) waveshape, following the standard IEC specifications. For typical high-voltage power line load impedances, it is shown that voltage magnitudes in the MV range can be induced across the line termination, in the case of a wave with a near-grazing incidence angle and with wave vector aligned along the horizontal conductor.

Facile metagrating holograms with broadband and extreme angle tolerance

The emerging meta-holograms rely on arrays of intractable meta-atoms with various geometries and sizes for customized phase profiles that can precisely modulate the phase of a wavefront at an optimal incident angle for given wavelengths. The stringent and band-limited angle tolerance remains a fundamental obstacle for their practical application, in addition to high fabrication precision demands. Utilizing a different design principle, we determined that facile metagrating holograms based on extraordinary optical diffraction can allow the molding of arbitrary wavefronts with extreme angle tolerances (near-grazing incidence) in the visible–near-infrared regime. By modulating the displacements between uniformly sized meta-atoms rather than the geometrical parameters, the metagratings produce a robust detour phase profile that is irrespective of the wavelength or incident angle. The demonstration of high-fidelity meta-holograms and in-site polarization multiplexing significantly simplifies the metasurface design and lowers the fabrication demand, thereby opening new routes for flat optics with high performances and improved practicality.

Active damping of sound transmission through an electrorheological fluid-actuated sandwich cylindrical shell

An exact model is proposed for sound transmission through a sandwich cylindrical shell of infinite extent that includes a tunable electrorheological fluid core, and is obliquely insonified by a plane progressive acoustic wave. The basic formulation utilizes Hamilton’s variational principle, the classical and first order shear deformation shell theories, the Kelvin–Voigt viscoelastic damping model (for the electrorheological fluid-core layer), and the wave equations for internal/external acoustic domains coupled by the proper fluid/structure compatibility relations. The Fourier–Bessel series expansions are used to arrange the governing (coupled) system equations in state-space form. The classical Sliding Mode Control law is then applied to semi-actively reduce sound transmission through the composite cylinder by smart variation of stiffness and damping characteristics of the electrorheological fluid-core actuator layer according to the control command. Numerical results present both the uncontrolled and controlled sound transmission loss spectra of the sandwich cylindrical shell at three angles of incidence for three distinct sets of material input parameters that represent the electric-field dependency of the complex shear modulus of the electrorheological fluid-core layer. The superior soundproof performance of electrorheological fluid-based sliding mode control system in avoiding the highly detrimental sound transmission loss dips occurring throughout the critical resonance and coincidence regions is demonstrated. Likewise, remarkable enhancements in the sound insulation characteristics of the electrorheological fluid-actuated structure utilizing the first or second electrorheological fluid material model are achieved within the stiffness-controlled region, especially at lower frequencies in near-grazing incidence situation. A number of limiting cases are introduced and validity of the formulation is confirmed by comparison with the available data.

Confinement loss in hollow-core negative curvature fiber: A multi-layered model

Simple structures are always a pursuit but sometimes not easily attainable. It took researchers nearly two decades for conceiving the structure of single-ring hollow-core negative curvature fiber (NCF). Recently NCF eventually approaches to the centre of intense study in fiber optics. The reason behind this slow-pace development is, undoubtfully, its inexplicit guidance mechanism. This paper aims at gaining a clear physical insight into the optical guidance mechanism in NCF. To clarify the origins of light confinement, we boldly disassemble the NCF structure into several layers and develop a multi-layered model. Four physical origins, namely single-path Fresnel transmission through cascaded interfaces, near-grazing incidence, multi-path interference caused by Fresnel reflection, and glass wall shape are revealed and their individual contributions are quantified for the first time. Such an elegant model could not only elucidate the optical guidance in existing types of hollow-core fibers but also assist in design of novel structure for new functions.

Assessment of modernized GPS L5 SNR for ground-based multipath reflectometry applications

Global Positioning System (GPS) signal-to-noise ratio (SNR) measurements can be employed to retrieve environmental variables in multipath reception conditions, whereby direct or line-of-sight transmission is received simultaneously with coherent reflections thereof. Previous GPS SNR multipath studies of soil moisture and snow depth have focused on the legacy GPS L1 and L2 bands. In the present paper, short-delay, near-grazing incidence multipath from the L5-band GPS SNR is assessed for its value in detecting soil moisture and snow depth. The L5 signal will become more important in the future because of compatibility and interoperability among the different global satellite navigation systems. The L5 results are compared with L2C estimates to determine whether the L2C–L5 differences (given their differing power budgets and their modulation properties) are significant. To address these questions, measurements and simulations were employed. A physically-based multipath simulator was enhanced to investigate the differences between parameter retrievals for the L2C and the L5 GPS signals. Parameter retrievals from synthetic observations for different scenarios were compared. Comparisons included varying reflector height, surface material, and surface roughness. Measurements from two GPS stations in Colorado, USA, were used to retrieve soil moisture and snow depth conditions. Over a 153-day period that encompassed the catastrophic 2013 Colorado flooding event, L2-derived volumetric soil moisture had an RMS difference of 0.042cm3/cm3 while the L5 RMS difference was 0.034cm3/cm3 with respect to in-situ data (values of volumetric soil moisture range between 0.04 and 0.34cm3/cm3). In a separate 483-day campaign, L5-derived snow depth estimates were compared to L2C-derived values and found strongly correlated, deviating from a one-to-one relationship by only 0.00001±0.0064cm/cm. These results indicate the absence of any detectable biases in L5 as compared to L2C for retrieving soil moisture and snow depth from GPS SNR multipath observations.

Alpha and recoil track detection in poly(methyl methacrylate) (PMMA)—Towards a method for in vitro assessment of radiopharmaceuticals internalized in cancer cells

A method for detection and characterization of single MeV α-particle and recoil tracks in PMMA photoresist by atomic force microscopy (AFM) analysis has been demonstrated. The energy deposition along the track is shown to lead to a latent pattern in the resist due to contrast reversal. It has been shown that the pattern, consisting of conical spikes, can be developed by conventional processing as a result of the dissolution rate of poly(methyl methacrylate) (PMMA) being greater than that for the modified material in the cylindrical volume of the track core. The spikes can be imaged and counted by routine AFM analysis. Investigations by angular-resolved near-grazing incidence reveal additional tracks that correspond to recoil tracks. The observations have been correlated with modelling, and shown to be in qualitative agreement with prevailing descriptions of collision cascades. The results may be relevant to technologies that are based on detection and characterization of single energetic ions. In particular, the direct visualization of the collision cascade may allow more accurate estimates of the actual interaction volume, which in turn will permit more precise assessment of dose distribution of α-emitting radionuclides used for targeted radiotherapy. The results could also be relevant to other diagnostic or process technologies based on interaction of energetic ions with matter.

Comparison of electrochromic properties of Ni1−xO in lithium and lithium-free aprotic electrolytes: From Ni1−xO pigment coatings to flexible electrochromic devices

Electrochromic nickel oxide (Ni1−xO) pigment powder which was made from nickel acetate reacting with H2O2/urea solution followed by heat treatment of xerogels at 400°C (24h) was milled with zirconia beads in pure water or water with added nickel oxyhydroxy precipitate acting simultaneously as the dispersant and the coating binder. The corresponding pigment particle suspensions were spin coated on FTO glass and flexible ITO-PET foils, respectively, and cured at 150°C. The optical properties of the deposited pigment coatings on FTO glass substrates were determined with an UV VIS spectrometer, providing total direct transmittance (TT), total diffuse transmittance (DT) and haze (in %). Haze increased with the coating thickness but did not exceed 4%. SEM micrographs of the deposited pigment coatings revealed that they consisted of agglomerated Ni1−xO pigment with voids.Potential cycling of Ni1−xO pigment coatings and Ni1−xO thin films was performed in 1M LiClO4/PC and 0.1M TBA+ triflate/PC electrolytes, revealing that the pigment coatings exhibited electrochemical and electrochromic activity also in lithium free (aprotic) electrolyte, while variations of the oxidation and reduction reaction peaks at various scan rates confirmed the presence of surface electrochemical reactions. The near-grazing incidence angle reflection–absorption spectroscopic technique (IR RA) was employed for identifying surface and inner-grain Ni–O stretching modes, the former being responsible for observed electrochemical properties, which was also demonstrated from the ex situ IR RA measurements performed for coatings charged/discharged in 1M LiClO4/PC and 0.1M TBA+ triflate/PC electrolytes. The results disclosed in this study suggest that the electrochromic effect was not in direct correlation with the de-insertion/insertion of small ions into the NiO (bunsenite) grains but rather the pigment coatings behave similarly to nickel oxide electrochemical capacitors. The electrochromic response of a flexible EC devices made either of PEDOT (self-standing foil) or WO3 and Ni1−xO pigment coatings glued with PMMA based electrolytes without and with lithium salt were recorded for the demonstration of the possible practical application of wet deposited Ni1−xO pigment coatings made as “electrochromic” paints.

The coloration and degradation mechanisms of electrochromic nickel oxide

Nickel oxide (NiO) is the most common low-cost high-performance anodic electrochromic material that is widely used in the applications for smart windows. The coloration mechanism is, however, still not well understood and we show that this is due to the evolving chemical nature of the film during the electrochromic process. Chemical bath deposited (CBD) NiO was studied using the near-grazing incidence angle Fourier transform infrared spectroscopy (NGIA FTIR) and endurance potential cycling. We will show that the initial hydration of NiO films toward Ni(OH)2 proceeds gradually through a combination of coloration from hydroxyl (OH−) ions and bleaching through H+ ions. This process increases the optical modulation of the deposited film. However, when the OH− ion diffusion is significantly enhanced, OH− ion incorporation during coloration will lead to water incorporation. The extensive intercalated networks can then isolate NiOOH grains that results in irreversible coloration and this is commonly reported as degradation. We will propose a model to show that an isolation process can explain this degradation and can be easily reversible by annealing. This understanding of the coloration and degradation mechanisms suggests that an optimum control of hydroxyl ions is critical for both efficiency and durability of NiO electrochromic devices.

The effect of impedance contrast upon surface motion due to scattering of plane harmonic P, SV, and Rayleigh waves by a randomly corrugated elastic inclusion

Scattering of a plane harmonic P, SV, or Rayleigh wave by a corrugated elastic inclusion completely embedded in a two-dimensional isotropic half space is investigated by using a direct boundary integral equation method. The corrugated scatterer is generated by superimposing a random perturbation of arbitrary amplitude to a smooth elliptical shape. The probability density function for the randomly corrugated inclusions is assumed to be of uniform or normal types. The displacement fields are evaluated along the half-space surface for a range of impedance contrasts, frequencies, five incident waves for circular- and elliptic-based rough inclusions. Subsequently, the roughness factor is introduced in terms of the surface response for the rough and the corresponding smooth inclusions. The results clearly show that this factor strongly depends upon the impedance contrast of the materials, the basic inclusion geometry, the type of incident wave, and the frequency. The peak values of the roughness factor are observed for the near-grazing SV incidence. Furthermore, the impedance contrast of the materials has a non-uniform effect upon the surface motion. While the smallest roughness factor is consistently observed for the minimal impedance contrast the peak values of the same factor may take place for different non-minimal impedance contrasts. For the range of frequencies considered here, the type of the random interface corrugation (random-uniform vs. random-normal) has a minimal effect upon the roughness factor.

Low frequency noise absorption by helium-filled duct lining

Low-frequency noise is hard to tackle by traditional sound absorption material because of its inherent high acoustic impedance and space limitation for installing such material. There is a significant sound reflection at the air-absorber interface due to the high impedance mismatch with ambient air and hence the sound wave does not have a chance to enter the absorber for dissipation. The performance of sound absorption material lining in the duct is even worse due to the grazing incidence on the air-absorber interface. The motivation of this study is twofold: (a) to extend the effective range of sound absorption to lower frequencies by filling helium gas in the porous material lining in the duct, and (b) to tackle low frequency noise with a thin layer of the porous material with near-grazing incidence. The work represents a further extension of our recent investigation of the sound propagation in helium-filled porous material. The acoustic impedance can be reduced by filling a low density gas such as helium in the porous material. This has been validated experimentally. Based on the acoustic properties of the helium-filled material found from the new function established in a recent publication by the authors of this paper, the sound absorption performance of helium-filled duct-lining (HDL) is investigated experimentally and theoretically in the current paper. This helps find out the optimal fibre diameter and the lowest frequency for achieving the 10 dB based on the expansion ratio of an empty expansion chamber of 6.5 which is high enough to assess the performance of silencer. There is good agreement between the theoretical prediction and experimental validation in terms of transmission loss, relative absorption coefficient and reflection coefficient. Helium-filled duct lining performs much better than air-filled duct lining in the thin and long configuration at low frequencies. It is because helium gas contributes large wave refraction that can enhance the sound absorption. This is demonstrated theoretically and experimentally with five different fibre diameter of porous materials.

Near-Field PML Optimization for Low and High Order FDTD Algorithms Using Closed-Form Predictive Equations

The convolutional perfectly-matched-layer (CPML) absorbing boundary condition is fully capable of handling near-field wave absorption that usually combines near-grazing wave incidence with wave evanescence. The appropriate choice of the various CPML parameters to realize this potential for any given simulation problem is a challenging task that is typically achieved through exhaustive and time-consuming searches that involve large numbers of full-scale simulations. The presented work here uses a previously developed predictive system of equations that accurately determines numerical reflections off the PML interface and embeds it into a global optimization routine that reliably computes the required optimum CPML parameters. This predictive system of equations has also been extended and validated for the M24 and FV24 integral-based high-order FDTD algorithms. With this approach, the task of selecting optimum CPML parameters that would usually take several days of intense computations can now be accomplished within a few minutes on an average personal computer.

Focusing a helium atom beam using a quantum-reflection mirror

We demonstrate one-dimensional (1D) focusing of a thermal helium atom beam by quantum reflection from a cylindrical concave quartz mirror at near-grazing incidence. The smallest width of the focus achieved is 1.8 μm, essentially limited by spherical aberration. The various effects that contribute to the finite focal width have been investigated. We propose to apply near-grazing reflection from two concave elliptical mirrors in a Kirkpatrick–Baez arrangement for two-dimensional (2D) focusing of a helium atom beam, paving the way for a helium atom microprobe.

Reflection and transmission matrices at a free-space-chiral interface based on the invariant constitutive relations for gyrotropic media and the Drude-Born-Federov constitutive relations

The expressions for the linear and cross-polarized reflection and transmission coefficients based on the invariant gyrotropic constitutive relations and the Drude-Born-Federov constitutive relations are compared. A physical interpretation for the first-order terms in the gyrotropic parameter and the chiral parameter is presented for normal and oblique angles of incidence. The analytical expressions for the linear cross-polarized or circular-like polarized reflection coefficients are proportional to the product of the gyrotropic measure, the tangent squared of the angle of refraction in the host medium, the round trip transmission coefficients for the horizontally and vertically polarized waves, and the polarization dependent reflection coefficients for a perfectly conducting mirror. These analytical results are consistent with the observed enhancement of the differential circular reflection for near-grazing incidence.

Optical High-Performance Computing—JOSA A and Applied Optics

The expressions for the linear and cross-polarized reflection and transmission coefficients based on the invariant gyrotropic constitutive relations and the Drude-Born-Federov constitutive relations are compared. A physical interpretation for the first-order terms in the gyrotropic parameter and the chiral parameter is presented for normal and oblique angles of incidence. The analytical expressions for the linear cross-polarized or circular-like polarized reflection coefficients are proportional to the product of the gyrotropic measure, the tangent squared of the angle of refraction in the host medium, the round trip transmission coefficients for the horizontally and vertically polarized waves, and the polarization dependent reflection coefficients for a perfectly conducting mirror. These analytical results are consistent with the observed enhancement of the differential circular reflection for near-grazing incidence.

Reflection and transmission matrices at a free-space-chiral interface based on the invariant constitutive relations for gyrotropic media and the Drude-Born-Federov constitutive relations

The expressions for the linear and cross-polarized reflection and transmission coefficients based on the invariant gyrotropic constitutive relations and the Drude-Born-Federov constitutive relations are compared. A physical interpretation for the first-order terms in the gyrotropic parameter and the chiral parameter is presented for normal and oblique angles of incidence. The analytical expressions for the linear cross-polarized or circular-like polarized reflection coefficients are proportional to the product of the gyrotropic measure, the tangent squared of the angle of refraction in the host medium, the round trip transmission coefficients for the horizontally and vertically polarized waves, and the polarization dependent reflection coefficients for a perfectly conducting mirror. These analytical results are consistent with the observed enhancement of the differential circular reflection for near-grazing incidence.

Detection of stationary foliage-obscured targets by polarimetric millimeter-wave Radar

This paper introduces a technique to detect the presence of foliage-obscured targets using millimeter-wave radars operating at near-grazing incidence. Radar return from a scene where a target is concealed behind foliage consists of both foliage backscatter return and a polarimetrically distorted and reduced target return. A data acquisition and signal processing technique that takes advantage of temporal decorrelations in foliage is proposed to enhance target detection and resolve it from foliage. Furthermore, an inversion technique is proposed to remove the polarimetric distortions in the target response. The techniques are verified through a series of indoor and outdoor experiments.

Ship detection and characterization using polarimetric SAR

Polarimetric information is investigated for ship detection and characterization at operational satellite synthetic aperture radar (SAR) incidence angles (20°‐60°). It is shown that among the conventional single-channel polarizations (HH, VV, or HV), HV provides the best ship‐sea contrast at incidence angles smaller than 50°. Furthermore, HH polarization permits the best ship‐sea contrast at near-grazing incidence angles. The wave polarization anisotropy is used for optimal information extraction from polarimetric SAR data. It is shown that fully polarimetric information permits a significant improvement in the ship‐sea contrast for relatively calm wind conditions, in comparison with conventional (i.e., scalar) single-channel polarizations (i.e., HH, VV, or HV). For rougher sea conditions, the effectiveness of polarimetric tools may be significantly degraded. Ship characterization is also investigated using the symmetric scattering characterization method (SSCM). Identification of ship targets with significant symmetric scattering can provide a useful ship pitch angle estimate under certain conditions.