Large Angles Of Incidence Research Articles

Sub-bandgap light absorption enhancement in germanium films through Berreman mode weak coupling to a microcavity mode

light absorption is a substantial problem that profoundly influences a wide range of disciplines. Whereas it is fundamentally restricted by the bandgap energy of the involved materials. Herein, we study the sub-bandgap light absorption in germanium films via Berreman mode (BE) and its enhancement through weak coupling to Fabry-Perot cavity mode. This enhancement is performed by integrating the semiconductor film into a microcavity structure and tune its resonance frequency to match the epsilon-near-zero (ENZ) wavelength of the film material in a planar multilayer structure. We ascertained that our approach of electric field confinement in the semiconductor layer could perform significant light absorption at large incidence angles. That provides a novel, general, and simple method to enhance the optical and optoelectronic responses of any ENZ material, especially semiconductors below their bandgap energies.

An advanced two-scale model of EM backscattering from rough surfaces

Based on the random and composite rough surface theory, an advanced two-scale model (ATSM) is proposed to calculate the electromagnetic scattering from the rough surface. The large-scale rough surface part is calculated by Kirchhoff Approximation Method (KAM). Compared with the previous two-scale method, the small-scale roughness is calculated by the Advanced Integral Equation Method (AIEM), which can improve the calculation accuracy and applicability range in terms of the roughness and frequency. By calculating the rough surface backscattering, it can be proved that the proposed two-scale method agrees well with the KAM results for near-vertical incidence and is identical to the AIEM results for large-angle incidence. It should be noted that the proposed method can make up for the drawback that KA decays too fast for swept incidence. The proposed ATSM is also compared with the Shooting and Bouncing Ray (SBR) method and the Poggio-MillerChang-Harrington-Wu-Tsai (PMCHWT) method to verify the accuracy.

Aligned polymer dispersed liquid crystal film for light enhancement of quantum dot backlight

Quantum dots (QDs) have been used to make backlight, which provides a superior color gamut, for liquid crystal flat panel displays. In the backlight system, quantum dots, embedded in a polymer film and illuminated by blue light, emit red and green light with narrow bandwidths. There is, however, a problem with the system in that the quantum dots emit light in all directions, and most of the emitted light is in directions with large incident angles and cannot exit the film due to the total internal reflection at the film-air interface and is wasted. We propose to use an aligned polymer dispersed liquid crystal (APDLC) film to reduce the total internal reflection in the QD backlight and thus improve the light efficiency. A regular PDLC film, where the embedded liquid crystal droplets are randomly oriented, exhibits isotropic scattering and is not a good candidate for the enhancement of light efficiency of QD backlight. Through a two-step polymerization, we successfully developed an aligned polymer dispersed liquid crystal (APDLC) film where the liquid crystal droplets are permanently unidirectionally aligned in the film’s normal direction. It exhibits selective scattering: it scatters light with large incident angles but not light with small incident angles. When the APDLC film is laminated on the QD backlight film, a significant enhancement of the light efficiency of the QD backlight is achieved. The APDLC film can also be used to increase the light efficiency of other flat panel displays, such as organic light emitting diode (OLED) display and micro-light emitting diode (MLED) display.

Dual-Band Single-Layer Fractal Frequency Selective Surface for 5G Applications

Due to the global growth in popularity of Fifth Generation (5G) cellular communications, the demand for shielding against it has risen for a variety of applications, mainly related to cybersecurity but also to isolation, calm areas and so on. This research paper aims to provide a suitable dual-band fractal FSS (Frequency Selective Surface) for the 5G lower band frequencies: 750 MHz and 3.5 GHz. The unit cell is in the shape of a bow tie, where each of the triangular parts are Sierpiński triangles. One major addition to the unit cell is a central metal strip to make the manufacturing of the FSS more feasible and to tune the operation frequencies and bandwidths. As with each different stage of a fractal antenna, the different stages of the fractal FSS design behave differently. For this application, stage 2 is sufficient, as we are able to cover frequency bands among those included in the FR1 5G spectrum. Some equations were derived using linear regression in order to provide specific design tools for building an FSS. These equations have high accuracy and can be used to adapt the proposed design to other frequencies. Some other parameters, which are not represented in the aforementioned equations, can also be adjusted for minor tweaking of the final design. This design performs well except under large incidence angles. This should be taken into account when proposing the installation of a structure based on it. A good agreement between simulation and measurement results is observed.

A dual-tunable ultra-broadband terahertz absorber based on graphene and strontium titanate

An electrically and thermally dual-tunable broadband terahertz absorber based on graphene and strontium titanate is designed and analyzed. The results show that by lifting the Fermi energy of graphene, absorption is significantly enhanced, particularly at higher frequencies, resulting in a broader absorption bandwidth. As the temperature of strontium titanate rises, the center absorption frequency shifts to the higher frequency and the bandwidth increases. At a Fermi energy of 1 eV and a temperature of 400 K, the device exhibits an ultra-broad bandwidth of 3.36 THz and remarkable peak absorption exceeding 99%. Moreover, the absorber is insensitive to incident angles, maintaining a stable broad-bandwidth beyond 3.3 THz within a large incident angle of 55° and 50° for TE and TM polarizations, respectively. The physical mechanisms are elucidated by impedance matching theory and electric field analyses. The structure shows great potential in tunable broadband terahertz absorbers and related applications.

Solute transport in permeable porous media containing a preferential flow feature: Investigation of non-Darcian flow effects

Fractures, faults, karstic features and/or interbedded layers of coarse sediment often manifest in the form of preferential flow features (PFFs). Previous investigations into the effects of PFFs on solute plume distributions within otherwise permeable host rocks have treated flow in PFFs as Darcian. However, flow through faults and fractures is known to exhibit non-Darcian flow behavior. The current study extends previous investigations of the effects of PFFs (e.g., a fault or fracture) on solute plumes in permeable aquifers that assume Darcian flow in the fault/fracture. The finite-element model COMSOL Multiphysics was applied to examine how non-Darcian flow in a single PFF influences flow fields and steady-state solute plume distributions in permeable aquifers, compared with the Darcian flow assumption. The Forchheimer equation was applied within numerical models to represent flow in the PFF for a variety of PFF-permeability contrasts, PFF apertures and matrix flow directions. The refraction of flow lines at matrix-PFF interfaces and the specific discharge in the PFF were found to be smaller in non-Darcian models, leading to larger peak solute concentrations (by up to 160%) and narrower plumes compared to Darcian models. In addition, the impact of non-Darcian flow on flow fields and solute plumes was greater with smaller matrix hydraulic conductivity, larger PFF aperture and larger incidence angle at the matrix-PFF interface. Upstream dispersion at the matrix-PFF interface, observed in previous matrix-PFF studies of solute transport as the result of steep solute concentration gradients opposing head gradients, was found to be reduced when non-Darcian flow was considered in PFFs. This study highlights that non-Darcian flow effects may be significant in the simulation of flow and solute transport within high-permeability PFFs embedded in otherwise porous aquifers.

The influence of oblique waves on the hydrodynamic efficiency of an onshore OWC wave energy converter

In this work, the maximum theoretical hydrodynamic efficiency of an onshore Oscillating Water Column (OWC) device is investigated in relation to the incidence angle of the wave and the configuration of the chamber. Linearized water wave theory is used, and the full solution of the associated boundary value problem (BVP) is achieved by using the simple matched eigenfunction expansion method (EEM) and the boundary element method (BEM) with second-order elements. This study is novel in using these techniques to address the influence of oblique water waves on the efficiency of an OWC with a thick front barrier. The hydrodynamic efficiency is analyzed for different directions of wave incidence and OWC chamber geometrical configurations. The semi-analytical and numerical approaches were found to be in good agreement with cases published in the specialized literature. The results show that both a thick front wall and a large incidence angle of the wave can significantly narrow the hydrodynamic efficiency band and modify the resonant frequency.

Microwave Metamaterial Absorbers with Controllable Luminescence Features.

Traditional microwave absorbers can hide target objects from the detection of radar waves without tackling optical waves. However, in actual scenarios, the objects might still be observed by a hyperspectral remote sensor or an imaging system from comparison with environmental optical features, in particular for variable backgrounds. For this aspect, a comprehensive stealthing technique able to deal with microwaves and optical features simultaneously adapting to the variable environment is highly desired. In this work, an ultrathin flexible metamaterial that can simultaneously realize wideband microwave absorption and controllable visible and near-infrared luminescence spectra is proposed. The designed artificial coat can absorb more than 80% of the incident energy in the X-band (8-12 GHz) within a large incidence angle range up to 54° at low polarization sensitivity, while its real-time visible and near-infrared luminescence spectrum can be electrically adjusted through an integrated emission system. The method proposed here can be extended to broader wave bands and find important applications in multifunctional stealthing technologies.

A W-Band Low-Profile Dual-Polarized Reflectarray With Integrated Feed for In-Band Full-Duplex Application

A W-band low-profile dual-polarized Cassegrain reflectarray with an integrated feed is designed in this work. High isolation, which is necessary for the in-band full-duplex (IBFD) applications, is realized between two feeding ports corresponding to the orthogonal polarizations. The antenna is composed of two parts, i.e., the dual-polarized planar-array feed and the Cassegrain reflectarray with a small focal-length to diameter ratio ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$F/D)$ </tex-math></inline-formula> . The dual-polarized planar array feed is appressed behind the main-reflectarray, and the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$F/D$ </tex-math></inline-formula> is reduced to 0.2 to realize the integration and the low-profile property. For the array feed, high isolation is achieved through the orthogonal-mode isolation and the differential signal cancellation. Its radiation pattern is also manipulated to fit the reflectarray considering the taper and spillover efficiency. For the reflectarray, the rotational symmetry configuration and the dual-layer-patch cell are applied to suit the dual-polarization operation and the large incident angle, respectively. The measured operation band, where the gain variation is within 1 dB, is 93.4–95.6 GHz. At the center frequency, the gains of the Cassegrain reflectarray for the two ports are 31.86 and 30.88 dBi, and the corresponding aperture efficiencies are 31.4% and 25.1%, respectively. The isolation between the two ports is higher than 55 dB within the operation band. This work is a low-profile candidate for W-band point-to-point IBFD communication.

Recent Advances in Copper Polycrystal Film’s Grain Boundaries Behavior and Its Influence in Properties with Molecular Dynamic Simulation

.Copper polycrystal film(CPF) is accepted as a promising material for electroplated film in semiconductor devices for its outstanding conductivity and ductility as well as the good resistance to elector-migration. However, the film material attains a rapid failure in the working environment, and hence the failure mechanism and the fabrication methods require more exploration. In previous studies, it is convinced that grain boundaries(GBs) movement and its interaction with twining boundaries(TBs) and dislocations have a great influence on the failure process. In this study, the applications of Molecular Dynamic(MD) Simulation in the research of CPF have been introduced. The GBs behaviour including deformation of the GBs and the interaction between GBs and TBs that is observed by dislocation extraction algorithm(DXA) has been summarized, and its relation to the properties such as yield strength and the roughness of growth has been discussed. And the best condition to construct a CPF with magnetron sputtering method is concluded to have substrates in 700K as well as low misorientation with grains under incident atoms of large enough kinetic energy and vertical incident angle.

A 2-D Clustering Algorithm for Data Reconstruction in Vertex Detector of ILC

On the vertex detector of the International Linear Collider, a large number of hits are generated by the charged particles coming from the beam background. These charged particles produce large angles of incidence and generate elongated clusters. The CMOS pixel sensor (CPS) which would contain on-chip artificial neural networks could tag and remove these clusters to reduce the data flow of the detector system. The clustering procedure is the first step of data preprocessing. The conventional clustering algorithm is not suitable for on-chip integration since it requires sequential and iterative processing. In this article, a 2-D real-time clustering algorithm is proposed. The clustering algorithm is tested by 4500 frames of pixel values (12 bit/pixel) from MIMOSA-18. The clustering algorithm is implemented using Very High-Speed Integrated Circuit Hardware Description Language (VHDL), synthesized for different windows, multiplexers, and analog-to-digital converter (ADC) resolution. Power consumption and the occupied surface of the clustering implementation are presented. This implementation provides a possibility to integrate the clustering procedure into a CPS.

Reflectarray Antenna Concept for a Snow Mass Measurement SAR Mission in Ku-Band on a Nanosatellite Platform

The design of a 1 × 0.33 m dual-polarized reflectarray antenna (RA) operating in Ku-Band (17.2 GHz) for a synthetic aperture radar (SAR) mission using a 12 U CubeSat is presented. The SAR RA has larger electrical size, larger aspect ratio, and smaller focal to aperture length ratio (F/L) compared to existing CubeSat RAs designed for communication payloads. Electrically small unit cells accounting for large incidence angle variations, improving phase resolution, and preventing grating lobes are designed. To illuminate the RA with the desired elliptical beam, a compact aperture-coupled microstrip patch array antenna was designed. Tests done on a CubeSat mockup integrating the dual-polarization feed source and a five-panel reflectarray showed good agreement between the simulated and the measured beamwidths, with low sidelobe and cross polarizations levels.

Ultra-wideband absorber for electromagnetic waves under large incident angle based on spoof surface plasmon polaritons

In this work, we proposed an ultra-wideband absorber using the dispersion characteristics of spoof surface plasmon polaritons (SSPP). The electromagnetic waves propagating on the SSPP structure have extremely slow group velocity and are confined in the sub-wavelength regions near the asymptotic frequency of the SSPP, and that electromagnetic energy can be absorbed by the high-lossy substrate and lumped resistors. The metallic strips of the SSPP structure are vertically bended and loaded with lumped resistors, and their lengths are linearly varied. This absorber performs excellent absorption ability, including ultra-wideband absorption, almost polarization insensitive performance, and high absorptivity under large incident angles for both transverse magnetic and transverse electronic waves. Based on the simulation analysis, an absorber sample was fabricated and measured, and the simulated and measured results are consistent with each other, validating the design method.

Investigating the absorption performance of a monolayer-coated absorber at oblique incidence

There is a widely observed phenomenon in the microwave absorption field that an absorber always exhibits good oblique incidence absorption capacity if it has high performance at normal incidence. However, if a certain angle is exceeded, this kind of effective absorptive capacity will no longer be maintained. Besides, an absorber performs differently for incident transverse electric (TE) and transverse magnetic (TM) waves: for the TE case, the absorber can no longer obtain effective absorption; for the TM case, another efficient absorption region was observed at higher frequencies even when the incident angle exceeded 80°. These phenomena are widely found in the literature, which demonstrates that they are caused by physical laws rather than material properties. To demonstrate the underlying reason, in this study, the common spherical carbonyl iron-polyurethane composite absorbers were fabricated as a typical example. Their absorbing performance was investigated via both simulation and experiment. All the phenomena mentioned above were observed, studied in detail by employing the multiple reflection model, and explained quantitatively. Further, along with establishing the underlying mechanism of electromagnetic wave transmission in the absorber, two formulas were deduced to predict: (a) the maximum incident angle for efficient absorption of the TE polarized wave; and (b) the required absorber thickness for obtaining efficient absorption for a large incident angle of the TM polarized wave.

Ultrawideband Terahertz Absorber with Dielectric Cylinders Loaded Patterned Graphene Structure.

In this paper, we theoretically designed and numerically analyzed an ultrabroadband meta-absorber with near unity absorptivity that works in terahertz spectrum. A wideband meta-absorber composed of bilayer patterned graphene and dielectric cylinder array with high symmetry was proposed. The wideband absorption mechanism benefited from two aspects. The first one was enhanced surface plasmons based on bilayer patterned graphene. And the second one was the coupling of continuous resonant modes within Fabry-Perot cavities to the enhanced surface plasmons in the graphene. An ultrawide bandwidth with absorptivity over were obtained from THz to THz. Simulated results showed that the proposed ultra-wideband absorbing structure also possessed high performance of polarization independence, flexible tunability, large incident angle insensitivity, and compact fabrication.

Calibration of a polarization image sensor and investigation of influencing factors.

Polarization measurements conducted with a polarization camera using the Sony IMX 250 MZR polarization image sensor are assessed with the super-pixel calibration technique and a simple test setup. We define an error that quantifies the quality of the polarization measurements. Multiple factors influencing the measurement quality of the polarization camera are investigated and discussed. We demonstrate that polarization measurements are generally consistent throughout the sensor if not corrupted by large chief ray angles or large angles of incidence. The central 600×400pixels were analyzed, and it is shown that sufficiently large f-numbers no longer influence measurement quality. We also argue that lens design and focal length have little influence on these central pixels. The findings of this study provide useful guidance for researchers using such a polarization image sensor.

Global Sensitivity Analysis of a Water Cloud Model toward Soil Moisture Retrieval over Vegetated Agricultural Fields

The release of high-spatiotemporal-resolution Sentinel-1 Synthetic Aperture Radar (SAR) data to the public has provided an unprecedented opportunity to map soil moisture at watershed and agricultural field scales. However, the existing retrieval algorithms fail to derive soil moisture with expected accuracy. Insufficient understanding of the effects of soil and vegetation parameters on the backscatters is an important reason for this failure. To this end, we present a Sensitivity Analysis (SA) to quantify the effects of parameters on the dual-polarized backscatters of Sentinel-1 based on a Water Cloud Model (WCM) and multiple global SA methods. The identification of the incidence angle and polarization of Sentinel-1 and the description scheme of vegetation parameters (A, B and α) in WCM are especially emphasized in this analysis towards an optimal estimation of parameters. Multiple SA methods derive identical parameter importance ranks, indicating that a highly reasonable and reliable SA is performed. Comparison between two existing vegetation description schemes shows that the scheme using Vegetation Water Content (VWC) outperforms the scheme combing particle moisture content and VWC. Surface roughness, soil moisture, VWC, and B, are most sensitive on the backscatters. Variation of parameter sensitivity indices with incidence angle at different polarizations indicates that VV- and VH- polarized backscatters at small incidence angles are the optimal options for soil moisture and surface roughness estimation, respectively, while VV-polarized backscatter at larger incidence angles is well-suited for VWC and B estimation and HH-polarized backscatter is well suited for roughness estimation. This analysis improves the understanding of the effects of vegetated surface parameters on multi-angle and multi-polarized backscatters of Sentinel-1 SAR, informing improvement in SAR-based soil moisture retrieval.

Broadband electromagnetic wave absorbing metamaterial based on FeSiAl alloy

FeSiAl alloy material is a typical soft magnetic material, which have important application potentials in the areas of electromagnetic wave absorption, and thus it is widely used in civilian and military field. Currently, researchers mostly adopts the changes of microstructure and doped components to improve electromagnetic wave absorbing properties of FeSiAl alloy materials. In this paper, a metamaterial designing method is used for the optimal design of a broadband electromagnetic wave absorbing material based on FeSiAl alloy. We investigate the geometrical dimensions and array period on the absorption performance about the FeSiAl metamaterial. The absorption mechanism of FeSiAl metamaterial is discussed through the propagation of electromagnetic field and Smith diagram. We find that the absorption performance can easily be regulated with the macrostructural size. Moreover, we create a broadband FeSiAl metamaterial. The designed FeSiAl metamaterial exhibit a −10 dB absorption bandwidth from 4 GHz to 8 GHz and 10 GHz to 15 GHz, and it is sensitive at large incidence angle, but do not sensitive within 45°.

Design of a Transparent Metamaterial Cross Polarization Converter With Large Incident Angle Range

In recent years, the metamaterial has become a popular and powerful tool in the electromagnetic polarization conversion design. In this paper, a flexible and transparent broadband cross-polarization-converter (CPC) was designed, fabricated and tested. ANSYS HFSS was used for the cross-polarization-converter simulation, and the Floquet port and master-slave boundary condition were set in the software. The multi-layer split-ring-resonator (SRR) structure was used to form the metamaterial for the CPC. As a result, >77% polarization conversion ratio (PCR) was achieved in the range of (7.8-18.2) GHz, with little change within 45° incident angle. The experimental results were consistent with the simulation data.

A highly selective absorber based on Archimedes-spiral-shaped metasurfaces

In this paper, a chiral metamaterial absorber based on an Archimedes spiral structure is designed whose operating band can be extended by adjusting array elements and array layout factors. Thanks to its circular dichroism, it is highly selective for left-handed and right-handed circular polarized (LCP and RCP) incident waves at 19.172 GHz, absorbing 95% for LCP, while RCP is basically not absorbed. Meanwhile, a 2 × 2 unit structure is constructed, achieving efficient absorption ranging from 17.55 to 18.13 GHz by overlapping the resonant frequency bands of four different structures, whose relative bandwidth is six times the original single frequency. Then, because of the particularity of the helix, the structure has the characteristics of large-angle incident stability and 360° insensitivity in polarization. Ultimately, we analyze the absorption mechanism arising from the current and electric field distribution of the topological arrangement. In the model improvement, the size can be shrunk proportionally to realize the absorption of 1.337 THz waves. This design can be used in military stealth, photon detection, energy collection, and other scenarios.