Change Of Incident Angle Research Articles

Angular variation of SAR polarimetric parameters over multiyear ice

ABSTRACT Backscatter and derived parameters from Synthetic Aperture Radar (SAR) may vary across the swath as the incident angle increases from the near to the far range, especially in the ScanSAR mode. Previous studies characterize this angular variation in the form of linear regression for commonly used ice types, namely first-year (FYI) and multi-year (MYI). The present study took advantage of the appearance of three grounded multi-year ice floes, viewed at different radar incidence angles, in a series of 22 RADARSAT-2 fully-polarimetric images, acquired over the Resolute Passage in the Canadian Arctic, during the period from 21 October to 28 December 2017. The ice properties remained invariant during the observation period; hence, variation of radar parameters of MYI can be attributed to change of incidence angle only. Quantification of angular variation of the selected SAR parameters was performed. They include conventional orthogonal backscatter coefficients, derived parameters from the incoherent decomposition of Cloude-Pottier, Yamaguchi, and Touzi, as well as a set of parameters from the compact polarimetry mode. Results specify the parameters that reveal variation with incidence angle and present the linear regressions equations. The explanation for the behaviour of each parameter, in terms of showing or not showing an angular trend, is offered. Results will be useful when using the polarimetric parameters in the sea ice classification scheme.

Radio absorption in structures like artificial magnetic conductors at large angles of incidence of TM-polarized waves

The frequency-angular characteristics of the reflection of TM-polarized waves from thin (thickness up to 1/200 wavelength) artificial magnetic conductor (AMC) and radio absorber (RA) based on band-reflecting and band-passing gratings are presented. It is shown that the operating frequency bands of the AMC and RA expand tens of times when the angle of incidence changes from 0 to 89 degrees. In this case, the value of the ratio (is the difference in wavelengths at the edges of the absorption band and is the thickness of the RA) increases to 30.

Hot spot localization in the field of view of the Kirkpatrick–Baez microscope

The Kirkpatrick–Baez (KB) microscope is an effective instrument for x-ray imaging of hot spot. However, the non-uniform distribution of response efficiency in the field of view is a drawback of the KB microscope. A more accurate hot spot image requires the correction of the measured image by combining the hot spot position and the response efficiency distribution. Here, we describe a method to locate the position of the hot spot in the field of view during hot spot imaging with a KB microscope. The position of the hot spot in the field of view can be obtained by measuring the grazing incidence angle change during hot spot imaging. In the experiment of hot spot self-emission imaging with a four-channel KB microscope, the location of the hot spot with an accuracy of 15 μm was realized, and the intensity corrected hot spot image was obtained. This will solve the problem of the non-uniform distribution of the response efficiency of the KB microscope and enable quantitative measurement of hot spot radiation intensity.

Sputtering yield increase with fluence in low-energy argon plasma-tungsten interaction

The increase of tungsten sputtering yield with fluence was investigated during plasma (≤100 eV) irradiation. This analysis focused on the combined effects of surface binding energy and surface morphology caused by Ar retention. As post-mortem analysis, Ar concentration was measured with SIMS (Secondary Ion Mass Spectroscopy) and TDS (Thermal Desorption Spectroscopy). The Ar concentration saturated at 21 % of W number density during the sputtering process. The corresponding change in surface morphology causes a change in the local ion incident angle, which leads to a sputtering yield increase of 10 %. The increased Ar concentration leads to a decrease in surface binding energy and a change in surface morphology which increases W sputtering yield from 0.02 to 0.03 by ion energy 80 eV. Over time, W sputtering yield reaches saturation as a function of saturated Ar concentration. This result implies that the synergistic role of Ar concentration and surface morphology on sputtering yield. This sputtering yield enhancement occurs more seriously in the realistic condition of plasma-facing materials that face low-energy plasma.

Programmable metasurfaces with high angular stability for arbitrarily-polarized obliquely-incident waves

Programmable metasurfaces show enormous potential in real-time electromagnetic (EM) manipulation and their stable responses to variable EM waves including incident angle and polarization changes are crucial for related applications. However, the demonstrated programmable metasurfaces are commonly considered in normal-incident waves with fixed polarization; how to achieve stable performance in more realistic scenarios of wide-angle and full-polarized wave incidences is still a challenge. Here, we propose and realize a wide-angle and full-polarization programmable metasurface, which can generate stabilized amplitude and phase responses at both transverse electric (TE) and transverse magnetic (TM) oblique incidences. These are achieved by introducing metallic walls around the metasurface element to reduce element coupling, which greatly improves the angular stability of its reflection responses. The mechanisms of angular insensitivity are analyzed comprehensively, and as a proof of concept, obliquely-incident beam steering, large-angle beam scanning and oblique vortex beam emitting are demonstrated on this programmable metasurface. Both the simulation and experimental results demonstrate that the programmable metasurface can operate well at both TE and TM wide-angle oblique incidences in the upper half space. Our work offers an actual route for improving the angular stability and polarization insensitivity of metasurfaces, which could push them one step closer towards more complicated applications.

UV-NIR polarization sensitive metamaterial absorber based on two-dimensional titanium grating

A UV-NIR metamaterial absorber (MMA) based on a two-dimensional titanium grating that is sensitive to polarization is proposed. The absorber is composed of a two-dimensional titanium grating shaped as a four-edge platform, a one-dimensional SiO2 grating, and an Au substrate. When the incident light is polarized in the X direction, the absorber achieves ultra-broadband absorption of 95.72 % in the range of 300–2400 nm. When the incident light is polarized in the Y direction, the absorber exhibits near-perfect ultra-narrowband absorption at 351 nm, and the Q factor and the sensitivity (S) of the narrowband absorption are 38.14 and 123.21 nm/RIU, respectively. The absorber also achieves polarization angle selective absorption at 400–2400 nm with an average extinction ratio of 25.73. Both the impedance matching theory and the energy distribution of the electromagnetic field explain the polarization-sensitive MMA well. The proposed absorber is resistant to changes in incidence angle and may be useful in polarization imaging, photoelectric detection, and optical sensors.

Enhancing mid-infrared surface phonon polariton modes: optimization of structural parameters in Nb-doped SrTiO3 with hole array structures for advanced infrared sensors

This study utilized the finite difference time domain method to investigate the mid infrared surface phonon polaritons and localized surface phonon resonances in undoped and niobium (Nb)-doped SrTiO3 (STO) with planar and holes array structures. Research has shown that Nb-doped STO operates in the Reststrahlen band of 8.06–18.48 µm, providing a wider spectral response than undoped STO (12.58–18.26 µm) and effectively covering the atmospheric window of long wave infrared. This indicates that the increase in virtual permittivity has the least impact on spectral broadening, indicating that the new infrared sensor technology has broad prospects. The optimization of structural parameters, including the period, filling factor, and depth of STO holes array, as well as the response to changes in incident light angle, is crucial for guiding the design of high-performance optoelectronic devices. In addition, this study explored the excitation of four resonant modes within a holes array and analyzed their relationship with array parameters to enhance the design of optoelectronic applications.

Temperature-Controlled and Adjustable Terahertz Device Based on Vanadium Dioxide

We propose a simple multifunctional terahertz absorber based on the simulation. The device consists of a gold layer, a SiO2 dielectric layer, and a VO2 top layer. The modulation mechanism of this device is to utilize the thermally induced phase transition characteristics of vanadium dioxide material. The simulation results show that when the temperature is 312 K, the device has the effect of complete reflection of terahertz waves. When the temperature is 345 K, the device has almost perfect absorption of terahertz wave in the range of 4.7–9.7 THz, and the spectral absorptivity is modulated in the range of 0~0.999. The electric field conditions at different temperatures were plotted to further explain the reasons for the performance transition of the device. The terahertz device was explained using impedance matching theory. In addition, the influence of different structural parameters on absorption rate was studied, providing reference for practical applications. At the same time, the device is polarization-insensitive and insensitive to the incident angle. When the incident angle changes from 0°to 45°, the device still has a stable absorption effect. The device has great application prospects in terahertz stealth, modulation, and other fields and provides ideas for the design of related devices.

Effect of three-phase-lag thermal and three-phase-lag diffusion models on waves at the boundary of elastic and thermoelastic diffusion medium

In this paper, we discuss the reflection and refraction of an incident P wave or [Formula: see text] wave at the interface of a plane. The plane, which is divided into two halves, is an elastic medium [Formula: see text] having an incident wave and a thermoelastic diffusion medium [Formula: see text] with TPLT (i.e., three-phase-lag thermal) and TPLD (i.e., three-phase-lag diffusion) models. It has been noticed that two waves are reflected and four are refracted in an isotropic thermoelastic diffusion medium. Out of the four refracted waves, three are longitudinal waves: a quasi-longitudinal wave [Formula: see text] a quasi-mass diffusion wave [Formula: see text], a quasi-thermal wave [Formula: see text] and one is a transverse wave [Formula: see text]. If we consider the above waves first, the amplitude and energy ratio are calculated by using the surface boundary conditions and then graphically represented to compare the change in energy and amplitude ratio with the change in incident angle for three particular cases. The conservation of energy is depicted by verifying that all the energy sums up to unity. The considered problem has its application in earthquake engineering, astronautics, rocket engineering, seismology and many more engineering areas.

Dumbbell shaped structure loaded modified circular ring resonator based perfect metamaterial absorber for S, X and Ku band microwave sensing applications

In this paper, a new metamaterial absorber (MMA) is presented that exhibits peak absorptions at 3.26 GHz, 11.6 GHz, and 17.13 GHz within S, X, and Ku bands. The unit cell of the proposed MMA is constructed on an FR4 substrate having an electrical dimension of 0.144λ × 0.144λ, where wavelength, λ is calculated at the lowest absorption frequency. The unique structural design of the unit cell consists of two concentric copper rings with which dumbbell-shaped structures are attached. The rotating symmetrical structural design of this MMA provides around 93.8%, 96.47%, and 99.95% peak absorptance in the mentioned frequencies, which is invariable with the change of incident angle as well as polarization angle. The metamaterial properties of the proposed absorber are studied along with the surface current analysis. The MMA shows single negative behaviour and it also exhibits high-quality factors (Q factor) of 21.73, 41.42, and 51.90 at maximum absorptance frequencies. The MMA is analysed by it's equivalent circuit to understand the resonance phenomenon, which is verified through simulation in Advanced Design Systems (ADS) software. The testing is done on the developed prototype of the proposed MMA. Measurement results are in close proximity to the simulation results. Due to its high Q factor, high EMR, and insensitivity to polarization and angle of incidence, it can be utilized as a part of miniaturized microwave device. In addition, the proposed MMA can exhibit high sensing performance and flexibility to differentiate different oils in S, X, and Ku bands.

A Study on Length Traceability and Diffraction Efficiency of Chromium Gratings

Measurement traceability is a prerequisite for achieving accurate and reliable results as well as technical standardization. The period of Chromium (Cr) gratings fabricated by atomic lithography can be directly traced back to natural constants. Applying the Cr grating to grating interferometry can achieve nanometer measurement traceability. This research aims to analyze the diffraction efficiency characteristics of self-traceable Cr gratings to provide a theoretical basis for the fabrication and application of Cr gratings. In this regard, we establish the theoretical model of the laser beam incident angle and grating diffraction efficiency using the rigorous coupled-wave method. Then, we analyze the influence of the laser beam incident angle on grating diffraction efficiency by simulation, verify the accuracy of the theoretical model, and finally build a measurement system for grating diffraction efficiency. Through experiments, we find that the diffraction efficiency of the grating shows a rapid increase to reach a stable maximum value followed by a decrease, when a laser beam with a wavelength of 405 nm is incident on the surface of a self-traceable grating in Transverse Magnetic (TM) polarization and the incident angle changes within an effective range. The experimental results are consistent with the trend of theoretical calculation results.

Dual-band terahertz metamaterial sensor and its sensing capacity enhanced with a central-relief design.

In this paper, a dual-band terahertz metamaterial sensor based on aluminum and silicon is proposed and simulated. The aluminum surface, which is deposited on a silicon substrate, is made of a C-shaped frame resonator, a rectangular beam, and a cross. The device is insensitive to the change of incident angle in the range of 0°-30°, which shows the great transmission stability of the sensor. By examining the resonance frequency shift, it is shown that 98.3 and 237.5GHz/RIU refractive index sensitivity can be obtained near 1.76 and 2.404THz transmission dips of the proposed structure, respectively. The two dips can be used to sense analytes in different refractive index ranges, respectively. For Dip 1 at 1.76THz, the range is 1.0-1.6. For Dip 2 at 2.404THz, the range is 1.6-2.0. Different from traditional multi-band metamaterial sensors, two dips generated by the proposed device can measure continuous and non-multiplexed refractive index ranges, respectively. Because the resonance frequencies of matters are different, such a characteristic enables the device to measure different types of analyte using the appropriate resonant peak. A central-relief design is then proposed based on perturbation theory to further improve its sensing performance. The aluminum cross is covered by polyimide, which can interfere with the scattering field on the metal surface and affect the transmission results. For both transmission dips, the optimized structure realizes higher sensitivities of 111.7GHz/RIU and 262.5GHz/RIU, respectively. More significantly, the optimized structure also has the characteristic of a wide and non-multiplexed refractive index range. In addition, the effects of analyte thickness and polyimide layer thickness on sensor performance are also discussed. The proposed structure opens up new prospects in the design of multiple-band terahertz metamaterial sensors. It can also meet the sensing needs of biomedical, environmental monitoring, and industrial manufacturing.

Design of a transmissive dual-frequency polarization rotator for linearly polarized electromagnetic wave

This paper presents the design of a transmission-type dual-frequency polarization rotator for linearly polarized electromagnetic wave. The rotator facilitates polarization conversion for incident waves at any azimuth angle ϕ at two specific frequency points, resulting in a transmission wave with a counterclockwise rotation of 2ϕ. The proposed polarization rotator structure comprises three metal layers separated by two dielectric substrates. It includes two large rectangular metal patches positioned orthogonally along the x- and y-axes, respectively, as well as two small rectangular metal patches also positioned orthogonally along the x- and y-axes, respectively. The upper and lower metal layers act as the receiving and radiation ends, respectively. The transmitting and receiving ends are interconnected through coaxial metal holes. Remarkably, polarization conversion efficiencies exceeding 0.95 and 0.94 at 12 and 17 GHz, respectively, are achieved, and these high conversion efficiencies remain consistent as the azimuth angle of incidence changes. Furthermore, experimental results obtained from fabricated samples align well with the simulation ones, thus validating the effectiveness of the proposed polarization rotator. The proposed transmissive dual-frequency polarization rotator exhibits good practical application prospects in mobile satellite communications, GNSS, WLAN, etc.

Improved Orthorectification and Empirical Reduction of Topographic Effects in Monostatic Mini-RF S-band Observations of the Moon

The Miniature Radio Frequency instrument (Mini-RF) on the Lunar Reconnaissance Orbiter obtained widespread synthetic aperture radar observations of the Moon in the S band (12.6 cm), including nearly complete coverage at both lunar poles. The currently archived monostatic data have spatial offsets from the lunar reference frame, making them more difficult to compare to other data sets. To address this issue, we have developed a new algorithm for spatially controlling the Mini-RF S-band monostatic data set and orthorectifying these data onto lunar topography. Additionally, as the influence of incidence angle changes on radar observations is well known, we describe an empirical approach to account for variations in observation geometry and surface topography. Individual radar swaths and mosaics produced using this method more clearly show the variability in scattering behavior due to changes in lunar regolith properties and suppress some of the behavior arising from these topographic effects alone. Once these terrain effects are taken into account, we find that areas of permanent shadow at both poles have a higher median radar reflectivity than nonpermanently shadowed regions, but the polarization behavior of shadowed versus unshadowed areas is largely similar. The higher radar reflectivity in permanent shadow is likely the result of physical or compositional differences in these unique environments, though the precise cause remains uncertain. The results here illustrate how reducing the influence of topography and geometry effects in Mini-RF radar data may enable better characterization of lunar geologic units, regolith structure, and potential areas hosting volatile deposits at the lunar poles.

Metasurfaces based terahertz and mid- and long-wave infrared high-efficiency beam splitting devices

The multi-mode composite imaging technology integrates the advantages of different sensors, and thus has the advantages of high image quality, strong information acquisition capability, high target detection and recognition ability, strong adaptability to complex environments, and high stability and robustness of the system. Among them, the terahertz and infrared composite imaging technology combines the characteristics of terahertz band and infrared band, has the advantages of wide spectrum coverage, high resolution and strong penetration, and has broad application prospects. As one of the key components of the common aperture composite imaging system, the efficient optical splitters in terahertz and infrared band are still lacking at present, and their performance needs to be improved urgently. In this paper, a kind of dichroic metasurface with a simple structure and high performance is proposed by combining simulation experiment and theoretical explanation. When used as a spectroscopic device at an incident angle of 45°, it achieves a transmission coefficient greater than 97% near the center frequency of 1.1 THz, and a reflection coefficient greater than 98% in a wavelength range of 3–5 μm for medium-wave infrared and 8–14 μm for long-wave infrared. The design has good robustness to structural mismatches and machining errors such as structural misalignment, structural fillet, small magnification scaling, and polarization insensitivity. When the incident angle changes in a range of 0–60°, the device still maintains excellent spectral characteristics. In this paper, based on Babinet theorem and equivalent circuit model, the electromagnetic response characteristics of the metasurface are analyzed theoretically, and the analysis results are in agreement with the simulation results. The results of this study prove the feasibility of metasurface as a spectral device in the multiwavelength composite imaging system of terahertz and infrared bands, and provide support for future studying new composite imaging detection technology. In addition, the metasurface structure described in this paper has broad application prospects in many fields such as multi-band infrared stealth, laser and pump light separation in lasers, and provides a valuable reference for designing terahertz and infrared spectroscopy in various scenarios. In the following figure, for S wave and P wave at an incident angle of 45°, panel (a) shows the reflection coefficients varying with the wavelength of metasurface and panel (b) displays terahertz transmission coefficient changing with frequency.

Ultra-wide-angle and broadband metamaterial absorber based on monopole top loading antenna and its reciprocity

In this paper, an ultra-wide-angle and broadband metamaterial absorber with a monopole top loading structure (MA-MTLS) was proposed. The resistive films printed on the dielectric are on the top layer. The dielectric square cylindrical shells with resistive films on the outside are in the middle, and a metal plate is on the bottom layer. The whole structure can be regarded as a top loading monopole antenna that has a good omnidirectional radiation capability. According to reciprocity, when a composite structure is used as an absorber, it will have a good absorption performance. The impedance matching characteristics were demonstrated by Smith chart and equivalent circuit, and then, the absorption performance was optimized. The simulated results show that the absorption of the MA-MTLS is more than 90% under TM polarized waves of 3.6–13.8 GHz as the incident angle changes from 0° to 80°. When the incident angle increases to 83°, the absorption is higher than 80% in the frequency band of 3.7–14.5 GHz. At the stage of experiment, the dielectric substrate was made by three-dimensional printing technology, the resistive films on the top layer were printed using screen printing techniques, and the resistive films on the outside of the square cylindrical shells were completed by pasting. The consistency between the experimental results and simulation data indicates that the design method is feasible and has good application prospects.

A tunable ultra-broadband and ultra-high sensitivity far-infrared metamaterial absorber based on VO2 and graphene.

We proposed a far-infrared tunable metamaterial absorber using vanadium dioxide (VO2) and graphene as controlling materials. The properties of the absorber are investigated theoretically using the finite-difference time-domain (FDTD) technique. It was found that when the Fermi energy level of graphene is fixed at zero, VO2 is in the insulated state, and the metasurface exhibits far-infrared broadband absorption performance, with absorptance exceeding 90% in the wavelength range of 12.6 μm to 23.2 μm. In addition, by elevating the Fermi energy level of graphene, the absorption bandwidth of the device is expanded continuously. When the VO2 is in the metallic state, the device can flexibly transform into a far-infrared narrowband absorber. The device also has the advantage of being insensitive to changes in polarization and incident angle. The origin of the absorption and the tuning principle of the device were analyzed and verified successfully by using an equivalent circuit model (ECM). Besides, we also studied the refraction index sensing characteristics of the absorber. Surprisingly, the absorber exhibits excellent sensing characteristics, and its sensitivity (S) reaches 14.108 μm per RIU and the figure of merit (FOM) is 6.13 per RIU.

Performance Analysis of a Single Light Source Bidirectional Visible Light Communication Reverse Reflection Link

Visible light communication has the advantages of large bandwidth, high security, and no RF interference, among which LED light sources are an important light source for indoor visible light communication. The use of LED as a light source for visible full-duplex communication is both to meet the lighting requirements and to ensure high-speed transmission of information. The uplink using the “cat’s eye” reverse modulation system can greatly reduce the system complexity of the reverse reflector. In order to analyze the factors affecting the optical power at the receiving end of the uplink of the indoor single light source visible light communication, this paper establishes the indoor visible light full-duplex communication system model and deduces the calculation method of the effective incidence angle of the uplink transmission light and the movable range of the reverse reflection end according to the model. The results show that when the link distance of the BK7 lens is 3 m, the lens aperture is increased from 100 mm to 150 mm, the lens focal length is increased from 100 mm to 150 mm, the travel distance of the reverse reflector is increased by 60%, and the effective range of the incidence angle is increased by about twice. In the absence of link loss, each 1 m increase in link distance increases the maximum travel distance of the reverse reflector by 0.8 m. Increasing the lens aperture, decreasing the focal length, and increasing the link distance can improve the movable range of the reverse reflector, and the effective incidence angle changes more gently with the position of the reverse reflector.

3D-printed stepped structure based on graphene-FeSiAl composites for broadband and wide-angle electromagnetic wave absorption

The design and preparation of absorbers with ultra-wide bandwidth and wide-angle absorption characteristics are crucial technologies for addressing electromagnetic (EM) pollution. In this paper, a stepped structure absorber was fabricated of graphene (GR)-FeSiAl/polylactic acid (PLA) composite by 3D printing technology. The absorber contains four layers, the bottom layer being a slab with periodic square holes, and the upper layer is a periodic distribution of three cubes with equal thickness and gradient changes in side length. It is shown that the stepped structure achieves a minimum reflection loss (RLmin) of −36.01 dB, with an effective absorption bandwidth (EAB, corresponding to the bandwidth of RL < −10 dB) of 12.75 GHz. These results were obtained when the unit cell had a side length of 22.5 mm, the slab thickness was 1.5 mm, and the cube had a thickness and edge length difference of 2.5 mm and 4.0 mm, respectively. Furthermore, the value of EAB is greater than 10 GHz for both transverse electric (TE) polarization and transverse magnetic (TM) polarization when the incident angle changes from 0° to 50°. The broadband and efficient EM wave (EMW) absorption characteristics of this absorber can be attributed to integration of excellent impedance matching and the multi-scale loss mechanisms.

Characterizing the channel dependence of vegetation effects on microwave emissions from soils

ABSTRACT The two vegetation transfer parameters of τ (Vegetation Optical Depth,VOD) and ω (Omega) could vary significantly across microwave channels in terms of frequencies, polarizations, and incidence angles, and their channel-dependent characteristics have not yet been fully investigated. In this study, we investigate the channel dependence of vegetation effects on microwave emissions from soils using a higher-order vegetation radiative transfer model of Tor Vergata. Corn was selected as the subject of investigation, and a corn growth model was developed utilizing field data collected from the multifrequency and multi-angular ground-based microwave radiation experiment from the Soil Moisture Experiment in the Luan River (SMELR). Upon compilation of the simulation dataset of microwave emissions of the corn field, the effective scattering albedo across different channels were calculated using the Tor Vergata model. Results show that vertical polarization of the vegetation optical depth is more affected by incidence angle changes, while horizontal polarization exhibits lower variations in vegetation optical depth due to incidence angle adjustments. The channel dependence of vegetation optical depth can be described as the polarization dependence parameter ( C P ) and the frequency dependence parameter ( C f ). These two parameters enable the calculation of vegetation optical depth at any channel under three adjacent frequencies (L-band, C-band and X-band). The effective scattering albedo of vegetation does not vary significantly with vegetation height or angle. It primarily depends on frequency and polarization, showing an overall increasing trend with increasing frequency. The effective scattering albedo with vertical polarization is slightly higher than that with horizontal polarization at higher frequencies, while both are lower in the L-band. This investigation is helpful for understanding the vegetation effects on microwave emissions from soils, ultimately advancing the accuracy of large-scale soil moisture retrieval in vegetated areas.