Effect Of Oblique Incidence Research Articles

Full telecomband covered half-wave meta-reflectarray for efficient circular polarization conversion

We report a full telecomband covered half-wave meta-reflectarray for efficient circular polarization conversion at near-infrared frequencies, which is composed of regular Z-shaped antenna array above a conductive ground layer separated with a dielectric spacer. The calculations of polarization conversion spectra demonstrate the ultra-broadband operation at 690 to 2010 nm continuously covering the whole telecom windows, with more than 90% conversion efficiency in full bandwidth. The relative bandwidth of as high as 98% is far higher than those reported previously never exceeding 50%. Sufficiently high conversion efficiencies of beyond 90% can be steadily performed for either left- or right-handed circular polarization incidence. The underlying mechanism is interpreted through the cross-coupling of multiple intrinsic resonances of meta-atoms. The effects of geometry and oblique incidence on the polarization conversion are discussed, showing the acceptable technical tolerance and wide operating angle. The proposed half-wave meta-reflectarray shows promising application in the field of on-chip integrated spin-based logic devices and optical communications.

Design of broadband metamaterial near-perfect absorbers in visible region based on stacked metal-dielectric gratings

A broadband near-perfect absorber in the visible region has been designed in this paper. This absorber consists of an alternating stack of metallic gratings and dielectric layers on the basis of a metallic substrate. Absorptivity and electromagnetic field distributions of the structure via the finite difference time domain (FDTD) method have been stimulated. The results show that the average absorptance of the absorber exceeds 95.2% in the wavelength range from 350 nm to 650 nm. The enhanced absorption can be ascribed to several mechanisms including surface plasmon polaritons, magnetic polaritons, and Fabry–Perot resonances. Besides, geometric effects on the resonance wavelengths have been investigated. Moreover, the effects of oblique incidence and polarization states on the spectral absorption have been analyzed. This designed absorber will be worth applying in solar energy harvesting via combining the study results mentioned above.

Realization of Focused Beam and Shaped Beam Transmitarrays Based on Broadband Unit Cells

A focused beam and a shaped beam four-layer transmitarray (TA) antenna operating in the $K$ -band have been designed, fabricated, and measured. Four-layer unit cells allowing a transmission phase range of 360° are used. The gain of the beam-focusing TA is 30.3 dB at 19 GHz with an aperture efficiency of 34.9%. Compared with TAs proposed in the literature, the presented antenna bandwidth is almost two times larger with a −1 dB gain bandwidth of 18% and a −3 dB gain bandwidth of 25.5%. A second TA radiating a shaped beam was designed, and its measured pattern correlates well with theoretical calculations. The sensitivity of the proposed unit cells to fabrication errors and the effect of oblique incidence are also studied.

Experimental study on the incident-angle-dependent laser coupling features of polystyrene targets

Laser-produced plasmas have attracted great interest due to their potential utility in wide-ranging applications, especially in the field of inertial confinement fusion (ICF). For direct-driven ICF, laser coupling with polystyrene targets is a crucial and fundamental problem. In addition, oblique incidence is also a common phenomenon for laser facilities with multiple beams. It is necessary to evaluate the effects of oblique incidence on the laser coupling features relevant to the direct-driven ICF. Experiments using an intense nanosecond flat-top laser at around W cm–2 to irradiate polystyrene planar targets from three different incidence angles have been performed on the Shenguang-III prototype laser facility. The time-integrated absolute values of the full aperture backscatter (FABS), near backscatter scattering (NBS), and the x-ray conversion efficiency (CE) have been measured quantitatively. According to the experimental results, with the increase of the incidence angle, the percentage of the stimulated Brillouin backscatter and the overall x-ray CE decreased while the stimulated Raman backscatter fraction rose. Theoretical analyses based on hydrodynamic simulations and linear theory were qualitatively consistent with the experimental results. In addition, the specularly reflected light was also observed at 30° laser oblique incidence.

Integrated optoelectronic model for organic solar cells based on the finite element method including the effect of oblique sunlight incidence and a non-ohmic electrode contact

We present an integrated optoelectronic model for organic solar cells (OSCs) based on the finite element method, which can numerically simulate the optical and electrical properties in the same calculation domain. In the optical model, the spatial distribution of optical absorption is calculated with respect to the incidence angle and light polarization. A glass factor is introduced to include the incoherent light interaction inside the thick glass substrate. In the electrical model, the current density–voltage (J–V) characteristics can be calculated by self-consistently solving the combined equations based on the Onsager–Braun charge-transfer exciton dissociation, drift-diffusion carrier transport, and non-ohmic contact models. The calculation results of the carrier density, the electric potential, and the electric field in the active layer are compared between the ohmic and non-ohmic contact models at the electrode–organic interface. We numerically calculate the angular and polarization dependences of the short-circuit current density, the open-circuit voltage, and the output electric power density at the spectral irradiance of the AM 1.5 spectrum. The calculation results are well matched with the experimental results at various incidence angles and light polarizations. The application of the proposed integrated optoelectronic model to OSCs will not be restricted to one-dimensional planar structures and can be extended to nonplanar or surface-textured structures.

Oblique incidence effect on steering efficiency of liquid crystal polarization gratings used for optical phased array beam steering amplification

A liquid crystal polarization grating (LCPG) is proposed that amplifies the steering angle of a liquid crystal optical phased array for non-mechanical beam steering, taking advantage of its high steering efficiency under normal incidence. However, oblique incidence may play an important role in the overall steering efficiency. The effect of oblique incidence on steering efficiency of a LCPG was analyzed by numerically solving the extended Jones matrix and considering propagation crosstalk. The results indicate that the outgoing laser beam is amplitude-modulated under the effect of oblique incidence and behaves as a sinusoidal-modulated amplitude grating, which diffracts certain energies to non-blazed orders. Over-oblique incidence may even eliminate the steering effect of the incident beam. The modulation depth of the induced amplitude grating was found to be proportional to the product of sinusoidal value of oblique incidence angle and the LC layer thickness, and inversely proportional to the periodic pitch length of the LCPG. Both in-plane incidence and out-of-plane incidence behave similarly to influence the steering efficiency. Finally, the overall steering efficiency for cascaded LCPGs was analyzed and a difference of up to 11 % steering efficiency can be induced between different LCPG configurations, even without considering the over-oblique incidence effect. Both the modulation depth and final steering efficiency can be optimized by varying the LC birefringence and layer thickness.

Design of narrow‐band dielectric frequency‐selective surfaces for microwave applications

Two types of narrow-band dielectric frequency-selective surfaces (DFSSs) have been designed at microwave frequencies. First, a DFSS showing total reflection has been analysed under guided-mode resonance conditions, based on a single dielectric grating which is illuminated by a TM polarised two-dimensional plane wave at Brewster-angle incidence, presenting extremely low-reflectance sidebands adjacent to the resonance peak. Second, a DFSS exhibiting total transmission at normal TE incidence has been designed, by superimposing the resonance condition of a dielectric grating on the classical high-reflectance response of a periodic (band-gap based) structure formed by alternating homogeneous dielectric layers. Finally, the oblique incidence and polarisation effects on the spectral response of the designed DFSSs have been also studied. In addition, dielectric ohmic losses and the problem of the finite size of the periodic structures have been accounted for in both structures. The obtained results have been successfully validated with the commercial software tool high frequency structure simulator.

Three-parameter error analysis method based on rotating coordinates in rotating birefringent polarizer system.

We propose error analysis using a rotating coordinate system with three parameters of linearly polarized light--incidence angle, azimuth angle on the front surface, and angle between the incidence and vibration planes--and demonstrate the method on a rotating birefringent prism system. The transmittance and angles are calculated plane-by-plane using a birefringence ellipsoid model and the final transmitted intensity equation is deduced. The effects of oblique incidence, light interference, beam convergence, and misalignment of the rotation and prism axes are discussed. We simulate the entire error model using MATLAB and conduct experiments based on a built polarimeter. The simulation and experimental results are consistent and demonstrate the rationality and validity of this method.

On the use of Gafchromic EBT3 films for validating a commercial electron Monte Carlo dose calculation algorithm

This study aims to investigate the effects of oblique incidence, small field size and inhomogeneous media on the electron dose distribution, and to compare calculated (Elekta/CMS XiO) and measured results. All comparisons were done in terms of absolute dose. A new measuring method was developed for high resolution, absolute dose measurement of non-standard beams using Gafchromic® EBT3 film. A portable U-shaped holder was designed and constructed to hold EBT3 films vertically in a reproducible setup submerged in a water phantom. The experimental film method was verified with ionisation chamber measurements and agreed to within 2% or 1 mm. Agreement between XiO electron Monte Carlo (eMC) and EBT3 was within 2% or 2 mm for most standard fields and 3% or 3 mm for the non-standard fields. Larger differences were seen in the build-up region where XiO eMC overestimates dose by up to 10% for obliquely incident fields and underestimates the dose for small circular fields by up to 5% when compared to measurement. Calculations with inhomogeneous media mimicking ribs, lung and skull tissue placed at the side of the film in water agreed with measurement to within 3% or 3 mm. Gafchromic film in water proved to be a convenient high spatial resolution method to verify dose distributions from electrons in non-standard conditions including irradiation in inhomogeneous media.

Effects of oblique incidence on terahertz responses of planar split-ring resonators

Effects of oblique incidence of terahertz waves on the response of planar split-ring resonators are investigated, both experimentally and by simulation. It is found that the incident angle dependent phase delay and coupling conditions of neighboring split-ring resonator (SRR) units play important roles and greatly change both the transmission and reflection spectra for the resonant feature of linear charge oscillations. Our results show that the SRR structure-supported magnetoelectric couplings at oblique excitation are trivial and can be ignored. A highly symmetric response is found in the cross-polarization effects, which may manifest the bianisotropic properties of the SRR system but this needs further study.

Effect of oblique incidence on silver nanomaterials fabricated in water via ultrafast laser ablation for photonics and explosives detection

Picosecond (ps) laser ablation of silver (Ag) substrate submerged in double distilled water was performed at 800nm for different angles of incidence of 5°, 15°, 30° and 45°. Prepared colloidal solutions were characterized through transmission electron microscopy, UV absorption spectroscopy to explore their morphologies and surface plasmon resonance (SPR) properties. Third order nonlinear optical (NLO) characterization of colloids was performed using degenerate four wave mixing (DFWM) technique with ∼40fs laser pulses at 800nm and the NLO coefficients were obtained. Detailed analysis of the data obtained from colloidal solutions suggested that superior results in terms of yield, sizes of the NPs, SPR peak position were achieved for ablation performed at 30° incident angle. Surface enhanced Raman spectra (SERS) of Rhodamine 6G from nanostructured substrates were investigated using excitation wavelengths of 532 and 785nm. In both the cases substrates prepared at 30° incident angle exhibited superior enhancement in the Raman signatures with a best enhancement factor achieved being >108. SERS of an explosive molecule 5-amino, 3-nitro, -1H-1,2,4-nitrozole (ANTA) was also demonstrated from these nanostructured substrates. Multiple usage of Ag nanostructures for SERS studies revealed that structures prepared at 30° incident angle provided superior performance amongst all.

THz Thermal Emission Control Via Electromagnetic Band Engineering

In the following, we report on the design and implementation of an inexpensive thermal source for THz radiation with a highly tunable radiation signature. It consists of five 350- μm-thick Si wafers, each spaced 1 mm apart. The thermal emission of this structure was calculated analytically and numerically. A comprehensive study was performed that included analyzing the oblique incidence, polarization, and design tolerance effects. The thermal emission was calculated using both Kirchhoff's law and directly using a Green's function method. It was measured using a Michelson interferometer. The predicted and measured thermal emission spectra had peaks around 150 GHz apart, with a narrow bandwidth of 50 GHz. The peaks were up to 80% of the expected blackbody levels and the bandgaps had emission levels of about 20% of those blackbody values. We demonstrate that, using these techniques, it is possible to achieve a desired THz thermal emission signature, with narrowband features that can be engineered to have specific angles of emission and polarization states.

3D FE modeling of oblique shot peening using a new periodic cell

Oblique incidence is often observed in the peening process due to the geometric complexity of some of the treated targets. Obliquity of the jet stream also exists as a result of the way the shots are propelled. It is therefore the purpose of this study to conduct a realistic 3D finite element (FE) analysis of the peening process involving a large number of shots impinging simultaneously at a rate sensitive target made from Ti-6Al-4V. A novel periodic cell model is developed and used to examine the effect of oblique incidence upon the induced plastic strains and residual stresses. Some aspects of the simulation are first validated against published work in literature. The periodicity of the model is also examined and verified. A parametric study is further conducted to investigate the effect of various parameters involved in peening process using the newly proposed model. Several conclusions are drawn concerning the effect of incident angle, shot diameter and friction coefficient upon the generated residual stress and plastic strain fields.

TM Mode Analysis of a Periodic Thick Mushroom Structure

We analyzed a periodic thick mushroom structure for use as an artificial magnetic conductor using mode-matching method. The fields in each region were represented by either Floquet modes or waveguide modes. By applying tangential electric and magnetic field continuity conditions and using matrix equations, unknown coefficients and dispersion diagram were calculated. The proposed model can account for the effects of oblique incidence. Simulation time using the method was much faster than the commercial tools. We found that the current method produces accurate results of reflection phase and dispersion diagram.

Enhanced optical transmission through asymmetric nanostructured gold films

We investigate optical properties of a gold structure comprising a two-dimensional array of gold nanoblocks placed on top of a thin gold film. We observe enhanced transmission through asymmetric nanostructured gold films, which can be attributed to the surface plasmon (SP) mode at the air-gold interface leaking to the substrate when the substrate index is larger than the superstrate index. When the substrate and superstrate are the same dielectric, the SPs at both superstrate-gold and gold-substrate interfaces strongly interact with each other and even and odd SPs are then excited. In addition, we investigate effects of oblique incidence and electronic interband transition on SP resonances. Our results provide a guideline for engineering novel devices with enhanced transmission based on nanostructures.

Simple Analytical Model of Propagation Through Thick Periodic Slot

Extraordinary transmission (ET) by a periodic conducting slot at normal incidence has potential applications in the optical and microwave frequencies. This paper presents a simple analytical model to account for the effects of oblique incidence of electromagnetic (EM) waves on a slotted periodic structure. The proposed analytical model, based on Floquet and circuit theory, provides physical insight to explain the reduction of ET frequency when the EM wave strikes the structure obliquely: ET occurs before the onset of the grating lobe. Higher evanescent Floquet modes contribute to the stored energy that represents fringing capacitance in terms of the incoming wave. Transmission bandwidth and material loading effects are also investigated. When applied on an artificial magnetic conductor (AMC), the model indicates that the thickness of the AMC can be dramatically reduced without any material loading. Results obtained by the analytical models are compared by full-wave simulations.

Oblique incidence and polarization effects in coupled gratings

Oblique incidence and polarization orientation of the input beam have dramatic effects on the spectral response of coupled dielectric waveguide gratings. Coupled gratings with small periodic perturbations can be described as a problem of two coupled resonances at strictly normal incidence, but we find that the device involves four coupled resonances when oblique incidence and polarization effects are included in the analysis. Very small deviations from normal incidence change qualitatively the spectral response and four peaks are observed, whereas only two peaks are present at normal incidence. Polarization misalignments produce a decrease of the reflectance of the resonances at normal incidence, but a simultaneous shift of the spectral position of the peaks is observed at oblique incidence.

Effects of normal and oblique incidence on zero-[formula omitted] gap in periodic lossy multilayer containing double-negative materials

In this paper the transmission of electromagnetic waves through a one-dimensional lossy photonic crystal consisting of layers with negative and positive refractive indices is investigated. The behavior and characteristics of the bandwidth, the depth and the central frequency of the zero-n¯ gap for different incidence angles, polarization, loss factor and ratios of thickness of the layers are studied. The results show that the gap is very sensitive to the incidence angle, polarization and the thickness ratio, but it is nearly insensitive to small loss factor. Such properties are quite useful in designing new types of edge filters and other optical devices in microwave engineering.

Oblique incidence effects in direct x‐ray detectors: A first‐order approximation using a physics‐based analytical model

The authors describe the modifications to a previously developed analytical model of indirect CsI:Tl-based detector response required for studying oblique x-ray incidence effects in direct semiconductor-based detectors. This first-order approximation analysis allows the authors to describe the associated degradation in resolution in direct detectors and compare the predictions to the published data for indirect detectors. The proposed model is based on a physics-based analytical description developed by Freed et al. ["A fast, angle-dependent, analytical model of CsI detector response for optimization of 3D x-ray breast imaging systems," Med. Phys. 37(6), 2593-2605 (2010)] that describes detector response functions for indirect detectors and oblique incident x rays. The model, modified in this work to address direct detector response, describes the dependence of the response with x-ray energy, thickness of the transducer layer, and the depth-dependent blur and collection efficiency. The authors report the detector response functions for indirect and direct detector models for typical thicknesses utilized in clinical systems for full-field digital mammography (150 microm for indirect CsI:Tl and 200 microm for a-Se direct detectors). The results suggest that the oblique incidence effect in a semiconductor detector differs from that in indirect detectors in two ways: The direct detector model produces a sharper overall PRF compared to the response corresponding to the indirect detector model for normal x-ray incidence and a larger relative increase in blur along the x-ray incidence direction compared to that found in indirect detectors with respect to the response at normal incidence angles. Compared to the effect seen in indirect detectors, the direct detector model exhibits a sharper response at normal x-ray incidence and a larger relative increase in blur along the x-ray incidence direction with respect to the blur in the orthogonal direction. The results suggest that the oblique incidence effect in direct detectors can be considered to be caused mostly by the geometry of the path where the x-ray beam and its secondary particles deposit energy in the semiconductor layer.

Directional Alignment of FeCo Crystallites in Si/NiFe/Ru/FeCoB Multilayer with High Anisotropy Field Above 500 Oe

In-plane magnetic anisotropy and crystal structure of FeCoB layer on Si/NiFe/Ru underlayer were investigated by using X-Ray Diffraction (XRD) measurement. A pole-figure measurement of XRD showed directionally tilted alignment of FeCo crystallites in Si/NiFe/Ru/FeCoB multilayered film with high in-plane anisotropy field H(k) but no directional alignment was found in FeCoB single layered film. The higher H(k) appeared in the Si/NiFe/Ru/FeCoB multilayered configuration with the thicker FeCoB layer. Since Ru crystallites in a multiunderlayer configuration exhibited no directional alignment, the surface structure of underlayer should be no main reason for the directional alignment of FeCo crystallites deposited on it. The dependence of hickness of FeCoB layer in Si/NiFe/Ru/FeCoB film on H(k) indicated that the in-plane magnetic anisotropy is caused by not only the structure of Ru underlayer but also oblique incidence effect of sputtered particles, which is attained in configuration of Facing Targets Sputtering (FTS) system. From these experimental results, remarkably high H(k) of 540 Oe was obtained.