Oblique Incidence Research Articles

A Tunable Dual‐Band Flexible Polarization Converter Based on Vanadium Dioxide Reflective Metasurface

A flexible dual‐band efficient polarization converter is realized by both coupled mode theory (CMT) analysis and numerical simulation in the microwave regime. The proposed structure can convert linearly polarized electromagnetic waves into their orthogonal components in the two sidebands. Moreover, a specular reflection performance is obtained in the central frequency band. The polarization conversion ratio (PCR) and operating bandwidth of dual band is dynamically modulated by vanadium dioxide (VO2). In the simulation results, it is shown that the proposed metasurface can achieve linear polarization conversion in 8.17–12.87 and 14.63–18.92 GHz in the insulating phase. PCRs of the dual band are 97.06% and 98.13%, the corresponding relative bandwidths (RBs) are 44.68% and 25.58%. In the metallic phase, the PCRs in 6.81–13.95 and 16.23–18.16 GHz are 91.12% (RB = 68.79%) and 96.92% (RB = 11.22%). The analyses of the phase difference (u–v coordinate system) and the surface current are given to explain the mechanisms of dual‐band and high PCR. The simulation results are in good agreement with those of CMT. Furthermore, the structure has a robust response to the oblique incidence angle (up to 30°) and has polarization insensitivity. The proposed metasurface has great potential applications in antenna radiation, biosensing, and stealth technology.

Analysis and optimization of aberration induced by oblique incidence for in-vivo tissue polarimetry.

For in-vivo polarimetry such as Mueller matrix endoscopy of human internal organ cavities, the complicated undulating tissue surfaces deliver an inescapable occurrence of oblique incidence, which induce a prominent aberration to backscattering tissue polarimetry. In this Letter, we quantitatively analyze such polarimetric aberration on polarization basic parameters derived from the Mueller matrix. A correlation heatmap is obtained as applicable criteria to select an appropriate incident angle for different polarization basic parameters. Based on the analyzing results, we propose two aberration optimization strategies of parameter selection and azimuth rotation, which are suitable for tissue samples with randomly and well-aligned fiber textures, respectively. Both strategies are demonstrated to be effective in the ex-vivo human gastric muscularis tissue experiment. The findings presented in this Letter can be useful to provide accurate polarization imaging results, widely applied on in-vivo polarimetric endoscopy for tissues with complicated surface topography.

Quasi-bound states in the continuum induced by C4v structure

We propose a method to achieve symmetry-protected bound states in the continuum (BICs) while maintaining the C4v structural symmetry, utilizing a periodically arranged array of topological Dirac semimetals (TDS) squares. By numerically controlling the metamaterial using four identical square films of TDS, we can manipulate both the resonance position and number. This phenomenon arises due to the inherent permittivity asymmetry of the metamaterial, causing the creation of quasi-BICs. When all four squares have the same Fermi energy, a BIC is formed. Different Fermi energies are assigned to the four squares, resulting in quasi-BICs with narrow resonances. The electric analysis in the x-y plane identified the dominant contributors as the electric quadrupole and octupole. Our findings provide a direct route to achieve high-quality (high-Q) quasi-BICs without any introduction of symmetry-breaking elements, such as transverse, in-plane, and out-of-plane components in the structure, or the use of oblique incidences within a non-breaking system.

On-demand Doppler-offset beamforming with intelligent spatiotemporal metasurfaces

Abstract Recently, significant efforts have been devoted to guaranteeing high-quality communication services in fast-moving scenes, such as high-speed trains. The challenges lie in the Doppler effect that shifts the frequency of the transmitted signal. To this end, the recent emergence of spatiotemporal metasurfaces offers a promising solution, which can manipulate electromagnetic waves in time and space domain while being lightweight and cost-effective. Here we introduce deep learning-assisted spatiotemporal metasurfaces to automatically and adaptively neutralize Doppler effect in fast-moving situations. A tandem neural network is used to establish a rapid connection between on-site targets and time-varying series of spatiotemporal metasurfaces, endowing the capability of on-demand beamforming with Doppler effects offset. Moreover, oblique incidence problems are also studied in practice, which can be used for relieving multipath effect. In the microwave experiment, we fabricate the intelligent spatiotemporal metasurfaces and demonstrate the potential to fulfill Doppler-offset beamforming under oblique incidence.

Direction independent broad-band wide angle metamaterial absorber for “K” band applications

Abstract In this work, the design of broadband, wide-angle, direction-independent metamaterial (MM) electromagnetic wave (EM) absorber for K-band frequency application is investigated and validated experimentally. The unit cell of the metamaterial absorber consists of four 90° rotated L-shaped metallic patches imprinted on a dielectric substrate backed by a metallic sheet. The structure yield absorption in the broad frequency ranges from 22.5 to 29.3 GHz for both TE and TM polarized waves with more than 90 % absorptivity having a wide fractional bandwidth of (6.8 GHz) 25.8 %. The structure is four-fold symmetric and hence yields polarization insensitivity for different angles of polarization under both TE and TM polarized waves. The structure is also investigated under oblique incidence where the 80 % absorptivity holds up to 45° incident angles for both TE and TM waves. The absorption mechanism is explained with the help of top and bottom surface current distribution, induced electric field, and parametric analysis. To verify the resonance in the structure, characteristic mode, and equivalent circuit analysis have been carried out and presented. A prototype of the absorber has been fabricated and simulated results are validated with measured results. Measured results are showing good agreement with the simulated responses. The novelty of the proposed absorber lies in its unique metallic pattern on a λ 0/8 (concerning the canter frequency of absorption bandwidth) thin FR-4 substrate while showing the wide absorption bandwidth and direction independence to normal and oblique incidence. The compact nature of the absorber and broadband response with good polarization insensitivity at normal and oblique incidence makes it commercially suitable for the reduction of radar cross section (RCS) in stealth applications at the K-band.

Low infrared emissivity and broadband wide-angle microwave absorption integrated bi-functional camouflage metamaterial with a hexagonal patch based metasurface superstrate.

This work proposed and demonstrated a bi-functional metamaterial to implement the multispectral camouflage in infrared and microwave bands. Aiming at integrating broadband, wide-angle and low infrared emissivity into one structure, the bi-functional structure is made up of three metasurface layers with different functions. Specifically, a metasurface superstrate based on hexagonal metallic patch was deployed to achieve a low infrared emissivity and a high transmittance of microwave simultaneously. In the framework of equivalent circuit model, the bi-functional structure was designed and optimized. A dielectric transition layer was introduced into the structure to obtain better microwave absorption performances. A sample of such structure was prepared based on optimized geometric parameters and tested. The simulated and measured results indicate that the novel hexagonal patch metasurface superstrate significantly reduces infrared emissivity and the measured emissivity of the structure is about 0.144 in 8-14µm infrared band. Meanwhile, the multilayered structure has a broadband absorption band from 2.32 GHz to 24.8 GHz with 7 mm thickness and is equipped with good angular stability under oblique incidence. In general, the method and specific design proposed in this work will benefit utilizing metasurface to implement bi-functional microwave and infrared camouflage materials with outstanding performances, which are promising for extensive applications.

Lattice Mie resonances and emissivity enhancement in mid-infrared iron pyrite metasurfaces.

High-refractive-index antennas with characteristic dimensions comparable to wavelength have a remarkable ability to support pronounces electric and magnetic dipole resonances. Furthermore, periodic arrangements of such resonant antennas result in narrow and strong lattice resonances facilitated by the lattice. We design iron pyrite antennas operating in the mid-infrared spectral range due to the material's low-energy bandgap and high refractive index. We utilize Kirchhoff's law, stating that emissivity and absorptance are equal to each other in equilibrium, and we apply it to improve the thermal properties of the iron pyrite metasurface. Through the excitation of collective resonances and manipulation of the antenna lattice's period, we demonstrate our capacity to control emissivity peaks. These peaks stem from the resonant excitation of electric and magnetic dipoles within proximity to the Rayleigh anomalies. In the lattice of truncated-cone antennas, we observe Rabi splitting of electric and magnetic dipole lattice resonances originating from the antennas' broken symmetry. We demonstrate that the truncated-cone antenna lattices support strong out-of-plane magnetic dipole lattice resonances at oblique incidence. We show that the truncated-cone antennas, as opposed to disks or cones, facilitate a particularly strong resonance and bound state in the continuum at the normal incidence. Our work demonstrates the effective manipulation of emissivity peaks in iron pyrite metasurfaces through controlled lattice resonances and antenna design, offering promising avenues for mid-infrared spectral engineering.

Ultrabroadband metastructure absorber with angular stability for conformal applications

The sustained growth and development of numerous microwave electronic facilities have boosted the demands for microwave absorbers with high performance in bandwidth, wide incident angle, and adaptability of the curved surface. To satisfy these requirements, a 3D-printed electromagnetic metastructure based on a new design methodology with comprehensive microwave absorption properties was proposed and manufactured in this work. The multi-resonant metastructure based on the composite of polyether-ether-ketone and flaky carbonyl iron was designed, and then the gradient impedance distribution was optimized via simulation and comparison of the broadband impedance matching and microwave attenuation properties in different polarization modes under oblique incidence, followed by structural parameters optimization to achieve the optimal absorption performance with broadband and wide incident angle. In the meantime, the conformal capacity of the metastructure under different types of curved surfaces was also investigated. Experimental results correlated well with simulations and demonstrated excellent absorption performance with effective absorption in the broad frequency range of 3.3–30 GHz, stable absorption in the wide incident angle of 0°–70° for both transverse electric and transverse magnetic polarization modes, and strong backward scattering suppression for different conformal states. This effective strategy will provide a novel and promising route toward the development of microwave absorption metastructures with comprehensive electromagnetic performance and accelerate the applications in electromagnetic protection.

Diffraction efficiency analysis of dual-layer diffractive elements with oblique incident angles.

This paper proposes a method that combines the effective area method and the oblique factors to analyze and design dual-layer diffractive optical elements at large incident angles. The method considers the effects of shadow and shield on the diffraction efficiency, as well as the correction of phase delay due to oblique incidence. The relationships among the diffraction efficiency, incident wavelengths, incident angles and period widths were analyzed with our method. A detailed comparison of the proposed method with the scalar diffraction theory is presented. The method was validated by numerical simulations of vector diffraction theory and showed similar diffraction efficiency distributions at different wavelengths and incident angles. The method provides a simple and efficient way to design and apply DLDOEs.

An analytical solution for the dynamic response of a seawater‐sloping seabed‐bedrock system under an oblique incident SV wave

AbstractThe characteristics of seabed ground motion are the theoretical basis for the study of seawater‐soil‐structure interactions and seismic‐induced geological hazards. There are numerous offshore marine structures located on sloping seabeds. Therefore, the sloping seabed's seismic response requires to be investigated. Engineers and scholars are more interested in the SV waves because they carry more energy than the P waves in earthquakes. An analytical solution for the steady‐state response of the inclined seafloor under an oblique incident SV wave is offered. The impacts of the seafloor angle on the fluid pressure and displacement of the saturated sediment layer are investigated under different incident wave frequencies, incident angles, and thicknesses of the seawater layer and sediment layer. By observing along the seabed surface, we discovered that the slope angle might cause considerable variances between different positions even at a small site. While examining the seabed at varying depths, we discovered that the incline angle has distinct impacts on different areas of the seabed. It has an amplifying effect at some depths and a reducing effect at other depths. Generally, as the slope angle increases, the distinction between the responses of the inclined and horizontal seabed becomes more prominent. Especially when the incidence angle of the SV waves is near the critical angle, the impacts of the slope angle become more significant. Even if the horizontal seabed model is employed with the same seawater depth, it is not suitable for analyzing the seismic response of a sloping seabed. Otherwise, significant errors would arise.

High Q-factor multifunctional plasmonic metasurfaces in the mid-infrared

As application scenarios grow in complexity, multifunctional integrated metasurfaces have been proposed as a potential development direction. We propose a multifunctional high Q-factor metasurface-based platform composed by size-scaled metasurface polarization selective absorbers (MPSAs) capable of polarization selective absorption, refractive index sensing and chiral response. By designing a gold step-shaped unit structure that can break the in-plane and out-of-plane symmetry, bound states in the continuums (BICs) with chiral response and a surface lattice resonance (SLR) are realized. The overall absorption amplitude of the quasi-BIC can reach a maximum value of 0.87 and a Q-factor of 153 while the SLR possesses a high Q-factor exceeding 1274 and a maximum absorption amplitude of 0.93. High extinction ratio up to 13 dB between the transverse magnetic and transverse electric polarization can be achieved. For refractive index (RI) sensing, the bulk RI sensitivities of the proposed MPSA for quasi-BIC and SLR are SBIC = 4000 nm/RIU and SSLR = 3740 nm/RIU, which leads to a high figure of merit (FOM) of FOMBIC = 161 /RIU and FOMSLR = 1247 /RIU, respectively. For chiral response, the maximum circular dichroism (CD) for the proposed MPSA is 0.88 by oblique incidence still maintaining a Q-factor exceeding 120, which indicates that it has potential application for chiral sensing. The proposed multifunctional metasurfaces-based platform possesses the potential to promising future applications for polarization manipulation, chiral biosensing, chiral drug analysis and quantum optics.

Wideband metasurface reflective linear-cross and linear-circular meander-line polarizer for WLAN and SatCom applications in S and C bands

The wideband multipurpose metasurface polarizer described in this article can convert linearly-polarized (LP) waves to cross-polarized (CLP) and circularly-polarized (CP) waves in S and C band applications. To achieve the desired polarization conversion, a meander-line with a non-uniform width unitcell is carved on a FR4-substrate. An airgap is then introduced between the dielectric and metal ground. Due to variations in the electrical length that the electromagnetic wave travels through the metasurface, this airgap is essential in determining whether the polarizer is a half- or quarter-waveplate. When the airgap was 9 mm, it was possible to record wideband LP-to-CLP at 2.87–7.12 GHz with a polarization conversion ratio (PCR) >90% and narrowband LP-CP at 2.45–2.69 GHz and 7.56–8.1 GHz with an axial-ratio (AR) <3 dB. By reducing the gap to 4.2 mm, the above band exhibits a wideband LP-CP conversion within bands of 3.02–3.35 GHz and 3.82–7.42 GHz with an incidence angle variation up-to 40°. Also, 3.18–3.33, 5.28–6.70, and 7.72–7.96 GHz bands exhibit PCR >90% over 40°. Surface impedance, surface current, and the transfer matrix technique were all used in the investigation of the conversion process for both normal and oblique incidence. RCS reduction, one of the applications, was also demonstrated.

Ultraviolet light scattering by a silicon Bethe hole

Abstract Bethe’s theory predicts that scattering by a small hole on a thin perfect electric conductor (PEC) is presented as radiation by an in-plane magnetic dipole of the incident magnetic field direction. Even in the near-infrared range where metals are no more PEC, the magnetic dipole radiation of Bethe holes has been demonstrated. However, such Bethe holes’ nature has not been addressed yet in the ultraviolet (UV) range where conductivity of metals becomes severely deteriorated. Meanwhile, UV plasmonics has been elevating its importance in spectroscopy and photochemistry, recognizing silicon (Si) as an alternative plasmonic metal featuring the interband transition in the UV range. In this work, we expanded the Bethe’s theory’s prediction to the UV range by investigating Si Bethe holes theoretically and experimentally in terms of the scattering pattern and polarization. Simulation results showed that the scattered field distribution resembles that of an in-plane magnetic dipole, and the dipole direction at oblique incidence is roughly given as the incident magnetic field direction with a deviation angle which can be predicted from the Fresnel equations. Simulation with various diameters showed that the magnetic dipole nature maintains with a diameter less than the quarter-wavelength and multipoles becomes noticeable for diameters larger than the half-wavelength. We performed scattering polarization measurement at 69-degree incidence, which confirms the theoretical analysis. The features of Si Bethe holes demonstrated here will be useful for designing UV plasmonic metasurfaces.

Assessment of acoustical materials sound absorption coefficient under oblique incidence plane wave and diffuse field using a virtual source antenna

The measurement of the absorption coefficient of acoustic materials is usually performed in an impedance tube under normal incidence on a small sample, or in a reverberant room under diffuse field incidence on a large sample of material. Both methods are prone to well-documented experimental limitations and errors. The approach proposed here uses a virtual source antenna (a point source moved at successive positions) over a material sample typically 1 to 4 m2 in size, and a fixed microphone pair directly above the material surface. Several methods have been tested to reconstruct the absorption coefficient under oblique plane wave or diffuse field incidence. Among these, the inversion of the Allard propagation model over an absorbing plane, in order to extract the material properties (complex wavenumber and density) from measured microphone transfer functions, is presented here. Absorption coefficients are shown for various acoustic materials.

Numerical investigations on the dynamic responses of metro tunnel-soil-ground building under oblique incidence of seismic wave

With the development of urban rail transits, the metro tunnels frequently underpass the above-ground buildings, and understanding the dynamic responses of metro tunnel-soil-ground buildings experiencing oblique incidence of seismic waves is of crucial importance. Considering that the seismic waves in the near-field seismic area are mostly obliquely incident to the ground at a certain angle, a two-dimensional numerical model for tunnel-soil interaction system and tunnel-soil-above ground structure was established by Abaqus software, in which the viscoelastic boundary and equivalent nodal forces were adopted to simulate the ground motion. The effects of tunnel burial depth, tunnel spacing, and tunnel frame spacing on the seismic responses of metro tunnel-soil and metro tunnel-soil-above ground building were analyzed under oblique incidence of SV wave with different incident angles. It was found that the tunnel depth has a significant effect on the lining dynamic bending moment and axial force peak, and its influence is closely related to the site category. The tunnel spacing has a minor impact on the dynamic response of the lining structure. The incident angle has a significant effect on the peak value of the dynamic shaft force of the lining. The application of the frame on the ground has a significant impact on the tunnel. The influence of above-ground buildings and tunnel burial depth should be considered when tunnel design is carried out.

Diffraction of a scalar plane wave oblique incidence on a flat screen with a one thin slit

In the framework of this work, we discuss the problem of diffraction of a plane and time-harmonic field on a flat screen with a single narrow slit. As a rule, this problem is considered for the case of normal incidence. At the same time, for a number of applied problems, in particular, the absorption of solar radiation by photocells, it is interesting to consider the general case of oblique incidence. In this work, the problem is solved in the Fraunhofer approximation. An analytical dependence of the intensity of the diffracted field on the direction of observation, the slit length, and the wavelength of the incident radiation is obtained.

Resonant Effects in Subwavelength Diffraction Gratings with Varying Period in the Case of Oblique Incidence

Resonant Effects in Subwavelength Diffraction Gratings with Varying Period in the Case of Oblique Incidence

Polarization-Insensitive and Oblique Incident Angle Stable Miniaturized Conformal FSS for 28/38 GHz mm-Wave Band 5G EMI Shielding Applications

This letter presents a miniaturized conformal dual-band mm-wave frequency selective surface (FSS) for fifth-generation (5G) electromagnetic interference (EMI) shielding applications. Two back-to-back resonators are separated with a 0.254 mm thin dielectric substrate. The proposed FSS exhibits two wide stopbands (S21<-10 dB) in the mm-wave 5G frequency spectrum. The stopband bandwidths (BW) are 42.8% at 28 GHz and 24.2% at 38 GHz. The periodicity of the unit cell is 0.26 λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> , where λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> is the wavelength of the lower stopband. The key features of the proposed FSS include a miniaturized unit cell structure for 28 GHz and 38 GHz frequency bands with a high oblique incident angle (OIA) stable filter behavior up to 60°. The proposed FSS also features polarization insensitivity and conformality. The simulated and measured results show good agreement for EMI shielding applications.

Flexible SiO2/rGO aerogel for wide-angle broadband microwave absorption

Graphene aerogel has been a promising microwave absorber due to ultralow density and strong dielectric loss ability. However, its intrinsic brittleness, and poor absorbing properties because of impedance mismatch severely hinders practical engineering applications. Herein, flexible silica fibers with excellent impedance matching properties were introduced into graphene honeycomb structures to co-construct a dual-structure aerogel (GS) by eco-friendly and low-cost technology. The unique dual-structure significantly enhances its resilience, enabling it to rebound after 1000 cycles of 75% compression. In addition, it can withstand 450 times of 180° folding and 360° rotation without damage, featuring remarkable flexibility and reliability. Furthermore, the simulation and experimental results indicate that GS has excellent wave absorbing performance. When the thickness is 6 mm, the effective absorption bandwidth is 32.55 GHz; when the thickness increases to 10 mm, the reflection loss is less than −8 dB in the range of 2.79–40 GHz. Moreover, the reflection loss remains almost constant when the oblique incidence angle ranges from 5° to 45°, which further demonstrates the critical role of impedance matching in enhancing the microwave absorption performance. These superior characteristics make GS an ideal candidate for many microwave absorption applications such as flexible electronics and aerospace.

Effect of a V‐shaped canyon on the seismic response of a bridge under oblique incident SH waves

AbstractTo elucidate the effect of canyon topography on dynamic response of a bridge, an analytical model for a simply supported bridge crossing a symmetric or non‐symmetric V‐shaped canyon in an elastic half‐space under SH waves is proposed. An exact solution to the dynamic canyon‐bridge interaction problem is derived using wave function expansion and verified by its comparison with past exact solution for a sole bridge model without a canyon. The wave propagation in the half‐space, wave scattering by the canyon and wave radiation from the bridge are all taken into account rigorously. Through a systematical parametrical study, it is found that the seismic response of the bridge beam and the wave‐facing side foundation will be significantly amplified. The degree of the topographic amplification is closely related to the geometry of the canyon, the mass and the stiffness of the bridge beam, and the frequency content and angle of the incident wave. The potential adverse effects of a V‐shaped canyon on a bridge should be considered in its seismic design.