Large Oblique Incident Angle Research Articles

Extremely Wideband Metamaterial Absorber Using Spatial Lossy Transmission Lines and Resistively Loaded High Impedance Surface

In this article, the design, principle, and characterization of a 3-D metamaterial absorber (MA) have been presented, which consists of the spatial lossy transmission lines (SLTLs) and the resistively loaded high impedance surface (HIS). The proposed MA possesses a fractional bandwidth of 193% with absorption rate of above 90% in the frequency range from 2 to 110 GHz under TM polarization while maintaining large oblique incident angle stability of over 50 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> . For demonstration, a prototype is fabricated, whose measurement results are in good agreement with the simulation results. Losses from the periodically patterned SLTLs and resistively loaded HIS are introduced by resistive graphene films using screen-printing technology, resulting in avoiding conventional soldering of numerous lumped resistors. An equivalent circuit model and a surface loss distribution are given to better illustrate the physical mechanism. Our findings establish general and systematic strategies for guiding the design of extremely wideband MA with large incident angle and pave the way for enhancing the microwave performance in applications of electromagnetic compatibility, imaging, radar detection, and stealth.

Oblique incidence-insensitive metamaterial absorber using strong magnetic coupling resonance for low-frequency microwave absorption

In this work, we presented an oblique incidence-insensitive metamaterial absorber (MA) based on the strong magnetic coupling resonance using the printed circuit board process, and the spacer is produced by a typical magnetic absorber material (MAM). As for reflection loss (RL) at oblique angles, it can achieve high-efficiency absorption for both transverse-electric (TE) and transverse-magnetic (TM) polarizations with a wide angular range from 0° to 60° in the low frequency of 2.33 GHz to 3.44 GHz. In addition, the magnetic coupling mechanism of the metamaterial absorber was deeply analyzed. Comparing the results of the simulation and experiment, it displays excellent absorption performance of the proposed absorber in TE and TM modes under large oblique incidence angles.

Planar ultrathin omni-directional perfect absorber utilizing amorphous silicon for photovoltaics

Resonant plasmonic metasurfaces and thin film stacks have been extensively studied for spectral control and perfect absorption enhancement functionality. Essentially, the plasmonic nanostructures or metallic films enable the optical field resonant and confinement at the nanoscale, and thus yield the Ohmic heat absorption in the nanoscale metals. However, typical perfect absorbers based on film coatings are usually sensitive to the variation of large oblique incident angles, and mostly lack the capability for direct conversion to photocurrents and photovoltaics. Here, we proposed a lithography-free perfect absorber design consisting of metallic and amorphous silicon (α-Si) films with deep-subwavelength thickness (∼ λ/20 - λ/100). The perfect absorptivity spectrum enjoys Omni-directional optical characteristics, which remains the high absorption for the normal incidence to large oblique incidence angles of ± 60°. Due to the strongly trapped resonance in the Fabry-Perot cavity, the majority of light absorption (∼89%) takes place in the core α-Si layer, which could enable the potential optoelectronic conversion to photocurrents and photovoltaics. Our proposed perfect absorber based on ultrathin α-Si films enjoys the great simplicity of design and manufacturing and suggests a variety of promising applications, including photovoltaics, optical sensors, solar cells, photodetectors, thermal bolometers, nano-imaging devices, color filters, and thermal emitters, etc.

Multichannel tunable narrowband mid-infrared optical filter based on phase-change material Ge2Sb2Te5 defect layers.

The defect mode, which exists at the defect layer of a one-dimensional photonic crystal (1DPhC) heterostructure, provides the possibility of realizing narrowband transmission owing to its strong electromagnetic field localized effects. In this study, we numerically investigate the single- and multichannel narrowband filters in the mid-infrared region based on the defect mode by adding the monolayer and multilayer phase-change material Ge2Sb2Te5 (GST) defect layer in 1DPhC. It can provide a promising avenue to tune the transmission spectra by changing the crystallization fraction X of the GST defect layer. The remarkable narrowband transmission enhancement can be acquired for both TM and TE polarizations in spite of the large oblique incident angle. Such a defect-mode-based 1DPhC heterostructure enables tunable operating wavelength by adjusting geometrical parameters to realize the spectral selectivity of the filter in the mid-infrared region. The significant improvement and tunability of the designed single- or multichannel filters can be applied to biochemical sensing and material characterization.

Nonlinear seismic responses of the powerhouse of a hydropower station under near-fault plane P-wave oblique incidence

Near-fault ground motion has unique motion characteristics that are mainly manifested as long periodic velocity and large displacement pulses. Engineering structures in near-fault areas are frequently damaged severely. Most previous studies have assumed that the seismic waves on a site in a near-fault area are vertical or approximately vertical incidence that are prone to large oblique incidence angles. First, the displacement expression of free surface is derived on the basis of the boundary conditions of free surface. The theoretical displacement curves of free surface in the range of 0°–90° are drawn on the basis of displacement expression. Then, the most unfavorable incidence angle (65°) is obtained. Subsequently, the equivalent nodal force formula on the artificial boundary node of plane P-wave oblique incidence is derived on the basis of the dynamic viscoelastic artificial boundary. Numerical verification shows that the proposed method has good precision. Ten near-fault ground motions are selected from the strong earthquake database of the Pacific Earthquake Engineering Research Center (PEER), in which velocity and non-velocity pulse ground motions have five groups each. Finally, the nonlinear seismic responses of the ground powerhouse of a hydropower station are investigated when the incidence angles of a near-fault plane P-wave are 15°, 30°, and 65°. Results show that the damage of the powerhouse under pulse-like ground motions are more serious than that under non-pulse ground motions when the incidence angles of seismic wave are the same. The superstructural interlayer displacement angles of the powerhouse and the relative displacements in the water flow direction between the upstream and downstream cattle legs increase with the ratio of the peak ground velocity to the peak ground acceleration (PGV/PGA). In comparison with the smaller incidence angle of seismic wave, the PGV/PGA of ground motion has a more significant influence on the seismic response of the powerhouse at a larger incidence angle of seismic wave.

In-situ impedance and absorption coefficient measurements using a double-layer microphone array

Acoustic impedance is typically measured using an impedance tube, which requires a material sample physically fitted to the tube. However, the impedance can vary greatly between the material mounted in the tube and the material located in a real environment, where the mounting conditions are likely to be different. Also, oblique incidence cannot be measured in an impedance tube. In this paper, we investigate the use of a double-layer microphone array for in-situ measurement of surface impedance and absorption coefficient. With the array positioned near the material surface, a source emits broad-band sound towards the array and the material. A measurement is taken, and the sound pressure and the surface-normal particle velocity at the material surface are calculated using Statistically Optimized Near-field Acoustical Holography (SONAH). From the surface pressure and velocity, the impedance across a selected area is calculated, and finally the absorption coefficient is calculated from the impedance. A set of tests has been performed on porous material samples in an anechoic chamber as well as in a fitted room. Different sample sizes and different sound incidence angles have been considered. The results show consistency between the measurements in the anechoic room and the ordinary room as well as good agreement with Miki’s model up to large oblique incidence angles.

Ultrawide Bandwidth Electromagnetic Wave Absorbers Composed of Double-Layer Frequency Selective Surfaces with Different Patterns

A novel design for an ultra-wide bandwidth and thin microwave absorber is introduced utilizing two frequency selective surfaces (FSSs) with different patterns of resonating frequencies. The circuit parameters, inductance and capacitance, of the three types of FSS (square loop, cross, square patch) were determined using an equivalent circuit and strip wire conductor model. The square loop FSS indicates a low frequency resonance (10 GHz) due to its high inductance and capacitance. On the other hand, the square patch of small inductance reveals a high resonating frequency (36 GHz). By optimizing the combination of the two FSSs, an ultra-wide absorption bandwidth (6.3–40.0 GHz for −10 dB reflection loss) was designed with a small total thickness of 5.5 mm, which is close to the theoretical limit. The free space measurement result with a test sample prepared by the screen printing method was in good agreement with the simulation result and verified the validity of the proposed design method. For these periodic array structures, however, the grating lobes were observed above the high frequency limit, and it needs to be emphasized that the further control of the unit cell periodicity is important, particularly for large oblique incidence angles.

Dynamic control of asymmetric electromagnetic wave transmission by active chiral metamaterial

The asymmetric transmission of electromagnetic (EM) wave can be fully manipulated by chiral metamaterials, but little can achieve real-time and high efficient tunability due to challenges in practically deployable solutions. Here, we proposed a new scheme for flexibly and dynamically controlling the asymmetric EM wave transmission at microwave frequencies using planar metamaterial of deep subwavelength thickness incorporated with active components of PIN diodes. The asymmetric transmission of linearly polarized EM wave exhibits a high efficiency and a pronounced real-time continuous tunability controlled by the external stimulation of voltage biasing. In addition, the asymmetric transmission effect can be well preserved at large oblique incident angle up to ±70°. The design principle and EM performance are validated by both full wave simulations and experimental measurements. Such dynamically controllable chiral metamaterial may provide robust and flexible approach to manipulate EM wave propagation, as well as to facilitate EM device integration to create diverse functionalities.

Blue-Phase Liquid Crystal Displays With Vertical Field Switching

A low-voltage, high-transmittance, hysteresis-free, and submillisecond-response polymer-stabilized blue-phase liquid crystal (BPLC) display with vertical field switching (VFS) and oblique incident light is demonstrated. In the VFS mode, the electric field is in longitudinal direction and is uniform. By using a thin cell gap and a large oblique incident angle, the operation voltage is significantly reduced which plays a key role to eliminate hysteresis and residual birefringence. The VFS mode is a strong contender for the emerging BPLC display and photonic devices.

Erratum: “Influence of a large oblique incident angle on energetic protons accelerated from solid-density plasmas by ultraintense laser pulses” [Appl. Phys. Lett. 90, 031503 (2007)

Erratum: “Influence of a large oblique incident angle on energetic protons accelerated from solid-density plasmas by ultraintense laser pulses” [Appl. Phys. Lett. 90, 031503 (2007)

Influence of a large oblique incident angle on energetic protons accelerated from solid-density plasmas by ultraintense laser pulses

The acceleration of energetic electron, proton, and heavy ion beams produced by ultrahigh-intensity laser pulses through thin plastic targets is studied using two-dimensional hybrid particle-in-cell simulation. When the laser is incident at a large angle, the proton beams accelerated from the front and rear surfaces of the target deviate from the normal direction because of the formation of non-Gaussian asymmetric sheath field at the target surfaces. In particular, the simulations clearly show that the proton beam in the backward direction can have higher Bragg peak energy than that of the forward direction if the incident angle is sufficiently large.

Characterization of a nematic PALC at large oblique incidence angles

Compared with conventional photometric methods of measuring cell parameters, including the cell gap and the pretilt angle of a nematic parallel-aligned liquid crystal (PALC) using multiple wavelengths at normal incidence, this research proposes the use of a phase-sensitive interferometric ellipsometer to determine cell parameters precisely based on a single wavelength at large oblique incidence angles. The advantage of this method is that it detects the phase difference using an optical heterodyne interferometer in which a common phase noise rejection mode is provided. Thus, there is a high signal-to-noise ratio (SNR) on the phase measurement. In addition, a range of large oblique incidence angles on the PALC is used so that a high sensitivity measurement of the cell parameters is obtained experimentally. During the measurements, the multiple reflections and spatial shifting effect of the emerging extraordinary ray (E-ray) and ordinary ray (O-ray) from the PALC at large oblique incidence angles are able to be reduced effectively by the use of retro-reflected geometry in the interferometer. The experimental results verify that the sensitivities for the cell gap and pretilt angle measurements are 0.3 nm and 0.01 degrees , respectively.