Large Angles Of Incidence Research Articles

Miura-ori based reconfigurable multilayer absorber for high-efficiency wide-angle absorption.

Radar stealth structures that can achieve high-efficiency wide-angle absorption are key components of future military equipment. However, it is difficult for both planar and three-dimensional (3D) absorbers to achieve efficient absorption in a large incidence angle range. The multilayer reconfigurable absorber component based on Miura origami provides a unique solution. First, the multilayer origami absorber is parameterized in the simulation software. Each origami structure is covered with resistive films that fit the panels. Geometric constraints are satisfied among the multilayer structures. They support reconfigurability in the range of continuous states (as opposed to discrete states), which is conducive to finding the folded state with a more efficient absorption rate within the frequency band. Secondly, the designed structure does not require a specialized mechanically supported multilayer origami absorber. In addition, the equivalent analogue circuit method is used to analyze the efficient absorption of multilayer origami under oblique incidence. Finally, our proposed absorber satisfies the requirements of multiple absorption metrics: broadband, high efficiency, wide incidence angle, and polarization insensitivity. As the validation, we simulated and fabricated a double-layer origami absorber. Our proposed origami absorber can maintain an absorption rate of more than 90% for both transverse electric (TE) and transverse magnetic (TM) polarizations in the operating frequency band (5-20 GHz) over a wide range of incidence angles (0°-70°). When the incidence angle qinc = 40°, the double-layer origami absorber (q1 = 90°, α1 = 60°, and a2 = 75°) can achieve at least 10 dB reflection reduction of -18 dB and -20 dB in TE and TM modes, respectively. The proposed origami absorber provides a reference for the design of other absorbers.

Ultra-broadband absorber based on algorithmic optimization of MgF2–Fe stacked structure

Broadband absorbers possess a wide array of potential applications, including electromagnetic cloaking, solar energy harvesting, and solar power generation. However, achieving excellent ultra-broadband absorption with plasma-metal materials remains challenging due to their inherently narrow bandwidth. To tackle this issue, we propose an ultra-broadband absorber, free from lithography, composed of a multilayer stack of magnesium-iron fluoride (MgF2–Fe). In this study, we optimized the structural parameters of the absorber using a particle swarm optimization algorithm to achieve theoretically optimal absorptivity. Numerical simulations revealed that the absorber achieved an impressive theoretical average absorption of 98.4% across a broad wavelength range from ultraviolet to near-infrared (321–1800 nm). Importantly, it exhibited stable absorption performance at large incidence angles (0–70°), while maintaining high absorption efficiency. Furthermore, it demonstrated polarization independence at perpendicular incidence angles. Subsequently, experimental preparations and tests were conducted on absorber samples to evaluate their solar energy absorption capacity. Both experimental and theoretical comparisons underscored the promising potential of this structure for applications in solar energy collection, conversion, and solar power generation.

Investigation into the Vortex Evolution characteristics of the Francis Turbine During the Runaway Process

Abstract The runaway process is a dynamic transition process that gradually deviates from the optimal operating condition. The flow passage components dissipate all of the energy corresponding to the hydraulic turbine’s head, causing the internal flow of the hydraulic turbine to gradually deteriorate and produce a large-scale vortex structure during the runaway process. A model Francis turbine is used as the research object in this paper to understand the evolution characteristics of an unsteady vortex during a runaway. The runaway process is investigated using high-precision simulation technology of gas-liquid two-phase flow, and the runaway speed is obtained, consistent with the experimental results. The results show that the cavitation volume increases as the rotating speed increases. The large incidence angle between the inlet flow and the blade causes large-scale flow separation in the runner, which leads to forming of a sheet vortex band near the hub of the runner hub and a columnar vortex in the blade passage of the runner blade suction surface. As the runaway process continues, the flow separation in the runner intensifies and the induced vortex size increases, obstructing the runner’s passage. In addition, pressure signal analysis reveals that the pressure fluctuation caused by the sheet vortex band is low-frequency. On the other hand, the pressure fluctuation caused by the columnar vortex is frequent. These two types of vortex structures with high-energy pressure fields degrade the flow in the runner and reduce the hydraulic performance of the turbine.

Effect of distributed Bragg reflectors on optoelectronic characteristics of GaN-based flip-chip light-emitting diodes

Distributed Bragg reflectors have been widely utilized in GaN-based flip-chip light-emitting diodes (FCLEDs) owing to their excellent reflection performance. Recently, wide reflected angle DBR (WRA-DBR) has been suggested to enhance the optical characteristics of GaN-based FCLEDs by incorporating multiple sub-DBRs with varying central wavelengths. However, the reflectivity of WRA-DBR decreases at large incident angle from 425 nm to 550 nm, which restricts further optical performance improvement of FCLEDs. Here, we demonstrate a quintuple-stack DBR comprised of five sub-DBRs. The quintuple-stack DBR possesses a high reflectivity (>97.5%) for incident angles below 50° within the blue and green light wavelength ranges. Compared to WRA-DBR, quintuple-stack DBR exhibits a higher reflectivity in wavelength range of 425 nm to 550 nm and thinner multilayer thicknesses. Furthermore, stronger electric field intensities exist in the top facet and sidewalls of FCLED with quintuple-stack DBR, revealing that quintuple-stack DBR is beneficial for enhancing the light extraction efficiency. As a result, the light output power of FCLED with quintuple-stack DBR is ∼3% higher than that of FCLED with WRA-DBR at 750 mA.

Enhanced Multi-Beam Echo Sounder Simulation through Distance-Aided and Height-Aided Sound Ray Marching Algorithms

The study proposes two innovative algorithms in the field of multi-beam echo sounder (MBES) simulation: distance-aided sound ray marching (DASRM) and height-aided sound ray marching (HASRM). These algorithms aim to enhance the efficiency and accuracy of MBES simulations, particularly when dealing with long-distance propagation and real-time processing limitations. DASRM addresses issues related to simulation accuracy by efficiently utilizing the KD-tree for spatial indexing and intersection detection instead of the signed distance field (SDF). Building upon the further analysis of DASRM, HASRM is proposed, which improves the search strategy for ray intersections and utilizes a height field pyramid for sampling and retrieval, thereby reducing memory usage while enhancing indexing efficiency. The experimental results demonstrate that both algorithms significantly outperform traditional methods in terms of simulation time, with HASRM exhibiting particular advantages in parallel computing due to its data structure and improved strategies. Additionally, DASRM is well suited for applications requiring complex scene construction, while HASRM proves especially effective in simulating MBES with a focus on underwater terrain due to its effectiveness in handling large incident angles and long-distance propagation.

Energy- and Angle-Resolved Scattering of Ne from Dodecane Liquid Surfaces: Theory Corroborating Experiment.

Motivated by recent experimental work by the Neumark group, we present here an all-atom molecular dynamics study of Ne scattering from a dodecane liquid surface with the objective of elucidating the fundamental aspects of gas-liquid dynamics. Using a fine-tuned force field, the GPU-accelerated simulations reproduced semiquantitatively the energy- and angle-resolved experimental results. The branching ratio between the impulsive scattering (IS) and thermal desorption (TD) channels exhibits a clear correlation with the incidence energy (Ei) and angle. Ne atoms with lower Ei values are more likely to be trapped, yielding an increased TD ratio. For a given Ei, a large incidence angle led to a higher IS ratio. The energy transfer between Ne atoms and liquid dodecane was found to be more sensitive to the deflection angle than to the incidence or reflection angle. With an increasing deflection angle, the fractional energy loss increases, suggesting that more kinetic energy is transferred to the liquid.

Polarization conversion and lateral shifts in multilayered structure with finitely-gapped topological surface states

Abstract The polarizatison conversion and the Goos–Hänchen (GH) shifts of the reflected electromagnetic wave for the multilayered structure made of topological insulator (TI) layers with finite surface energy gap are investigated. The transfer matrix formalism is adopted to analyze the reflection of electromagnetic wave through the multilayered structure, and the influences of surface energy gap, thickness and number of the TI layers are discussed. We find that maximum polarization conversion rate can be obtained with appropriate surface energy gap of TI, and within a certain range of finite energy gap, the polarization conversion effect is stronger than that for the case under the infinite surface energy gap limit. Greater polarization conversion rate for TI with small surface energy gap can be found than that for TI with larger energy gap in some range of layer numbers. At large incident angles the GH shifts vary considerably with the layer number for TI with relatively larger energy gap. Result of the combined influence of surface energy gap and layer number shows that, there exists both the positive and negative enhancement peaks of the GH shifts, and for smaller energy gap, fewer TI layers are required to obtain the transition between positive and negative GH shifts.

Multi-functional metasurface: ultra-wideband/multi-band absorption switching by adjusting guided-mode resonance and local surface plasmon resonance effects

This study introduces an innovative dual-tunable absorption film with the capability to switch between ultra-wideband and narrowband absorption. By manipulating the temperature, the film can achieve multi-band absorption within the 30–45 THz range or ultra-wideband absorption spanning 30–130 THz, with an absorption rate exceeding 0.9. Furthermore, the structural parameters of the absorption film are optimized using the particle swarm optimization (PSO) algorithm to ensure the optimal absorption response. The absorption response of the film is primarily attributed to the coupling of guided-mode resonance and local surface plasmon resonance effects. The film’s symmetric structure enables polarization incoherence and allows for tuning through various means such as doping/voltage, temperature and structural parameters. In the case of a multi-band absorption response, the film exhibits good sensitivity to refractive index changes in multiple absorption modes. Additionally, the absorption spectrum of the film remains effective even at large incidence angles, making it highly promising for applications in fields such as biosensing and infrared stealth.

Ultra-broadband, high absorption, polarization-insensitive microwave absorbers designed based on multi-scale fractal metasurfaces

Microwave absorbers play an essential role in radar stealth applications. However, conventional microwave absorbers are hindered by limitations such as narrow bandwidth, large size, and inadequate absorption rates. Leveraging metasurfaces presents an ideal approach for creating microwave absorbers, but metasurface-based broadband absorbers typically entail intricate structures, natural absorbing materials, or electronic components, posing challenges in terms of fabrication and expansion. Here, we numerically demonstrate a simple multi-scale fractal metasurface microwave absorber, with the full width at half-maximum of band exceeding one optical octave. By integrating the localized surface plasmon resonance absorption band and the Salisbury-Screen-type absorption band, we achieve an average absorption of 90% ranging from 11.5 GHz to 27.1 GHz. Furthermore, this absorber exhibits excellent polarization insensitivity and maintains high absorption even at large incident angles. The device, offering both broadband absorption and a straightforward structure, holds significant promise for practical applications and widespread adoption.

All-Silicon Polarization-Insensitive Metamaterial Absorber in the Terahertz Range.

All-silicon terahertz absorbers have attracted considerable interest. We present a design and numerical study of an all-silicon polarization-insensitive terahertz metamaterial absorber. The meta-atoms of the metamaterial absorber are square silicon rings which can be viewed as gratings. By properly optimizing the structure of the meta-atom, we achieve a broadband absorptivity that is above 90% ranging from 0.77 THz to 2.53 THz, with a relative bandwidth of 106.7%. Impedance matching reduces the reflection of the terahertz waves and the (0, ±1)-order diffraction induce the strong absorption. The absorption of this absorber is insensitive to the polarization of the terahertz wave and has a large incident angle tolerance of up to 60 degrees. The all-silicon metamaterial absorber proposed here provides an effective way to obtain broadband absorption in the terahertz regime. Metamaterial absorbers have outstanding applications in terahertz communication and imaging.

Robust broadband dual-polarized rectenna with high conversion efficiency

In this paper, a robust wide-beam dual-polarized broadband rectenna with high conversion efficiency is proposed. First, a metal wall-loaded broadband cross-dipole antenna is designed as the receiving antenna. With the metal wall, the bandwidth of the receiving antenna is greatly improved. The measured results show that the −10 dB impedance bandwidth is 820 MHz from 1.65 to 2.47 GHz, and the 3-dB beamwidth is over 80° within the operating bandwidth. Besides, a novel impedance matching network is introduced to help the rectifier achieve high conversion efficiency in a wide bandwidth. The measured results show that the RF-DC conversion efficiency is maintained above 50 % with a maximum of 78 % within the bandwidth from 1.56 to 2.53 GHz when the load impedance is 840 Ω and the input power is 12.0 dBm. Finally, the receiving antenna and rectifier are integrated and measured. The results illustrate that the proposed design can be robustly against dynamic ambient power density, large oblique incidence angle, and arbitrarily polarized wave. Specifically, the measured conversion efficiency is over 49 % at an ambient power density of 200 μW/cm2 within the incident angle from −40° to 40° under arbitrary polarization. Hence, it could be a feasible solution to wireless energy harvesting (WEH) applications.

An improved algorithm based on equivalent sound velocity profile method at large incident angle

An improved algorithm based on equivalent sound velocity profile method at large incident angle

Narrowband Mg/SiC multilayer mirror working as high-harmonic selector at 30.4 nm wavelength

Multilayers (MLs) are a good choice for high-harmonic generation (HHG) sources for selecting single-wavelength radiation. MLs working around 30.4 nm with a significantly reduced bandwidth based on high Bragg order reflection have been designed and fabricated. The narrowband MLs were characterized using grazing incidence x-ray reflectometry, extreme ultraviolet reflectivity, and intrinsic stress measurements. The results indicate that Mg/SiC MLs designed with 3rd Bragg order have narrowest bandwidth of 0.71 nm (full width half maximum) with a spectral resolution (λ/Δλ) of 42, and reflectance of 30% under near normal incidence geometry. Based on these results, the simulation of narrowband MLs working at large incident angles demonstrates that 3rd Bragg order ML has a better spectral selectivity compared with 1st Bragg order ML for HHG sources.

Experimental and numerical research on blended blade and end wall technique in a compressor cascade

In this paper, to explore the effects and mechanisms of the blended blade and end wall (BBEW) technique on corner separation control, two different BBEW configurations, including the side blended blade and end wall (S-BBEW) and full blended blade and end wall (F-BBEW), are studied and compared through experimental and numerical analyses. First, for a linear cascade, the S-BBEW and F-BBEW designs are carefully built. Then, both numerical simulations and experiments are conducted for each design, demonstrating a significant reduction in the loss coefficient. The results indicate that the S-BBEW is effective in the control of boundary layer development and delays the initial point of separation, especially at the design working condition. Moreover, the F-BBEW design nearly eliminates the corner separation by the control of both boundary layer development and secondary flow, which provides a higher performance improvement, especially at a large incidence angle.

Pn‐Wave Receiver Function

AbstractThe receiver function (RF) method is the most widely adopted method for imaging crustal structures using earthquake data. Through attenuation during long‐distance propagation, high‐frequency components are scarce in teleseismic waveforms, resulting in low‐frequency RFs and low‐resolution crustal images. The Pn‐wave contains more high‐frequency components because of the short epicentral distance. To improve the resolution of crustal structure studies, we propose the Pn‐wave receiver function (PnRF) method. Unlike other near‐earthquake phases, the Pn‐wave can be considered a plane wave in the crust beneath seismic stations, and interference from other phases can be avoided at epicentral distances of 5–15°. PnRFs calculated from both numerical synthetic data and observational data at broadband seismic stations show that all converted waves are present in PnRFs at the predicted time according to the theory of plane waves. PnRFs calculated by observational data of a dense nodal array clearly show not only the converted wave from the Moho but also the converted wave from the crustal interface, which is too weak to be observed in tele‐RFs because the Pn‐wave has a larger incident angle and higher frequency than the teleseismic P‐wave. When used in conjunction with dense nodal array observations, the PnRF method has the potential to image crustal structures with a high resolution close to that of the deep seismic reflection method.

A Metastructure Based on Amorphous Carbon for High Efficiency and Selective Solar Absorption.

Efficient solar thermal conversion is crucial for renewable clean energy technologies such as solar thermal power generation, solar thermophotovoltaic and seawater desalination. To maximize solar energy conversion efficiency, a solar selective absorber with tailored absorption properties designed for solar applications is indispensable. In this study, we propose a broadband selective absorber based on amorphous carbon (a-C) metamaterials that achieves high absorption in the ultraviolet (UV), visible (Vis) and near-infrared (NIR) spectral ranges. Additionally, through metal doping, the optical properties of carbon matrix materials can be modulated. We introduce Ti@a-C thin film into the nanostructure to enhance light absorption across most of the solar spectrum, particularly in the NIR wavelength band, which is essential for improving energy utilization. The impressive solar absorptivity and photothermal conversion efficiency reach 97.8% and 95.6%, respectively. Notably, these superior performances are well-maintained even at large incident angles with different polarized states. These findings open new avenues for the application of a-C matrix materials, especially in fields related to solar energy harvesting.

Multi-Mode Long-Wavelength GaAs/AlGaAs Quantum Well Infrared Photodetectors for Circular Polarization Detection

We present an integrated device combining a double L-shaped chiral metasurface with long-wavelength GaAs/AlGaAs quantum well infrared photodetectors (QWIPs), achieving a circular polarized extinction ratio (CPER) as high as 45 in the long-wavelength infrared range of 7–9 μm. The unit of the chiral metasurface array consists of two structurally identical L-shaped gold structures with central symmetry. The CPER of the double L-shaped QWIPs is 14 times higher than that of a single L-shaped QWIP. The device operates in three modes within the detection band: the microcavity mode, the surface plasmon polariton (SPP) mode, and the hybrid mode. The double L-shaped chiral structure selects and reflects a small portion of left-handed circularly polarized light (LCP), while the majority enters the device and excites SPP modes with the bottom gold grating layer, leading to an absorption enhancement. In contrast, right-handed circularly polarized light (RCP) is mostly reflected with limited excitation of SPP waves. QWIPs exhibit a peak absorption of 0.8 and a coupling efficiency of 2700% in the active region of the quantum well due to the combined effects of the microcavity and SPP modes, in which the SPP mode plays a dominant role. The proposed device maintains high circular polarization discrimination capability under large incident angles and can be applied in spectral imaging.

An ultra-broadband high-performance solar energy perfect absorber from deep ultraviolet to mid-infrared

Metamaterial absorbers possess potential applications in a large number of fields, including solar energy harvesting, photovoltaics, stealth technology and sensors, owing to their capability to achieve perfect absorption of electromagnetic waves. In this study, a solar metamaterial absorber comprising a TiN substrate, a SiO2 spacer layer and an array of patterned-layer TiN-SiO2 double circular employing the finite-difference time-domain (FDTD) method is proposed to achieve perfect absorption in ultra-broadband. Optimization results indicate that the broadband absorption spectrum of the absorber extends from deep ultraviolet to mid-infrared wavelengths with an average absorption of 98.1%. The distribution of electric and magnetic fields suggests that the absorber achieves high absorption due to the interaction between surface plasmon resonance (SPR), cavity resonance (CR) and magnetic resonance (MR). Furthermore, the proposed absorber is insensitive to large-angle incidence and polarization-independent. Moreover, the absorber can be produced at a low cost due to its tolerance of the geometrical errors. The newly proposed metamaterial absorber is expected to be applied in solar energy related fields.

Flat-band Friedrich-Wintgen bound states in the continuum based on borophene metamaterials.

Many applications involve the phenomenon of a material absorbing electromagnetic radiation. By exploiting wave interference, the efficiency of absorption can be significantly enhanced. Here, we propose Friedrich-Wintgen bound states in the continuum (F-W BICs) based on borophene metamaterials to realize coherent perfect absorption with a dual-band absorption peak in commercially important communication bands. Metamaterials consist of borophene gratings and a borophene sheet that can simultaneously support a Fabry-Perot plasmon resonance and a guided plasmon mode. The formation and dynamic modulation of the F-W BIC can be achieved by adjusting the width or carrier density of the borophene grating, while the strong coupling leads to the anti-crossover behavior of the absorption spectrum. Due to the weak angular dispersion originating from the intrinsic flat-band characteristic of the deep sub-wavelength periodic structure, the proposed plasmonic system exhibits almost no change in wavelength and absorption at large incident angles (within 70 degrees). In addition, we employ the temporal coupled-mode theory including near- and far-field coupling to obtain strong critical coupling, successfully achieve coherent perfect absorption, and can realize the absorption switch by changing the phase difference between the two coherent beams. Our findings can offer theoretical support for absorber design and all-optical tuning.

High-performance absorber with substitutable materials for short-wave infrared sensing

The optical absorption device plays a crucial role as a component of the infrared astronomical telescope and possesses a significant impact on astronomical observations. A simple metamaterial absorber with substitutable middle materials is made for short-wave infrared sensing. The absorber is designed as a hollow square column, using a patterning approach for the top-layer structure of metamaterials. The absorption characteristics are verified using the impedance matching method, which involves extracting S-parameters and then performing inverse calculations to determine the absorber’s equivalent impedance. The result shows the highest absorption peak is at 3.25 μm, reaching 99.71%, with an impressive average absorption rate of 99.01% between 1.52 and 3.66 μm. The results demonstrate that this absorber shows polarization insensitivity while maintaining high absorption even at large angles of incidence. The distribution of the electromagnetic field within the absorber, the electromagnetic losses within individual layers, and their impact on the absorptive performance are analyzed in detail. Polarization angles, transverse magnetic polarization, and transverse electric polarization are further explored. The parameters of each layer have been discussed. An investigation of the intermediate dielectric layer has been conducted. The proposed absorber shows the potential to achieve exceptional absorption performance under various dielectric conditions, rendering it a promising candidate for use in astronomical observation, medical tests, infrared detection, invisible short-wave infrared systems, radar and various optical devices.