Wide Range Of Incident Angles Research Articles

Absorption and Reflection of Switchable Multifunctional Metamaterial Absorber Based on Vanadium Dioxide

A dynamically tunable terahertz broadband absorber based on the metamaterial structure of vanadium dioxide (VO2) is proposed and analyzed. The absorber consists of two patterned VO2 layers and a metal bottom layer separated by two polytetrafluoroethylene (PTFE) dielectric layers. Simulation results show that the absorption exceeds 90% in the frequency range of 2.4–11 THz with a relative bandwidth of 128.4% under normal incidence. When VO2 is in the metal phase, the designed absorber functions as an ideal absorber. The absorption rate can be flexibly adjusted from 2% to 99% as vanadium dioxide transitions from the insulator phase to the metal phase. Therefore, the newly developed broad structure has the capability to seamlessly transition between functioning as an absorber or reflector through modifications in the conductivity of VO2 from the insulator phase to the metal phase. Moreover, further insight into the underlying physical processes can be gained by studying the insensitivity of the proposed absorber to the polarization of incident light and its ability to achieve high absorption across a wide range of incident angles. Impedance matching theory and electric field distribution of the absorber are investigated. The THz absorber has many potential applications in fields such as THz sensors, modulation, and switches.

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.

Quasi-BICs enhanced second harmonic generation from WSe2 monolayer.

Quasi-bound states in the continuum (quasi-BICs) offer unique advantages in enhancing nonlinear optical processes and advancing the development of active optical devices. Here, the tunable robust quasi-BICs resonances are experimentally achieved through the engineering of multiple-hole Si-metasurface. Notably, the quasi-BICs mode exhibits flat bands with minimal dispersion at a wide range of incident angles, as demonstrated by the angle-resolved spectroscopy measurements. Furthermore, we demonstrate a giant second-harmonic generation (SHG) enhancement by coupling a WSe2 monolayer to the quasi-BICs hosted in the metasurface. Leveraging the strong local electric field and high state density of the observed quasi-BICs, the SHG from the WSe2 monolayer can be enhanced by more than two orders of magnitude. Our work paves the way for effectively enhancing nonlinear optical processes in two dimensional (2D) materials within the framework of silicon photonics and is expected to be applied in nonlinear optical devices.

Wide-band microwave absorber with polarization insensitivity and incident angular stability using resistive ink for stealth application

A wide-band microwave absorber using five layers of resistive ink patterns is designed with a periodicity of 0.15λL, where λL denotes the wavelength at the lowest operating frequency. Symmetrically shaped resonators are patterned on five layers of FR4 substrates with air spacers between them. The fractional bandwidth of the absorber is 140% with absorption greater than 90% attained in the operating frequency range from 3.38 GHz to 19.5 GHz. Cross-polarized absorption of the structure is found to be less than 0.1%. Absorption is stable for a wide range of incident angles up to 40∘. Absorption behavior is found to be stable for both TE and TM polarized incident waves under normal incidence. Further, the mono-static radar cross-section is analyzed and found to be reduced below -40 db in the entire operating frequency.

Wide-angle ultrabroadband absorption in black phosphorus/dielectric multilayer anisotropic metamaterials

An ultrabroadband far-infrared absorber is achieved using an anisotropic metamaterial composed of alternating black phosphorus (BP) and dielectric films arranged in a trapezoidal structure. We numerically demonstrate that ultrabroad bandwidths (with >90% absorptivity) can be achieved with the strong anisotropic dielectric response of BP, namely 63.6 μm along the armchair direction and 53.6 μm along the zigzag direction. Importantly, the high absorption is maintained across a wide range of incident angles. Our simulation results align well with analytical calculations based on the effective medium theory, considering the multilayer structure as an effective homogeneous metamaterial with anisotropic permittivity. From the distribution profiles of magnetic fields, we observe tight trapping of different wavelengths at varying widths of the trapezoidal absorber, revealing the slow-light effect underlying the broadband absorption. Our study holds significant potential for device applications, such as BP-based broadband infrared photodetectors.

Efficient and high-quality absorption enhancement using epsilon-near-zero cylindrical nano-shells constructed by graphene

This paper presents a detailed scattering analysis of a hollow-core plasmonic-shell cylindrical wire to design an efficient, compact, narrowband, and reconfigurable optical absorber. The shell is formed by a thin graphene material, investigated in its epsilon-near-zero (ENZ) plasmonic region. Compared to the graphene plasmonic resonances in the terahertz(THz)/far-infrared (FIR) frequencies, the ENZ plasmonic resonances offer a blue shift in the operating frequency of the second-order plasmonic resonances by increasing the geometrical dimensions. This feature is successfully used to design efficient optical wave absorbers with absorption cross-sections much larger than geometrical and scattering cross-sections. The observed blue shift in the resonance spectrum, which is the key point of the design, is further verified by defining each particle with its polarizability and fulfilling the resonant scattering condition in the framework of Mie’s theory. Furthermore, graphene relaxation time and chemical potential can be used to manipulate the absorption rate. Observed resonances have narrow widths, achieved with simple geometry. To consider more practical scenarios, the one-dimensional arrangement of the cylindrical elements as a dense and sparse array is also considered and the design key point regarding graphene quality is revealed. The quality factor of the sparse array resonance is 2272.8 and it demands high-quality graphene material in design. It is also observed that due to the use of small particles in the design, the near-field and cooperative effects are not visible in the absorption cross-section of the array and a clear single peak is attained. This polarization-insensitive absorber can tolerate a wide range of incident angles with an absorption rate above 90%.

Vanadium dioxide-based terahertz metasurface device with switchable broadband absorption and beam steering functions

A simple terahertz (THz) metasurface device with dual functionalities of broadband absorption and beam steering is proposed in this paper. The device exploits the phase change properties of vanadium dioxide (VO2), transitioning between its insulating state and metallic states to achieve functional switching. In the metallic state, the device exhibits broadband absorption, surpassing 90% absorption in the 1.03–2.07 THz range. The absorption function maintains exceptional performance across a wide range of incident angles and possesses polarization insensitive properties. In the insulating state of VO2, the metasurface converts into a THz reflective beam steering device. By employing phase coding, 1-bit coding metasurfaces based on phase gradients are designed, enabling dual-beam and four-beam reflection. Building upon this foundation, 2-bit coding metasurfaces facilitate anomalous reflection of a single beam in distinct directions. Consequently, the device not only toggles between broadband absorption and beam steering but also satisfies the design criteria for 3-bit coding metasurfaces. The modulation characteristics of this device serve as a valuable design reference for multifunctional THz devices in diverse scenarios.

Metasurfaces for sound absorption over a broad range of wave incidence angles

Control of reflected waves that meet surfaces from oblique directions is crucial, for instance, in closed spaces. Metasurfaces composed of Helmholtz resonators can be efficient and compact absorbers but have limited ability to achieve high absorption over a wide incidence angle range, especially when designed for high performance in the region approaching grazing incidence. In turn, sonic crystals can be used to manipulate wave propagation direction at low frequencies. We propose a type of absorber that combines a surface of 2D Helmholtz resonators and a 2D sonic crystal with cylindrical scatterers arranged in a hexagonal lattice. The combined effect of both structures yields a metasurface that can achieve high absorption over a broad range of incidence angles. Here, an analytic model to estimate the behavior of the absorbers for wavelengths that are much longer than the unit cell dimensions is presented. The model is used in combination with an optimization strategy to realize designs for single frequency and octave-band performance. The test cases show that surfaces with absorption coefficient values above 0.9 for the range of incidence angle extending from 0∘ until 83∘ can be realized. The performance of the absorbers is verified with a finite element model and experimentally.

Nonlocal, Flat-Band Meta-Optics for Monolithic, High-Efficiency, Compact Photodetectors.

Miniaturized photodetectors are becoming increasingly sought-after components for next-generation technologies, such as autonomous vehicles, integrated wearable devices, or gadgets embedded on the Internet of Things. A major challenge, however, lies in shrinking the device footprint while maintaining high efficiency. This conundrum can be solved by realizing a nontrivial relation between the energy and momentum of photons, such as dispersion-free devices, known as flat bands. Here, we leverage flat-band meta-optics to simultaneously achieve critical absorption over a wide range of incidence angles. For a monolithic silicon meta-optical photodiode, we achieved an ∼10-fold enhancement in the photon-to-electron conversion efficiency. Such enhancement over a large angular range of ∼36° allows incoming light to be collected via a large-aperture lens and focused on a compact photodiode, potentially enabling high-speed and low-light operation. Our research unveils new possibilities for creating compact and efficient optoelectronic devices with far-reaching impact on various applications, including augmented reality and light detection and ranging.

Versatile terahertz metasurface: dynamic switching between electromagnetically induced transparency and perfect absorption

This paper introduces a versatile metasurface based on vanadium dioxide (VO2) and graphene that seamlessly transitions between electromagnetically induced transparency (EIT) and multi-band absorption through VO2’s phase change property. When VO2 is in a dielectric state, the device can generate EIT. This configuration allows dynamic tuning of the central frequency by adjusting the graphene’s Fermi levels (E f ), achieving a remarkable group delay of 1.42 ps. When VO2 is in a metallic state, the structure facilitates a Fabry–Perot resonance between the VO2 layer at the bottom and the graphene layer at the top, leading to exceptional light absorption. Specifically, absorptivity of 99.8% and 99.4% is achieved at 1.66 THz and 2.87 THz, respectively. In addition, these two resonance peaks can also be dynamically adjusted by modulating E f . Furthermore, the device serves as a highly sensitive sensor with sensitivity up to 0.65 THz/RIU. Notably, both absorption and EIT modes are polarization-insensitive and exhibit tolerance to a wide range of incidence angles. Consequently, the proposed device holds significant promise across various applications within the electromagnetic field, including tunable devices, absorbers, sensors, slow-light devices, and so on.

Deep learning based inverse design of metasurface absorber for maximizing solar spectral absorption

The contemporary global challenges of energy crisis and global warming demand efficient utilization of solar thermal energy resources. In this study, a novel approach is employed wherein micro-nano-structures are meticulously designed in both the top and bottom layers of a bilayer quadrangular prismatic metasurface absorber (BQPMA). This method results in a remarkable weighted absorption of solar radiation in the extensive 280–4000 nm band range, attaining an impressive 97.75%. The innovative design process incorporates inverse design coupled with deep learning techniques, enabling rapid and precise selection of structural parameters with a minimal error of merely 0.06%. The extensive broadband absorption observed within the solar spectral range can be primarily attributed to the light trapping and moth-eye effects facilitated by Si3N4 and Ti quadrangular prismatic structures. Notably, the proposed BQPMA exhibits robust ultra-broadband absorption across a wide range of incidence angles, achieving approximately 92% absorption even at 70°. The proposed BQPMA device is ideal for solar thermal applications (such as solar thermal systems, thermal photovoltaics and thermoelectronic devices, etc.) due to the high temperature resistance of materials such as titanium nitride and its excellent spectral properties.

Design and Optical Performance Evaluation of the Three-Dimensional Solar Concentrators with Multiple Compound Parabolic Profiles and Elliptical and Rectangular Receiver Shapes

The compound parabolic concentrator (CPC) is a core technology in the field of solar concentration. Nevertheless, it only has one degree of freedom in the choice of its half-acceptance angle. In this study, extending the idea of the three-dimensional CPC, a design method for new kinds of concentrators having a CPC shape at each profile with various acceptance angles in all directions is proposed. The feature of this method is that the receiver can take any shape. Here, elliptical and rectangular receivers are assumed, and the shape and concentration performance of the concentrators with multiple CPC profiles and an elliptical receiver (MultiPro-ECPCs) and the concentrators with multiple CPC profiles and a rectangular receiver (MultiPro-RCPCs) are derived. The new designs are compared to the conventional CPC and a mirrorless flat receiver through ray-tracing simulations in terms of energy distribution on the receiver, optical efficiency, and optical concentration ratio based on axial and solar angles. The results show that in terms of optical efficiency, the MultiPro-RCPCs cover a wider range of incident angles after the 3DCPC. In terms of the optical concentration ratio, the MultiPro-ECPC with a longitudinal half-acceptance angle of 15° has the highest peak value of 19.5, followed by the MultiPro-RCPC. This study enlightens that with the concentration system settings adapted to the acceptance range of the proposed concentrators, a higher concentration can be achieved with the MultiPro-ECPC and MultiPro-RCPC compared to the conventional CPC.

Photocontrolled ultra-broadband metamaterial absorber around the terahertz regime.

In this paper, we innovatively stack multiple resonant units of photoconductive silicon to design an ultra-broadband metamaterial absorber. By manipulating the conductivity of the silicon with a pump beam, adjustments are made to the amplitude of the wide absorption spectrum spanning 6.6 THz, enabling functional switching from total reflection to near-perfect ultra-broadband absorption. By integrating vanadium dioxide as an intermediary layer, a dual-mode switchable absorber is realized, offering dual control functionalities. Temperature changes enable the absorber to switch between dual-band absorption and ultra-broadband absorption, while variations in pump beam intensity allow for further amplitude adjustments within the absorption spectrum. Impedance matching theory and near-field analysis provide the necessary physical foundation for understanding broadband absorption. Structural parameters, incident angle, and polarization angle of the incident electromagnetic waves are also studied to demonstrate the device's robustness. Our proposed absorbers not only greatly broaden the absorption bandwidth of silicon-based absorbers, but also offer versatility, polarization insensitivity, and robustness over a wide range of incidence angles. Moreover, our design ideas are useful for broadening the bandwidth and enhancing absorption, which enables wider applications in ultra-broadband terahertz absorption and promises extensive prospects.

Direct numerical simulation of the drag, lift, and torque coefficients of high aspect ratio biomass cylindrical particles

Biomass straw fuel has the advantage of low-carbon sustainability, and therefore, it has been widely used in recent years in coupled blending combustion with coal-fired utility boilers for power generation. At present, the drag force FD, the lift force FL, and the torque T evaluation model are very limited. In this study, within a wide range of Reynolds numbers (10 ≤ Re ≤ 2000) and incident angles (0° ≤ θ ≤ 90°), the computational fluid dynamics open source code OpenFOAM-body-fitted mesh method is used to carry out the direct numerical simulation of the flow characteristics of large cylindrical biomass particles with a high aspect ratio of L/D = 9:1. The results show that (1) the projected area of the cylinder begins to decrease after reaching the maximum at θ = 15°, while the change in the incident angle causes the formation of a smaller recirculation zone on the leeward side of the structure, and the effect of the pressure difference on the drag coefficient (CD) is reduced. (2) The lift coefficient (CL) displays a parabolic symmetric distribution when θ = 45°, and then the distribution becomes asymmetrical when Re > 100. The torque coefficient (CT) exhibits a similar trend. (3) Based on the simulation data and the literature data, new models for CD, CL, and CT for cylinders with L/D = 9:1, 10 ≤ Re ≤ 2000 and 0° ≤ θ ≤ 90° are obtained, and the mean square errors are 2.4 × 10−2, 1.4 × 10−2, and 6.4 × 10−2, respectively. This new model can improve the accuracy and adaptability of the universal model of gas–solid dynamics for biomass particles.

Wind Effects on Re-Entrant Wing Faces of Plus Plan-Shaped Building

The wind flow patterns and pressure distribution around a plus-shaped building are significantly influenced by re-entrant corner dimensions and building height. The present study aims to find the wind effects and study the pressure distribution with different flow patterns. The building has a plan area of approximately 300 square meters and a height of 50 m, maintaining equal lengths for the re-entrant corners. The research study is performed using the ANSYS CFX academic version. The study employs diverse visualizations, featuring the pressure coefficient (CPe), vertical and specific surface streamlines, and pressure contours. Wind incidence angles for the study are varied from 0° to 90° at an interval of 15°. Building dimensions are scaled down as per the ASCE wind tunnel test manual, and the length scale is kept at 1:100 for CFD investigation. This approach yields crucial insights into the circulation of wind patterns and the distribution of pressure across a wide range of wind incidence angles. The influence of streamlines in the bottom portion of the building model is relatively minimal. In contrast to the upper portion, the bottom exhibits robust vortices, particularly for the wind angles varying from 0° to 30°. Positive pressure is observed on the windward face, and the leeward face and side face exhibit negative pressure. This study furnishes valuable insights into flow patterns and pressure distribution across a wide range of wind incidence angles from 0° to 90° at an interval of 15°. These findings contribute significantly to a thorough comprehension of the wind flow patterns and pressure distribution around the plus-shaped building model.

Frequency‐independent Sound Absorbing Metamaterials

AbstractInspired by the concept of frequency‐independent antennas in the electromagnetics, which theoretically exhibit a frequency‐independent bandwidth, this paper extends the concept to acoustics by adopting a log‐spiral and its modified shape to introduce a class of metamaterials called frequency‐independent sound‐absorbing metamaterials. Without the requirement of multiple resonators, these metamaterials achieve an ultra‐broadband absorption spanning from 550 Hz for a wide range of incident angles up to 80°, using a single unit cell structure owing to Fabry–Pérot resonance, viscous boundary losses, and thermoviscous loss of the foam. Numerical and experimental analyses are provided to demonstrate the working principles of the proposed structures. Their performance is compared with archimedean spiral, and the influence of air and porous material parameters on the performance of the metamaterial is discussed in detail for normal and oblique incident waves. The underlying principle is discussed in detail, highlighting the distinctions between the proposed structures and state‐of‐the‐art broadband metamaterials. The proposed structures represent a pioneering advancement in the field by offering compact and ultra‐broadband sound‐absorbing metamaterials that are suitable for practical applications.

Vanadium dioxide-based ultra-broadband metamaterial absorber for terahertz waves

An ultra-broadband, polarization-insensitive terahertz metamaterial absorber based on vanadium dioxide resonance structure is proposed in this paper. The device boasts advantages such as a straightforward design, broad absorption bandwidth, and substantial fractional bandwidth. Numerical simulations show that vanadium dioxide enables the absorber to achieve near-perfect absorption (with a rate greater than 95%) in its metallic state. It demonstrates an average absorption rate exceeding 96.5% across an ultra-broadband range (4.02 THz to 11.95 THz). The absorption bandwidth reaches 7.93 THz, with a relative bandwidth of 99.3%, representing a notable improvement over comparable absorbers. Furthermore, due to its highly symmetric pattern design, the device exhibits excellent polarization insensitivity and maintains stability even within a wide incident angle range. The operational mechanism of the absorber is elucidated through the application of impedance matching theory and near-field distribution analysis. The designed absorber structure makes vanadium dioxide materials more widely applicable in terahertz and ultra-broadband absorber devices, holding promising application prospects in fields such as security imaging, energy harvesting, and so on.

Multi-scale anisotropy in NE China: Evidence for localized mantle upwelling

It is commonly proposed that the subduction of the Pacific plate has been responsible for widespread Holocene intraplate volcanism across NE China and the Korean Peninsula. Yet, how this process drives volcanism and even if it plays a critical role remains a topic of vigorous debate. In this study, using seismic data from four networks across NE China and northern Democratic People's Republic of Korea (DPRK), we analyze shear wave splitting in converted P to S-waves at the Moho (Pms), S-waves from the subducted slab interface (local S), and SKS phases. The Pms phases show a relatively weak crustal anisotropy (<0.25 s), with fast polarization directions aligned sub-parallel to major tectonic features. For the local S and SKS phases, fast polarization directions show significant lateral variations. We further perform a quantitative inversion to show that the depth of the anisotropy is ∼150 km, thus driven by flow within the asthenosphere associated with Pacific subduction. However, the presence of many null SKS splitting phases, together with scattered local S anisotropy across a wide range of incidence angles suggests a localized region of vertical flow directly beneath Changbaishan volcano. Such patterns correspond well to regional upper-mantle seismic velocity structure, and suggest that a localized upwelling with a relatively deep origin drives volcanism in the Changbaishan region. Furthermore, we infer that this mantle upwelling is deflected to the SW beneath Changbaishan and spreads asymmetrically at the base of the lithosphere, possibly because of the long history of volcanism in the region.

Anisotropic honeycomb stack metamaterials of graphene for ultrawideband terahertz absorption.

Graphene aerogels have implied great potential for electromagnetic wave absorption. However, the investigation of their design for broadband absorption in the terahertz (THz) range remains insufficient. Here, we propose an anisotropic honeycomb stack metamaterial (AHSM) based on graphene to achieve ultrawideband THz absorption. The absorption mechanism is elucidated using the effective medium method, offering deeper physics insights. At low THz frequencies, the impedance matching from the air to the AHSM can be improved by reducing the chemical potential of graphene for high absorption. There is a suppression of absorption at the intermediate frequencies due to constructive interference, which can be avoided by shortening the sizes of honeycomb edges. With the aim to elevate absorption at high frequencies, one can increase the stack layer number to enhance multiple reflections and destructive interference within the metastructure. Based on the above principles, we design an AHSM that achieves a broadband absorbance of over 90 % from 1 THz to 10 THz. This absorption can tolerate a wide range of incident angles for both TE and TM wave excitations. Our research will provide a theoretical guide to future experimental exploration of graphene aerogels for THz metamaterial absorber applications.

Ultra-broadband solar energy absorber based on Ti and TiN from visible to mid-infrared

In this work, a metamaterial absorber based on titanium (Ti) and titanium nitride (TiN) is proposed. Theoretical calculation by finite-difference time-domain (FDTD) method shows that the absorber could achieve a bandwidth of 3720 nm in the spectrum range of 280 nm − 4000 nm. The average absorptivity of the absorber can reach 96.9%. The physical mechanism of the absorber is analyzed from the impedance matching theory and electromagnetic field distribution. Furthermore, simulation results illustrate that the absorber is polarization-insensitive and allows a wide range of incident angles. The absorption spectrum is basically consistent with that of air mass 1.5 (AM1.5). And it can achieve high thermal conversion efficiencies over a wide range of temperatures. These results indicate that the absorber has a good development prospect in solar energy collection.