Wide Range Of Incident Angles Research Articles

Rapid design of hybrid mechanism metasurface with random coding for terahertz dual-band RCS reduction.

In this paper, a hybrid mechanism metasurface (HMM) employing 1-bit random coding is proposed to achieve polarization-insensitive and dual-wideband monostatic/bistatic radar cross section (RCS) reduction under a wide range of incident angles. The anisotropic unit cell is designed by the combination of the multi-objective particle swarm optimization (MOPSO) algorithm and Python-CST joint simulation, which facilitates the rapid acquisition of the desired unit cell with excellent dual-band absorption conversion capability. The unit cell and its mirrored version are used to represent the units "0" and "1", respectively. In addition, the array distribution with random coding of the units "0" and "1" is optimized under different incident angles, polarizations and frequencies, which enables better diffusion-like scattering. Simulation results demonstrate that the proposed coding HMM can effectively reduce the monostatic/bistatic RCS by over 10 dB within the dual-band frequency ranges of 2.07-3.02 THz and 3.78-4.71 THz. Furthermore, the specular and bistatic RCS reduction performances remain stable at oblique incident angles up to 45° for both TE and TM polarizations.

Optically transparent conformal ultra-broadband metamaterial absorber based on ITO conductive film

An optically transparent metamaterial absorber with polarization insensitivity and wide-angle absorption is proposed. The absorber, which consists of an indium tin oxide resistive film and a low-loss substrate, is optically transparent and conformal. By tuning the reflection response of the frequency-selective surface and the thickness of the spacer layer, the whole structure achieves an absorption of more than 97% in the range of 6.54–18.66 GHz. Numerical simulation results show that the absorber can still maintain an absorption rate of more than 85% in a wide range of oblique incidence angles from 0° to 60°. In addition, the intrinsic physical mechanism of the absorber is elucidated using the impedance matching theory and the distribution of surface currents. The ratio of dielectric loss and ohmic loss is also quantitatively analyzed. Finally, the reflection and transmission coefficients of the sample were measured in a microwave anechoic chamber, which showed a good agreement with the simulation results. This design uses a low-resistivity resistive film as a frequency-selective surface, which was rarely involved in previous studies, and provides a new idea for future design and application.

Ultra-broadband and polarization-insensitive terahertz metamaterial absorber based on undoped silicon

Terahertz (THz) absorbers with high absorptivity across a wide frequency band have attracted considerable interest. In this paper, we present the design of an ultra-broadband THz metamaterial absorber with high absorptivity based on a single-layer square silicon array and a silicon film backed with metallic gold. It is numerically demonstrated that the absorption exceeds 90% covering the frequency range of 0.62–8.75 THz (with an effective absorption bandwidth of 8.13 THz) when the conductivity of semiconductor (undoped silicon) is 600 S/m. Diffraction and impedance-matched induce strong absorption and the corresponding broad bandwidth. The proposed ultra-broadband THz absorber is polarization-insensitive at normal incidence. It can operate over a rather wide range of incidence angle up to 60 degrees for transverse magnetic (TM) terahertz wave. This work provides a way to achieving ultra-broadband terahertz absorption, which is very useful in the development of terahertz imaging, high sensitivity sensors and detection.

Wave velocity measurement in the through-thickness direction of the anisotropic material plate with ultrasonic polar scan

Ultrasonic polar scan (UPS) records the amplitude in transmission for a wide range of incidence angles, providing a UPS image with characteristic contours reflecting the acoustic parameters of the material. This study focused on a newly developed wave velocity measurement of the DD6 single-crystal material plate by analyzing the UPS image to realize the ultrasonic thickness measurement of the DD6 single-crystal material plate accordingly. Firstly, considering transducer contour size compensation, the UPS images are obtained by numerical simulation on the DD6 single-crystal material plates with different crystal orientations. Secondly, the fitting equations are obtained by analyzing the UPS images to calculate wave velocity in the thickness direction. Finally, a 5-joint UPS scanner is designed to validate the accuracy of wave velocity simulation results experimentally, and it indicates a good agreement with conventional ultrasonic velocity measurement. The measurement results demonstrated that the UPS method could evaluate the wave velocity in the through-thickness direction of the DD6 single-crystal material plate with high precision.

High Transmission in layered composite with graphene and hyperbolic metamaterials

The transmission characteristics are presented for the sandwich composite structure containing hyperbolic metamaterials (HMMs) and graphene. The numerical results show that the high transmittance (larger than 0.9) within a ultra wide incident angle range (0 ∼ 68.1°, 0 ∼ 75.7°) can be obtained by optimizing the chemical potential (μc=0.5,0.6eV) for both s-polarized and p-polarized waves under the certain incident frequency (30THz). Meanwhile, to acquire the transmission enhancement in a wide incident angle range, the thickness of HMMs and the number of graphene layers should also be chosen appropriately. Additionally, the broadband and wide-angle of the transmission enhancement for s-polarized and p-polarized waves can be acquired by adjusting the incident angles and frequencies, which has the potential application for bandpass and high-pass filters.

Thermal and Optical Analyses of a Hybrid Solar Photovoltaic/Thermal (PV/T) Collector with Asymmetric Reflector: Numerical Modeling and Validation with Experimental Results

This study presents a combined thermal and optical, three-dimensional analysis of an asymmetric compound parabolic collector (ACPC) with an integrated hybrid photovoltaic/thermal (PV/T) receiver with the aim of establishing a sustainable approach in two ways: firstly, by determining the optimal tilt angle for operations, and secondly, by introducing an innovative simulation method which reduces computational cost while calculating thermal performance. Initially the Incident Angle Modifier (IAM) was calculated for a wide range of incident angles, and the ray-tracing results were verified using three different simulation tools (Tonatiuh, COMSOL, and SolidWorks) with mean deviations being lower than 4%. The optimal tilt angle of the collector was determined for seven months of the year by conducting a detailed ray-tracing analysis for the mean day of each month considering whole day operation. In the thermal analysis part, the authors introduced novel numerical modeling for numerical simulations. This modeling method, designed with sustainability in mind, enables lighter computational domains for the air gap while achieving accurate numerical results. The approach was established using two distinct simulation tools: COMSOL and SolidWorks. From the optical analysis, it was found that in all months examined there is a four-hour time range around solar noon in which the optimum tilt angle remains constant at a value of 30°. The numerical models constructed for the thermal analysis were verified with each other (6.15% mean deviation) and validated through experimental results taken from the literature regarding the examined collector (<6% mean deviation). In addition, the two simulation tools exhibited a deviation of around 6% between each other. Finally, the thermal performance of the collector was investigated for the mean day of September at solar noon by adopting the optimal tilt angle for that month according to the optical analysis, considering inlet temperatures from 20 °C up to 80 °C.

An ultra-thin absorber in microwave range: 50 GHz band-width, absorption over 80 %

An ultra-thin microwave absorber with a 34 GHz bandwidth more than 90% absorption in the frequency range of 33.5 GHz - 67.5 GHz, and 50 GHz bandwidth, more than 80 % absorption is proposed. The functionality of the device was analyzed using an equivalent circuit model (ECM) by exploiting the impedance matching concept in the transmission line theory. By changing the chemical potential of the graphene, following manipulating characteristics of the graphene surface conductivity, can achieve several absorption responses in wideband range frequencies. Additionally, the proposed absorption is stable in a wide range of incident angles. These advantages make the proposed absorption attractive for several applications such as optical sensors and detectors.

An X-shaped triple split ring resonator-based metamaterial perfect absorber with quad-band incident and polarization angle insensitivity for C, X, and Ku band applications

In this study, an X-shaped triple split ring resonator-based metamaterial perfect absorber with quad-band incident and polarization angle insensitivity for C, X, and Ku band applications has been proposed. The metamaterial perfect absorber consists of a prearrangement with metallic patch, metal ground plane and a middle Rogers RT5870 dielectric substrate with 1.575 mm thickness. The numerical methods implemented using finite integration technology (FIT) demonstrate that the metamaterial perfect absorber (MPA) exhibits an absorption capacity exceeding 99% in all frequency bands, precisely matching the impedance of free space. These frequency bands are located at 4.37 GHz, 7.08 GHz, 9.38 GHz, and 10.65 GHz with the absorption levels of approximately 99.8%, 99.9%, 99.9%, and 99.9% respectively for both transverse electric (TE) and transverse magnetic (TM) modes. Additionally, the MPA demonstrates quality factors of 21.92, 7.2, 13.42, and 15 for both modes. The investigation of the electromagnetic (EM) field with surface current distribution has provided information about the mechanism of such quad-band absorbance. The numerical analysis reveals that the metamaterial perfect absorber (MPA) can achieve remarkable absorbance outcomes across a wide range of incident angles, up to 800s, while also being insensitive to polarization up to 300 for both modes. A comparable circuit analysis demonstrates the superior performance of the MPA, indicating its potential for excellent functionality within the advanced design system (ADS) software. Furthermore, the suggested structures, modelled using the high-frequency structure simulator (HFSS), closely align with the maximum absorbance observed at each resonance peak in the computer simulator technology (CST) simulator results. The proposed MPA possesses high-performance characteristics that enable its application in various domains such as satellite communications, raw satellite feeds, and radar systems.

Vanadium dioxide based bifunctional metasurface for broadband absorption and cross-polarization conversion in THz range

Achieving switchable and reconfigurable functions based on a single metasurface has become one of the research hotspots among scholars in recent years. Based on the excellent phase transition properties of vanadium dioxide (VO2), a THz bifunctional metasurface device (BMD) is designed that can realize broadband absorption and broadband cross-polarization conversion. When VO2 behaves in the metallic state, the BMD acts as a broadband metamaterial absorber (MA), and its absorbance is larger than 90 % in the range of 1.97–4.63 THz with a relative bandwidth (RB) of 80.61 %. In addition, the MA is insensitive to the polarization angle and can maintain high absorbance over a wide range of incident angles for both y- and x-polarized THz waves. When VO2 behaves in the insulated state, the BMD will act as a broadband linear polarization converter (PC). It has a polarization conversion ratio (PCR) of greater than 90 % with a RB of 92.31 % in the frequency range of 1.54–4.18 THz. The excellent absorption and polarization conversion characteristics of the proposed BMD make it possess potential values in different THz areas with sub-wavelength device scales.

Diffraction Neural Network for Multi‐Source Information of Arrival Sensing

AbstractAll‐optical diffractive neural networks (DNNs) based on passive structures have been shown to perform complicated functions by optical acceleration, thus reforming in situ applications requiring digital computing processing. Its stable, compact, and passive features make it suitable for many conventional applications in complex electromagnetic environments. Here, a DNN based on a multilayer passive metasurface array is presented to estimate the information of arrival (IOA) over a wide range of frequency bands and incident angles. Unlike traditional methods, multiple correlated or coherent electromagnetic signals interfere with each other, resulting in difficult separation which is a fundamental obstacle in multi‐source IOA detection. Here, the proposed diffraction system creates separate virtual channels to isolate and process different incoming waves. It recognizes and classifies the beams, focuses them on the output plane divided by the arrival angle and working frequency, and displays the IOA and number of sources in real time at the speed of light. Furthermore, the massive‐parallelism and data‐post‐processing‐free strategy allows for broader applications in harsh environments, such as seismic detection and underwater circumstances.

A triple‐band frequency selective surface for 5G millimeter‐wave communications

AbstractIn this letter, a new triple‐band millimeter‐wave frequency selective surface (FSS) is presented. The proposed FSS structure consists of three slot‐based resonance mechanisms on the single layer of Rogers Diclad880 dielectric. It shows a high compactness with a unit cell surface area of 5.4 × 5.4 mm2. The proposed FSS exhibits a triple‐band bandpass filter characteristics for the allocated 5G millimeter‐wave frequency regions with sufficient bandwidths corresponding to 23.5–24.6, 27–28.5, and 37.5–41 GHz, respectively. Simulation outcomes reveal that the FSS poses polarization insensitive behavior, and maintains high transmission skill for a wide range of incident angles up to 40°. A 40 × 40 FSS panel is fabricated, and the experimental verification is carried out. The outcomes prove that the proposed FSS is suitable for multiband 5G millimeter‐wave filtering applications.

Multifunctional and switchable metamaterial for terahertz polarization modulation in the reflection mode.

In this paper, a broadband multi-layered active metamaterial design is investigated, which can achieve a high polarization conversion efficiency over a wide band of frequencies in the terahertz regime. The design can be switched to an efficient metamaterial absorber using the phase transition property of vanadium dioxide (V O 2). Additionally, the designed structure can convert the linear polarization of the incoming wavefronts to its cross-polarization and linear polarization to circular polarization in the reflection mode. The broadband characteristic is achieved due to the strong anisotropic behavior of the metasurface. The structure is robust to a wide range of incident angles as well. The proposed switchable multifunctional design can contribute to the development of active plasmonic polarization devices and metamaterial absorbers.

Compact single-band and multiband terahertz plasmonic absorbers using hybrid graphene-metal resonators with switching and modulation capability

THz thermal and single-pixel imaging need compact tunable absorbers with features like high modulation depth and multi bandwidth in the case of multispectral imaging. Here we are going to propose compact switchable absorbers based on a hybrid graphene-metal resonator that is designed through a genetic algorithm and numerical method. The proposed absorbers have a thickness as small as λmax/120 and can acquire perfect modulation depth. The special shape of the proposed resonator helps to gather 6 resonance bands in a very compact unit-cell. A theoretical formula is proposed to predict the resonance frequencies of the absorber based on Fabry-Perot concepts and graphene-metal plasmons. The role of local and propagating surface plasmon and their hybridization in the absorption mechanism is clarified. The modulation capabilities and tunability of the absorbers are analyzed. The absorbers work very well in a wide range of incident angles. Based on numerical analysis the hybrid resonators were capable of suppressing parasitic resonance modes making the designed absorbers ideal for emitter applications. The proposed absorbers have applications in THz thermal imaging, modulators, and emitters.

Programable structural color generation by phase-change-medium-enabled pixelated hyperbolic metamaterial

Phase-change-medium-enabled hyperbolic metamaterial structures were investigated for generating a wide range of colors by applying external electrical pulses to the embedded graphene-based micro-heater system. The stochastic particle swarm optimization method was implemented to accelerate the design process, followed by the transfer-matrix method to observe the effects of incidence polarization and angle. The results revealed a wide range of colors attainable via the constitutive properties of the designed pixels, which remains insensitive to the incidence polarization, and under a wide range of incidence angle. The investigated programable structure can find potential use in reflective displays and holographic devices for security systems.

Perfect absorber with high sensitivity based on hexagonal star graphene surface

Graphene absorbers have attracted wide attention due to their high absorption rate and narrow bandwidth in the terahertz field. In this study, we proposed a dual-band terahertz tunable perfect absorber designed with a hexagonal star ring and a circular ring monolayer graphene metasurface, which exhibits tunable, polarization-insensitive, and high sensitivity characteristics. The graphene absorber was simulated by using the finite element method and validated by using the impedance matching method. Simulation results show that this structure has two perfect absorption peaks at 4.8 and 8.04 THz. The variation of the dielectric layer material was analyzed, and the parameters of the structure and the intrinsic graphene parameters were adjusted to control the absorption efficiency of the dual-band absorption peaks. Simulation results show that the high symmetry of the structure brings incident and polarization-insensitive properties, maintaining absorption higher than 98% over a wide range of incident and azimuth angles. The sensing performance of the device was investigated by varying the ambient refractive index, and the highest sensitivity parameter S of the absorber reached to 2.375 THz/RIU. These results demonstrate that this high-sensitivity sensor has great potential for terahertz imaging and microstructure detection.

Convolution Operation on Pancharatnam-Berry Coding Metasurfaces in Visible Band

Coding metasurfaces bring photonic design into a new era for their convenience in manipulating electromagnetic waves in a programmable way. Different coding patterns can be further convoluted to give rise to nontrivial effects such as multiple beam steering, simultaneous control of surface and space waves, and the integration of multiple functionalities. However, previous experimental works have been limited to low frequencies. Extending convoluted coding metasurfaces to optical frequency can significantly reduce the size of structures, and therefore facilitate their applications in optical circuits. Here, we experimentally demonstrate a Pancharatnam-Berry metasurface designed by convoluting two distinct coding patterns. Such a metasurface exhibits integrated functionalities: the optical spin Hall effect and spin to orbital angular momentum conversion, which are robust over a broad visible band and a wide range of incident angles. Our work represents an important step towards multifunctional coding metasurfaces at optical frequency based on convolution operation.

3D printed propeller-like metamaterial for wide-angle and broadband microwave absorption

• A broadband wide-angle 3D meta-absorber was proposed based on the reciprocity theory. • The absorber was designed by characteristic mode analysis and dispersion engineering. • The absorber was fabricated of CB-PP composite by 3D printing technology. • Broadband and wide-angle absorption of the absorber was verified experimentally. It is a challenge to design an absorber which can simultaneously satisfy comprehensive demands of broad absorption band, wide incident angle range, and low-profile. In this work, we designed a metamaterial absorber (MMA) based on the antenna reciprocity theory to achieve the above goals. Firstly, a three-dimensional (3D) propeller-like structure with reference to a typical magneto-electric dipole (MED) antenna was proposed and analyzed by the characteristic mode (CM) theory to realize near-omnidirectional radiation. Then, the radiation-absorption conversion was realized by introducing lossy materials into this structure, and the absorption performance was further improved by optimizing the dispersion feature of the lossy materials. Finally, the propeller-like metamaterial absorber with a thickness of 0.113 λ L was manufactured efficiently and integrally through 3D printing technology. Simulation results showed that the proposed absorber can achieve broadband absorption with the efficiency more than 90% in the frequency band of 3.4-10 GHz. It also has excellent wide-angle absorption capacity, with Transverse Electric (TE) polarization of 0° to 50° and Transverse Magnetic (TM) polarization of 0° to 80°. With the increase of incident angle, the upper limit of absorption bandwidth can be gradually extended to 18 GHz. Moreover, the effectiveness in the range of 0° to 60° incident angle is verified by measuring the reflectivity of the 3D printed absorber.

An efficient underwater absorber using pentamode metamaterials for broadband frequency

To further design and develop broadband and efficient underwater sound absorption structure, we propose a hybrid acoustic structure composed of a rubber matrix layer, cylindrical cavity, pentamode metamaterial (PM) layer, impedance matching layer, and steel backing plate. Thanks to the special properties of the PMs, the hybrid acoustic structure obtains excellent broadband sound absorption performance (SAP) in the frequency range of 500–10000 Hz than that of the traditional cavity acoustic structure. To this end, we study the design principle of PM structure and its metal–water characteristics and analyze the sound absorption mechanism at the working frequency. In addition, the research shows that the material and geometric parameters of the structure have a significant impact on the SAP. By the optimization of structural material parameters, the average absorption coefficient at 500–10000 Hz can further increase to above 0.781. Finally, we also demonstrate that the structure still has good SAP under a wide range of oblique incidence angles. The research in this paper opens up bright perspectives for the design of underwater anechoic coatings for multifunctional applications.

Design of All‐Metal 3D Anisotropic Metamaterial for Ultrabroadband Terahertz Reflective Linear Polarization Conversion

Herein, a novel and simple design of all‐metal 3D anisotropic metamaterial (3DAMM) is proposed and investigated numerically, which can achieve a high‐efficient and wide‐angle ultrabroadband reflective linear–linear and dual‐band linear–circular polarization conversion in the terahertz (THz) region. The 3DAMM is composed of a periodic array of copper stand‐up split ring resonator (SRR) adhered on a copper film ground plane. Incident ultrabroadband linear polarization waves can be converted to its orthogonal counterpart after reflection due to the three neighboring plasmon resonances of the 3DAMM structure. The proposed design demonstrates a high conversion efficiency of over 90% within a relative bandwidth of 88.7% in the range of 0.62–1.61 THz, and its efficiency is validated for both transverse electric and transverse magnetic modes and a wide range of incident angles (0°–50°). This ultrabroadband and high‐efficient linear polarization conversion of the 3DAMM is attributed to multiple plasmon resonances within the operating frequency range. The surface current distributions on the unit cell show that the ultra‐broadband conversion behaviors result from the decomposed electric field components coupling with different resonance modes. Further simulation results suggest that the polarization conversion properties of the 3DAMM can be adjusted by changing the geometric parameters of the unit cell. The proposed 3DAMM‐based reflective linear polarization convertor can find potential applications in remote sensors, imaging systems, and reflector antennas at THz frequencies.

Strategically constructed high-reflectivity multiple-stack distributed Bragg reflectors for efficient GaN-based flip-chip mini-LEDs

Full-angle distributed Bragg reflectors (DBRs) consisting of numerous sub-DBRs with discrete central wavelengths have been developed to enhance performance of GaN-based flip-chip mini light-emitting diodes (FC mini-LEDs). However, relatively low reflectivity of full-angle DBRs at large angle incidence restricts further enhancement in performance of FC mini-LEDs. Here, we introduce a reflectivity optimization strategy for constructing high-reflectivity multiple-stack DBRs by rationally engineering the number of sub-DBRs and adjusting central wavelength distribution of sub-DBRs. Based on the reflectivity optimization strategy, we devise a Ti3O5/SiO2 quintuple-stack DBR which is composed of five sub-DBRs. Our quintuple-stack DBR maintains a high reflectivity (>97.5%) over a wide range of incident angles of light. Notably, compared with the full-angle DBR, our quintuple-stack DBR exhibits higher reflectivity at large angle incidence and thinner multilayer thickness. Furthermore, we demonstrate two types of GaN-based blue FC mini-LEDs with indium-tin oxide (ITO)/quintuple-stack DBR and ITO/full-angle DBR p-type ohmic contacts. Benefiting from superior reflection performance, blue FC mini-LED with ITO/quintuple-stack DBR achieves an enhancement of ∼5.8% in light output power at 10 mA, in comparison with blue FC mini-LED with ITO/full-angle DBR. Our work signifies an advancement towards high-reflectivity DBRs, which enables higher-performance FC mini-LEDs.