Wide Incident Angle Research Articles

Switchable and tunable terahertz metamaterial absorber based on graphene and VO2 resonator with ultra-broadband and multi-band absorption

Vanadium dioxide (VO2) and graphene have been combined to create new design possibilities for high-performance metamaterial devices, which offer an alluring foundation for electromagnetic absorption applications. The structure under investigation, which is based on patterned graphene-vanadium dioxide and metamaterial absorbers, is capable of achieving both broadband and multi-band absorption. Also, the offered structure is tunable and switchable in the proposed wide-band range. We theoretically suggest a switchable and tunable graphene and VO2 resonator-based Broadband and Multi-band terahertz (THz) metamaterial absorber. The proposed absorber can achieve more than 99% absorption while operating in multi-band absorption mode at 2.26, 6.68, and 7.71 THz. Moreover, it can achieve absorption of more than 90% when operating in broadband mode, which has a bandwidth of 8.74 THz within the frequency range of 2.76–11.5 THz. In the 3.12–11.01 THz frequency band, this absorption percentage is 98%. The absorption spectra can be tailored by altering the chemical potential of the graphene layer and the conductivity of VO2. The suggested absorber also has a wide incidence angle insensitivity. As a result, the suggested absorber can be considered a good choice for various THz frequency range applications.

Optimization of a Ge2Sb2Te5-Based Electrically Tunable Phase-Change Thermal Emitter for Dynamic Thermal Camouflage.

Controlling infrared thermal radiations can significantly improve the environmental adaptability of targets and has attracted increasing attention in the field of thermal camouflage. Thermal emitters based on Ge2Sb2Te5 (GST) can flexibly change their radiation energy by controlling the reversible phase transition of GST, which possesses fast switching speed and low power consumption. However, the feasibility of the dynamic regulation of GST emitters lacks experimental and simulation verification. In this paper, we propose an electrically tunable thermal emitter consisting of a metal-insulator-metal plasmonic metasurface based on GST. Both optical and thermal simulations are conducted to optimize the structural parameters of the GST emitter. The results indicate that this emitter possesses large emissivity tunability, wide incident angle, polarization insensitivity, phase-transition feasibility, and dynamic thermal camouflage capability. Therefore, this work proposes a reliable optimization method to design viable GST-based thermal emitters. Moreover, it provides theoretical support for the practical application of phase-change materials in dynamic infrared thermal camouflage technology.

Ultra-broadband and wide-angle plasmonic absorber based on all-dielectric gallium arsenide pyramid nanostructure for full solar radiation spectrum range

Broadband and plasmonic absorbers (PAs) are key components for numerous applications in the fields of energy harvesting, thermal photonics, optoelectronic devices, and enhanced spectroscopy. In this work, we propose a simple and cost-effective metamaterial plasmonic absorber (MPA) design based on an all-dielectric GaAs pyramid nanostructure for the full solar radiation spectrum range. Simulation results demonstrate that the absorbance of the proposed MPA exceeds 85% in the full solar spectrum range, and it exceeds 90% from 0.25 μm to 3.1 μm with a relative bandwidth of approximately 170.74%, with an absorption peak up to 99% at resonance wavelength. The high absorption is attributed to the excitation of wave-guide mode, surface plasmon polariton (SPP) resonance modes combined with coupling effect and intrinsic lossy of dielectric GaAs and metallic tungsten (W). Furthermore, the proposed MPA is polarization-independent and wide incident angles for both transverse electric (TE) and transverse magnetic (TM) modes. Additionally, the MPA can tolerate certain structural parameter errors during practical fabrication applications. Furthermore, the photophysical properties of this MPA for solar cell applications are investigated, revealing the achievement of high short-circuit current density. Our proposed design is highly promising for applications in solar energy harvesting, thermoelectric, and thermal emitter applications.

Enhancing 5G propagation into vehicles and buildings using optically transparent and polarisation insensitive metasurfaces over wide-incidence angles

This article introduces two transmissive metasurfaces applied to normal windows, aiming to improve the 5G outdoor-to-indoor (O2I) coverage. These windows can be utilized in various settings, such as vehicles or buildings. The proposed unit cells, designed to be wide-incident angle and polarization insensitive, are implemented in both single-glazing and double-glazing glasses, arranged in a periodic structure to form the transmission surfaces. Both metasurfaces maintain optical transparency by incorporating Indium Tin Oxide (ITO) as the conductive element in each unit cell. These engineered transmission surfaces enhance the 5G signal indoor coverage at the 3.5 GHz band across a broad range of incident angles. While multi-layer structures typically exhibit heightened sensitivity to the angle of incidence, the proposed two-layered transmissive surfaces demonstrate substantial angular stability, reaching up to 65 and 75 degrees for double- and single-glazed glass, respectively. To achieve this wide and stable angular response, evolutionary optimization techniques were employed to fine-tune the proposed unit cells. Both designs exhibit a high transmission coefficient across the operating frequency for a variety of incident angles, surpassing those reported in the existing literature. Experimental evaluations of the fabricated prototypes indicate that both metasurfaces hold significant potential for enhancing signal propagation into buildings and vehicles.

An acceleration method employing sparse sensing matrix for fast analysis of the wide-angle electromagnetic problems based on compressive sensing

Abstract The electromagnetic scattering problem over a wide incident angle can be rapidly solved by introducing the compressive sensing theory into the method of moments, whose main computational complexity is comprised of two parts: a few calculations of matrix equations and the recovery of original induced currents. To further improve the method, a novel construction scheme of measurement matrix is proposed in this paper. With the help of the measurement matrix, one can obtain a sparse sensing matrix, and consequently the computational cost for recovery can be reduced by at least half. The scheme is described in detail, and the analysis of computational complexity and numerical experiments are provided to demonstrate the effectiveness.

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.

Topological Flexible 3D Microwave Perfect Absorber

AbstractIn this work, an approach to achieve perfect absorption and microwave absorption enhancement is proposed by combining traditional wave‐absorbing materials with metamaterials. Specifically, an absorbing basement membrane (BM) is integrated with 3D woodpile (WP) photonic crystals, both of which are composed of flexible silicon materials as matrix and carbonyl iron powder (CIP) as absorbent. Direct ink writing (DIW) is used to fabricate the 3D photonic crystals with either single or coupled concentrations of absorbent. Experiments show that the 10‐dB bandwidth of the integrated absorber can reach 10 GHz with a wide elevation angle of incidence. The absorber composed of BM and WP with coupled concentrations of absorbent leads to perfect absorption with a maximum absorption of over 100 dB at an elevation angle of 30°. By changing the concentration of absorbent in BM, the frequency of perfect absorption can be engineered in the range of 6–16 GHz. Such absorption singularity reveals an experimentally observable topological charge that confirms the robustness of the singularity of perfect absorption. The topologically protected flexible 3D microwave absorber offers exceptional performance for versatile scenarios and enriches the studies on topology arising from scattering singularity.

Dual-band polarization insensitive graphene-based reconfigurable absorber in a terahertz regime.

In this paper, a dual-band graphene-based absorber is structured and investigated in a terahertz frequency domain. The designed absorber contains a plus-shaped graphene sheet, as well as four dual concentric graphene-based rings. It is designed according to the SiO2-based substrate. A dual concentric graphene ring provides more than 90% absorption from 2.36-3.24THz, while a plus-shaped graphene sheet is accountable for absorption from 5.0-5.32THz. The maximum absorption is found at 2.56, 3.08, and 5.16THz with 99.81%, 97.34%, and 99.74%, respectively. The variation in the chemical potential of the graphene material offers re-configurability in the dual absorption bands. Due to the four-fold symmetry of the designed structure, the absorptivity value is approximately constant for various polarizations with a wide incident angle (more than 60°). The aforementioned features of the proposed graphene-based absorber make it appropriate in a terahertz frequency regime.

Five-Band Tunable Terahertz Metamaterial Absorber Using Two Sets of Different-Sized Graphene-Based Copper-Coin-like Resonators

In this paper, a five-band metamaterial absorber with a tunable function in a terahertz band is proposed, which consists of a gold grounding layer, a polyimide dielectric layer, and a periodic patterned graphene layer. The patterned graphene layer is constructed from two sets of copper-coin-shaped structures of different sizes. The designed absorber achieves absorptions of 96.4%, 99.4%, 99.8%, 98.4%, and 99.9% at 4.62 THz, 7.29 THz, 7.70 THz, 8.19 THz, and 8.93 THz, respectively, with an average absorption intensity of 98.78%. The physical mechanism of this five-band absorber was explained by the impedance matching principle and electric field distribution. The absorption performance of the five-band absorber can be effectively tuned by changing the geometry of the patterned graphene array and the thickness of the dielectric layer. Given that the resonant frequency of the absorber varies in proportion to the Fermi level, by varying the Fermi level of the graphene hypersurface, we can achieve the continuous tuning of the absorption performance over a wide frequency range. The five-band absorber has a stable absorption performance over a wide incidence angle of 0–65°, and by combining the merits of high absorption, dynamic adjustability, and a large number of absorption peaks, the given absorber could have great potential for applications in nondestructive testing, imaging, communication, sensing, and detectors.

Wavelength division multiplexing visible light communication with wide incident angle enabled by perovskite quantum dots

In this paper, a wavelength division multiplexing (WDM) visible light communication (VLC) system based on the perovskite quantum dots (QDs) with two different fluorescence wavelengths is demonstrated. This system consists of a transmitter, free space channel and a receiver. The NRZ-OOK (non-return-zero on-off-keying) signals are transmitted in this channel to achieve WDM visible light communication at a transmission distance of 60 cm, with a maximum transmission capacity of 100 Mbps in the field of view of 80°. In addition, computational temporal ghost imaging (CTGI) algorithm is used to reduce the bit error rate (BER) of the received signals and improve maximum transmission angle to 120°. The proposed WDM system paves the way for visible light communication in free space.

Ultra-broadband and wide-angle plasmonic light absorber based on all-dielectric gallium arsenide (GaAs) metasurface in visible and near-infrared region

High-performance and cost-effective broadband light absorbers are crucial for various optical applications, including solar energy harvesting, thermophotovoltaic systems, and integrated photoelectric devices. In this study, an ultra-broadband and wide-angle plasmonic light absorber (PLA) based on an all-dielectric gallium arsenide (GaAs) metasurface in the visible and near-infrared regions is proposed and numerically investigated. The unit-cell of the proposed PLA is consisted of all-dielectric GaAs cylinder resonator (CR) adhered on an ultra-thin GaAs spacer layer backed with a tungsten (W) substrate. Numerical simulation results show that the absorption peak of the proposed PLA is up to 99.85 %, 99.9 % and 99.98 % at 0.452 μm, 0.712 μm and 1.596 μm, respectively. In addition, the ultra-broadband strong absorption of over 90 % also can be achieved from 0.39 μm to 2.09 μm with a relative bandwidth of 137.1 %. The ultrabroadband strong absorption of the proposed PLA is further demonstrated by the equivalent circuit model (ECM), which displays excellent agreement with the results of the full-wave numerical simulation performed using the finite element method (FEM). The simulated electromagnetic (EM) field distributions reveal that the ultra-broadband strong absorption of the designed PLA is mainly from the excited propagating surface plasmon (PSP), located surface plasmon (LSP), cavity resonance modes combined with coupling effect between adjacent CRs. The proposed PLA is also polarization-insensitive and wide incident angles for both transverse electric (TE) and transverse magnetic (TM) modes. The absorption properties of the designed PLA can be adjusted by varying geometric parameters of the unit-cell. The PLA achieves a remarkable short-circuit current density of over 64.75569 mA/cm2 and exhibits nearly perfect solar energy trapping due to the effective coupling of multiple resonances, resulting in enhanced photovoltaic performance. Thus, the proposed PLA can be potential application in the photodetectors, thermal imaging, and solar energy harvesting devices.

A metal-free polarization-insensitive ultrathin and ultra-wideband terahertz absorber with wide incident angle

In this work, a novel metal-free absorber structure design is proposed with an ultra-thin geometry and ultrawideband response in the lower terahertz region. The unit cell of the proposed absorber is a simple tri-layered structure with silicon as a substrate sandwiched between the silicon resonator layer at the top and a graphite layer at the bottom. The absorber displays a near-perfect absorption for a wider range of frequency spectrum between 2.7 THz to 9.2 THz covering a bandwidth of 6.5 THz and a relative bandwidth of 102%. The proposed absorber exhibits a multi-band response in close proximity when coupled by a mode-hybridization technique resulting in an ultra-wide-band response. The interaction between the resonating modes of the silicon disc resonator layer and multiple modes in the dielectric cavity is responsible for this ultra-wideband absorption response. The performance of the proposed absorber is analyzed and optimized for various geometrical parameters. Also, the frequency response tunability behavior is reported in contrast to certain geometrical parameter variations. The simulation results are validated using the equivalent circuit modeling technique. The absorber response is polarisation insensitive owing to its symmetrical structure and is angularly stable for a wider angle of incidence up to 70 degrees. The resonator-free bi-layered structure of the proposed absorber can be utilized as a narrowband narrow-scale sensor. The sensing performance of the proposed absorber is analysed across a wide frequency range which makes it a suitable candidate for bio-sensing applications.

Ultra-broadband transmission absorption of the all-dielectric water-based metamaterial

In this paper, a structurally simple transmissive all-dielectric metamaterial absorber is designed, fabricated and measured. The unit cell consists of photosensitive resin and water, which the water layers composed of rectangle loop, rectangular block and traffic circle. It achieves over 90% absorption of electromagnetic waves in the frequency range between 11.0 and 27.3 GHz. Meanwhile, three absorption peaks are located at 12.5, 18.4, and 26.0 GHz, which the absorptive are 97.4%, 98.3%, and 98.5%, respectively. In addition, the characteristics of the designed metamaterial reveal wide-incident angle absorption and polarization-insensitive properties. The experimental sample is measured by using the free-space method, and the experimental results are verified to be in well agreement with the simulation data. Observing the distribution of electric and magnetic fields at different resonance frequencies, the physical mechanism of the absorption is attributed to strong magnetic resonance. Therefore, the proposed metamaterial has potential for widespread use in the fields of civilian and military.

Dynamically tunable multifunctional terahertz absorber based on hybrid vanadium dioxide and graphene metamaterials.

In this work, in pursuit of a multifunctional device with a simple structure, high absorption rate, and excellent bandwidth, a tunable broadband terahertz (THz) absorber based on vanadium dioxide (V O 2) and graphene is proposed. Due to the phase transition of V O 2 and the electrically tunable properties of graphene, the structure realizes single broadband and dual-band absorption characteristics. When graphene is in the insulating state (E f=0e V) and V O 2 is in the metallic state, the developed system has more than 90% absorption and a wide absorption band from 1.36 to 5.48THz. By adjusting the V O 2 conductivity, the bandwidth absorption can be dynamically varied from 23% to more than 90%, which makes it a perfect broadband absorber. When graphene is in the metallic state (E f=1e V), V O 2 is in the insulating state, and the designed device behaves as a tunable and perfect dual-band absorber, where the absorptivity of the dual-band spectrum can be continuously adjusted by varying the Fermi energy level of graphene. In addition, both the broad absorption spectrum and the dual-band absorption spectrum maintain strong polarization-independent properties and operate well over a wide incidence angle, and the designed system may provide new avenues for the development of terahertz and other frequency-domain tunable devices.

A wide-angle incidence, broadband rectified metasurface for radar frequency band EM energy harvesting and wireless power transfer

Traditional power transmission networks with complex physical wiring have struggled to meet the power needs of emerging portable and decentralized devices. Recently, rectified metasurfaces for electromagnetic (EM) energy harvesting have attracted great attention from researchers due to their customizability and application flexibility. However, many previous structures mainly focused on harvesting efficiency and ignored harvesting bandwidth. Here, we propose a broadband rectified metasurface structure for radar frequency band EM energy harvesting and wireless power transfer. The cell structure consists of a centrosymmetric hyperbolic ring (CHR) structure that can exhibit an EM wave absorptivity of greater than 80 % at wide incidence angles. The AC energy of each cell is exported through metal vias, and the energy is pooled through the feed networks. Finally, the rectification circuit is designed to complete the AC-to-DC conversion and the DC energy harvesting. Broadband EM energy harvester can be achieved when the incidence angle is 0° to 60°. The half-power bandwidth (HPBW) can be up to 55 % with a normal incidence. To verify the rectified metasurface function, a prototype with 6 × 6 metasurface array is fabricated and tested. When the rectified metasurface operates at 6.1 GHz, RF-to-DC energy harvesting efficiency of greater than 35 % can be achieved in the incidence angle range of 0° to 60° and maximum efficiency is 57 %. It makes it possible to self-power sensor network nodes working in the vicinity of communication base stations or radar base stations.

Design and investigation on polarization insensitive dual band graphene based tunable absorber for THz applications

In this communication, a metal-free graphene-based Absorber is designed and examined in the THz regime. The graphene layer is coated over the SiO2 substrate, while the silver layer is used on the other side of the substrate. The designed absorber works in two frequency bands i.e. 2.2–2.91 THz (11–14 μm) and 4.85–5.83 THz (51–62 μm), having more than 90% absorption. Peak absorption is obtained 2.55 THz and 5.05 THz with 97.78% and 99.71% absorption. With the help of circular sectoring, the designed absorber shows wide polarization and wide incident angle (up to 60°) insensitivity for both TE and TM modes. Change in chemical potential of graphene provides the tunable characteristics in the designed absorber. All these outcomes confirm that the designed metal-free graphene-based metamaterial absorber works efficiently in THz regime and can be used in different applications such as, sensing and imaging.

Harmonizing terahertz waves: unleashing the versatility of a multi-mode metamaterial absorber

An absorber composed of stacked graphene rings, vanadium dioxide (VO2) patches and metal reflectors separated by dielectric layers is proposed and numerically simulated using the finite element method (FEM). This absorber exhibits a multitude of absorption modes, including ultra-wideband, double-narrow band, and switchable multi-band perfect absorption. The physical mechanism behind perfect absorption is thoroughly investigated through the utilization of impedance matching theory, multiple interference theory (MIT), and coupled mode theory (CMT). Detailed analysis of the electric field further reveals the underlying physical phenomena. Notably, the absorber showcases the ability to dynamically adjust absorption performance by modifying the conductivity of (VO2) and the Fermi level of graphene, while maintaining consistently high absorption levels. The proposed multi-mode absorber maintains good performance within wide incidence angle. Based on the above advantages, this absorber holds promising potential for advancements in energy harvesting and sensor technologies.

Electromagnetic and microwave absorption properties of flexible fabric coatings containing Ag decorated spherical graphene powders with FSS incorporation

Flexible textile microwave absorbing materials own the advantages of flexibility, foldability and cuttability, have become a research hotspot in electromagnetic stealth technology. In this study, a microwave absorbing coating was prepared on a fabric substrate using Ag decorated spherical graphene (SG) as the absorbent. XRD results demonstrated the metallic Ag ion was successfully reduced. TEM photographs showed the silver nanoparticles were stuck to the spherical graphene. The complex permittivity was enhanced after compositing with Ag particles. Along with the increase of silver content from 0 to 20 wt%, the complex permittivity rises from 3.29–0.42j to 6.93–2.22j owing to its higher electric conductivity and stronger relaxation effect. Meanwhile, the flexible coating containing Ag decorated spherical graphene (Ag@SG) also exhibited better microwave absorption performance. The minimum reflection loss (RLmin) for the coating with a thickness of 3.0 mm was −15 dB. And the effectively absorbing broadband (EAB) is located at 9.08–11.52 GHz. In addition, the microwave absorbing performance was greatly enhanced after being combined with periodic frequency selective surface (FSS). The EAB was obtained over the entire X-band frequency range, and the RLmin value achieved −41.72 dB at 10.25 GHz. What's more, the coating owned an EAB in a wide range incident angle of 60°–90°. This work was help to developing high performance flexible fabric coating containing Ag decorated spherical graphene with FSS incorporation.

An elliptical nanoantenna array plasmonic metasurface for efficient solar energy harvesting.

Plasmonic metasurfaces with subwavelength nanoantenna arrays have attracted much attention for their ability to control and manage optical properties. Solar absorbers are potential candidates for effectively converting photons into heat and electricity. This study introduces a novel ultrathin metasurface solar absorber employing elliptical-shaped nanoantenna arrays. We theoretically and numerically demonstrate a near-perfect broadband absorber with over 90% absorption efficiency in a wide range of wavelengths of 300-2500 nm, using finite element (FEM) and finite-difference time-domain (FDTD) methods. The proposed nanostructure configuration enhances light absorption by exciting localized surface plasmon resonances (LSPRs) between elliptical-shaped nanoantenna gaps at many wavelengths, maintaining stability at wide incident angles and insensitivity to light polarization. Compared to other state-of-the-art absorbers with a thickness of less than 300 nm, the designed nanostructure with 260 nm thickness achieves over 90% optical absorption across a broad range of wavelengths of 300-1116 nm in air (or vacuum) environments and performs effectively under water conditions for solar energy harvesting in a range of wavelengths of 300-1436 nm, and therefore can serve as a solar evaporator. Combining refractory plasmonic titanium nitride (TiN) and semiconductor gallium nitride (GaN) nanostructures holds great potential for efficient optoelectronic and photocatalytic applications, especially in harsh and high-temperature environments like thermophotovoltaic systems. The TiN-based metasurface absorber, with its ultrathin nanostructure and suitable spectral absorption in ultraviolet-visible-infrared spectra, offers scalability and cost-effectiveness. The findings in this work will deepen our understanding of LSPRs and pave a novel path for efficient solar energy conversion.

Ultrawideband Polarization Conversion Metasurface with Wide Incidence Angle Suitable to Reduce RCS of Planar and Curved Surfaces

Ultrawideband Polarization Conversion Metasurface with Wide Incidence Angle Suitable to Reduce RCS of Planar and Curved Surfaces