Wide Incident Angle Research Articles

Tunable perfect dual-narrowband absorber based on graphene-photonic crystal heterostructure

A dual-narrowband absorber in the terahertz (THz) band is proposed and numerically analyzed, which consists of a sheet of graphene on a photonic crystal (PC) heterostructure. We find a huge enhancement of absorption because optical Tamm states (OTSs) are excited at the interfaces of graphene-PC and PC heterostructure. The full width at half maximum (FWHM) of the absorption peak near the wavelength of 300 μm can be as narrow as 1.25 μm, and narrower FWHM can be obtained by adapting the wavelength and incident angle of the light beam. The wavelengths of absorption peaks and the absorptivities can be tuned by controlling the Fermi energy of graphene and the periods of PCs. In addition, the absorber can maintain high absorptivity over a wide incident angle range for both TE and TM polarization. The proposed optical absorber has potential applications in THz biosensors, filters, and switches.

A High‐Performance Nonlinear Metasurface for Spatial‐Wave Absorption

AbstractNonlinear absorption is a significant topic in classic nonlinear sciences. Today, a growing interest in this area is drawn by increasingly severe interferences associated with strong microwaves. Although conventional absorbers may help mitigate the harmful effects, they unavoidably cause a huge loss to small useful signals. Here, a novel nonlinear absorbing metasurface is proposed targeting spatial waves. Based on an active nonlinear mechanism, the input impedance of the metasurface is highly dependent on the wave intensity. When the waves get stronger, the impedance‐match between the meta‐surface and free space is improved, so they are preferably allowed to enter the structure and be dissipated inside. A prototype is fabricated and measured in a microwave anechoic chamber, and its intriguing characteristics such as wide incident angle scope, dual polarizations, and wide instantaneous bandwidth are demonstrated, which are in good agreement with theoretical and simulated anticipations. With an ultralow profile and a simple direct‐current‐supplying strategy, the proposal can find potential applications in various scenarios to reinforce the protection of the weak against the strong.

Ultralight and Low-Cost Structural Absorbers With Enhanced Microwave Absorption Performance Based on Sustainable Waste Biomass

As a practical microwave absorber, besides satisfying the basic broadband absorption performance, the density, cost, and process complexity are considerable factors in radar stealth and electromagnetic protection fields as well. Here, a simple and economical structural absorber (SA) is proposed to remarkably broaden the absorption bandwidth of biochar from banana peel. By optimizing the size parameters of the unit cell, the optimal SA shows more than 90&#x0025; absorption in the frequency range from 3.4 to 18 GHz by employing a lightweight porous carbon derived from sustainable biomass. The lightweight porous biochar provides the SA low-density properties with a surface density of only 0.574 <inline-formula> <tex-math notation="LaTeX">$\text{g}\cdot $ </tex-math></inline-formula>cm<inline-formula> <tex-math notation="LaTeX">$^{-2}$ </tex-math></inline-formula>. In such a SA, the greatly enhanced absorption performance mainly originates from the synergistic effect of microscopic and macroscopic scales. By optimizing the pattern design of the SA, the effective impedance is matched with free space in a wide frequency range, thus dissipating more electromagnetic waves through multiple resonances and edge scattering effects. In addition, whether transverse electric or transverse magnetic polarizations, stable broadband absorption can be maintained in a wide incidence angle. Thus, this work provides a new prospect for waste utilization of banana peel and a facile low-cost route to greatly extend the microwave absorption bandwidth.

Design of a multiband graphene-based absorber for terahertz applications using different geometric shapes

By integrating multiple graphene-based geometric shapes, a unique absorber is proposed in this research, which achieves absorption peaks at 10 resonant frequencies, with an average absorption rate of 97.23% at terahertz frequency. The proposed structure consists of circular, triangular, square, pentagonal, and hexagonal ring-shaped absorbers to achieve multiband absorption. The proposed absorber is polarization-insensitive and has a wide incidence angle tolerance. Furthermore, increasing the graphene chemical potential value increases absorption while also tuning the resonance frequency. Additionally, this proposed configuration provides ultrawideband response at first band, with an absorption bandwidth percentage of 88.52. Two graphene-based absorbers are designed: one with geometric shapes that are in contact with one another and the other with geometric shapes that are not in contact with one another. Several investigations are exhibited on these two models to find the perfect combination of absorbers. The unique feature of this research is its novel, to the best of our knowledge, design and modeling, which provide absorption peaks at 10 resonant frequencies and also tuning of resonant frequency. All of these characteristics and findings distinguish the proposed multiband graphene-based absorber, which is well suited for terahertz applications.

Controlling Graphene Sheet Resistance for Broadband Printable and Flexible Artificial Magnetic Conductor-Based Microwave Radar Absorber Applications

Current artificial magnetic conductor (AMC) designs use metallic patterns on rigid substrates and focus on shapes and sizes of AMC structures, rather than on material performance, which has hindered operation bandwidth and design flexibility. Here, we introduce printed graphene AMC-based broadband and flexible microwave radar absorbers, which not only redirect but also absorb the incident wave so to broaden the operation bandwidth. Contrasting to other reported works, the phase characteristics of the AMCs are realized through the control of the surface resistance provided by printed graphene laminates. We produced a variety of AMC structures, composed of printed graphene circular ring arrays with exactly the same shape and size, but different sheet resistances. By carefully designing the sheet resistance of printed graphene laminates, the optimized anti-phase reflection cancellation between AMCs can be achieved. With printed graphene AMCs and flexible dielectric substrate, the absorber presented in this work has a broadband effective absorption (above 90% absorptivity) from 7.58 GHz to 18, is polarization insensitive under normal incident, and can work at relatively wide incident angles. Furthermore, this absorber is capable of bending easily with notable performance, which makes it ideal for applications with irregular and uneven shapes.

Transparent ultra-broadband absorber based on a tree seedling composite structure

In this work, we design, analyze and process an ultra-broadband absorption optical transparent metamaterials absorbing device. The simulation results show that within the 13.68–71.6 GHz frequency range, the devicereaches more than 90% absorptivity, and that transverse magnetic waves and transverse waves under a large angle of polarization and incident angle can obtain high absorption performance. Therefore the device has the perfect performance of polarization insensitivity and wide angle of incidence. The absorbing mechanism is analyzed by using the equivalent medium theory, and the theoretical results are in good agreement with the simulation results. Because of its advantages of wideband absorption, high transparency, lightweight, and easy availability, the deive has important application value in practical civil and military fields, such as electromagnetic interference, electromagnetic compatibility, super lens, invisibility cloak, solar cell device design and so on.

Performance of an emulsion telescope for gamma-ray observations in the GRAINE2018 balloon-borne experiment

Abstract The Gamma-Ray Astro-Imager with Nuclear Emulsion (GRAINE) project is aimed at the precise observation of astronomical gamma-ray sources in the energy range of 10 MeV–100 GeV using a balloon-borne telescope utilizing a nuclear emulsion, which can help realize precise imaging with high angular resolution (1.0○ at 100 MeV), polarization sensitivity, and large aperture area (10 m2). In 2018, the third balloon experiment was carried out as a demonstration of the detection of the brightest known astronomical gamma-ray source, the Vela pulsar, with an aperture area of 0.38 m2. In these data, some gamma rays were produced by the π0 → 2γ decay, which was caused by the hadronic interactions of cosmic rays in the detector. These could be used to calibrate the reconstructed angle, energy, and so on. In this study, we establish a method of searching for hadronic interactions and concomitant gamma rays with high statistics and purity. Our analysis indicates that the performance of our detector for gamma rays is as expected in wide incidence angle and energy ranges. We plan to commence scientific observations using the proposed system with the verified high angular resolution and largest aperture area in 2022 or later.

Design and analysis of a wideband microwave absorber based on graphene-assisted metamaterial

Microwave absorption materials or structures show great application prospects in the design of 5 G systems. A graphene-assisted wideband metamaterial microwave absorber with polarization insensitivity and wide incidence angle is proposed based on the round-with-square-hole (RSH) shaped monolayer graphene structure in this paper. The calculation results of the device over an ultra-wideband (16–100 GHz) yields good absorption above 90% when the chemical potential of graphene is 0.1 eV. The fractional bandwidth is 145% for both transverse electric and transverse magnetic polarizations with normal incidence. The influences of the geometry structure parameters and graphene chemical potential on absorption characteristics have been investigated. The structure exhibits polarization independent and wide incidence angle properties since the geometrical symmetry structure. In addition, the structure provides excellent performance when the whole thickness of the device is less than 1 mm. Mechanism of the wideband operation is deciphered based on the normalized relative impedance, surface current and electric field distributions. The novel microwave absorber demonstrates great potential in various fields of electromagnetic stealth, electromagnetic interference shielding, 5 G systems, and so on.

Wide incident angle insensitive ultrawideband mid-infrared perfect absorber

A wide incident angle insensitive compact ultrawideband absorber for infrared frequency spectrum is designed and analyzed in this paper. The absorber is based on gold-dielectric-gold three layer structure. This absorber has absorption of more than 90% in the frequency range of 14.297–32.161 THz (fractional bandwidth of 76.903%). It has three peaks at frequencies of 15.41 THz, 21.73 THz and 30.66 THz with absorption values of 99.8%, 99.99% and 99.66% respectively. In addition to this, this absorber is found to be insensitive to the incident angles over a wide range of 45o. The overall volume of the absorber is 5 µm × 5 µm × 1.94 µm. It has substrate thickness of ~λo/7 and periodicity of ~λo/3 at 23.229 THz. Due to this ultra wide bandwidth, wide incident angel insensitiveness and compact dimensions, this absorber will find its suitability in various THz or Mid-infrared applications.

Broadband and wide-angle metamaterial absorber based on the hybrid of spoof surface plasmonic polariton structure and resistive metasurface.

Electromagnetic (EM) wave absorber with broad and robust absorption performance over wide incident angle range is persistently desired in specific applications. In this work, we propose and demonstrate a broadband and wide-angle metamaterial absorber (MA) based on a hybrid of stereo spoof surface plasmonic polariton (SSPP) structure and planar resistive metasurface. At first, we design a broadband SSPP absorber by adjusting the dispersion and loss of the artificial plasmonic structure (PS) simultaneously. Furthermore, owing to utilize its spatial phase manipulation ability, we integrate a resistive metasurface on top of the PS to construct a modified circuit analog (CA) absorber with a dispersive metamaterial spacer. The absorption mechanism of the hybrid structure is analyzed theoretically. The results indicate that the hybrid MA is equipped with broad and robust absorption performance over a wide incident angle range due to the synergistic absorption of the PS and metasurface. Finally, a prototype of the hybrid MA is fabricated by silk-printing technic and its absorption performances are measured. The experimental results can verify the theoretic ones and indicate that proposed hybrid MA can achieve 90% absorptivity from 3.9 GHz to 10.6 GHz with thickness of 7.0 mm, which is only 106% times of the ultimate thickness corresponding to the absorption performance of MA. In general, the concept and design offer a distinct approach of utilizing SSPP to design absorbers with excellent performances from radio frequency to optic band, which are promising for extensive applications.

Ultra-broadband absorber based on cascaded nanodisk arrays

An ultra-broadband perfect absorber consisting of cascaded nanodisk arrays is demonstrated by placing insulator–metal–insulator–metal nanodisks on insulator–metal film stacks. The absorber shows over 90% absorption in a wavelength range between 600 nm and 4000 nm under transverse magnetic (TM) polarization, with an average absorptivity of 91.5% and a relative absorption bandwidth of 147.8%. The analysis of the electric field and magnetic field show that the synergy of localized surface plasmons, propagating surface plasmons, and plasmonic resonant cavity modes leads to the ultra-broadband perfect absorption, which accords well with the results of impedance-matched analysis. The influences of structural parameters and different metal materials on absorption performance are discussed. Furthermore, the absorber is polarization-independent, and the absorption remains more than 90% at a wide incident angle up to 40° under TE polarization and TM polarization. The designed ultra-broadband absorber has promising prospects in photoelectric detection and imaging.

A Broadband Switchable Metamaterial Absorber/Reflector Based On Multi-Laps Graphene Sheets in the Terahertz Band

Switchable metamaterial absorbers/reflectors (MAs/MRs) are important bifunctional electromagnetic devices and have been the subject of numerous scientific studies. However, there is a lack of bifunctional devices that operate in the terahertz band. Here, we theoretically propose a broadband switchable MA with many excellent properties, such as good thermal stability, high insensitivity to inferior film quality of the graphene, excitation polarization and wide incident angles, and outstanding structural parameter tolerance. The bandwidth of the proposed broadband MA is 3.4 THz with an absorptivity over 90% in the frequency band of 1.6-5 THz. The proposed absorber can switch to a reflector with a reflectivity over 93% by tuning the chemical potential of the graphene and reducing the temperature. Therefore, the switching intensity of the proposed MA exceeds 83%. The physical mechanisms of the broadband absorption of the proposed structure are investigated using the impedance matching theory and the multiple reflection interference theory. The reflection mechanism of the proposed broadband reflector is discussed by analyzing the effective parameters. The absorption and switching mechanism are theoretically investigated by performing detailed numerical calculations to analyze the surface loss intensity, electric field, and magnetic field. These findings can accelerate the development of terahertz broadband switchable devices.

Design, development and characterization of wide incidence angle and polarization insensitive metasurface absorber based on resistive-ink for X and Ku band RCS reduction

This article deals with the design, development and characterization of a broadband, wide incidence angle and polarization-insensitive metasurface, which consists of periodic resistive-ink pattern printed on top of FR-4 dielectric substrate as a RADAR absorber. Geometrical dimensions of the proposed unit cell have been optimized numerically using CST-Microwave Studio to achieve 99% absorptivity for X and Ku frequency spectrum (8–18 GHz) under the normal angle of incidence and to achieve almost 90% absorptivity under the oblique angle of incidence up to 50° within X-Band to Ku-Band spectrum. Furthermore, it is reported that by having fourfold rotation symmetry, the designed structure is insensitive to the polarization of the incident wave. An equivalent circuit analytical modeling has also been performed to understand and verify numerical findings that are nearly in agreement. The designed absorber is fabricated and characterized using ABmm VNA and free space measurement method, for both normal and oblique angle of incidence, and it is found that measured characteristics are in agreement with numerical-analytical findings. Furthermore, the monostatic and bistatic Radar Cross Section reduction capability of the designed absorbing structure for both planar and curved surface (cylinder) is studied extensively to assess its potential application in X-Band to Ku-Band Spectrum.

Correction of thickness uniformity of multilayer fluoride coatings on steep spherical substrates using shadowing mask combinations in planetary rotation systems.

In this study, the deposition angle parameter was introduced into the traditional deposition model to eliminate the coating thickness errors caused by large deposition angles and differences in the microstructures of coatings from the expected values. Based on the different thickness distributions of MgF2 and LaF3 coatings, a method that involves switching shadowing masks and a stepwise numerical optimization algorithm were proposed to control each coating thickness distribution accurately. When designing shadowing masks for a multilayer antireflective (AR) coating with a wide incident angle range, the correlation between the thickness distributions of the MgF2 and LaF3 layers was added to the merit function to ensure consistency in the two distributions. Two-layer deep-ultraviolet (DUV) AR fluoride coatings and a six-layer DUV low-polarization antireflection fluoride coating at 193 nm were fabricated on spherical substrate holders using thermal evaporation. In the experiment, when the ratio of the clear aperture to the radius of curvature was increased from -1.68 to 1.63, the thickness uniformities of the MgF2 and LaF3 layers of the two-layer DUV AR coatings on the fused silica substrates increased to a minimum of 98.49%. Owing to the transmission spectra and the incidence angle-resolved transmission, uniform optical performance and low-variation polarization transmittance were also achieved for the 193 nm AR coating deposited on the fused silica substrates.

Application of Two-Dimensional Compressive Sensing to Wavelet Method of Moments for Fast Analysis of Wide-Angle Electromagnetic Scattering Problems

To efficiently solve the electromagnetic scattering problems over a wide incident angle, a novel scheme by introducing the two-dimensional compressive sensing theory into the wavelet method of moments is proposed. In this scheme, a linear system of equations with multiple right-hand sides in wavelet domain is formed firstly, and one side of the bilateral sparse transform to the induced current matrix is simultaneously accomplished and then the bilateral measurement of the induced current matrix is operated by the linear superposition of the right-hand side vectors a few times and the extraction of rows from the impedance matrix. Finally, after completing the other side of the bilateral sparse transform, the wide-angle problems can be solved rapidly by two times of recovery algorithm with prior knowledge. The basic principle is elaborated in detail, and the effectiveness is demonstrated by numerical experiments.

Design of a type of broadband metamaterial absorber based on metal and graphene

In this paper, we demonstrate a kind of broadband metamaterial perfect absorber using both graphene and metal resonator elements. Through step by step design and simulation, wider absorption band from about 4.22 THz to 7.48 THz with average absorption rate up to 98.21% is achieved in the absorption spectrum. In addition, the absorber has characteristics of polarization insensitivity and wide incident angle due to its inherent rotational symmetry. Moreover, the absorption band can be adjusted by changing the chemical potential of the graphene. The superiorities of broadband, high absorption rate, polarization independent and wide-angle characteristics make it have potential application prospects in electromagnetic wave absorbing, signal sensing and detection, and other optoelectronic devices.

Broadband optical negative refraction based on dielectric phase gradient metagratings

In this work, we design and study a dielectric phase-gradient metagrating (DPGM) with SiGe whose operating frequency is in the visible band. It is found that this DPGM can enable negative refraction for incident light over a wide incident angle, and due to the tolerances in the metasurface design, this negative refraction has a broadband response from 400 nm to 470 nm. The underlying mechanism is due to the m-dependent diffraction law. An angularly asymmetric transmission phenomenon is observed in the designed DPGM, and the physical mechanism behind the phenomenon is revealed. Our results pave the way to achieving negative refraction in visible frequencies.

Numerical Investigation of Graphene and STO Based Tunable Terahertz Absorber with Switchable Bifunctionality of Broadband and Narrowband Absorption.

A graphene metamaterial and strontium titanate (STO)-based terahertz absorber with tunable and switchable bifunctionality has been numerically investigated in this work. Through electrically tuning the Fermi energy level of the cross-shaped graphene, the bandwidth of the proposed absorber varies continuously from 0.12 THz to 0.38 THz with the absorptance exceeding 90%, which indicates the functionality of broadband absorption. When the Fermi energy level of the cross-shaped graphene is 0 eV, the proposed absorber exhibits the other functionality of narrowband absorption owing to the thermal control of the relative permittivity of STO, and the rate of change of the center frequency is 50% ranging from 0.56 THz to 0.84 THz. The peak intensity of the narrowband absorption approximates to nearly 100% through adjusting the Fermi energy level of the graphene strips. The calculated results indicate that it is not sensitive to the polarization for wide incidence angles. The proposed absorber can realize tunable bifunctionality of broadband absorption with a tunable bandwidth and narrowband absorption with a tunable center frequency, which provides an alternative design opinion of the tunable terahertz devices with high performance for high-density integrated systems.

Optimal design of multilayer diffractive optical element in wide angle of incidence

We propose a method for designing the multilayer diffractive optical elements (MLDOEs) to improve the polychromatic integral diffraction efficiency (PIDE) in wide angle of incidence. By comparing and analyzing the characteristic angle weighted PIDE (CAW-PIDE) in the whole angles of incidence, the optimal microstructure height combination of the MLDOE can be obtained. The optimization process and simulation examples for the single-band and multi-band MLDOE are given. Compared to the conventional method, the PIDE is obviously improved in wide angle of incidence by our method. This optimization method can be applied to different working wavebands, and the image quality of the hybrid optical system with the MLDOE can be improved in wide angle of incidence.

Tunable Triple-Band Terahertz Absorber Based on Bulk-Dirac-Semimetal Metasurface

A tunable triple-band absorber based on bulk-Dirac-semimetal (BDS) metasurface is proposed in the terahertz (THz) range. The design is comprised of a BDS film with a pattern of dual ring slots and a dielectric layer stacked on a fully reflective gold ground. And the electric or magnetic resonances result in the incident plane waves at different frequencies to be effectively absorbed by the BDS film, which leads to the high absorption at 1.56 THz, 3.28 THz, and 3.95 THz resonant frequencies. The tunability of the resonant frequencies depends on various Fermi levels. Moreover, the physical mechanisms of the absorber with good stability over wide incident angles and polarization insensitivity are expounded. The proposed BDS-based absorber has great potentials in sensors, thermal emitters, and energy harvesting, etc.