Wide Incident Angle Research Articles

A bi-tunable switchable polarization-independent dual-band metamaterial terahertz absorber using VO2 and Dirac semimetal

A bi-tunable switchable dual-band THz absorber using vanadium dioxide (VO2) and Dirac semimetal films (DSFs), which is independent on polarization, is proposed. The frequency and intensity of absorptances of the two absorption peaks can be dynamically changed by tuning the Fermi energy of the DSF or the VO2 conductivity. Moreover, the absorber can alternate between an absorption function (with an absorptance of 99.4%) and a reflection function (with a reflectance of 77.4%) at the frequencies of absorption peak B in case of VO2 transitions from metallic state to insulator state. In addition, the proposed absorber exhibits advantages such as polarization-independent nature and wide incident angle tolerance. This study provides references to future studies of THz bi-tunable dual-band or multi-band polarization-independent absorbers.

Tunable wide-angle perfect absorber based on black phosphorous-dielectric-metallic hybrid architecture

Black phosphorus (BP), as a newly emerging two-dimensional (2D) material, has received extensive interest due to its unique optical and electronic properties. In this work, we theoretically propose a monolayer BP-dielectric-metallic hybrid architecture, which exhibits a tunable nearly perfect absorption efficiency with ~99% at the mid-infrared wavelengths. The calculated results by finite-difference time domain (FDTD) simulations match well with the coupled-mode theory. The mechanism of the perfect absorption is due to the strong electromagnetic coupling between magnetic polaritons and BP surface plasmons as well as the guided mode. In addition, the hybrid system exhibits a strong tolerance for the absorption performance and remarkable property of polarization-sensitivity under wide incident angle range of 0°–60° attributable to the anisotropy of BP. The tunable characteristics can be achieved via changing the doping level of BP and the related geometrical parameters. These results suggest great potential applications in designing BP-based optoelectronic devices at the mid-infrared regime.

Single-peak-regulation and wide-angle dual-band metamaterial absorber based on hollow-cross and solid-cross resonators

A dual-band microwave metamaterial absorber with single-peak regulation and wide-angle absorption has been proposed and illustrated. The designed metamaterial absorber is consisted of hollow-cross resonators, solid-cross resonators, dielectric substrate and metallic background plane. Strong absorption peak coefficients of 99.92% and 99.55% are achieved at 8.42 and 11.31 GHz, respectively, which is basically consistent with the experimental results. Surface current density and changing material properties are employed to illustrate the absorptive mechanism. More importantly, the proposed dual-band metamaterial absorber has the adjustable property of single absorption peak and could operate well at wide incidence angles for both transverse electric (TE) and transverse magnetic (TM) waves. Research results could provide and enrich instructive guidances for realizing a single-peak-regulation and wide-angle dual-band metamaterial absorber.

Design of an ultra-thin, multiband, micro-slot based terahertz metamaterial absorber

An ultra-thin, multiband, micro-slot based metamaterial absorber is presented in this paper. The proposed unit cell is compact which is in the form of single-layer gold patch-gallium arsenide-ground package, involving four identical micro-slots. Using the micro-slots, the fundamental circular patch gains a multiband resonation skill, and the absorber operates at 1.16, 2.73, and 4.57 THz bands with near-perfect absorptivity. The absorption mechanism is discussed based on the electric and surface current distributions, and all outcomes are also validated by the effective medium approach. The proposed absorber is ultra-thin with a thickness of 2.6 µm, corresponding to ∼λ/100 at its lowest operation frequency. The unit cell indicates polarization-independent, and wide incident angle absorption spectra up to 40°, hence, the proposed absorber is a promising candidate as an absorbing platform for THz band imaging and sensing applications.

Integrated effect of hierarchical structure combining isotropic worm-like pit with anisotropic inverted nanopyramid for quasi-omnidirectional c-Si solar cell

The significance of efficient antireflection over a wide light incident angle (θ) range without sacrificing surface passivation too much increases with the rapid development of photovoltaic industry. Here, worm-like pit (WLP)/inverted nanopyramid (INP) hierarchical structure was realized on c-Si by constructing INP on modified WLP with large lateral size. The INP was obtained through Ag assisted chemical etching based nanopore reshaping in nanostructure rebuilding (NSR) solution. The anti-reflectance performance especially the omnidirectional ability accompanied with the surface passivation effect of both micro pyramid (MP) and WLP/INP was systematically investigated and compared. Additionally, the hierarchical structure based c-Si cell with an efficiency up to 18.5% was achieved, which is 0.12% absolutely higher than that of MP based counterpart (18.38%). Most significantly, the WLP/INP based cell can always possess a higher electric output power over the θ range from −80° to 80° with a highest relative power enhancement of 3.0% compared with that of the MP based one. The above surface processing is facile and scalable, providing an alternative way for the obtaining of c-Si solar cells with large electrical energy output during practical application.

Wideband THz Absorber: By Merging the Resonance of Dielectric Cavity and Graphite Disk Resonator

A graphite/dielectric cavity resonator-based absorber is numerically analyzed and implemented with wideband absorption. A graphite disk resonator is inserted in the low permittivity dielectric THz cavity resonator for the generation of multiple modes. The modes generated in dielectric cavity are merged with the resonating mode of graphite disk resonator for achieving the wideband absorption. The proposed absorber provides the frequency bandwidth of 129.34% (0.65 - 3.03 THz) for absorption >90%. The utilization of symmetrical geometry makes the absorber insensitive to polarization angle (φ) of the incident wave. Also, its performance remains stable over a wide incident angle (θ) covering the range 0° <; 8<; 50°. The operation and response of the proposed absorber is verified by implementing an equivalent circuit model. The proposed absorber can be utilized as sensor for THz medium sensing and imaging applications. A narrowband absorber can also be implemented using the proposed technique which can be utilized in sensing applications of micro scale objects. In addition, the proposed research work provides a technique for the implementation of a metal-free compact structure which can be useful in future micro/nanoscale systems.

Wideband, Very Low RCS Engineered Surface With a Wide Incident Angle Stability

In this communication, broadband, low radar cross section (RCS) engineered surface with the ability of operating over a wide range of oblique incident angles is presented. Two types of artificial magnetic conductor (AMC) unit-cells with a 180° ± 37° reflection phase differences are developed. Perforated superstrate is loaded on the top of conventional metallic patch-based AMC unit-cells which contributes to oblique incidence stability up to 60°. Moreover, it results in a wide operational frequency range of 16.5–58 GHz under normal incidence. To satisfy the periodicity condition, an appropriate number of proposed AMC unit-cells are considered as a super-cell. Then for the first time, the type and size of super-cells are optimized simultaneously. In consequence, maximum RCS reduction with a very uniformed scattered pattern is achieved. The superiority of the new arrangement over conventional coding arrangements is investigated for both monostatic and bistatic RCS results. It is shown that new structure results in a wider bandwidth, greater RCS reduction level and more uniformed scattered patterns. The proposed surface exhibits a very broadband of 111.5% for −10 dB normalized RCS reduction under normal incidence. A prototype is fabricated, and the experimental results showed promising agreement with the simulated ones.

Graphene-Assisted Narrow Bandwidth Dual-Band Tunable Terahertz Metamaterial Absorber

A tunable graphene terahertz metamaterial absorber is designed by treating a monolayer continuous dumbbell-shaped structure graphene layer, which can simultaneously realize the narrow bandwidth and dual-band absorption with transverse magnetic (TM) polarization and wide incident angle operation. Two pronounced absorption peaks are caused by the novel toroidal dipole phenomenon and magnetic plasmon polariton. By optimizing the geometry parameters, the relative narrow bandwidths of the two absorption peaks are 26.4 GHz and 23.5 GHz at frequencies of 0.2242 THz and 0.5302 THz, respectively, with the absorption rate above 99.6%. Since the continuous dumbbell-shaped graphene structure is treated in the absorber, a more convenient way to realize the tuning ability by treating a bias voltage compared to the absorbers with discrete graphene structures. The designed device can work at a wide incident angles (up to 80°) under TM polarization with the absorption above 88% at low frequency. The simulation results are basically in good agreement with the results of the equivalent circuit model. This work offers huge potential applications in terahertz imaging, detecting and sensing, especially in the 6G communication systems.

Bifunctional terahertz absorber with a tunable and switchable property between broadband and dual-band.

In this paper, we propose a terahertz bifunctional absorber with broadband and dual-band absorbing properties based on a hybrid graphene-vanadium dioxide (VO2) metamaterial configuration. When VO2 is in the insulating state and the Fermi energy of graphene is set to 0.8 eV, the designed device behaves as a tunable perfect dual-band absorber. The operating bandwidth and magnitude of the dual-band spectrum can be continuously adjusted by changing the Fermi energy of graphene. When VO2 is changed from insulator to metal, the designed system can be regarded as a broadband absorber, it has a broad absorption band in the range of 1.45-4.37 THz, and the corresponding absorptance is more than 90%. The simulation results indicate that the absorptance can be dynamically changed from 17% to 99% by adjusting the conductivity of the VO2 when the Fermi energy of graphene is fixed at 0.01 eV. Besides, both dual absorption spectrum and broad absorption spectrum maintain a strong polarization-independent characteristic and operate well at wide incident angles. Furthermore, we have introduced the interference theory to explain the physical mechanism of the absorption from an optical method. Therefore, our designed system can be applied in many promising fields like cloaking and switch.

Robust Conformal Perfect Absorber Involving Lossy Ultrathin Film

Perfect absorbers have been extensively investigated due to their significant value in solar cell, photodetection, and stealth technologies. Various subwavelength structures have been proposed to improve the absorption performances, such as high absorptance, broad band, and wide absorption angle. However, excellent performances usually put forward higher requirements on structural designs, such as varying the geometry sizes or shapes to fit different center wavelengths, which inevitably increases the fabrication burden. Here, a planar sandwich structure involving a layer of highly lossy material is proposed to achieve a robust perfect absorption with 95% absorptance ranging from the visible to near infrared range. Such an excellent absorption performance is also polarization-independent and applicable to a wide incident angle. Furthermore, the proposed design can also be applied to conformal surfaces with a 90% fluctuation over a steep surface. We believe that the proposed perfect absorber with distinguished performances can find wide application.

Tunable broadband terahertz absorber using a single-layer square graphene patch with fourfold rotationally symmetric groove

We present a broadband and tunable graphene-based absorber in the terahertz (THz) region, which is composed of a monolayer of graphene square-shaped patch with fourfold rotationally symmetric groove and a metallic ground film separated by a silicon dioxide (SiO2) spacer layer. The bandwidth of 90% absorbance for the structure reaches up to 1.66 THz with a central frequency of 2.53 THz under normal incidence, and the broadband absorption still remains 80% over a wide incident angles up to 50° for both transverse electric (TE) and transverse magnetic (TM) modes. Furthermore, the peak absorption of the proposed absorber can be tuned from 15% to near 100% by varying the graphene's Fermi energy from 0 to 1.0 eV via adjusting the bias voltage of the graphene layer or chemical doping. Due to the geometrical symmetry, the proposed structure is insensitive to the polarization state of incidence wave. Benefits from these outstanding performances, we believe that our proposed absorber may have great applications in tunable THz filtering, imaging, and cloaking.

A new compact dual-band perfect absorption ultrathin planar metasurface energy harvester in X- and V-bands with a wide incident angle

In this paper, we design and simulate a new dual-band ultrathin metasurface energy harvester (EH) in X- and V-bands. One of the significant advantages of the proposed EH is the efficiency, which is more than 70% over 170° of the TM-polarized oblique incident angles at f = 10 GHz. The full-wave simulation results show that the maximum harvesting efficiencies of 93.4% (in 10.1 GHz) and 84.3% (in 42.86 GHz) are obtained by the TM 75°-polarized and the normal incident waves, respectively. Besides, half-power bandwidths (ratios) are about 1.12 GHz (11.08%) and 2.36 GHz (5.5%), respectively. The designed unit-cell is miniaturized using a C-shaped slot with the dimensions of 0.1λ0 × 0.1λ0 × 0.008λ0 at the lowest resonance frequency (λ0 = 30.12 mm). To analyze the physical mechanism of energy harvesting, the full-wave simulation results of the current and electric field distributions at two resonance frequencies are presented. We demonstrate how the rotation of the load around the metallic via can affect the resonance frequency and the other characteristics of the EH at the X-band. We validate the load’s rotation effects using the waveguide measurement method as well. Finally, we show that the location of the metallic via has a greater impact compared to the load’s rotation using the full-wave numerical simulation at the X-band.

An ultrathin planar acoustic metasurface diffuser with narrowband uniform reflection

Diffuse reflection of sound is desirable in many practical scenarios, such as architectural acoustics, but most existing designs of acoustic diffusers have bulky size, corrugated profile, or limited spatial resolution. We design an ultrathin planar acoustic diffuser for producing narrowband diffuse reflection with high uniformity via precise modulation of reflected wavefront and propose a metasurface-based implementation comprising a monolayer of Helmholtz-like resonators much smaller than the working wavelength in all three dimensions. Our design is benchmarked with a commercialized Schroeder diffuser and is numerically proven to be capable of scattering the illuminating wave more uniformly than the conventional mechanisms based on the quadratic residue sequence over wide incident angles. We anticipate our design with simplicity and capability to find important applications in diverse scenarios.

MoS2-based broadband and highly efficient solar absorbers.

In order to obtain broadband, highly efficient, wide-angle, and polarization-insensitive solar absorbers, we propose a universal configuration consisting of monolayer molybdenum disulfide (MoS2) and the metal-insulator-metal structure, which gives rise to significant absorption enhancement of the MoS2 layer. Light trapping structures with silver square-, circle-, and crossed-shaped resonators are investigated. The localized surface plasmon resonances among the silver resonators induce prominent interaction between the incident photon and MoS2 layer, contributing to efficient absorption of light energy. Simulation results show that the absorber made of square patches enables the best performance and realizes absorptance higher than 90% from 400 to 666 nm and an average absorptance greater than 91% in the range of 400-700 nm. The average light absorption within the MoS2 layer reaches 74% in the visible spectrum, which is one of the highest levels for the existing MoS2-based absorbers. Meanwhile, the polarization-independent designs exhibit good angle tolerance within 50° incidences. Such a universal structure can also obtain broadband and highly efficient absorption by using other transition metal dichalcogenides such as MoSe2, WS2, and WSe2, which indicates that the configuration has great applicability in solar energy absorption of 2D materials. The proposed solar absorbers with simple configuration and broadband absorption in wide incident angles have potential in applications such as solar cells, photovoltaic devices, and blackbody materials.

Photonic crystal with tunable air layers based asymmetric transmission film for space solar power station

In this paper, high-contrast asymmetric transmission film based on periodic one-dimensional photonic crystal with tunable air layers is presented for the spherical concentrator of the orb-shape membrane energy gathering array based space solar power station concept. As the thicknesses of the air gaps in the photonic crystal structure are precisely controlled, the optical transmission properties of the photonic crystals structure dynamically vary, allowing transmission of propagation waves in the forward direction but reflection in the reverse direction. The geometric parameters of the proposed photonic crystal structure are optimized to obtain both high forward transmittance and reverse reflectance over a broad operating wavelength range from 400 nm to 1200 nm and wide angle of incidence from 0° to 60°. The numerical simulations demonstrate that the average forward transmittance, reverse reflectance and the total solar ray collection ratio of the optimal structure amount to 99.3%, 95.2% and 94.5%, respectively.

A dynamically tunable and wide-angle terahertz absorber based on graphene-dielectric grating

We theoretically and numerically demonstrate a tunable and wide-angle terahertz absorber, which is composed of multilayer graphene-dielectric grating and bottom metal substrate. Numerical simulation shows that the proposed absorber has the advantage of dynamically tunable range from 1.015 THz to 1.165 THz when the chemical potential of graphene increases from 10 meV to 150 meV. The absorption efficiency can reach a high value of 99%. To show the working mechanism of absorption, the near field distributions of magnetic components are presented at the absorption wavelength. We also demonstrate that the tunable range of absorption can be engineered by designing the geometry parameters. In addition, it is shown that the designed absorber can maintain the good performance of absorption over a wide incident angle from [Formula: see text] to [Formula: see text] under TM-polarization.

Wide Bandwidth Angle- and Polarization-Insensitive Symmetric Metamaterial Absorber for X and Ku Band Applications

In this paper, a wide bandwidth angle- and polarization-insensitive symmetric metamaterial (MM) absorber for X and Ku band is proposed. For both normal and oblique incidence in TEM mode, the proposed unit cell shows high absorption at different polarizing angles due to structural symmetry. A four-fold resonator was introduced in the unit cell to enhance the bandwidth. The performance of the proposed absorber is determined by both full-wave simulations and measurements. The simulated and measured absorptions are almost similar at normal incidence with 94.63%, 95.58%, 97% and 75.58% at 11.31 GHz, 14.11 GHz, 14.23 GHz, and 17.79 GHz respectively. At 45° for these frequencies, the absorptions are 95.47%, 97.2%, 97.12% and 75.29% respectively. For 90°, the absorptions are similar to those for 45° except 98.15% for 14.21 GHz. At all these angles and resonance frequencies, either permittivity or permeability was found negative, as a result, the refractive index was negative revealing metamaterial characteristics of the unit cell. Along with high absorptivity and wide incidence angle insensitivity up to 90°, a total of 1.42 GHz of absorption bandwidth was achieved, which is better than recent similar works with FR4 substrate.

Hybrid-phase approach to achieve broadband monostatic/bistatic RCS reduction based on metasurfaces

In this paper, we propose an approach to design metasurfaces to realize superior performance for monostatic and bistatic radar cross section (RCS) reduction within a wide bandwidth. By engineering four subarrays to exhibit different hybrid focusing-linear phases within a metasurface, nearly uniform diffusive scattering can be inherently guaranteed. As an illustration, we design and fabricate a proof-of-prototype metasurface and experimentally demonstrate its wave-diffusion performances. Numerical and experimental results both show that the thin metasurface enables a monostatic/bistatic 10 dB RCS reduction over a broad bandwidth ranging from 7.2 to 17.2 GHz. In addition, more than 10 dB RCS reduction performance is preserved within a band of 7.8 ∼ 14.2 GHz even for off-normal incidence up to 60°. Our approach features broadband, polarization insensitive, wide incident angles and easy design without time-consuming optimization, promising great potentials in applications of low-scattering stealth.

Broadband Terahertz Near-Perfect Absorbers.

Broadband terahertz (THz) absorbers are highly desired in detection, modulation, receiving, and imaging devices. We report the design and successful implementation of a novel broadband THz metasurface with a near-perfect absorption. Different from the traditional metal/dielectric/metal three-layer structures, the as-designed THz absorber has one more metal layer and a dielectric spacer on top, both of which are 200 nm thick. Although the total thickness increased by ∼7%, the near-perfect THz absorption band significantly broadened by 4×, achieving a broadband absorption of 270 GHz. Broadband, polarization-insensitive, and near-perfect THz absorptions were also observed over wide incident angles in these meta-absorbers, where the electric field and power loss were mainly concentrated in the additional thin dielectric layer. Such a broadband THz absorption was achieved through electromagnetic coupling between the top and middle metal layers and the resultant overlapping of the resonance frequencies. This strategy can be adapted to other spectrum-shaping devices.

Planar microwave retroreflector based on transmissive gradient index metasurface

In this paper, a novel planar microwave retroreflector based on a transmissive gradient metasurface combined with a curved metal mirror is proposed and demonstrated. The transmissive metasurface can efficiently converge a wide-angle incident wave to a pre-designed curved metal mirror behind it with a proper distance, which acts as an effective reflective surface that can greatly enhance the backscattering of the incident wave with a wide-angle view. According to the full-wave simulations, the proposed metasurface retroreflector can perform an excellent retroreflective effect for incident microwaves of angle view between −30° and 30° range. A prototype was fabricated and the experimental results verify that the metasurface retroreflector can realize the monostatic radar cross section (RCS) enhancement with a continuous wide incident angle view from −30° to 30° at 10 GHz within a stable 3 dB RCS level. It is further demonstrated that the excellent wide-angle backscattering performance (absolute RCS enhancement value, operational bandwidth and/or incident angle view) of the proposed microwave metasurface retroreflector is competitive against the traditional trihedral corner reflector with comparable dimensions, thus opening up new possibilities to substitute the traditional bulky radar retroreflector by using a planar compact metasurface structure for microwave engineering. The presented microwave metasurface retroreflector is promising to develop into a low-profile, light weight and planar radar retroreflector which possesses tremendous RCS backscattering enhancement and wide-angle view operation range.