Wide Range Of Incident Angles Research Articles

Switchable and tunable terahertz metamaterial absorber with broadband and multi-band absorption.

In this paper, we propose and demonstrate a switchable terahertz metamaterial absorber with broadband and multi-band absorption based on a simple configuration of graphene and vanadium dioxide (VO2). The switchable functional characteristics of the absorber can be achieved by changing the phase transition property of VO2. When VO2 is insulating, the device acts as a broadband absorber with absorbance greater than 90% under normal incidence from 1.06 THz to 2.58 THz. The broadband absorber exhibits excellent absorption performance under a wide range of incident and polarization angles for TE and TM polarizations. Moreover, the absorption bandwidth and intensity of the absorber can be dynamically adjusted by changing the Fermi energy level of graphene. When VO2 is in the conducting state, the designed metamaterial device acts as a multi-band absorber with absorption frequencies at 1 THz, 2.45 THz, and 2.82 THz. The multi-band absorption is achieved owing to the fundamental resonant modes of the graphene ring sheet, VO2 hollow ring patch, and coupling interaction between them. Moreover, the multi-band absorber is insensitive to polarization and incident angles for TE and TM polarizations, and the three resonance frequencies can be reconfigured by changing the Fermi energy level of graphene. Our designed device exhibits the merits of bi-functionality and a simple configuration, which is very attractive for potential terahertz applications such as intelligent attenuators, reflectors, and spatial modulators.

Surface-based Ku- and Ka-band polarimetric radar for sea ice studies

Abstract. To improve our understanding of how snow properties influence sea ice thickness retrievals from presently operational and upcoming satellite radar altimeter missions, as well as to investigate the potential for combining dual frequencies to simultaneously map snow depth and sea ice thickness, a new, surface-based, fully polarimetric Ku- and Ka-band radar (KuKa radar) was built and deployed during the 2019–2020 year-long MOSAiC international Arctic drift expedition. This instrument, built to operate both as an altimeter (stare mode) and as a scatterometer (scan mode), provided the first in situ Ku- and Ka-band dual-frequency radar observations from autumn freeze-up through midwinter and covering newly formed ice in leads and first-year and second-year ice floes. Data gathered in the altimeter mode will be used to investigate the potential for estimating snow depth as the difference between dominant radar scattering horizons in the Ka- and Ku-band data. In the scatterometer mode, the Ku- and Ka-band radars operated under a wide range of azimuth and incidence angles, continuously assessing changes in the polarimetric radar backscatter and derived polarimetric parameters, as snow properties varied under varying atmospheric conditions. These observations allow for characterizing radar backscatter responses to changes in atmospheric and surface geophysical conditions. In this paper, we describe the KuKa radar, illustrate examples of its data and demonstrate their potential for these investigations.

Numerical study on the angular light trapping of the energy yield of organic solar cells with an optical cavity

A limiting factor in organic solar cells (OSCs) is the incomplete absorption in the thin absorber layer. One concept to enhance absorption is to apply an optical cavity design. In this study, the performance of an OSC with cavity is evaluated. By means of a comprehensive energy yield (EY) model, the improvement is demonstrated by applying realistic sky irradiance, covering a wide range of incidence angles. The relative enhancement in EY for different locations is found to be 11-14% compared to the reference device with an indium tin oxide front electrode. The study highlights the improved angular light absorption as well as the angular robustness of an OSC with cavity.

A Multi-Functional Reconfigurable Metasurface: Electromagnetic Design Accounting for Fabrication Aspects

In this paper we present the theoretical considerations and the design evolution of a proof-of-concept reconfigurable metasurface, primarily used as a tunable microwave absorber, but also as a wavefront manipulation and polarization conversion device in reflection. We outline the design evolution and all considerations taken into account, from the selection of patch shape, unit cell size, and substrate, to the topology of the structure that realizes the desired tunability. The presented design conforms to fabrication restrictions and is co-designed to work with an integrated circuit chip for providing tunable complex loads to the metasurface, using a commercially available semiconductor process. The proposed structure can perform multiple tunable functionalities by appropriately biasing the integrated circuit: Perfect absorption for a wide range of incidence angles of both linear polarization states, accommodating a spectral range in the vicinity of 5 GHz, with potential also for wavefront control, exemplified via anomalous reflection and polarization conversion. The end vision is for such a design to be scalable and deployable as a practical HyperSurface, i.e., an intelligent multi-functional metasurface capable of concurrent reconfigurable functionalities: absorption, beam steering, polarization conversion, wavefront shaping, holography, and sensing.

Broadband and wide angle quarter-wave plate based on single-layered anisotropic terahertz metasurface

Broadband control of polarization state is essential for terahertz applications ranging from non-destructive testing to imaging in medicine and wireless communications. We present a single-layered terahertz anisotropic metasurface quarter-wave plate consisting of hybrid metallic wire grating arrays, which enables broadband linear to pure circular polarization conversion with an ellipticity over 0.99 and 43% bandwidth of the central wavelength. The broadband effect is robust across a wide range of incident angles. Interestingly, angle-tunable arbitrary polarization states are implemented by changing the angle of incidence at other frequencies. Such single-layered broadband quarter-wave plate provides a route to develop broadband polarization conversion components, facilitating compact applications of terahertz technologies.

Polarization-Insensitive Metasurface for Harvesting Electromagnetic Energy with High Efficiency and Frequency Stability over Wide Range of Incidence Angles

In this paper, a polarization-insensitive metasurface, harvesting electromagnetic (EM) energy with high efficiency and frequency stability over a wide range of incidence angles, is proposed. The previously reported metasurfaces suffer from their maximum efficiencies shifting with the frequency when the incidence angle increases. By introducing a square-shaped metal via ring around the elements, the mutual coupling among adjacent cells is reduced so that the proposed metasurface can maintain maximum efficiency at the fixed operation frequency over a wide range of incidence angles. Furthermore, with one single harvesting via in the proper position for the connection of a harvesting load, the metasurface can collect EM energy effectively with both transverse electric (TE) and transverse magnetic (TM) polarizations in one single harvesting load. Compared with the reported metasurfaces, this proposed metasurface has a higher efficiency and fixed operation frequency within a wide incidence range. The energy distribution, harvesting efficiency, and surface current are simulated to investigate the operation mechanism of the proposed metasurface. The simulation results show that the maximum harvesting efficiency is 91% at 5.8 GHz for both TE and TM polarizations at the normal incidence. When the incident angle increases to 75°, the maximum efficiency is achieved at 5.79 GHz (0.19% shift), and the maximum efficiencies of TM and TE polarizations are 91% and 68%, respectively. A 5 × 5 array is fabricated and tested. The experimental results are in good agreement with the simulated ones.

Design and simulation of a wideband, wide-angle and polarization-insensitive microwave absorber based on pattern optimization of resistive films

A wideband, thin, and multilayer (planar) microwave absorber is designed in this paper. The absorber is a periodic array of cells which includes three patterns of resistive film on two dielectric substrates. To design the array’s cell, a pattern optimization procedure based on the application of a binary optimization algorithm was utilized. The outcome of this procedure is a pixelated pattern of resistive film yielding the maximum possible absorption bandwidth (BW) and a high insensitivity to the polarization of the incident wave. Our pattern-optimized design achieved a 90%-absorption rate over a wide frequency BW of 1.6 to 23.7 GHz (175% fractional BW) under normal incidence. Moreover, the absorber operates over a wide range of incidence angles for obliquely incident waves.

Modulation of photoelectric properties of indium tin oxide thin films via oxygen control, and its application to epsilon-near-zero properties for an infrared absorber

Indium tin oxide (ITO) films with tunable epsilon-near-zero (ENZ) properties have attracted great interest among researchers. In this study, ITO films with tunable photoelectric properties were prepared by a magnetron sputtering technique under different oxygen flow rates, and their structures and optical properties are studied. The ENZ wavelength shift and electron accumulation layer were observed from their permittivity and carrier concentrations, which were calculated by a graded model combined with an oscillator model. This demonstrated that the variable carrier concentrations induced the tunable ENZ property of the ITO films. Meanwhile, the interface of the film and the substrate was found to have a significant influence on the loose lower layer by researching the profile of permittivity and carrier concentrations. Furthermore, the simulation of the transfer matrix method indicated that using ENZ materials as substrates can achieve zero reflection at an ENZ wavelength. ITO films serve as absorbers based on this characteristic. An absorption device was fabricated; its largest absorption rate was 97.41% at 1576 nm, and it maintained high absorption over a wide range of incident angles. The absorption mechanism in the ENZ film is illustrated by absorption maps and electric field distribution.

Broadband absorption enhancement with ultrathin MoS2 film in the visible regime* *Project supported by the National Natural Science Foundation of China (Grant No. 61405217), the Zhejiang Provincial Natural Science Foundation, China (Grant No. LY20F050001), the Anhui Polytechnic University Research Startup Foundation, China (Grant No. 2020YQQ042), and the Pre-research Project of Natural Science Foundation of Anhui Polytechnic University, China (Grant No.

The broadband absorption enhancement effect in ultrathin molybdenum disulfide (MoS2) films is investigated. It is achieved by inserting the MoS2 film between a dielectric film and a one-dimensional silver grating backed with a silver mirror. The broadband absorption enhancement in the visible region is achieved, which exhibits large integrated absorption and short-circuit current density for solar energy under normal incidence. The optical properties of the proposed absorber are found to be superior to those of a reference planar structure, which makes the proposed structure advantageous for practical photovoltaic application. Moreover, the integrated absorption and short-circuit current density can be maintained high for a wide range of incident angles. A qualitative understanding of such broadband absorption enhancement effect is examined by illustrating the electromagnetic field distribution at some selected wavelengths. The results pave the way for developing high-performance optoelectronic devices, such as solar cells, photodetectors, and modulators.

Multi-layer light trapping structures for enhanced solar collection.

Light trapping is a commonly used technique for enhancing the efficiency of solar collection in many photovoltaic (PV) devices. In this paper, we present the design of multi-layer light trapping structures that can potentially be retrofitted, or directly integrated, onto crystalline or amorphous silicon solar panels for enhanced optical collection at normal and extreme angle of incidence. This approach can improve the daily optical collection performance of solar panel with and without internally integrated light trapping structure by up to 7.18% and 159.93%, respectively. These improvements predict an enhancement beyond many research level and commercially deployed light trapping technologies. We further enhance this performance by combining our multi-layer optics with high refractive index materials to achieve a daily optical collection of up to 32.20% beyond leading light trapping structures. Our additive light trapping designs could enable the upgradeability of older PV technologies and can be tailored to optimally operate at unique angular ranges for building exteriors or over a wide range of incidence angle for applications such as unmanned aerial vehicles.

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.

Moth-eye structured mold using sputtered glassy carbon layer for large-scale applications

Surface reflection is a problem in applications such as solar panels, smartphones, and tablet computers. To prevent such reflection, we previously developed a moth-eye structure that can be fabricated with a single film. This antireflection structure (ARS) suppresses reflection over a wide range of light wavelengths and incident angles. The production of large-area ARS films, which can be achieved using ultraviolet nanoimprint lithography (UV-NIL), is required for solar panels and displays. In our previous study, a moth-eye structure was fabricated by irradiating a glassy carbon (GC) substrate with an oxygen ion beam. However, it is difficult to fabricate a large GC substrate because GC is made using powder metallurgy. To overcome this problem, we used a sputtered GC layer as the ARS master mold material. In this study, we demonstrate that a moth-eye structure can be formed by irradiating sputtered GC with an oxygen ion beam using an inductively coupled plasma (ICP) system. We formed a moth-eye structure on sputtered GC using two-step ICP etching. The fabricated moth-eye structured mold had a reflectance of approximately 0.1% in the visible-light region. In addition, a moth-eye film transferred using UV-NIL with the fabricated moth-eye structured mold had a low reflectance of approximately 0.4% in the visible-light region. • A GC layer was formed uniformly on the glass substrate with sputter-coated titanium. • A moth-eye structured mold was fabricated by irradiating the sputtered GC layer with an oxygen ion beam using an ICP system. • An optimized moth-eye structure was formed on the sputtered GC layer using two-step ICP etching. • A transparent moth-eye film with low reflectance was fabricated using UV-NIL from the fabricated moth-eye structured mold.

Tailoring radiative properties of a complex trapezoidal grating solar absorber by coupling between SPP and multi-order MP for solar energy harvesting

The coupling resonance effect between surface plasmon polariton (SPP) and multi-mode magnetic polariton (MP) is very complicated. In this study, we propose a complex trapezoidal grating solar absorber (CTGSA) with nanoscale structure, which is made of two tungsten trapezoids and a silicon dioxide spacer lying on the tungsten substrate, to study the coupling resonance between SPP and multi-mode MP in detail, not just the fundamental mode. The finite-difference time-domain method is employed to calculate spectral absorptivity and electromagnetic distribution. The coupling between SPP and MP of different modes can be clearly shown through the electromagnetic field, which is conducive to the investigation of the physical mechanisms of the four close-to-unity peaks in the absorptivity curve. The effects of geometric parameters on the resonance conditions are also explored. The coupling between SPP and MP renders the manual control of the absorptance of the structure possible. Calculations of directional absorptance at peak wavelengths for solar radiative intensity show that the absorptance of the proposed CTGSA is stable for a wide range of incident angles. Abnormal behaviors of directional absorptance are observed due to Wood’s anomaly. Besides, for steam generation application, the solar-thermal conversion efficiency is up to 74%, which has a practical significance. By providing a detailed analysis of the coupling of SPP and multi-order MP based on electromagnetic distribution, this paper will fill the gap in the field of MP resonance generated in the space between trapezoidal ridges and present a nanoscale structure with remarkable properties for harvesting broadband solar energy, which can provide a useful reference for optimizing design of solar absorbers.

Wide-angle microwave absorption properties of multilayer metamaterial fabricated by 3D printing

Conventional absorbing materials have always been challenged by the technical difficulty of effectively absorbing (<−10 dB) oblique incident electromagnetic waves in a wide frequency domain. In this study, 3D printing technology was used to combine metamaterials with multilevel gradient absorbing materials. To provide good impedance matching, the multilayer structure absorber was designed with a gradient index of characteristic impedanceby adjusting the geometric parameters of the unit cell and the distribution of spherical carbonyl iron powderenhancedpolyether ether ketone (CI-PEEK). The results show that the prepared 3D-printed multilayer metamaterial effectively absorbed electromagnetic waves across a wide range of incident angles (0°–50°) in the bandwidth of 8–18 GHz. Compared with the current technology on wave absorbing materials, this work showed a considerable improvement in the effective absorption of wide-angle electromagnetic waves and the integrated manufacturing of absorption composite materials with the desired characteristics. Thus, this study has important research significance and engineering value.

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, 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.

Preliminary results from the cosmic data taking of the BESIII cylindrical GEM detectors

BESIII is a multipurpose spectrometer optimized for physics in the tau-charm energy region. Both the detector and the accelerator are undergoing an upgrade program, that will allow BESIII to run for 5 to 10 more years. A major upgrade is the replacement of the inner drift chamber with a new detector based on Cylindrical Gas Electron Multipliers to improve both the secondary vertex reconstruction and the radiation tolerance. The CGEM-IT will be composed of three coaxial layers of cylindrical triple GEMs, operating in an Ar + iC4H10 (90:10) gas mixture with field and gain optimized to minimize the spatial resolution. The new detector is readout with innovative TIGER electronics produced in 110 nm CMOS technology. The front-end is a custom designed 64 channel ASIC featuring a fully digital output and operated in trigger-less mode. It can provide analog charge and time measurements with a TDC time resolution better than 100 ps, which will allow operating in μTPC mode. With planar prototypes, we measured an unprecedented spatial resolution below 150 μm in a 1 Tesla magnetic field in a wide range of incident angles of the incoming particle. Before the installation inside BESIII, foreseen in 2021, a long standalone data taking is ongoing at the Institute of High Energy Physics in Beijing; currently, the first two cylindrical chambers are available for the test, and are used to complete the integration between the detector and the electronics and to assess the required performance. In this proceeding, a description of the CGEM-IT project, the TIGER features and performance, and the results of the analysis of first cosmic ray data taking will be presented. Focus will be given on the strip analysis, from which it is possible to measure the basic properties of the detector, and the cluster analysis, where a comparison with the results with planar prototypes will be discussed. The first preliminary results on efficiency and spatial resolution will be also presented.

Tunable bifunctional polarization-independent metamaterial device based on Dirac semimetal and vanadium dioxide.

In this study, a tunable bifunctional polarization-independent metamaterial device based on Dirac semimetal films (DSFs) and vanadium dioxide (VO2) is investigated. At the VO2 insulator state, a polarization-independent electromagnetically induced reflectance effect can be achieved via destructive interference between bright and dark modes. When VO2 transitions to a metallic state, the proposed device behaves as a dual-band polarization-independent absorber with 99.9% and 94.5% absorptance at 9.06 and 10.9 THz, respectively, and is insensitive over a wide range of incidence angles. In both cases, refractive index sensing is achieved, and the response can be dynamically tuned by changing the Fermi energy of the DSF.

Freestanding and Supported MoS2 Monolayers under Cluster Irradiation: Insights from Molecular Dynamics Simulations.

Two-dimensional (2D) materials with nanometer-size holes are promising systems for DNA sequencing, water purification, and molecule selection/separation. However, controllable creation of holes with uniform sizes and shapes is still a challenge, especially when the 2D material consists of several atomic layers as, e.g., MoS2, the archetypical transition metal dichalcogenide. We use analytical potential molecular dynamics simulations to study the response of 2D MoS2 to cluster irradiation. We model both freestanding and supported sheets and assess the amount of damage created in MoS2 by the impacts of noble gas clusters in a wide range of cluster energies and incident angles. We show that cluster irradiation can be used to produce uniform holes in 2D MoS2 with the diameter being dependent on cluster size and energy. Energetic clusters can also be used to displace sulfur atoms preferentially from either top or bottom layers of S atoms in MoS2 and also clean the surface of MoS2 sheets from adsorbents. Our results for MoS2, which should be relevant to other 2D transition metal dichalcogenides, suggest new routes toward cluster beam engineering of devices based on 2D inorganic materials.

Soil moisture retrievals using L-band radiometry from variable angular ground-based and airborne observations

Surface soil moisture is a vital variable in the process of energy exchange between the land and atmosphere. Monitoring the surface soil moisture at the local and global scales has become feasible due to the development of microwave remote sensing. With the new development of potential satellite missions, it is important to evaluate existing soil moisture retrieval algorithms, which will greatly contribute to the improvement of current soil moisture products and the development of new methodologies. This paper compared the performance of four well-known soil moisture retrieval algorithms with L-band radiometry at fixed incidence angles, including the single channel algorithm at horizontal polarization (SCA-H), the single channel algorithm at vertical polarization (SCA-V), the dual channel algorithm (DCA), and the land parameter retrieval model (LPRM). The experimental data used for evaluation was from the Soil Moisture Experiment in the Luan River (SMELR), which consists of variable angular ground-based and airborne observations covering a wide range of incidence angles (22.5°–55°) at L-band. The microwave radiative transfer models are set to be consistent to guarantee that the four different algorithms are comparable. The results showed that the retrieval accuracy of the SCA-H and SCA-V is significantly affected by the input vegetation optical depth, and the calibration of vegetation parameters should be considered in the implementation of the SCA-H and SCA-V. The DCA does not rely on the auxiliary vegetation information and can also achieve good performance for both the soil moisture and vegetation optical depth. However, its retrieval requires a penalty on parameter constraints since the input brightness temperatures at horizontal and vertical polarization are correlated. The LPRM has a poor performance at incidence angles less than 30°, as it analytically utilizes the polarization difference in the brightness temperature, which is quite small at lower incidence angles. The accuracy of four soil moisture algorithms achieve their best performances at intermediate incidence angles of 40° to 45°, and is slightly degraded when the incident angles increased to larger than 50°, which is contributed to the increasing vegetation effects and depolarization that leads to an information loss. These findings provide quantitative evidence to help understand the differences in various current soil moisture algorithms and further promote the development of new methodologies for future soil moisture missions.