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In this paper, ultra-wideband and wide-angle radar cross section (RCS) reduction of a concave structure is designed and realized based on a chessboard polarization conversion metasurface (CPCM), employing an ultra-wideband polarization conversion metasurface (PCM) composed of a single layer of square split-ring resonators. The concave structure, which is equivalent to an octagonal-like prism, is divided into eight regions. To achieve perfect phase cancellation in the non-central region, it can be equivalent to oblique incidence when the central region is under normal incidence, and phase compensation of the unit cell of metasurfaces in the non-central region is considered. The simulated results demonstrate that the RCS reduction of the proposed concave structure is less than −10 dB in the frequency ranges of 8.8 GHz to 35.75 GHz with fractional bandwidths of 120.99% and exceeds −30 dB at numerous resonant frequencies such as 9.52 GHz, 13.89 GHz, 23.45 GHz, and 35.2 GHz under normal incidence. The experimental results are in good agreement with the simulations. Furthermore, the RCS reduction characteristics of the proposed concave structure at different azimuth angles are also evaluated. Numerical calculations and experiments show that the wide-angle RCS reduction from 0° to 34° is achieved. To the best of the information we have, this is the first time that the chessboard metasurfaces, which consist of several polarizing reflectors, have been employed to obtain broadband and wide-angle RCS reduction for the concave structure. This technique validates the novelty and effectiveness of wide-angle and ultra-wideband RCS reduction of the concave structure. [ABSTRACT FROM AUTHOR]

In this paper, it is proposed to employ phase cancellation metasurface (PCM) to substantially reduce the RCS of an electrically large open-ended cavity (EOC) by scattering the energy to the nonthreatening region, achieving broadband and ultra-wide-angle RCS reduction of the cavity. The PCM, constituted by the collection of polarizing reflectors, is loaded on the inner and outer walls of the composed cavity structure in accordance with the orthogonality principle to obtain a low-RCS cavity. The main benefits of using the polarizing reflector to build a PCM are as follows: On the one hand, it reduces the number of units that make up the PCM, which in principle only requires half of the quantity. On the other hand, it is possible to form a phase difference of 180 degrees that is completely stable. Both the simulation and measured results show that the RCS reduction of 10 dB for normal incidence relative to the original state of the cavity (that is, the metal cavity) over a frequency range of 6.3 GHz to 13.2 GHz under TE polarization and 7.8 GHz to 13.4 GHz under TM polarization has been obtained. Furthermore, the RCS reduction performances of the cavity loaded with the composed PCM under variable azimuth angles are also studied. Numerical and experimental results demonstrate that the ultra-wide-angle RCS reduction from -180° to 180° is acquired. To the best of our knowledge, this is the first time that the PCM, which is made up of a number of polarizing reflectors, has been used to get broadband and ultra-wide-angle RCS reduction for the EOC. This method, which is simple, effective, and low cost, has great application prospects in the RCS reduction design of the cavity. [ABSTRACT FROM AUTHOR]

Beam steering plays a vital role in light detection and ranging (LiDAR). However, the scanning speed is below megahertz (MHz) in current LiDAR systems. Frequency‐gradient metasurface has been reported to achieve gigahertz (GHz) beam steering with 25° scanning angle in two dimensions. However, the fabrication of millions of form‐varying nanoscale meta‐atoms poses challenges in operation efficiency, massive production, and overall cost‐effectiveness when the size goes beyond millimeter scale. Here, a dynamic beam steering comprising periodic grating nanostructures fabricated through nanoimprint lithography with an ultrafast scanning rate up to GHz (≈81 ps) and ultra‐wide three‐dimensional (3D) fields of view up to 53° × 23° is reported, which shows the powerful acquisition of 3D space data. The detection distance of object reaches to ≈230 m. This work demonstrates a wide‐angle and high‐speed beam steering, and paves a robust way to realize cost‐efficient, high‐volume manufacturing, and powerful 3D all‐solid‐state LiDAR for next‐generation intelligent vision and self‐driving technologies. [ABSTRACT FROM AUTHOR]

In this paper, the soft and hard composite patterns for electromagnetic scattering control at both normal and grazing incident angle is proposed. The soft and hard parts are composed of dielectric substrate sandwiched by transverse or longitude metal strips with metal via and the perfect electrical conductor ground, and these two parts are combined at a certain inclination angled to form the soft and hard composite patterns. The simulation results demonstrate that the proposed composite patterns have the monostatic radar cross‐section (RCS) reduction of 20.79 and 18.36 dB at 11.24 GHz under normal incidence for transverse electric polarization and transverse magnetic polarization, respectively. Also, this structure achieves 2.78 and 5.43 dB monostatic RCS reduction under grazing incidence from 60° to 90°. The simulation results are in good agreement with the experimental ones. The physical mechanisms of the proposed composite patterns can be elucidated by the equivalent impedance method based on its near‐field distribution. The proposed composite patterns can be functionalized by changing the electromagnetic parameters of the soft and hard surfaces, which has a great potential to be used as the traveling wave antenna enclosure for scattering control. [ABSTRACT FROM AUTHOR]

An ultra‐broadband double‐layer microwave metamaterial absorber (MMA) with reflectivity below −10 dB ranging from 1.4 to17.31GHz is presented in this article. The MMA has a fractional bandwidth of 170% and a total thickness of 20.8 mm. By analyzing the function of different layers, the MMA can be simplified to the combination of a dual‐band (DB) absorber and a single‐band (SB) absorber. Layer I consists of a square loop loaded with asymmetric resistors and achieves a DB absorption by exciting high‐order modes covering the low‐frequency and high‐frequency. Layer II is composed of double‐ring structures integrated with resistors and realizes an SB absorber in the middle frequency. To understand the physical mechanism in detail, the equivalent circuit model (ECM) is proposed and illustrates a good impedance match within the entire absorption bandwidth with free space. An in‐depth analysis has been carried out by illustrating several key dimensions. To verify our idea, a prototype has been designed, fabricated, and measured. Measurement results show an ultra‐broadband absorption with the reflectivity below −10 dB ranging from 1.7 to 17.27 GHz under the normal direction, which agrees well with simulation and ECM results. [ABSTRACT FROM AUTHOR]

In this study, a new closed-form expression is developed to determine the equivalent electromagnetic parameters (permittivity and permeability) of biaxial anisotropic honeycomb radar-absorbing materials (RAMs). Based on the equivalent medium theory, the nonmagnetic substrate and absorbing coating of the honeycomb structure are equivalent to one kind of material, and then, the effective electromagnetic properties of irregular hexagonal (compressed or stretched) honeycombs are obtained by using the layered anisotropic model. In order to validate the effectiveness and correctness of this new closed-form expression, experimental tests and numerical simulation are adopted. The results show good agreement among the experimental tests, numerical simulation, and theoretical analysis within the specified frequency range (2–18 GHz), which confirms the validly of the proposed new method. Compared with the previous layered anisotropic model, the modified new model is more accurate for irregular hexagonal honeycomb RAMs. Furthermore, it can predict the effective permittivity (permeability) precisely without experimental measurements or numerical simulation, and it is of great significance for the design of honeycomb RAMs. [ABSTRACT FROM AUTHOR]

In this work, an ultra-thin microwave absorber with a Jerusalem cross (JC) resonator exhibiting a broad low-frequency absorption bandwidth was successfully designed and fabricated. The system is composed of a perfect electric conductor-magnetic-periodic metal three-layer structure. The top metal layer consists of a periodic JC, which can be used to obtain dual-bands and low-frequencies in L-band, S-band, and C-band applications. The simulations show that the magnetic absorber with a thickness of 2 mm exhibits two absorption peaks at 1.4 and 4.7 GHz, with a reflectivity of −10 and −8 dB, respectively. The L-band RL (reflection loss) of −4 dB bandwidth has 91% (1–1.91 GHz), whereas the S-band and the C-band of −4 dB bandwidth have 47% (2.87–5.68 GHz). The total thickness of the absorber measures only 1/100λ at 1.5 GHz. When a magnetic harmonic TM is polarized at an incidence angle of 75°, an average RL of −10 dB in the 1–8 GHz range is obtained. The equivalent L–C circuit and the power loss density clarify the mechanism behind the absorption. Such a low-frequency absorber can be used in various applications to quench microwave energy. [ABSTRACT FROM AUTHOR]

A stretchable microwave metamaterial absorber with specific pattern design is proposed and fabricated in this paper to improve the stability of microwave absorption for deformation. Compensation mechanism is revealed by the electrical current loop for resonance absorption excitation in an equivalent circuit model. The proposed metamaterial absorber with a slotted cross pattern is compared with the conventional cross pattern absorber. A stretchable silver conductive ink validates the silk screen printing of resistive patterns on flexible polydimethylsiloxane substrate and hence realizes the stretching experiments. In a 25% stretch, the resonant frequency of the compensated absorber shifts only 0.5% for TM polarization, comparing to the 8% shifting of the conventional structure. [ABSTRACT FROM AUTHOR]

Microwave absorbing honeycomb composites with grading coating thickness are prepared by impregnation of aramid paper frame within water solutions of conductive carbon blacks. Based on effective medium method, grading impedance in the composites can be theoretically expressed by a linear approximation of the inversely tapered effective permittivity at wave propagation direction, which provides a feasible way to optimize the design of microwave absorbing honeycomb structures. The consistent result of calculations and experimental measurements for positive impedance grading design confirms the broadband absorption effect in high frequencies. [ABSTRACT FROM PUBLISHER]

Fe84.94Si9.68Al5.38 flaky particles have been obtained by ball attrition method. The volume fraction dependence of microwave permeability has been measured for composites filled with these particles. Effective medium theories have been employed to extract the apparent microwave permeability. It is found that the flaky Fe–Si–Al particles have large apparent microwave permeability and large microwave magnetic loss. For example, at 0.5 GHz, the real and imaginary parts of apparent permeability are 10 and 5, respectively. The extracted microwave permeability is found to be consistent with generalized Snoek's limit. Due to high microwave magnetic loss, the flaky Fe–Si–Al particles can be useful for technical applications in the field of electromagnetic compatibility. Mössbauer spectroscopy measurements show that the hyperfine magnetic field and isomer shift have wide distribution, indicating that Fe atoms in the flaky Fe-Si-Al alloys have complicated environments. [ABSTRACT FROM AUTHOR]

Radar absorbing honeycomb composites with different coating thicknesses are prepared by impregnation of aramid paper frame with solutions containing conductive carbon blacks (non-magnetic) and PI (polyimide). Expressions for the effective permittivity of the composites are studied and validated both in theory and experiment. It is found that a theoretical equivalent panel with given permittivity can be obtained to represent the honeycomb structure in the quasistatic approximation, which provides a feasible way to optimize the design of radar absorbing honeycomb structure by connecting the effective electromagnetic parameters with the unit cell dimensions. The effective permittivity is measured by a network analyzer system in the frequency range of 8–12 GHz and compared with the theoretical result. [ABSTRACT FROM PUBLISHER]

In the 4π confocal microscope developed by S. W. Hell et al, laser light coherently illuminates both sides of a thin sample through a pair of high-NA objectives, effectively producing a single standingwave fringe of 2-photon fluorescence excitation with weak side lobes. Developed initially for biological applications, the 4π microscope of Hell et al demonstrated 75 nm axial resolution with 810 nm light. We have constructed a 4π confocal multiphoton microscope for 3d analysis of bandedge/excitonic photoluminescence in thin films. Excitation is with 130 fs pulses from a Ti:sapphire laser. Instrumental features and preliminary tests with rhodamine and GaN and InN films are reported. [ABSTRACT FROM AUTHOR]

The permeability spectra of NiZn spinel ferrites with different amounts of SiO2 (0,0.3,0.5 wt %) aggregated at the grain boundaries have been resolved into two contribution components: domain wall resonance and spin rotation relaxation. The fitting results of permeability dispersion show that the spin rotation relaxation mechanism makes major contributions to the dispersion. With increasing the contents of SiO2, the fitting results of permittivity dispersion spectra show a wider distribution of relaxation time and a longer relaxation time. [ABSTRACT FROM AUTHOR]

The permeability spectra of NiZn spinel ferrites with different amounts of SiO2 (0,0.3,0.5 wt %) aggregated at the grain boundaries have been resolved into two contribution components: domain wall resonance and spin rotation relaxation. The fitting results of permeability dispersion show that the spin rotation relaxation mechanism makes major contributions to the dispersion. With increasing the contents of SiO2, the fitting results of permittivity dispersion spectra show a wider distribution of relaxation time and a longer relaxation time. [ABSTRACT FROM AUTHOR]

Fe-Si-Al particles with irregular shapes have been successfully milled into flaky shapes by attrition for 10, 20, and 30 h, respectively. The attrition time has very small effect on the static magnetic properties, but shows strong effects on the high frequency properties (permittivity and permeability). With increasing the attrition time, the values of permeability can be obviously enhanced, especially at lower frequencies. The high frequency permittivity values of flakes milled for 30 h are much larger than those of particles milled for 10 or 20 h. A multiple relaxation processes model has been used to study the permeability dispersion behaviors. Analysis show the variation of shape anisotropy could be the main cause for the observed changes in permeability dispersion behaviors. However, the electromagnetic wave absorption properties of composites filled with these flaky shape Fe-Si-Al particles are found deteriorated when the particles have been milled for longer time. [ABSTRACT FROM AUTHOR]

A dual-band and high-efficiency reflective linear polarization converter composed of a layer of slotted metal wires has been proposed. Both the simulated and experimental results indicate that the structure can convert a linearly polarized wave to its cross-polarized state for two distinct frequency bands under normal incidence: 9.8-15.1 and 19.2-25.7 GHz. This phenomenon is attributed to a resonance that corresponds to the “trapped mode” at 15.8 GHz. This mode is stable with structural parameters and incident angle at a relatively wide range, and thus becomes promising for dual-band (also multiband) devices design. By surface current distribution and electric field analysis, the operation mechanism has been illuminated, especially for the “trapped mode”, identified by the equally but also oppositely directed currents in each unit cell. [ABSTRACT FROM AUTHOR]

Fe-Si based alloys have been widely studied because of the excellent high frequency, soft magnetic properties, which are determined by the hyperfine structure of alloys. The evolution of the hyperfine structure and magnetic characteristics in an Fe-Si-Cr alloy with increasing heat treatment temperature have been studied. Fe-21.5 at.% Si-2 at.% Cr alloy powder was prepared and observed. The Fe-Si based alloys have three-step phase transitions under heat treatment. Hyperfine magnetic fields are increased after A2 ordered as the localized degree of 3d electrons decreased. Quadrupole splitting in alloys mostly originate from the paramagnetic phase, and the asymmetry of charge could increase the polarization response under high frequency microwaves. This could increase the microwave permittivity and paramagnetic phase ratio, which can contribute to better soft magnetic properties. The Mössbauer effective thickness of a Fe-Si-Cr alloy particles sample is calculated as 7.3345. Theoretical magnetic moments from first principle calculations and the Mössbauer result can be obtained and are consistent with experiment results. The formation of the DO3 phase could deteriorate soft magnetic properties of samples. High permeability of Fe-Si-Cr alloys can be obtained under heat treatment of the critical temperature when the DO3 phase starts to form. [ABSTRACT FROM AUTHOR]

Highlights: [•] We fabricated Hitperm amorphous ribbons with melt-spinning quenching method. [•] Vertical variation structures were demonstrated on section morphology of ribbons. [•] Different crystalline structures observed on the both sides of Al addition Hitperm. [•] Al was precipitated from the upper side while Cu precipitated from the wheel side. [•] Al addition embrittle ribbons without causing significant changes of static magnetic properties. [Copyright &y& Elsevier]

• Al 2 O 3 /ZrB 2 thin films exhibit a favorable thermal stability at 600 °C for 30 h. • Al 2 O 3 /ZrB 2 thin films exhibit a high average infrared reflectance of 0.86 after annealing at 750 °C for 2 h. • Al 2 O 3 thin film can effectively prevent diffusion of oxygen atoms during the heat treatment process. In the present research, ZrB 2 thin films and Al 2 O 3 /ZrB 2 multilayer were deposited on silicon substrates using magnetron sputtering method. To investigate the thermal stability of the thin films, films were annealed at temperatures of 600 °C, 700 °C and 800 °C, respectively. Results from X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared spectrometer (FTIR) suggested that ZrB 2 thin films were severely oxidized at 700 °C for 2 h which caused the obvious increase of infrared emissivity. In contrast, double-layer-structure Al 2 O 3 /ZrB 2 thin films exhibited a low infrared emissivity of 0.16 after annealing at 750 °C for 2 h. Similarity, ZrB 2 thin films showed a rise of infrared emissivity after annealing at 600 °C for 10 h. However, double-layer-structure Al 2 O 3 /ZrB 2 thin films exhibited a favorable thermal stability at 600 °C for 30 h with a low average infrared emissivity of 0.13 in the waveband of 2–14 μm. Compared with ZrB 2 thin films, double-layer-structure Al 2 O 3 /ZrB 2 thin films could work as low infrared emissivity layers at higher temperature for longer time for the reason that the addition of Al 2 O 3 thin film can effectively prevent diffusion of oxygen in air during the heat treatment process. [ABSTRACT FROM AUTHOR]

The present study reports the preparation and characterization of Cr 2 O 3 –Al 2 O 3 composite pigments with low visible and high near-infrared (NIR) reflectance. The complex pigments were prepared by Solid-State Reaction Sintering Method. The effects of calcination temperature and Al 2 O 3 content on reflectance were investigated by SEM, X-ray diffraction, XPS and UV-VIS-NIR spectroscopy. And the resistance of the complex Cr 2 O 3 –Al 2 O 3 pigments were further discussed to analyze the carrier concentration. The results reveal that the complex Cr 2 O 3 –Al 2 O 3 pigments having good optical performance and acid-alkali resistance can be prepared by the calcination of the precursor at about 1000 °C. The prepared complex Cr 2 O 3 –Al 2 O 3 pigments will be beneficial in applications such as cool materials used for buildings with energy saving performance. [ABSTRACT FROM AUTHOR]