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Three dimensional Not-And (3D NAND) flash memory devices are scaling in the vertical direction to more than 200 oxide/sacrificial wordline nitride layers to further increase storage capacity and enhance energy efficiency. The accurate measurement of the thicknesses of these layers is critical to controlling stress-induced wafer warping and pattern distortion. While traditional optical metrology in the UV-vis-NIR range offers a non-destructive inline solution for high volume manufacturing, we demonstrate in this paper, that mid-IR metrology has advantages in de-correlating oxide and nitride thicknesses owing to their unique absorption signatures. Furthermore, because of the depths sensitivity of oxide and nitride absorptions, the simulated measurement results show the ability to differentiate thickness variations in the vertical zones. Good blind test results were obtained with a machine learning model trained on pseudo-references and pseudo spectra with added skew. [ABSTRACT FROM AUTHOR]

In this paper, a switchable multi-functional wideband metasurface device based on vanadium dioxide (VO2) in the terahertz (THz) region is numerically proposed. By controlling the phase of VO2, the functions of the proposed structure can be reversibly switched between a perfect absorber and two different polarization conversions. When VO2 is in the metallic phase, the absorption response can be achieved with an absorption rate above 0.9 in the range of 1.42 - 4.13 THz for wide-angle incidence, contributed by the highly lossy feature of VO2 metallic. Meanwhile, when VO2 acts as an insulator phase, the proposed metasurface exhibits linear-to-linear polarization conversion in the range of 1.27 - 3.53 THz and linear-to-circular polarization conversion in the frequency ranges of 1.08 - 1.24 THz and 3.65 - 5.27 THz simultaneously. This work opens new avenues for realizing multi-functional wideband metasurface devices in the terahertz range.

This letter reports a SnS2 and ZnO nanocomposite (NC) prepared by dispersion method. The nanocomposite shows promising characteristics for optoelectronic application. SnS2:ZnO NC shows a wide absorption spectrum covering ultraviolet (UV)-visible-near infrared (NIR) regions. Hence, using the proposed nanocomposite a broadband photodetector with a structure comprising Al/ SnS2:ZnO/PEDOT:PSS/ Indium Tin Oxide (ITO) is fabricated. At a bias voltage of 1 V, the measured responsivity values (A/W) of the proposed device are 140.41, 848.63, and 1094.48 at 350 nm (UV), 750 nm (visible) and 900 nm (NIR), respectively. [ABSTRACT FROM AUTHOR]

We are developing a 70-dB SNR 1,000 frame-per-second absorption imaging system, and it enables us to visualize gas concentration distribution in a vacuum chamber for semiconductor manufacturing. This article presents the measurement and analysis of temporal and steady-state NO2 gas concentration distribution using this system. In the steady-state gas flow condition which supplied from a shower plate, the spectrometry of NO2 gas shows a good linear relationship between the absorbance and the partial pressure with the system. The system enables the visualization of steady-state of NO2 gas concentration distribution which supplied from one hole of shower plate, and the results are compared with simulation to discuss the reasonability of the measurement. The system also enables to visualize the temporal NO2 gas concentration distribution supplied from a single nozzle, and the advantages of the system’s features of 70-dB and 1,000 fps are discussed. [ABSTRACT FROM AUTHOR]

Glancing angle deposition (GLAD) technique has been carried out to deposit the titanium dioxide (TiO2) nanowires (NW) on the silicon (Si) substrate. The TiO2 NW is coated with a gold (Au) thin film (TF) and annealed at a very high temperature. The morphological characteristics have been reported using field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM) analysis. The structural characteristics have also been reported using the X-ray diffraction (XRD) analysis. In the visible region, an overall 2.57 fold optical absorption enhancement has been observed. The annealed Au TF/TiO2 NW based device shows boosted photosensitivity (56 fold), improved rectification ratio (2.5 fold), low ideality factor (1.46), high detectivity (3 × 109 Jones), and low noise equivalent power (NEP) (2.92 × 10−9 watt). All the improved electrical performances ensure that the annealed Au TF/TiO2 NW based device can be commercially used as a reliable photodetector. [ABSTRACT FROM AUTHOR]

Photodetectors and imagers with short-wave infrared (SWIR) sensitivities at wavelengths beyond the bandgap of silicon (Eg ∼ 1.1 eV) are currently subject to a rapidly expanding application space. Despite continued technological advancement, linear dynamic range and high costs associated with processing III–V semiconductors still limit their widespread usage. Solution-processed colloidal quantum dots (CQDs) – with tunable bandgap and strong light-matter interaction – are highly suitable as infrared absorbers for cost-effective photodetection. Here, we demonstrate a SWIR photodiode architecture with n-type zinc oxide (ZnO) nanomembrane and p-type lead sulfide (PbS) CQD thin film vertically stacked together. In the PbS CQD/ZnO nanomembrane p-n photodiode, the incident SWIR photons can be absorbed in the PbS CQD film and the photoexcited electron-hole pairs are rapidly separated by the built-in electric field. The fabricated heterojunction p-n photodiodes show a broad short-circuit photocurrent response with a peak responsivity of ∼75 mA/W and a maximum linear dynamic range of ∼100 dB. Finally, to demonstrate the uniformity and scalability, 16 × 16 photodetectors arrays (256 pixels) have been realized showing a very uniform photocurrent response with Isc in the range of 100 nA. As the increased maturity of large-scale film fabrication of PbS CQD along with CMOS readout integrated circuit technology, focal plane arrays can be realized based on PbS CQD/ZnO nanomembrane heterostructures. [ABSTRACT FROM AUTHOR]

We perform first-principles calculations, to investigate the effect of doping of metal dopants (silver, gold, aluminum, and copper) on the electronic, structural, and optical properties of strontium titanate SrTiO3. We further explore the impact of varying strains (compressive and tensile) on the optical and electronic properties of pristine and doped SrTiO3. Our results suggest that the undoped SrTiO3 exhibit a wide bandgap of 2.94 eV with high optical absorption only in the ultraviolet (UV) region. On doping the SrTiO3 by metal dopants, the bandgap is significantly narrowed and most of the absorption peaks are found to shift towards the lower energies (red-shift). Subsequently, the absorption is enhanced in the visible region (∼380 to 800 nm) of the optical spectrum. The decrease in the bandgap is highest for the dopant Cu. Further, on the application of the biaxial strain (tensile and compressive) on the doped-SrTiO3, the optical absorption is enhanced in the visible region, particularly for the high compressive strain of 20%. The calculated refractive index and dielectric function for all the structures of SrTiO3 are found to follow the trend of absorption. The enhanced absorption in the desired visible region through doping and strain may find prospective applications in the fabrication of photovoltaic, optoelectronic, and solar devices. [ABSTRACT FROM AUTHOR]

An efficient light management technique is proposed and investigated by developing a TCAD model for microcrystalline silicon based thin film solar cell. The model includes physical insight related to some optical phenomenon like scattering and diffusive reflection of light as well as some electrical phenomena like carrier transport at the heterointerfaces. Two different structures, which comprises of sawtooth and grating-type back nanotextured in combination with random front nano-textured absorber layers are considered in this work. It has been observed that sawtooth or grating type back nano-textured absorber layer results in enhanced absorption of incident photons as compared to the planar back absorber layer. Also, the grating type back-textured absorber provides better performance compared to its sawtooth counterparts. Finally, the depth and period of grating are optimized in order to maximize the power conversion efficiency of grating-back-textured with random-front-textured absorber. The optimized structure results in an efficiency of 12.325% which corresponds to an improvement of 3.975% compared to the random front and back nano-textured thin film solar cell. [ABSTRACT FROM AUTHOR]

Marine data products from the Ocean and Land Color Instrument (OLCI-A) operated onboard the Copernicus Sentinel 3A satellite, were assessed using in situ reference measurements representative of various European Seas. Results from the evaluation of spectral inherent optical properties indicated substantial accuracy for the satellite-derived absorption coefficient of pigmented particles $a_{\text {ph}}^{\text {OLCI-A}}(\lambda)$ , the diffuse attenuation coefficient of downward irradiance $K_{d}^{\text {OLCI-A}}(\lambda)$ , and the backscattering coefficient of marine particles $b_{\text {bp}}^{\text {OLCI-A}}(\boldsymbol {\lambda })$. Conversely, large underestimates were shown for the spectral absorption coefficient of non-water constituents $a_{\text {nw}}^{\text {OLCI-A}}(\lambda)$ and the cumulative absorption coefficient of colored dissolved organic matter and of non-pigmented particles $a_{\text {dg}}^{\text {OLCI-A}}(\lambda)$. [ABSTRACT FROM AUTHOR]

Spectral indices are a well-established remote sensing tool showcased in many studies pertaining to fields such as agriculture, forest ecology, geology, soil sciences, vegetation, water resources, and urban development. In the case of geology, several indices already exist for the purpose of highlighting regions with interesting lithologies, such as iron oxides, aluminous clays, carbonates, and mafic to ultramafic minerals, based on signature absorption features in their reflectance spectra. However, in the literature, most indices often depend on the few, wide spectral bands used by spaceborne multispectral imagers such as Landsat 8 OLI, Terra ASTER, and Sentinel-2 MSI. Hyperspectral imagers are able to glean more detailed compositional information about a rock surface by operating with a myriad of narrow spectral bands, though the usage of these bands in spectral indices for mining and exploration purposes remains largely unexplored. We propose a new bastnäsite index (BI) capable of detecting the presence of rare earth elements (REEs) and mapping their relative abundances across rock surfaces. Using four reference bands and four absorption bands in the visible to near-infrared wavelength range, we apply this novel technique in a laboratory setting to ore samples extracted from the carbonatite body localized in the Sulfide Queen mine in Mountain Pass, CA, USA. Visual inspection is used to compare an index map to corresponding reflectance spectra for areas of known high and low REE abundances, demonstrating its potential as a fast and straightforward means to assess REE ore grade remotely. [ABSTRACT FROM AUTHOR]

Plasmonic metasurfaces operating in the long-wavelength infrared region (LWIR) are primarily employed for bio-sensing and imaging applications. The key factor affecting the capability of LWIR metasurfaces is absorption efficiency, whereas significant advances have been made to attain higher absorption intensities, using metal-insulator-metal (MIM) stack structures, which require a complex nanofabrication process and repetition of the patterning process to develop each layer. This study experimentally demonstrates the integration of metasurfaces with plasmonic resonators. The metasurfaces and plasmonic resonators are developed through electron beam lithography (EBL) on the same plane of the Silicon substrate. The fabricated prototype has broad incident angle stability at a bandwidth of $2\mu \text{m}$ with near-perfect absorption at $8\mu \text{m}$ resonance wavelength when characterized through the Attenuated Total Reflectance - Fourier Transforms Infrared Spectroscopy (ATR-FTIR) setup. The metasurface device achieves tunable resonance behavior when the incident angles reach up to 70°. The proposed integration of metasurface with plasmonic resonators are well-suited for enhanced biosensing applications with metasurface devices.

Hybrid DC circuit breakers (HDCCBs) have excellent performance in terms of breaking speed and low losses and have become the mainstream technology for dc fault isolation in dc grids. However, because of the breaking limitation of the solid-state branch in HdcCBs, the existing HdcCBs are still weak in breaking high current. Although the breaking ability can be improved by series and parallel devices, the cost will increase rapidly, and it is very difficult to share current by paralleling a single type of device. This article proposes a novel HdcCB topology that combines two types of power electronic devices in solid-state branches to meet the requirement of a large breaking capacity in dc grids. The structure and breaking principles of this novel topology, which aims to break high current, are introduced. The design methods of the key parameters are presented. Then, the current breaking ability of the novel topology is verified by developing a prototype completing a 10 kV/60 kA breaking test in 3 ms. Finally, this novel topology is compared with other dc circuit breakers at the same breaking level, which shows that this novel topology has obvious advantages in cost and volume with fewer devices. [ABSTRACT FROM AUTHOR]

Acute myocardial infarction can be diagnosed by measurement of myoglobin concentration in urine. In this article, a label-free sensor is proposed based on the photothermal effect of biomolecules, which is used for the immediate detection of myoglobin concentration in urine. The sensor was fabricated by a tapered microfiber (TMF) coated with the ultraviolet-curable polymer (UVCP) that has a high thermo-optical coefficient. The photothermal signal of biomolecules is only related to their selective absorption, while the scattering and reflection losses do not generate the photothermal signal. Therefore, specific identification could be realized by detecting the photothermal signal of myoglobin biomolecules. The results showed that the UVCP could enhance the detection sensitivity of photothermal signal due to the high thermo-optic coefficient. The sensitivity of the device is −267.652 nm/(mg/mL) in the concentration range of 0.0001–0.10 mg/mL, the limit of detection (LOD) is 74.7 ng/mL, and the response time is only 30 s. It can meet the demand for detecting the myoglobin concentrations in the urine analysis of related diseases, which has potential applications in the early diagnosis and quantitative analysis of conditions. [ABSTRACT FROM AUTHOR]

In this paper, we introduce a novel method for reconstructing surface normals and depth of dynamic objects in water. Past shape recovery methods have leveraged various visual cues for estimating shape (e.g., depth) or surface normals. Methods that estimate both compute one from the other. We show that these two geometric surface properties can be simultaneously recovered for each pixel when the object is observed underwater. Our key idea is to leverage multi-wavelength near-infrared light absorption along different underwater light paths in conjunction with surface shading. Our method can handle both Lambertian and non-Lambertian surfaces. We derive a principled theory for this surface normals and shape from water method and a practical calibration method for determining its imaging parameters values. By construction, the method can be implemented as a one-shot imaging system. We prototype both an off-line and a video-rate imaging system and demonstrate the effectiveness of the method on a number of real-world static and dynamic objects. The results show that the method can recover intricate surface features that are otherwise inaccessible. [ABSTRACT FROM AUTHOR]

Recognizing wet surfaces and their degrees of wetness is essential for many computer vision applications. Surface wetness can inform us slippery spots on a road to autonomous vehicles, muddy areas of a trail to humanoid robots, and the freshness of groceries to us. The fact that surfaces darken when wet, i.e., monochromatic appearance change, has been modeled to recognize wet surfaces in the past. In this paper, we show that color change, particularly in its spectral behavior, carries rich information about surface wetness. We first derive an analytical spectral appearance model of wet surfaces that expresses the characteristic spectral sharpening due to multiple scattering and absorption in the surface. We present a novel method for estimating key parameters of this spectral appearance model, which enables the recovery of the original surface color and the degree of wetness from a single multispectral image. Applied to a multispectral image, the method estimates the spatial map of wetness together with the dry spectral distribution of the surface. To our knowledge, this is the first work to model and leverage the spectral characteristics of wet surfaces to decipher its appearance. We conduct comprehensive experimental validation with a number of wet real surfaces. The results demonstrate the accuracy of our model and the effectiveness of our method for surface wetness and color estimation. [ABSTRACT FROM AUTHOR]

Despite the wide applications of Short-Wave Infrared (SWIR) imaging, standard imaging materials require lattice matching and complex fabrication techniques which are relatively expensive. In this work, 50 nm thick amorphous indium telluride (In2Te3) films are RF sputtered on a silicon (Si/SiO2) substrate at 300 °C. The films are used to fabricate back-gated field-effect photodetectors (PDs). The In2Te3 detectors exhibit a broad wavelength response from 406 nm to 1600 nm and a photo responsivity of 0.44 AW− under 1310 nm excitation at 5 V bias. Furthermore, an n-type behavior with an electron field-effect mobility of 1.15 cm2V−1s−1 is observed. We also evaluated the frequency response of the PDs under 1310 nm modulated light, a 3dB cut-off frequency of ~0.97 MHz is measured. Further, the device exhibits specific detectivity of $1.32\times 10^{{9}}$ Jones and very good stability at ambient conditions. [ABSTRACT FROM AUTHOR]

This article uses the first principle calculations, to investigate the electronic (and optical) attributes of NOx-adsorbed gas molecules. To begin, the electronic properties demonstrate the NOx molecules’ effect on the Mn-doped MoSe2 monolayer (ML) in terms of adsorption energy [ ${E}_{{({\mathrm {ads}}{)}}}$ ], charge transfer (${Q}_{\text {T}}{)}$ , sensitivity ($\eta {)}$ , and total density of states (TDOS). Second, the shift in the absorption spectrum ($\alpha {)}$ of NOx gas molecules adsorbed on the Mn-doped MoSe2 ML demonstrates the optical property. In addition, the adsorption of NO2 gas on the Mn-doped MoSe2 ML exhibits improved interaction, with the highest adsorption energy of −4.01 eV and a charge transfer of $- 0.06{e}$ , respectively. Later, the increased interaction is justified by the TDOS behavior, which exhibits an increase in impurities near the Fermi level in comparison with NO and $\text{N}_{{2}}\text{O}$ gas molecules. In addition, the activated Mn atom has a higher sensitivity to the NO2 gas molecule, measuring 56.24%. Finally, the NO2 gas molecule is more amenable to infrared (IR) detection than the Mn-doped MoSe2 ML, as the absorption spectrum shifts to the IR region. As a result, the NOx gas-adsorbed Mn-doped MoSe2 ML is well suited for IR photodetector applications due to its high-IR light absorption molecules. [ABSTRACT FROM AUTHOR]

In this work, a highly sensitive and selective colorimetric sensor assay for the detection of Hg2+ ions is presented utilizing an anti-aggregation sensing approach with AuNPs and an amine-based sensor (S). The addition of sensor (S) to gold nanoparticles (AuNPs) leads to their aggregation and, thus, color variation from red to blue. Whereas, in the presence of Hg2+ ions, i.e., target analyte, sensor (S) preferably binds with Hg2+ ions over AuNPs, thus inhibiting their aggregation and subsequent color change. This approach is optimized in terms of various analytical parameters, such as incubation time, pH effect, and S titration, to obtain high sensitivity and selectivity. The UV–Visible studies validate the performance of developed approach for Hg2+ ions detection, and the limit of detection was calculated to be 17 nM, which is lesser than its permissible limit (30 nM) in drinking water. The interference studies also confirm the develop approach selectivity for Hg2+ ions even in the presence of coexisting ions in aqueous media. The experimental results were further correlated with the computational density functional theory (DFT) studies. The presented method is not only cost-effective but also eliminates tedious synthetic schemes and need of laborious manpower. [ABSTRACT FROM AUTHOR]

In this study, we proposed an electrical readout surface plasmon resonance (SPR) chemical sensor coupled with visible light to avoid significant light absorption of water. A gold (Au) diffraction grating with a submicrometer pitch, designed using the rigorous coupled-wave analysis (RCWA) method to have a submicrometer pitch and 30-nm-tall Au grating, was fabricated on an n-type silicon (n-Si) substrate by nanofabrication/microfabrication. The hot electrons generated by SPR on the grating are converted to photocurrent at the Schottky barrier at the Au/n-Si interface. The Schottky barrier height was estimated based on the $I-V$ characteristics of the device to be $q{\Phi}_{\mathrm{B}} = 0.72$ eV. The peak shift of photocurrent was observed to be consistentwith the SPR theory when the concentration of glucose aqueous solution changed the refractive index on the Au grating. Assuming that the incident wavelength and angle were fixed, a noise analysis revealed a refractive index resolution of $3.4-4.0 \times 10^{-5}$ refractive index unit (RIU). Since this structure eliminated bulky optics in SPR experiments, it would contribute to the realization of a thin single-chip label-free chemical sensor. [ABSTRACT FROM AUTHOR]

A strategy for folded reflectarray with low detectability and enhanced gain is presented. For this purpose, the folded reflectarray is engineered by combining a polarization-conversion focusing metasur face, a frequency selective rasorber (FSR), and a diffusion metasurface. The FSR is composed of a lossy layer and a single-polarized I-shaped slotted frequency selective surface (FSS) between which the diffusive metasurface is inserted for in-band and out-of-band cross-polarized low radar cross-sectional (RCS) reduction. Therein, the antenna exhibits a transmission window and two out-of-band absorptions due to the FSR for copolarized radiation. In contrast to detection, the system operated for both wave incidence. For cross-polarized wave incidence, a wideband RCS reduction is enabled due to the wave-diffusive function of the inserted metasurface. The half profile is guaranteed since the wave bounces two times between the FSS and focusing metasurface. The numerical and experimental results coincide well, both indicating that the low-RCS folded reflectarray achieves a peak gain of 23.5 dB at 10.2 GHz, corresponding to an aperture efficiency of 37.8%. Besides, the antenna achieves 10 dB monostatic RCS reduction within 4.6–8.8 GHz and 9.6–13.8 GHz under copolarization detection and a 10 dB monostatic RCS reduction from 8.4 to 19 GHz under cross-polarization detection. Moreover, good invisibility is also achieved for bistatic RCS reduction. [ABSTRACT FROM AUTHOR]

In this letter, a broadband and low-profile frequency selective rasorber with an ultraminiaturized metal-graphene structure is proposed. The rasorber is composed of two cascaded layers separated by an air spacer, a top lossy layer and a bottom bandpass layer. By supplying four branches of multiple vias connected stubs across a circular graphene film in the lossy layer, extra inductance and capacitance effects are introduced while guaranteeing the impedance match with the free space at the low frequency range. As a result, a broad absorption bandwidth is ensured from 1.96 to 10.2 GHz with the thickness of only 0.076 λl at the lowest absorption frequency, and the passband is at 12.83 GHz with the insertion loss of 0.23 dB. Besides, a miniaturized unit is achieved with the period of 0.039 λl, and the oblique angle incidence response is up to 45° for TE and TM modes. To validate the design, a prototype is fabricated and measured, and the screen-printing process of graphene resistive film as omnidirectional resistance is adopted to further reduce the profile. The measured results agree well with the simulation. [ABSTRACT FROM AUTHOR]

For the issue of stealth and electromagnetic protection of weapons and equipment in strong electromagnetic pulse (EMP) environment, a transparent double-sided ultrawideband metasurface is proposed to realize wave absorption/absorption-scattering integrated function in two directions. The interactions between backscattered waves produced by the meta-atoms are manipulated and optimized simultaneously by hybrid far-field scattering pattern analysis and genetic algorithm to achieve ultra wideband destructive interference. Both simulation and experimental results demonstrate that the polarization-independent −10 dB radar cross section (RCS) reduction performance can be achieved for more than 128% of the fractional bandwidth (4–18.25 GHz) at forward incidence, and the absorption more than 90% in the range of 2.75–12.75 GHz (fractional bandwidth more than 129%) at backward incidence. The scattering and absorbing characteristics of the metasurface are still well maintained in the range of 0°–50° oblique incidence. The method of this letter has an important prospect in related applications of strong EMP stealth and protection technology under the integration of information equipment in the future. [ABSTRACT FROM AUTHOR]

This letter presents a thermal-switchable absorbing metasurface (TSAM) based on the phase-change material vanadium dioxide (VO2). A thin film of VO2 was deposited on a sapphire substrate by a magnetron sputtering and then sliced into tiny chips, which can be regarded as thermal-switchable resistors. The surface structure of the TSAM element is realized by loading four VO2 chips into a square metallic loop. Polytetrafluoroethylene is used as the substrate due to its high thermal conductivity compared with foam. The TSAM is on the “absorbing” mode when it is heated underneath and is switched into the “reflecting” mode at room temperature. The absorbing mode is characterized by a wideband absorption (8–22 GHz). A TSAM prototype was fabricated and measured to verify this design. [ABSTRACT FROM AUTHOR]

Wider absorption bandwidth is usually accompanied by more complex and difficult designs and more computational costs, which have become an important challenge in the field of electromagnetic scattering. In this communication, the backend-balanced-impedance (BBI) new design concept is put forward for the first time which considers the backplane as an impedance that can be designed to compensate for the impedance of a known absorber for expanding the absorption bandwidth. Based on the design concept, an ultra-wideband absorber derived from the known absorber is proposed simply and efficiently. The matched BBI for the known absorber is obtained and developed using reverse calculation. To explain the operating principle, a simple equivalent circuit (EC) model of the ultra-wideband absorber from the BBI is established. Furthermore, the electric field and surface loss distribution are systematically analyzed to demonstrate physical pictures of the BBI. The simulated and measured results show that the designed ultra-wideband absorber from the BBI can get an absorption band in the 0.87–9.28 GHz range (165.8%) for at least 10 dB of reflection reduction at the normal incidence with a thickness of only $0.0858\lambda _{L} $ , where $\lambda _{L} $ denotes the wavelength at the lowest operating frequency. [ABSTRACT FROM AUTHOR]

Insects are exposed to environmental radio frequency electromagnetic fields (RF-EMFs), which are partially absorbed by their body. This absorption is currently unknown for most insect types. Therefore, numerical simulations were performed to study the far-field absorption of RF-EMFs by different insect types at the frequencies between 2 and 120 GHz, which are (expected to be) used in (future) wireless communication. The simulations were done using anatomically accurate as well as spheroid models of the insects. The maximum absorbed power, which ranged from 7.55 to 389 nW for an incident electric field strength of 1 V/m for the studied insect types, was obtained at wavelengths comparable to the insects’ size. We created a log-linear model that can estimate absorbed power in insects with an average relative error of $ < 43\%$ between 6 and 120 GHz using only the insects’ volume and the frequency as an input using the simulation results. In addition, our simulations showed a very high correlation ($r > 0.95$) between the absorbed power predicted with anatomically accurate insect models and those predicted with spheroid models at the frequencies between 6 and 24 GHz. This suggests that such models could be used to evaluate the RF-EMF exposure of insects in future studies. [ABSTRACT FROM AUTHOR]

Signals acquired by optoacoustic tomography systems have broadband frequency content that encodes information about structures on different physical scales. Concurrent processing and rendering of such broadband signals may result in images with poor contrast and fidelity due to a bias towards low frequency contributions from larger structures. This problem cannot be addressed by filtering different frequency bands and reconstructing them individually, as this procedure leads to artefacts due to its incompatibility with the entangled frequency content of signals generated by structures of different sizes. Here we introduce frequency-band model-based (fbMB) reconstruction to separate frequency-band-specific optoacoustic image components during image formation, thereby enabling structures of all sizes to be rendered with high fidelity. In order to disentangle the overlapping frequency content of image components, fbMB uses soft priors to achieve an optimal trade-off between localization of the components in frequency bands and their structural integrity. We demonstrate that fbMB produces optoacoustic images with improved contrast and fidelity, which reveal anatomical structures in in vivo images of mice in unprecedented detail. These enhancements further improve the accuracy of spectral unmixing in small vasculature. By offering a precise treatment of the frequency components of optoacoustic signals, fbMB improves the quality, accuracy, and quantification of optoacoustic images and provides a method of choice for optoacoustic reconstructions. [ABSTRACT FROM AUTHOR]

Image contrast in multispectral optoacoustic tomography (MSOT) can be severely reduced by electrical noise and interference in the acquired optoacoustic signals. Previously employed signal processing techniques have proven insufficient to remove the effects of electrical noise because they typically rely on simplified models and fail to capture complex characteristics of signal and noise. Moreover, they often involve time-consuming processing steps that are unsuited for real-time imaging applications. In this work, we develop and demonstrate a discriminative deep learning approach to separate electrical noise from optoacoustic signals prior to image reconstruction. The proposed deep learning algorithm is based on two key features. First, it learns spatiotemporal correlations in both noise and signal by using the entire optoacoustic sinogram as input. Second, it employs training on a large dataset of experimentally acquired pure noise and synthetic optoacoustic signals. We validated the ability of the trained model to accurately remove electrical noise on synthetic data and on optoacoustic images of a phantom and the human breast. We demonstrate significant enhancements of morphological and spectral optoacoustic images reaching 19% higher blood vessel contrast and localized spectral contrast at depths of more than 2 cm for images acquired in vivo. We discuss how the proposed denoising framework is applicable to clinical multispectral optoacoustic tomography and suitable for real-time operation. [ABSTRACT FROM AUTHOR]

For massive dosimetry analysis of finite composite materials (CMs), an effective approach is proposed based on the artificial intelligence (AI) computational algorithm. Machine learning (ML) method is used to analyze systematic energy characteristics of finite CM for dosimetry exposure. To prove the validity of our algorithm, we analyzed a finite CM in a specific scenario and calculated its exposure dosimetry. The dataset for algorithm training is obtained through numerical analysis. In order to verify the efficiency of the proposed method, the accuracy of this algorithm and its time consumption are calculated. Five ML methods are subsequently used to compare the average absorption of the energy resonance exposure in the finite CM. The time consumption and error distribution in test sets prove the high efficiency of AI computational algorithm to analyze the energy exposure of finite CM. The proposed AI algorithm is useful for the massive numerical analysis in the research of electromagnetic (EM) exposure. [ABSTRACT FROM AUTHOR]

Here, we have fabricated high-performance near-infrared (NIR) tunnelling heterojunction photodetectors (THPDs) by in-situ synthesizing three-dimensional (3D) graphene on Si with the insertion of a high- $\kappa $ tunnelling layer. Combining the high light-harvesting ability of 3D-graphene and the effective dark current suppression of the high- $\kappa $ tunnelling layer, the trade-off between the ultrafast response and ultra-sensitivity in graphene-based PDs is successfully bridged. Our device exhibits ultra-high responsivity (11.2 A/W), excellent specific detectivity (${5}.{9} \times {10}~^{{10}}$ Jones), and ultra-fast response ($168~\mu \text{s}$) at the communication wavelength (1550 nm). This work provides a universal strategy to fabricate high-performance and low-cost graphene/silicon PDs in the communication wavelength. [ABSTRACT FROM AUTHOR]

In this article, a unique structure of InP/InGaAs uni-traveling-carrier photodiode (UTC-PD) is proposed. Compared with the one-sided junction photodiode, the UTC-PD has the advantages of a simpler epitaxial layer structure while maintaining the characteristics of high bandwidth and high output power loss profile. Simulated results show that a large built-in electric field can be generated under illumination, which aids UTC carrier velocity. The merits of the new structure are compared to a standard UTC-PD photodiode in terms of improved electric field and carrier concentration response. It is demonstrated that the photogeneration and light absorption of UTC-PDs are improved by incorporating a step-like internal texture of cones and dots profile in the photo absorption layer. The simulated device shows a peak electrical 3-dB bandwidth of 65 GHz at a low light intensity and reverse bias voltage. The performance characteristics of 1-D and 2-D UTC-PD simulations, including internal electric field distribution, energy band diagram, carrier concentration, and power loss distribution, are carefully studied. A theoretical discussion of the working principle and key performance characteristics of a UTC-PD enhanced by heterojunction design is reported. The entire physical environment is modeled and simulated through the finite element method (FEM) using commercial software. The proposed photodiode structure configuration is designed and optimized for photodetectors for high RF frequencies at different light intensities. To the best of our knowledge, the obtained bandwidth and electric field response is the fastest among those reported for other various higher wavelength photodiodes. [ABSTRACT FROM AUTHOR]

Optical spectroscopy supporting studies of the absorption and emission of light by matter is a widely used tool in analytical science. Recently, optical spectrometers have shown tremendous improvement on spectral resolution, accuracy, sensitivity, as well as miniaturization. However, most of them still only focus on single or few sensing modes, thus limiting their practical applications especially in complex sensing conditions. To extend applications, integrating with absorption, emission, and surface plasmon resonance (SPR) spectroscopy detecting systems, a multipurpose optical fiber smartphone spectrometer (MOSS) is designed to provide a point-of-care testing platform for on-site biochemical sensing. The MOSS is compact only with the size of $167\times 75\times85$ mm3 but has excellent sensing performance. It reaches extremely high spectral resolution as $\approx 10$ nm and has been successfully applied in nitrite, mercury ion, and sucrose sensing. In addition, the MOSS is equipped with inner power supply and user-friendly smartphone application, especially suiting for on-site applications. Since the MOSS can support high-quality multipurpose spectral detection but only using compact and cost-effective devices, it can be a potential point-of-care testing platform especially for biochemical sensing applications. [ABSTRACT FROM AUTHOR]

High-speed, high-photoresponse, low-dark current photodetectors with monolithic integration compatibility on a single silicon chip have a vital role in fast optical communication. Schottky barrier junctions provide ultrahigh frequency bandwidth, which makes them suitable for a high-speed data rate optical transition system. In this article, we proposed the novel design of a fast silicon Schottky photodiode (SPD) with reduced dark current and enhanced photocurrent to produce an acceptable photo-to-dark current ratio. A differential Schottky diode is used in series with the designed SPD to reduce the final dark current delivered to the next signal conditioner electronics. A periodic plasmonic array is applied on the metallic Schottky contact layer to localize the illuminated light in the depletion region of Schottky contact and increase light absorption, which results in enhanced photocurrent. The simulation of the device shows a photo-to-dark current ratio of ${2.7} \times {10}^{{4}}$ at room temperature and 1-V biasing voltage, which is much greater than the reported data. Moreover, the design results in a complementary metal–oxide–semiconductor (CMOS)-compatible photodiode (PD) with 28-time reduced dark current and 3.3-time photocurrent enhancement with an ultrafast impulse response of 3 ps and a response time of about 18 ps that are attractive for ultra-fast data communication applications in 850-nm region. [ABSTRACT FROM AUTHOR]

In this letter, a Jaumann-like rasorber is examined, which offers wideband absorption, angular stability, and polarization insensitivity. The structure is defined by three layers with two resistive layers placed on top of a lossless layer. Also, all three layers use capacitors to ensure compactness for the metasurface rasorber. These features provide broad angular stability up to 55 $^{\circ }$ while also maintaining low insertion losses of about 1 dB and only 0.6 dB for measured broadside incidence. The rasorber is also shown to work as a radome in the near field of a planar antenna array. [ABSTRACT FROM AUTHOR]

A miniaturized frequency-selective rasorber (FSR) with absorption bands on two sides of the passband is proposed in this letter. The FSR consists of one lossy frequency-selective surface (FSS) layer and one lossless FSS layer, and the equivalent circuit model of FSR is also analyzed. The lossy layer is composed of four electric-field-coupled (ELC) resonators loaded with four resistors, and the lossless layer comprises a square slot structure. By embedding the ELC resonators into the square ring, a passband can be added betweven the absorption bands, and the size of the overall structure can be reduced. Under normal incidence, the −10 dB absorption band is from 4.28 to 8.98 GHz and from 11.19 to 13.77 GHz. The transmission operates at 10 GHz and has a small insertion loss with 0.25 dB. Furthermore, an FSR prototype was fabricated and measured. The measured results are consistent with the simulation ones. The proposed FSR can be applied to radar radome to enhance the anti-interference and stealth performances. [ABSTRACT FROM AUTHOR]

In this letter, a terahertz metamaterials broadband perfect absorber based on molybdenum disulfide (MoS2) is proposed. The absorber is composed of a single layer of MoS2 and a metal plate separated by thin polyimide. The single-layer MoS2 structure consists of four rectangular strips transformed by fillets. Numerical simulations using the full-vector finite element method show that the new hybrid mode can be excited and the electric fields of the modes can be redistributed due to the introduction of fillet MoS2 metamaterial structure, which effectively improves the absorption performance of devices. In the range of 1.8 to 2.2 THz, the absorptivity is close to 100%, and the absorption curve is much flatter, which provide a useful reference for terahertz modulation equipment. [ABSTRACT FROM AUTHOR]

In this article, an ultrawideband radar scattering cross-sectional (RCS) reduction metasurface is proposed combining with absorption and phase cancellation mechanisms. A two-lossy-layer absorber is designed first, which is responsible for low-frequency RCS reduction from 1.3 to 13 GHz (163.6%). The equivalent circuit of the proposed absorber is extracted, proving that the wideband characteristic is due to the occurrence of multiple resonant points. To further improve the RCS reduction bandwidth, the absorber unit cells are arranged with three different heights. The proposed metasurface consists of 3 $\times $ 3 lattice and each lattice has $5\times $ 5 absorber unit cells. The reflection phase cancellation can be achieved owing to the wave path difference by introducing a certain height difference among subregions. Resultantly, high-frequency RCS reduction from 13 to 40 GHz can be realized since the backward scattering can be canceled effectively. Finally, an extraordinary ultrawideband RCS reduction from 1.5 to 40 GHz (185.5%) can be acquired successfully. A prototype with the thickness of 0.104 $\lambda _{L}$ (the wavelength of the lowest working frequency) is fabricated and measured for demonstration. The measured results prove that the proposed design has numerous advantages including minimization, ultrawideband RCS reduction, wide angle stability, and polarization insensitivity. [ABSTRACT FROM AUTHOR]

In this work, the authors report the design, fabrication, and measurement of an optically transparent microwave absorber with tunable absorption frequency based on a patterned graphene-indium tin oxide (ITO) hybrid structure. This transparent tunable absorber consists of periodically patterned graphene sandwich structure (GSS), patterned ITO layer, polyvinyl chloride (PVC) substrate, and ITO ground plane from top to bottom. The frequency of absorption peak can be dynamically controlled from 13.1 to 10.9 GHz by regulating the bias voltage between the graphene sheets from 0 to 30 V, in which the peak absorption rates remain above 99.7%. An inhomogeneous doping model for graphene patterns is introduced to interpret the usage of relatively large tuning voltage. The engineered absorber also exhibits polarization-insensitive characteristics attributed to the symmetrical design of the hybrid structure. This tunable device with optical transparency achieved by integrating different transparent materials has broad prospects in the application of multispectral and smart stealth technology. [ABSTRACT FROM AUTHOR]

This work presents an ultrawideband and high-absorption structure (UHAS) obtained by a fast design method. The proposed analytic approach can simply design single-FSS-layer absorber with ideal performance stability under oblique incidence. With this method, a composite material reinforced circuit-analog (CA) absorber is designed, which unit cell of CA sheet is a square ring loaded with chip resistors. At normal incidence, both TE and TM polarization simulation results demonstrate that the reflection coefficient below −10 dB is from 5.8 to 22.2 GHz; meanwhile, the reflection coefficient less than −20 dB covers a bandwidth of 7.4–19.1 GHz; also, there is a −30 dB absorption band in 8.6–17.2 GHz. Furthermore, under TE polarization, experimental results indicate that UHAS can maintain a −20 dB absorption band from 7.4 to 18.0 GHz within 40° oblique angle of incidence; −10 dB absorption is from 6.1 to 18.0 GHz within 50° oblique angle of incidence. The good agreement between simulation and measurement validates the proposed method and UHAS. Finally, the UHAS, with a protective layer, will be more stable in practical applications. [ABSTRACT FROM AUTHOR]

The active and dynamic controls of the terahertz (THz) waves are highly demanded due to the rapid development of wireless communication systems. Recently, reconfigurable intelligent surface (RIS) has gained significant attention due to their tremendous potential in controlling electromagnetic (EM) waves. Particularly, RIS-based wireless communications are promising for improving the system’s performance by properly designing the reflection coefficient of the unit cell. In this work, we investigate a graphene-based RIS for active and dynamic controls of THz waves. The RIS design consists of a rectangular graphene meta-atoms periodic array placed over a metallic grounded silicon substrate. An equivalent circuit modeling of the RIS design and its solution is provided. The RIS performance is numerically analyzed. The graphene-based RIS achieves nearly 100% reflection in the operational frequency ranging from 0.1 to 4 THz. The perfect reflection is insensitive to the polarization and the incident angles. Moreover, 100% absorption in the graphene-based RIS is achieved by electrically reconfiguring the meta-atom response via the chemical potential of the graphene. The graphene RIS also achieves the anomalous reflection performance by controlling the number of unit cells and phase gradient of RIS. In view of the effective THz wireless communication environment, this article finally presents a simple RIS-aided communication model to study the impact of the proposed graphene-based RIS on the signal-to-noise ratio (SNR). The results reveal that the proposed THz RIS with graphene meta-atoms is promising for efficient and intelligent THz wireless communications. [ABSTRACT FROM AUTHOR]

The limitation in enhancing the absorption bandwidth of a microwave absorber with a thin thickness in the ultrahigh-frequency (UHF) band has become an important challenge in the electromagnetic scattering field. Therefore, in this study, a new design concept of a single-layer impedance-well (IW) passive absorber is put forward for the first time, which exhibits enhanced absorption bandwidth and effectively approaches the Rozanov thickness limit for passive absorbers. The design concept proposed in this study is explained by using the transmission line theory and circuit analysis. Furthermore, the input impedance and electric field distribution of the IW absorber are systematically analyzed to demonstrate the existence of the IW effect and the predicted functionalities of the IW absorber. The simulated and measured results reveal that the designed absorber with the IW effect can obtain an absorption band over 0.45–2.34 GHz range with a bandwidth ratio (BWR) of 5.2 (135.5%) at a minimum reflectivity reduction of −10 dB under normal incidence. In addition, the thickness is reduced to $0.069 \lambda _{{\mathrm {L}}}$ , which $\lambda _{{\mathrm {L}}}$ is the wavelength at the lowest operating frequency. [ABSTRACT FROM AUTHOR]

Due to the refraction and absorption of light by water, underwater images usually suffer from severe degradation, such as color cast, hazy blur, and low visibility, which would degrade the effectiveness of marine applications equipped on autonomous underwater vehicles. To eliminate the degradation of underwater images, we propose a target oriented perceptual adversarial fusion network, dubbed TOPAL. Concretely, we consider the degradation factors of underwater images in terms of turbidity and chromatism. And according to the degradation issues, we first develop a multi-scale dense boosted module to strengthen the visual contrast and a deep aesthetic render module to perform the color correction, respectively. After that, we employ the dual channel-wise attention module and guide the adaptive fusion of latent features, in which both diverse details and credible appearance are integrated. To bridge the gap between synthetic and real-world images, a global-local adversarial mechanism is introduced in the reconstruction. Besides, perceptual information is also embedded into the process to assist the understanding of scenery content. To evaluate the performance of TOPAL, we conduct extensive experiments on several benchmarks and make comparisons among state-of-the-art methods. Quantitative and qualitative results demonstrate that our TOPAL improves the quality of underwater images greatly and achieves superior performance than others. [ABSTRACT FROM AUTHOR]

The active mode-locking technique of quantum cascade lasers is highly affected by the bias current level and the magnitude of the modulated signal because of the spatial hole burning effect. The laser generates a stable pulse train near the threshold current and for a high modulation magnitude. We show in this paper an improvement of these conditions with an integration of a single-layer graphene which works as a saturable absorber. The light-matter interaction in the laser active region is described by the two-level Maxwell-Bloch equations and in graphene layer by the Maxwell-Ampere equation and Maxwell-Bloch equations. These system equations are solved using the Finite-Difference Time-Domain (FDTD) method which has the main advantage of not involving any approximation. The weakly coupled splitting scheme is adopted for the time discretization of Maxwell and Bloch equations. The graphene saturable absorption is modelled by a saturable conductivity in Maxwell-Ampere equation and with the dipole moment and carrier density in Maxwell-Bloch equations. For a QCL structure with a gain recovery time of 50 ps, simulation results show better stability of the mode-locking over a broad range of injection current and for smaller magnitude of the modulated signal. QCL structure with gain recovery time close to 1 ps reveals the generation of two pulses per roundtrip linked to spatial hole burning. The carrier grating strength can be reduced by changing the boundary conditions of the cavity such as with a high reflectivity mirror in front of graphene layer. [ABSTRACT FROM AUTHOR]

A simple concentric circular ring resonator-based nonreciprocal deca-band absorber is demonstrated in this article. The design primarily consists of two frequency selective surfaces (FSSs) among one is a compliment to the other printed on either side of the 2.4 mm thin FR-4 dielectric substrate. It exhibits near-unity absorption effectively at ten different resonant frequencies over X-Ka bands with an average absorption efficiency of 94.6%. Surface plasmonic resonances are the reason behind the multiband absorption and are illustrated with current profiles at resonant frequencies. The performance is angular stable for both transverse electric and transverse magnetic oblique incidences up to 60o. The design is compact with a structural periodicity of 0.210 ${{\boldsymbol{\lambda }}}_{\boldsymbol{o}}$ and thickness 0.07 ${{\boldsymbol{\lambda }}}_{\boldsymbol{o}}$ , where ${{\boldsymbol{\lambda }}}_{\boldsymbol{o}}$ is the lowest resonant peak's free-space wavelength. The 20×20 array of the design prototype is fabricated, and the measured results agree with the full-wave analysis. Bistatic radar cross section (RCS) analysis of the proposed absorber is evaluated along with a conventional microstrip patch antenna at X-band. Since the absorber design is based on complementary FSSs, it can also be utilized as a reflector for the not absorbing frequencies. The novelty lies in the utilization of the dual-side of the substrate made this design compact with a greater number of absorption peaks and good angular stability, making it potential for real-time RCS, and electromagnetic interference and shielding applications. [ABSTRACT FROM AUTHOR]

This article presents the design and analysis of a dual-polarized low-profile circuit analog absorber with wideband absorption characteristics. A lossy frequency selective surface consisting of cross dipole with central resonant node element is used as the top layer, while there is a continuous copper ground as the bottom layer. The novelty of the design lies in the use of the resonant node as it improves the absorption bandwidth by providing an additional resonance without compromising the low-profile nature of the design. It has a thickness of 0.077 $\lambda_{L}$ (where $\lambda {\_}L$ is the wavelength corresponding to the lowest operating frequency). The absorber also exhibits a stable absorption characteristics up to 30 $^\circ$ angle of incidence and can avoid grating lobes within the operating bandwidth up to 40.79 $^\circ$ angle of incidence. An equivalent circuit model is proposed to discuss the operating principles of the design. The validation of the design is done experimentally using the time-gating technique, and a good agreement between the measured and the simulated results is observed. The 10-dB reflection reduction bandwidth and the thickness-to-bandwidth ratio of the absorber are obtained as 108.67% (from 3.58 to 12.10 GHz) and 0.104 from full-wave simulation and 115.67% (from 3.5 to 13 GHz) and 0.098 from measurement, respectively. [ABSTRACT FROM AUTHOR]

The radome absorber, always be defined as rasorber, is investigated based on the SiO $_{2{\text{f}}}$ /SiO $_{2}$ fiber-reinforced ceramic matrix composite (CMC) for high-strength environment. The prominent advantage of using CMC as the substrate is that it can significantly improve the power capacity and environmental tolerance of the rasorber. The surface roughness and the dielectric stability are considered key factors in the electromagnetic-material codesign. Silica sol is used to densify the surface of fiber CMC for screen printing, and the process is characterized by scanning electron microscopy. The passband can be controlled by integrating p-i-n diodes on the screen-printed silver paste. The equivalent ground condition for the bandpass reflector is theoretically analyzed, and ideal integration of absorbing and transmitting functions is achieved. Afterward, a CMC-based rasorber is manufactured. A dual-polarized transmission window at 1.6 GHz can be triggered, while a low radar cross section (RCS) band is designed from 2.7 to 4 GHz. Transmission and RCS characteristics of the CMC rasorber under 25–700 $^{\circ }$ C environment are both analyzed. The tensile, compressive, and flexural strength of the proposed CMC rasorber are tested, and the electromagnetic properties are verified using compact antenna test range. [ABSTRACT FROM AUTHOR]

This article presents the design and analysis of a flexible wideband absorber comprising a resistive ring-shaped frequency-selective surface (FSS). In this article, a novel conductive material is used for the designing and fabrication of the absorber. The structure consists of two different-size rings arranged diagonally as an FSS and printed on a flexible photo paper substrate material using screen printing technology and backed with a copper ground plane. The unit cell dimension is 0.64 $\,\lambda _{l}$ × 0.64 $\,\lambda _{l}$ × 0.088 $\,\lambda _{l}$ , where $\lambda _{l}$ is the wavelength at the lowest absorption frequency. The fractional bandwidth of the proposed absorber is 86.2%, which covers $X$ and Ku bands completely and the lower portion of the $K$ -band. The proposed absorber shows more than 90% absorption in the frequency range of 8–20.12 GHz at different angles of polarization. The effect of various design parameters of the FSS, such as period of unit cell, thickness of the substrate, interelement spacing, and width of the rings, is examined and verified based on the simulation, circuital, and experimental model. [ABSTRACT FROM AUTHOR]

In this article, a novel design of compact frequency-selective rasorber exhibiting absorption–transmission–absorption characteristics and having high passband selectivity has been proposed. A square-loop resonator with mounted lossy elements serves as the elemental broadband absorber from 4 to 12 GHz. The transmission frequency at 8.4 GHz is realized by printing a Minkowski fractal-shaped resonator with a high-Q factor on the resistive layer, which provides a transmission pole at the frequency corresponding to the passband of a double-cross-slot-shaped bandpass layer. The functioning of the proposed frequency-selective rasorbers (FSR) is explained by a corresponding equivalent circuit model. The proposed FSR exhibits higher selectivity at the passband, thus making it a suitable candidate for shielding the narrowband radiating system. Measurements performed on the fabricated prototype of a 17 × 17 unit cell array provides experimental validation. Further, a low radar cross-section antenna is realized by integrating a patch antenna with the proposed FSR, which achieves an average out-of-band monostatic radar cross-section (RCS) reduction of 11.92 and 5.04 dB in the lower (4–7.5 GHz) and upper (9.2–10.8 GHz) absorption bands, respectively, while maintaining the other antenna parameters. Furthermore, the bistatic total RCS reduction of 77% and 60% are achieved in the lower and upper frequency bands, respectively. [ABSTRACT FROM AUTHOR]

A high selectivity frequency selective rasorber (FSR) with an ultraminiaturized unit based on 2.5 dimensional (2.5-D) parallel resonator (PR), exhibiting low insertion loss passband between two absorption bands, is investigated. The lossy unit is realized by inserting a 2.5-D strip-type PR structure into the center of each side of the metal square ring and loaded with resistors connected at the four corners. The novel 2.5-D PR consists of interdigitated capacitors and strip metal wire connecting the other side of the lossy layer obtained by using metallized vias. The 2.5-D PR can effectively alleviate the congestion of the single-sided structures and achieve a high degree of miniaturization by means of tortuous extension inductance structure; as an additional feature, the values of L and C can be independently adjusted to determine the passband frequency allowing to provide additional degree of freedom to the design. An equivalent circuit model is proposed to analyze its operating principle. The dimensions of the miniaturized unit are 0.13λf × 0.13λf × 0.16λf (being λf the free space wavelength at the passband). A transmission window with low insertion loss of 0.125 dB is obtained at 4.65 GHz under normal incidence. The fractional bandwidth for −10 dB reflection is about 96%. The miniaturized FSR satisfies the characteristic of polarization insensitivity (TE and TM) and angular insensitivity (up to 45°). A prototype of miniaturized FSR has been manufactured and measured, showing a reasonable agreement with simulations. [ABSTRACT FROM AUTHOR]

Metamaterials have attracted enormous interest in the perfect absorber. In sharp contrast to most previous studies focused on improving absorption coefficient and expanding absorption bandwidth, this article presents an optimal design approach that enhances the robustness of absorption bandwidth to several hard-to-control variables. The split ring resonator (SRR) was used as an optimizable object to illustrate this approach. Unlike the traditional design method for metamaterial absorbers, which pays attention to the construction of equivalent circuit models and the effect of lumped parameters on the performance index, this article uses the decoupling analysis to establish the mapping between structural parameters and the performance index. The concept of factor effect and interaction effect is introduced to describe the decoupling analysis, and the impact of each factor on the signal-to-noise ratio (SNR) of absorption bandwidth is analyzed. Compared to the original design, the robustness of the optimal design is improved. The predicted result obtained by decoupling analysis and the simulated result shows a good agreement. [ABSTRACT FROM AUTHOR]

This article develops a broadband absorber by fully using the radiation features of monopole antennas for ultrawide-angle absorption application. A unit cell (UC) based on monopole antennas is proposed, and its working polarization is analyzed with the antenna reciprocity method and image theory. A wide-angle impedance matching layer is also introduced to further enhance the absorption at large angles. The optimized monopole-based UC reveals 90% absorptivity from 5.5–14.76 GHz at the incident angle of 80°. To the best of our knowledge, this is the first time to report an absorber that can work perfectly under such ultrawide angles in a broadband manner. An 11 × 36 array is configured and simulated to verify the effectiveness of the monopole-based UC. The monostatic and bistatic scattering are both reduced significantly as well as other diffuse scattering directions from the simulated results. A prototype of 110 mm × 360 mm is fabricated and measured. Time gating methods are adopted to diminish the direct coupling effect. The measurements are in good agreement with the full-wave simulation results. [ABSTRACT FROM AUTHOR]