Abstract:In this work, we propose an optimum unit cell arrangement to obtain near absolute polarization insensitivity in a metal-insulator-metal (MIM) based ultra-broadband perfect absorber. Our findings prove that upon utilizing this optimum arrangement, the response of the absorber is retained and unchanged over all arbitrary incidence light polarizations, regardless of the shape of the top metal patch. First, the impact of the geometry of the top nanopatch resonators on the absorption bandwidth of the overall structure is explored. Then, the response of the MIM design for different incidence polarizations and angles is scrutinized. Finally, the proposed design is fabricated and characterized.

Precise control and stabilization of the operating temperature environment of spacecraft and satellites during their life cycle is of paramount importance to increase device reliabilities and reduce the thermomechanical constraints. Optical solar reflectors are the physical interface between the spacecraft and space, and they are broadband mirrors for the solar spectrum, while having strong thermal emission in the mid-infrared part of the electromagnetic spectrum. Strong light−matter interactions in metamaterials and metasurfaces offer significant advantages compared to the conventional methods in performance, weight, launch, and assembly costs. However, the fabrication complexity of these metastructures due to necessitating lithography hinders their upscaling, reproducibility, large-area compatibility, and mass production. In this regard, we propose a facile, lithography-free fabrication route, exploiting oblique deposition to design a metasurface based on disordered and densely packed Indium Tin Oxide (ITO) nanorod forests. The excellent light trapping capability of the nanorod forests, randomness in the geometrical dimensions of these nanorods, combined with the lossy plasmonic nature of ITO in the thermal-infrared range led to strong coupling of thermal radiation to broad plasmonic resonances and, consequently, an experimental emissivity of 0.968, in a very wide range from 2.5 to 25 μm. In the solar spectrum, the low-loss dielectric characteristic of ITO resulted in an experimental solar absorptivity as small as 0.168. Our proposed design with high throughput, robustness, low cost, and high performance, therefore, shows great promise not only for space missions, but also for promoting environmentally friendly passive radiative cooling for our planet and thermal imaging in the field of security labeling.

Colorimetric detection of target molecules with insensitivity to incident-light polarization has attracted considerable attention in recent years. This resulted from the ability to provide rapid output and reduced assay times as a result of color changes upon altering the environment that are easily distinguishable by the naked eye. In this paper, we propose a highly sensitive refractive-index sensor, utilizing the excitation of guided modes of a novel two-dimensional periodically modulated dielectric grating-waveguide structure. The optimized nanosensor can numerically excite guidedmode resonances with an ultranarrow linewidth (full width at half-maximum) of 0.58 nm. Sensitivity is numerically investigated by considering the deposition of dielectric layers on the structure. For a layer thickness of 30 nm, the maximum sensitivity reached as high as 110 nm/refractive index unit (RIU), resulting in a very high figure of merit of 190. The fabricated devices with 30 nm aluminum oxide and zinc oxide coatings achieved a maximum sensitivity of 235.2 nm/RIU with a linewidth of 19 nm. Colorimetric detection with polarization insensitivity is confirmed practically by a simple optical microscope. Samples with different coatings have been observed to have clearly distinct colors, while the color of each sample is nearly identical upon azimuthal rotation. Excellent agreement is obtained between the numerical and experimental results regarding the spectral position of the resonances and sensitivity. The proposed device is, therefore, highly promising in efficient, highly sensitive, almost lossless, and compact molecular diagnostics in the field of biomedicine with personalized, label-free, early point-of-care diagnosis and field analysis, drug detection, and environmental monitoring.

This work analyzes the effect of various gate structures on the DC and radio frequency (RF) performance of AlGaN/GaN high-electron mobility transistors (HEMTs). AlGaN/GaN HEMT devices with a 3 μm drain-to-source spacing, 125 μm gate width and 0.3 μm gate length in various gate structures were fabricated to achieve the desired frequency response with a robust, high yield, and repeatable process. The maximum drain current (I DS , max), maximum DC transconductance (g m), pinch-off voltage (V th), current-gain cutoff frequency (f T), maximum oscillation frequency (f max), and RF characteristics of the devices in terms of the small-signal gain and RF output power (P out) at 8 GHz were investigated. The results showed that the output power is increased by 1 dB when the gate structure is changed from field plate to gamma gate. The V th , g m , f T and f max values are maximized when the thickness of the passivation layer between the gate foot and the gate head is minimized. It is shown that the I DS,max is decreased and P out is increased when the gate recess etching process is performed.