Ultra-Broadband Absorber with Large Angular Stability Based on Frequency Selective Surface.

In this paper, a low-profile, double-layer absorber with ultra-broadband absorption and large-angle stability is proposed. In order to improve the angular stability, the square ring with concave-convex deformation is designed. It can expand the current path to realize the miniaturization of the absorber, which decreases the influence of oblique incident on absorption. The equivalent circuit model provides detailed resonance and admittance analysis, showing the existence of three resonances working together to achieve broadband absorption. The simulated results illustrate that the designed unit can achieve above 80% absorption within 2.09-18.1 GHz. The angular stability is up to 50° under TE/TM polarization with a period of 0.07 λL (the wavelength of the lowest operating frequency). The 300 mm × 300 mm prototype absorbers were fabricated for demonstration, with a total thickness of 0.096 λL. The measurement results are consistent with the simulated results, which shows that the designed absorber unit can achieve ultra-broadband and large-angle absorption. The performance of devices can be widely applied in infrared detection, radiation refrigeration, and stealth technology.

High numerical aperture microwave metalens.

The numerical aperture (NA) of a lens determines its focusing resolution capability and the maximum light collection or emission angle. In this Letter, an ultrathin high NA metalens operating in the microwave band is designed and demonstrated both numerically and experimentally. The proposed element is constructed by a multi-layer complementary split ring resonator, which can cover full 2π phase shift simultaneously with high transmission magnitude by varying its radius gradually. The numerical and experimental results reveal that the designed ultrathin (thickness is only ∼0.23λ) metalens can focus normal incident microwave efficiently to a spot of full width at half-maximum (FWHM) as small as ∼0.54λ with a corresponding high NA exceeding 0.9. Besides, the high NA metalens also possesses a relatively large focusing efficiency with a peak 48% within considered broad frequency range from 7.5 to 10 GHz. The performances of the presented metalens can be comparable or even superior to nowadays high-quality optical metalenses and represent an important step to develop a high-performance metalens in low spectrum. Besides, it can greatly facilitate the development of some novel miniaturized devices like a high-gain low profile scanning antenna, an ultra-compact retroreflector, and cloaks.

Design of a Tunable Absorber Based on Active Frequency-Selective Surface for UHF Applications.

An ultrathin tunable absorber for the ultrahigh frequency (UHF) band is presented in this paper. The absorber is a single-layer structure based on the topology of a Salisbury screen, in which the conventional resistive layer is replaced by an active frequency-selective surface (AFSS) loaded with resistors and varactors. The reflectivity response of the absorber can be controlled by adjusting the reverse bias voltage for the varactors, which is verified by both simulated and measured results. The experimental results show that the reflectivity response of the absorber can be modulated below -10 dB over a frequency band ranging from 415 to 822 MHz. The total thickness of the absorber, 10 mm, is equivalent to only λ/72 of the lower limit frequency. The absorbing mechanism for the designed absorber is illustrated by simulating the volume loss density distributions. A detailed analysis is also carried out on the basis of these parameters, such as the AFSS shape, resistor, thickness of the foam, thickness and permittivity of the dielectric substrate, and incident angles, which contribute to the reflectivity of the AFSS absorber.

Ultra-wideband, Wide Angle and Polarization-insensitive Specular Reflection Reduction by Metasurface based on Parameter-adjustable Meta-Atoms.

In this paper, an ultra-wideband, wide angle and polarization-insensitive metasurface is designed, fabricated, and characterized for suppressing the specular electromagnetic wave reflection or backward radar cross section (RCS). Square ring structure is chosen as the basic meta-atoms. A new physical mechanism based on size adjustment of the basic meta-atoms is proposed for ultra-wideband manipulation of electromagnetic (EM) waves. Based on hybrid array pattern synthesis (APS) and particle swarm optimization (PSO) algorithm, the selection and distribution of the basic meta-atoms are optimized simultaneously to obtain the ultra-wideband diffusion scattering patterns. The metasurface can achieve an excellent RCS reduction in an ultra-wide frequency range under x- and y-polarized normal incidences. The new proposed mechanism greatly extends the bandwidth of RCS reduction. The simulation and experiment results show the metasurface can achieve ultra-wideband and polarization-insensitive specular reflection reduction for both normal and wide-angle incidences. The proposed methodology opens up a new route for realizing ultra-wideband diffusion scattering of EM wave, which is important for stealth and other microwave applications in the future.