We present a wideband rectifying metasurface (RMS) with enhanced system efficiency for wireless power transmission and energy harvesting. The RMS consists of periodic arrays with integrated diodes, with a high input impedance matched with the diodes, thus eliminating the matching network between metasurface (MS) and rectifier. Besides, a unique harmonic feedback network is embedded in each unit cell, rectifying the high-order harmonic generated by the diode repeatedly, improving the total efficiency over a wide bandwidth. The converted DC power is channeled to one single load through the DC-paths formed by the branches of MS and a series of inductors, avoiding additional DC-combination network. The MS can effectively capture EM energy at an operating band of 2-3.5 GHz, and the simulated radiation-AC efficiency is up to 99% at 2.8 GHz. A finite 8 × 8 array is fabricated and measured. The total conversion efficiency reaches a peak value of 60.5% at 2.7 GHz, and is greater than 40% from 2.3 GHz to 2.9 GHz, exhibiting a wide bandwidth of 23.1% compared with existing state-of-the-art studies. The integrated MS-based RF energy harvester with advantages of simple structure is of great significance for wireless power transfer and energy harvesting applications.

This article presents a multifunctional dual-band energy selective surface (ESS) based on a two-layer resonant circuit model with high-power microwave (HPM) protection ability for two bands. An equivalent circuit model (ECM) engineering multiple stopbands and passbands with combinations of parallel and serial resonators is proposed for dual-band design. By loading diodes in the resonator accordingly, the operating band can be switched from low insertion loss (IL) state to high shielding effectiveness (SE) state power-dependently. Additionally, the independence of two bands is greatly improved by utilizing two separate layers to mount the diodes, attributing to the enhanced difference of induced voltage between diodes in different layers resulting from shielding effect of top layer. Various functions in two-frequency co-incidence scenarios are analyzed, demonstrating the advantage of independent modulation. Based on the ECM, a two-layer ESS is designed. Two wide bands with IL<1dB under low-power incidence and SE>20dB under HPM are achieved, due to second-order resonance produced by double-layer configuration.  The multiple nonlinear functions of the design are verified by field-circuit co-simulations. Several prototypes are measured, exhibiting 0.6&25.2 dB and 0.7&27.3 dB of IL&SE at two central operating frequencies for low-power and high-power incidences, respectively, with good agreement with the simulations.

In this paper, we present a tunable bandpass frequency-selective surface (FSS) with wide tuning range. The FSS is composed of a metallic rectangular waveguide array implanted in the double-sided printed circuit board (DSPCB) and an array of embedded varactors with bias network in the backside of the DSPCB. The waveguide array has a frequency-selective transmission peak due to tunneling effect and the transmission frequency is tuned by changing the capacitance of varactors through different bias voltages. One advantage of this configuration is that the varactors together with the bias network are designed on the same layer, which can effectively reduce the overall thickness into only 0.006λc (λc is the free-space wavelength at the highest transmission frequency). On the other hand, the insertion loss associated with the parasitic resistance of varactors is greatly decreased, owing to the novel integrated design of biasing lines with varactors and the nearly-full-metal waveguide array. A prototype is fabricated and measured, showing that by changing the varactor capacitance from 0.12 pF to 1 pF, the passband exhibits a 2.32:1 frequency tuning range from 5.8 GHz to 2.5 GHz with an insertion loss of 0.2 dB-2.6 dB. A polarization-insensitive FSS is also studied in simulations.

An ultrathin and simple frequency-selective rasorber (FSR) with a passband located within a wide absorption band is proposed. The ultrawide absorption band is obtained by employing commercial magnetic materials in the absorption channel and the passband is realized using epsilon-near-zero (ENZ) tunneling waveguides. The attractively ultrathin and simple feature is achieved by utilizing tunneling effect at the cutoff frequency of metallic waveguides with arbitrary length, permitting the overall thickness shrink into to the same as that of the absorber.