A novel design method for ultra-broadband energy selective surface (ESS) is presented in this paper based on the equivalent theory from filter circuits to metamaterial units. This method provides a new way of designing ESS in ultra-broadband with low insertion loss and high shielding efficiency from the perspective of filter circuit topology. It is illustrated and verified with a three layer ultra-broadband ESS. Firstly, a third-order filter with a switching bandpass to bandstop in C to Ku band is designed and optimized. Then, the filter is mapped to a physical model of a three-layer ESS which has a nonlinear transmission response for low-intensity radiation field (LIRF) and high-intensity radiation field (HIRF) in C/X/Ku-band. The three ESS prototypes are fabricated and measured in three different band waveguides to prove the design method of the ESS proposed in ultra-broadband. Simulated and measured results demonstrate that low insertion loss and high shielding efficiency at the same time can be obtained under low and high incident power levels, respectively, where the 1 dB loss and 23 dB shielding bandwidth cover from 5.4 GHz to 14.45 GHz. Furthermore, the insertion loss is less than 0.5 dB in the frequency band of 5.45 GHz to 13.8 GHz and maximum shielding effectiveness reaches 25 dB at 10 GHz.

In this article, a wideband energy selective surface (ESS) with quasi-elliptic response and high shielding effectiveness is proposed for providing out-of-band rejection and high-power microwave protection at the same time for wideband systems. By cascading three layers of structures and loading diodes, wideband quasi-elliptic response and nonlinear transmission property are realized. The ESS has a wide pass-band and high out-of-band rejection simultaneously with turned off diodes under a low-power incidence. On the contrary, under a high-power incident wave, diodes are turned on adaptively, leading to a high shielding in wideband. An equivalent circuit model is first designed in which a three-order Chebyshev filter with a serial LC and diodes is schemed. ESS structure is then constructed based on the circuit model. Field-circuit cosimulation is employed to analyze the power-dependent transmission property. In addition, two prototypes are designed and measured in rectangular waveguides, where it can be seen that the simulation and measurement findings agree well. The proposed ESS has a wide pass-band of 6–10 GHz and great out-of-band rejection with one transmission zero at 11.6 GHz under a low-power incidence, whereas high shielding in wideband is observed for high-power incidence.

In this paper, a flexible and frequency-reconfigurable band-stop Frequency Selective Surface (FSS) is studied theoretically and experimentally. The flexible FSS is fabricated through screen printing, a low-cost, high-efficiency additive manufacturing methodology. With the load of bias voltage controllable varactor, resonance frequency tuning from 3.51GHz to 6.02 GHz has been measured to prove the frequency reconfigurability of the FSS under bias of 0-20V. Outstanding wide angular stability under oblique incidence has also been examined both in theory and measurement. The FSS resonance frequency is completely fixed under incidence from 0°-60°, and the angular stability maintains under different bias voltages. The mechanical flexibility, frequency-reconfigurability and angular stability makes it meaningful in applications of flexible/conformal coverage for aiming objects, including electromagnetic scattering control, EMC (Electromagnetic Compatibility), etc.

This article presents a detailed analysis of the response threshold of energy selective surface (ESS) based on semiconductors. ESS could adaptively protect electronic equipment from high-power microwave (HPM) when the field intensity of incident waves surpasses the response threshold, at which the semiconductors loaded on ESS are triggered on by the induced voltages or currents. The effects of various design parameters and the implied physical mechanism are investigated using electromagnetic simulations and experiments. With the help of the equivalent-circuit model (ECM), methods of modulating response threshold are proposed. According to our research, the response threshold of ESS is closely related to the quality factor $Q_{p}$ of the corresponding ECM. ESS with a larger $Q_{p}$ is supposed to be triggered on at a lower power level. The proposed method is used to design ESS with different response thresholds through adjustment of geometry parameters instead of replacing semiconductors. Prototypes are fabricated and measured under different power levels. The results agree well with simulations, proving the proposed method is effective and useful in the design of ESS.

A terahertz metamaterial with switching characteristics from optical buffering to absorbing performance is realized by incorporating a phase-change film of vanadium dioxide. By introducing the electromagnetically induced transparency behavior based on simple strip pairs, the slow light effect with group delay up to 3.5 ps is obtained. When vanadium dioxide is in the insulator state, the remarkable delay can be observed as the incident pulse transmits through the designed structure. Once the vanadium dioxide film is tuned to the metallic state, the metamaterial is switched to a terahertz absorber and the maximum absorption rate of 94% is observed at 1.04 THz. The switching mechanism is discussed by analyzing the electric field and power loss distributions, as well as the impedance matching principle. Moreover, the buffering capability and the absorption performance both remain noticeable within a wide range of the incidence angle. This work offers a strategy for the function-switching metamaterial which provides potential applications in terahertz detecting, switching and slow light devices.