In this Letter, a new frequency-selective absorber (FSA) is presented that has wide-band transmission/absorption/reflection reduction responses. The presented absorber is completed by lossy cross-dog bone and lossless second-order frequency-selective surfaces (FSSs). The modal interaction pole generated by the lossy array is employed to achieve a wide-band transmission. Three absorption bands are combined to realize a continuous stop-band with high absorptivity. Furthermore, the improved absorber structure is investigated to reduce the strong reflection between the absorption and transmission bands. The 10 dB reflection reduction is obtained with a fractional bandwidth of 126.4%. Meanwhile, the - 3 dB fractional pass-band width is 41.3% with minimum insertion loss (IL) of 0.37 dB. The absorption bandwidth of 91.5% is provided with at least 90% absorptivity. The distance between the lossy and lossless layers is only 0.08 $\lambda_L$ L at the lowest operating frequency. Finally, the proposed design is validated by a good agreement between the numerical simulations and experimental measurements.

Fano resonance presents an asymmetric line shape formed by an interference of a continuum coupled with a discrete autoionized state. In this paper, we show several simple circuits for Fano resonances from the stable-input impedance mechanism, where the elements consisting of inductors and capacitors are formulated for various resonant modes and the resistor represents the damping of the oscillators. By tuning the pole-zero of the input impedance, a simple circuit with only three passive components e.g. two inductors and one capacitor, can exhibit asymmetric resonance with arbitrary Q-factors flexiblely. Meanwhile, four passive components can exhibit various resonances including the Lorentz-like and reversely electromagnetically induced transparency (EIT) formations. Our work not only provides an intuitive understanding of Fano resonances, but also pave the way to realize Fano resonaces using simple circuit elements.

In this letter, a broadband switchable active frequency selective surfaces (AFSSs) is presented, the proposed structure is comprised of three metallic layers and two Rogers RT5880 substrates. In order to realize broadband transmission, a second-order bandpass filter is constructed and the multilayer coupling effect is utilized. And a modified equivalent circuit model of the PIN diode is proposed to reduce the simulation error and improve the design accuracy. By controlling the bias voltage of PIN diodes, two completely different functions are realized. At the function of dual-polarized shielding, the shielding bandwidth is 2-18 GHz, and the transmission bandwidth is 8.2-10.5 GHz with a fractional bandwidth of 24.6% at the function of dual-polarized transmission. A prototype has been fabricated and measured, and the measurement results are in good agreement with the simulation results, indicating that the AFSSs has good polarization stability for X-band radar applications.

Parity-time (PT) symmetric structures present the unidirectional invisibility at the spontaneous PT-symmetry breaking point. In this paper, we propose a PT-symmetric circuit consisting of a resistor and a microwave tunnel diode (TD) which represent the attenuation and amplification, respectively. Based on the scattering matrix method, the circuit can exhibit an ideal unidirectional performance at the spontaneous PT-symmetry breaking point by tuning the transmission lines between the lumped elements. Additionally, the resistance of the reactance component can alter the bandwidth of the unidirectional invisibility flexibly. Furthermore, the electromagnetic simulation for the proposed circuit validates the unidirectional invisibility and the synchronization with the input energy well. Our work not only provides an unidirectional invisible circuit based on PT-symmetry, but also proposes a potential solution for the extremely selective filter or cloaking applications.

In this letter, a novel synthesis method for broadband multilayer absorber based on multi-objective genetic algorithm (MOGA) is proposed. The inhomogeneous medium can be equivalent to homogeneous medium using the effective medium theory, and the absorption of the multilayer inhomogeneous medium absorber can be calculated. A MOGA is introduced to optimize the geometric parameters of the multilayer inhomogeneous medium absorber. Compared with the design results of two existing methods, the total thickness of the structure designed using the MOGA synthesis method is reduced by 10.8% and 9.1%. In addition, a broadband and wide-angle absorber with 90% absorption bandwidth covering 2–18 GHz is designed using this method, and the total thickness is only 0.133 wavelength and has excellent oblique incidence stability, which has potential uses in microwave and visible light applications.