Vortex beams have been extensively realized using different approaches. Typically, the efficiency and bandwidth of a vortex beam are limited by impure copolarized components and the intrinsic dispersion of passive resonant structures. Here, we propose a strategy to generate wideband vortex beams by using a Pancharatnam–Berry metasurface in which two orthogonal reflections exhibit a broadband out-of-phase difference. To achieve this, a broadband strategy based on multimode operation and dispersion engineering methods was established. A dual-layer meta-atom is proposed; each layer comprises of five metallic dipoles, and the geometrical parameters are carefully adjusted to tune the resonant frequencies. Because the dipole orientations in each layer are orthogonal, the reflection responses under the two orthogonal polarizations can be independently engineered. Both numerical and experimental results indicate that our method not only enables a high-efficiency spiral beam conversion over a broad range of 6.95–18 GHz (>82%) but also causes a polarization-insensitive effect; thus, it can be adapted for any linear or circular polarization.

A hybrid design method for broadband RCS reduction based on reflection diffusion is proposed and successfully demonstrated. To this end, we first analyze the scattering behavior of full parabolic-phased metasurface and find out its weaknesses in achieving perfect diffusive patterns. Then, we alleviate the issue of full parabolic-phased method through hybridizing two strategies, i.e., concave/convex chessboard arrangement and random focal lengths. The proposed hybrid design makes the phase profile aperiodic without any mirror symmetry, which is the key to achieving perfect diffusive scattering behavior within a wide operation band. Numerical and experimental results show that our proposed design features broadband, polarization insensitivity, wide incidence angle and can efficiently decrease the RCS more than 10 dB within 7.8-23.2 GHz. Our approach comprehensively solves the issues of narrow band, high bistatic RCS value, time-consuming optimization process, and sensitivity to different polarization and incident angles, promising great potential in stealth applications.

In this article, the frequency-selective rasorber (FSR) built on multilayer resistive sheets, which achieves a broad absorption band, was proposed. First, the detailed analysis of the absorption and transmission performance for FSR with the single-layer/multilayer resistive sheets based on the equivalent circuit model (ECM) was demonstrated, which proved that the FSR with the multilayer resistive sheets can obtain a broader absorption band. Second, the multilayer resonator (MR) of a smaller size and higher design freedom compared with single-layer resonator was proposed. Then, the FSR on the basis of multilayer resistive sheets using MR was designed. It consists of the three-layered metallic structures and separated by an air spacer. For the top and middle layers, the lumped-resistor-loaded metallic dipole with the MR inserted into the center was applied to generate a wide absorption band and transparent window, respectively. For the bottom layer, the square-slot frequency-selective surface was optimized to match the transmission band of the resistive sheet. Simulated and measured results show that the broad absorption band and transmission band with low insertion loss have been realized, and the proposed FSR yields the widest relative bandwidth $\vert S_{11}\vert \le -10$ dB of 145.2%.

This paper aims to achieve a high-transmittance varactor-tunable frequency selective surface (FSS). First, the impedance condition for the high-transmittance FSS is deduced. Next, a square-slot FSS loaded with varactors, owns the empirical formulas for equivalent circuit parameter extraction, is selected to demonstrate the proposed method for realizing a high-transmittance varactor-tunable FSS. Several conclusions are deduced and verified through the full-wave simulation. To illustrate the effectiveness of the proposed method, two FSSs operating at the same frequency are designed. The simulation and measurement results show that the FSS with the higher transmission rate follows the deduced conclusions. Note that the transmission rate of the varactor-tunable FSS is more than 77.8% (S21 = 1.09dB) from 4.84 to 5.9GHz when the bias voltage applied to the varactors is tuned from 5 to 20V. Compared to the reported tunable FSSs, the varactor-tunable FSS achieved in this paper obtains the highest transmission rate over the tunable transmission band.

Spectrum efficient data transmission is of key interest for high capacity optical communication systems considering the limited available bandwidth. Transmission of the high speed signal with higher-order modulation formats within the Nyquist bandwidth using coherent detection brings attractive performance advantages. However, high speed Nyquist signal generation with high order modulation formats is challenging. Electrical Nyquist pulse generation is restricted by the limited sampling rate and processor capacities of digital-to-analog convertor devices, while the optical Nyquist signals can provide a much higher symbol rate using time domain multiplexing method. However, most optical Nyquist signals are based on direct detection with simple modulation formats. Here we report the first experimental demonstration of high speed all optical Nyquist signal generation based on Sinc-shaped pulse generation and time-division multiplexing with high level modulation format and full-band coherent detection. Our experiments demonstrate a highly flexible and compatible all optical high speed Nyquist signal generation and detection scheme for future fiber communication systems.

This paper presents a dual-mechanism method to design a single-layer absorptive metasurface with wideband 20 dB RCS reduction by simultaneously combining the absorption and phase cancellation mechanisms. The metasurface comprises two kinds of absorbing unit cells with 10 dB absorption performance but different reflection phases. The impedance condition for 20 dB RCS reduction is theoretically analyzed considering both the absorption and the phase cancellation based on the two unit cells, and the relationship between the surface impedance and the reflection phase/amplitude is revealed. According to these analyses, two unit cells with absorption performance and different reflection phases are designed and utilized to realize the absorptive metasurface. Numerical and experimental results show that the single-layer absorptive metasurface features wideband 20 dB RCS within 11.5–16 GHz with a thickness of only 3 mm.