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In this paper, we demonstrate an intensity-tunable THz bandpass filter by introducing liquid crystal (LC) integrated with asymmetric frequency selective surface (FSS) and subwavelength metal gratings. Here, the tunable THz filter is derived from the inner polarization state conversion in composited devices, and the incident linear polarization can be converted into 90° orthogonal components. By controlling the LC orientation under the applied electric field with the metamaterial electrodes, the polarization conversion process can be actively modulated; thus, the polarization-dependent and tunable THz bandpass filter is achieved. Based on the multilayer design and the inner Fabry–Perot-like resonance mechanism, the LC-integrated metamaterials filter presents better filtering performance than the single FSS filter, and the Q-value is improved from 7.7 to 13.8 at the working frequency. Our simulated work paves the way for the design of new and efficient THz filters.

Recently, the terahertz (THz) chiral field control opens a new window to THz devices and their applications. In this Letter, the active manipulation for THz chiral states based on the cholesteric liquid crystal (CLC) has been demonstrated by THz time domain cross-polarization spectroscopy. The results show that the CLC has strong THz optical activity and circular dichroism (CD) effect, and the strongest THz CD of 22 dB and a polarization rotation angle of 88.4° occur around the phase transition temperature T_S−N=250K. Rising to a room temperature of 300 K, the CLC turns from a chiral state to an isotropic state for THz waves with the phase transition processes of CLC molecules. Therefore, this CLC device can be performed as a thermally active THz circular polarizer, which brings potential applications in THz polarization imaging, broadband communication, and spectroscopy.

We experimentally demonstrate an active terahertz (THz) resonance switch induced by the polarization conversion in a compound metasurface, which is a LC layer sandwiched by a metallic wire grating and resonance metamaterial (LCGM). Here, the liquid crystal (LC) plays the role of polarization conversion, which can induce the TE resonance. Moreover, there exists a localized resonance between metallic grating and metamaterial layers, and then the excited resonance will be greatly enhanced. The results show that the high extinction ratio of the resonance switch exceeds 30 dB at 0.82 THz. This work will bring new ideas for the research in developing THz phase, polarization, and switch devices with LC and metasurface.

We investigated THz nonreciprocal circularly polarized transmission in thin longitudinally magnetized InSb film, especially focusing on its non-eigen nonreciprocal transmission mechanism at room temperature. Then, based on this effect, we presented a THz isolator for linear polarized waves. The nonreciprocal transmission of the InSb film in this device is converted and enhanced by a pair of orthogonal artificial birefringence gratings. After the optimization, the isolation reaches 24 dB, and the insertion loss is <0.5  dB at room temperature and a low magnetic field.

A terahertz modulator based on the insulator–metal transition (IMT) in a photonic crystal waveguide (PCW) coated by vanadium dioxide (VO_2) film is proposed. The numerical simulations show that a dielectric state and a metallic state with quite different photonic band structures and transmission properties in the proposed PCW are reciprocally converted because of the IMT of VO_2, and the pass-bands of this PCW are greatly shifted from 0.68 to 0.8 and 1.02 to 1.25 THz to 0.8–1.45 THz. This PCW significantly enhances the modulation depth and sensitivity compared with bare VO_2 film. Extensive investigation demonstrates that the thickness of VO_2 film greatly affects the IMT process in the PCW, and limits the ultimate modulation depth of the device. The proposed modulation scheme will be of great significance for potential THz applications.

We numerically investigate the photonic bandgaps as well as the transmission properties of a two-dimensional magnetic photonic crystal (PC) in the terahertz (THz) region by the plane wave expansion and finite-difference time-domain methods. The calculation predicts a magnetic PC waveguide that can work as a tunable filter with a bandwidth larger than 0.1?THz, and its central frequency is from 0.83 to 1.03?THz. It also shows that a magnetic PC can be used as a polarization controller with three functions, including controllable polarizing, polarization beam splitting, and −π–π phase shifting.

The state conversion and terahertz (THz) wave modulation based on a plasmonic device composed of silicon column arrays with vanadium dioxide (VO_2) coating were experimentally demonstrated. For double 45° tilted optical pumping, a state conversion from dielectric photonic crystal (PC) to metallic PC was demonstrated due to the insulator–metal transition (IMT) of VO_2 with the pump power increasing. In this process, a broadband intensity modulation with 70% modulation depth was achieved. Furthermore, for normally incident optical pumping, another state conversion from dielectric PC to plasmonic device was also demonstrated due to the partial IMT of VO_2, and the out of plane PC resonance gradually changed to be plasmonic resonances. This device and its modulation scheme will be of great significance for potential THz applications.

In the use of the local extended Kalman filter algorithm for network training and pruning, the authors propose replacing the monotonical increasing sigmoid activation function with a periodic function. Computer simulations show that the rate of convergence is substantially improved and that more insignificant weights can be pruned.

A local extended Kalman filter training and pruning approach is proposed to train feedforward networks with the goal of reducing the computational complexity and storage requirement in large-scale practical problems. The performance of the proposed algorithm is demonstrated for the problem of handwritten digit recognition.