We present a terahertz (THz) amplitude modulation method with near perfect E-field amplitude modulation depths that is based on impedance matching in VO₂ thin films during the thermally induced insulator-metal transition (IMT). It was observed that the impedance matching-induced THz amplitude modulation was sensitive to the resistance switching characteristics of the VO₂ thin films. By designing the VO₂ thin films to have four orders of magnitude of change in resistance during the IMT, we experimentally achieved an E-field amplitude modulation depth of 94.5% (intensity modulation depth of 99.7%) between the insulator phase of VO₂ and the impedance matching state, and an E-field amplitude modulation depth of 97.6% (intensity modulation depth of 99.94%) between the impedance matching state and the metallic phase of VO₂ at 0.5 THz. The experimental results were consistent with the results of simulations based on the transmission matrix model.

A cylindrical lens telescope tilted-pulse-front pumping scheme was proposed for high energy terahertz (THz) pulse generation. This scheme allows higher pump energy to be used with lower saturation effects under high pump fluence, and higher THz generation efficiency was achieved within large range of pump energy. The optimum pump pulse duration and crystal cooling temperature for THz generation in LiNbO₃ (LN) crystal were also researched systematically. Excited by 800-nm laser, up to 0.19 mJ THz pulse energy and 0.27% conversion efficiency was demonstrated under 800-nm 400-fs laser excitation with ~100-mJ pulse energy and 150-K LN cooling temperature.

The combination of VO₂ and metasurfaces has opened an attractive route to dynamically control terahertz (THz) waves based on the giant conductivity change. However, the high-precision control of microfabrication and single performance of conductivity change limit the multifunctional application of VO₂-based metasurfaces. Here, we proposed a VO₂-metallic hybrid metasurface by in situ depositing high-quality VO₂ thin films onto a metasurface composed of asymmetric Fano resonance units. It exhibits agile frequency and amplitude modulation for THz transmission across tuning the dielectric constant and conductivity of VO₂. The metallic metasurface is designed as a matrix to achieve high transmission at 0.61 and 0.78 THz due to the split-ring resonance. During the thermally triggered phase transition of VO₂, we found that the resonance frequency and amplitude can be tuned dominantly by the change of dielectric constant and conductivity, respectively. In particular, the increased dielectric constant enables red shift of the frequency by around 0.48 THz and the conductivity increases lead to a giant THz amplitude modulation of 88%. These results provide a route for developing VO₂-based THz smart devices combined with functional metasurfaces and hold great promise for applications in THz sensor and modulation.

Perfect absorbers with broadband absorption of terahertz (THz) radiation are promising for applications in imaging and detection to enhance the contrast and sensitivity, as well as to provide concealment. Different from previous two-dimensional structures, here we put forward a new type of THz absorber based on sawtooth-shape doped-silicon with near-unit absorption across a broad spectral range. Absorbance over 99% is observed numerically from 1.2 to 3 THz by optimizing the geometric parameters of the sawtooth structure. Our absorbers can operate over a wide range of incident angle and are polarization insensitive. The underlying mechanisms due to the combination of an air-cavity mode and mode-matching resonance on the air-sawtooth interface are analyzed in terms of the field patterns and electromagnetic power loss features.

In this work, we propose what we believe to be a novel reordered Hadamard basis method, namely frequency-domain modulus-correlation Hadamard (FMH), which ensures uniform increment of spatial frequencies in both two orthogonal directions. Both simulated and experimental results reveal that the FMH can significantly decrease required sampling ratio for reconstructing high-quality images, compared with the Sylvester Hadamard and the Walsh Hadamard. By integrating a pseudo-background post-processing technique, we achieve a remarkably low sampling ratio of 4.27% for terahertz image reconstruction, with the Pearson correlation coefficient exceeding 0.9. This advancement resulted in nearly a 5-fold increase in the acquisition rate of near-field terahertz single-pixel imaging, compared with the Walsh Hadamard.