Terahertz parametric source is a laser driven terahertz wave radiation source, which has many advantages such as high coherence, wide tunability and room temperature operation, etc. After a brief introduction to basic principle of terahertz parametric sources, this paper mainly summarizes the recent representative research results about terahertz parametric sources, including: 1) Commonly used nonlinear crystals including lithium niobate, potassium titanium phosphate, potassium titanyl arsenate, rubidium titanium phosphate; 2) Large pulse energy terahertz parametric sources. The main generation methods include using surface-emitted structure, using ring cavity, increasing the damage threshold of the nonlinear crystal, etc. The largest reported terahertz energy reaches 17 μJ; 3) High average power terahertz parametric sources. The main generation methods include deploying laser diode side-pumped laser as the pumping source, balancing the increase of the pulse energy and pulse repetition rate simultaneously, etc. The highest terahertz average power reported reaches 367 μJ till now; 4) Theoretical simulation on different kinds of terahertz source structures. Mainly based on the coupled wave equations, thetheoretical models for terahertz parametric generator, injection-seeded parametric generator, intracavity-pumped and extracavity-pumped terahertz parametric oscillators are introduced respectively.

Recently, broadband THz wave generation from a liquid target excited by femtosecond laser has been experimentally demonstrated, and it is found that liquid has unique properties as a terahertz wave radiation source. Liquid has the comparable material density to that of solid, which means that laser pulses will interact with three orders more molecules for liquid source than for gas source. In the other hand, compared with solid, the fluidity of the liquid allows each laser pulse to interact with a fresh area of the target, which means that intense laser pulse is not an issue of material damage or degradation. These characteristics make liquid very promising for the study of high-energy density plasma and even becoming the next generation of THz source. In this paper, the influences of the shape and type of liquid target, incidence position and angle of laser beam, pulse duration and energy on THz wave generation are reviewed.

Terahertz(THz) imaging is one of the important directions for the application of THz technology. THz imaging systems based on semiconductor photonics devices such as THz quantum cascade lasers(QCLs) and THz quantum well photodetectors(QWPs) have the advantages of compact structure, high spatial resolution, and high imaging signal-to-noise ratio, and have become the research highlights in the related field. In this paper, the research progress of far-field and near-field imaging systems based on THz QCLand THz QWP devices is systematically reviewed, the composition, mechanism, and performance of these THz imaging systems are carefully described, the performance parameters of each kind of imaging system are summarized, the methods to further improve theperformance of THz imaging system are proposed and their application prospects are also discussed.

Terahertz(THz) wave is an electromagnetic frequency band that has not been fully developed and utilized, but has shown important application value in material science and device testing. However, due to the limit of far-field diffraction, it is difficult for THz wave to be focused into new materials and nano-devices with nanometer or even atomic scale, which greatly hinders the development of THz science and its applications. Recently, to improve the spatial resolution of THz spectroscopy and imaging, near-field microscopy coupled with THz has been developed, which can achieve nanometer to angstrom spatial resolution. This paper summarizes the principle, development process and application examples of THz coupled near-field microscopy, including scanning near-field microscopy and scanning tunneling microscopy, and discusses the future opportunities and challenges in THz near-field microscopy.

Since the discovery of infrared radiation, scientists have been trying to apply infrared technology to the fields of earth observation, space remote sensing and space exploration. At present, the second and third generation of infrared detectors have entered large-scale applications, and the third generation of high-end infrared detectors is gradually breaking through. With the development of material preparation technology, nano-processing technology, integration technology and related cross-disciplines, forward-looking new materials, new technologies and new concepts have begun to appear. Infrared-terahertz detectors has also begun to change gradually from the traditional form of single detection, passive detection and separate detection to the direction of multi-dimensional detection, autonomous detection and intelligent chip integration. On the basis of introducing the physical mechanism of photoelectric detector, this paper summarizes the application and development of infrared-terahertz detection technology in the field of astronomical remote sensing, then focuses on the three expected revolutionary development direction of infrared-terahertz detectors, including the integration of light field based on artificial microstructure, the on-chip intelligence based on three-dimensional stacking technology and the application of new low-dimensional materials, and finally looks forward to the future development trend of detectors towards ultra-high performance, multi-dimensional sensing, intelligence and integration of sensing, memory and computing.

Vanadium dioxide(VO2) is an archetypal strongly correlated-electron material. When the phase transition threshold is reached, there will be a reversible transition from the insulating monoclinic phase to the metallic rutile phase for VO2. The transition can be induced mainly by thermal, optical, electrical, magnetic field, and strain. The abrupt change of VO2 phase can occur in subpicosecond time scales, along with the significant change of optical reflectivity, refractive index, magnetic susceptibility, and other physical quantities. In particular, the resistivity of VO2 will change in three to five orders of magnitude before and after the phase change, which makes VO2 has great application prospects in the fields of intelligent energy-saving windows, photoelectric detection, photoelectric storage, optical switches, and other fields. This review first describes the phase transitions mechanism of VO2, which is driven by the electron correlation or the lattice structure alone or both. Then it focuses on employing ultrafast time-resolved techniques, particularly terahertz time-domain spectroscopy techniques, to study the phase transition dynamics process of VO2 thin films. Finally, the application research of terahertz modulators, terahertz filters, terahertz switches, and other devices based on VO2 thin films are introduced.

The hypersonic vehicles will produce plasma sheath when flying in the near-space, and the existence of plasma sheath will seriously interfere with the communication between the ground and the vehicles.To solve this problem, based on the calculation of the spherical cone flow field, a plasma sheath model is established according to the temperature and pressure distribution of the flow field and the air dissociation reaction model. And then based on thefinite-difference time-domain(FDTD) method, the transmission characteristics of terahertz waves in the plasmon sheath at different flight speeds and incident angles are studied. It is shown that THz wave can effectively penetrate the plasma sheath at different flight speeds and incident angles. This study is of great significance for the communication between near-space and hypersonic vehicles.

High-sensitive detector arrays in terahertz(THz) band have great application value in many fields. Quantum capacitance detector(QCD) is one kind of direct detectors with single-photon detection and array capabilities in THz band, which uses frequency division multiplexing, and the array readout technology is relatively simple. For high-sensitive detector arrays, reliable readout technology is the basic guarantee for its superior performance. In this study, QCD signal is measured and characterized by homodyne readout technique. As microwave in-phase quadrature-phase(IQ) mixer is an important component of the homodyne readout circuit, the IQ mixer is characterized and calibrated, and its measurement reliability is improved by eliminating the impact of its imbalance on signal readout. Under this calibration method, the quantum capacitance response signal of QCD is measured, and the results show that the QCD response signal is periodic, which is highly consistent with the expected quantum capacitance effect. In addition, the constructed readout circuit can also be used for signal readout of extremely low temperature(below 15 mK) detectors such as microwave kinetic inductance detector(MKID) arrays, which lays a good foundation for the development of high-sensitive THz detector arrays and has good application value.

Due to its high carrier frequency, large bandwidth and rich spectral information, terahertz waves have been widely concerned for their potential in high-speed communication, molecular detection, and biomedical imaging. Terahertz modulator is a key device in terahertz detection system, but the currently reported modulators cannot have the characteristics of high efficiency, high speed and low insertion loss at the same time. Therefore, an electronically controlled terahertz modulator based on a GaAs Schottky diode combined with a surface plasma gate array structure is proposed and designed. The device superimposes the electric field enhancement effects of the resonant cavity and the metal gate array on each other, which significantly improves the modulation performance of the device and achieves multi-frequency modulation in the range of 0.4 to 1.4 THz with a maximum modulation depth of about 80%, insertion loss lower than 10 dB, and modulation speed greater than 100 kHz.

Two-dimensional(2D) PtSe2 has the unique properties such as large-range tunability in band gap and high air stability, holding a great promise in the development of novel optoelectronic devices. In this work, the ultrafast photocarrier dynamics in 2D PtSe2 with different thicknesses have been studied by using time-resolved terahertz(THz) spectroscopy. It is found that both the amplitude of transient THz photoconductivity and its dependence on the excitation fluence of the material show a significant nonlinear increase with the increase of sample thickness. The dynamical parameters including photocarrier density, scattering time and backscattering factor are obtained through analyzing the THz frequency-dependent conductivities. In combination with the excitation-wavelength dependent THz relaxation dynamics, it can be inferred that the competition between bound excitons and free carriers is mainly responsible for the nonlinear thickness dependence. In addition, the exciton effect and thickness-induced semiconductor-semimetal transition in PtSe2 are also revealed using the optical pump-optical probe spectroscopy. This work demonstrates the effective regulation on the nonequilibrium ultrafast dynamics of PtSe2 through varying the thickness of materials, and provides an important guideline for the optoelectronic applications of noble-metal based 2D materials.