Significance Terahertz wave generation technology has rapidly become efficient recently and is the key, foundational technology to realize a wide application of terahertz band in comprehensive research. The terahertz band, located in the transition region from traditional electronics to photonics, has low energy, water sensitivity, special penetrability and many other unique properties. With respect to these characteristics, terahertz wave has widely used applications in biomedical diagnosis, safety inspection, nondestructive testing, terahertz communication and radar. Terahertz parametric and difference frequency radiation sources, based on optical nonlinear frequency conversion technology, can generate wideband tunable, monochromatic terahertz waves. Terahertz parametric and difference frequency radiation sources also have the advantage of compact structure and utilization at room temperatures(18-30 degrees C). With the improvement of laser technology and crystal growth technology, terahertz parametric radiation source and terahertz difference frequency radiation sources are developing rapidly to expand frequency tuning range, improve output energy, narrow terahertz wave linewidth resulting in a series of new technologies, such as ring cavity, circulating pump, and pulse -seeded injection. Progress Compared with congruent MgO-doped lithium niobite(MgO: CLN) crystal, the upper tuning frequency limit of terahertz parametric oscillator based on near-stoichiometric MgO-doped lithium niobite(MgO: SLN) crystal can be increased from 3 THz to 4.64 THz (Fig. 4). Terahertz parametric oscillator based on KTiOPO4 (KTP) and KTiOAsO4 (KTA) crystal can further improve the frequency tuning upper limit of terahertz wave; even though there are gaps in the frequency tuning range, continuous tuning cannot be achieved ( Fig. 6). Terahertz parametric oscillator based on ring cavity can broaden the frequency tuning range of terahertz wave and improve the output energy ( Fig. 7). Terahertz parametric oscillator based on pump recycling technology can improve the pump efficiency and greatly increase the output energy of terahertz wave in the entire frequency tuning range (Fig. 9). Comparing the pump source with ns pulse width, the terahertz parametric radiation source based on sub-ns pump laser can not only improve the pump peak energy but also effectively suppress the stimulated Brillouin scattering in nonlinear crystal, considerably improving the output energy. Pulse-seeded injection technology not only further improves the frequency tuning upper limit of the terahertz parametric oscillator based on LiNbO3 crystal to 5. 15 THz but also addresses the disadvantage of low output energy in high frequency band to maintain high output energy in a wider range (Fig. 11). Based on dual KTP-optical parametric oscillation (KTP-OPO) technology, terahertz difference frequency radiation source based on inorganic crystal such as GaSe can achieve high repetition rate terahertz wave output (Fig. 12), which can be used in near-field microscopy, rapid scanning THz spectroscopy and other occasions, requiring high repetition rate of terahertz wave. Terahertz difference frequency radiation source based on 4'-dimethylamino-N-methy1-4-stilbazolium tosylate (DAST), 4-N, N-dimethylamino-4'-N'-methyl-stilbazolium 2, 4, 6-trimethylbenzenesulfonate(DSTMS), N-benzy1-2-methyl-4-nitroaniline(BNA), and other organic crystals can achieve ultra-wideband terahertz wave output in the range of 1 THz to 30 THz. Owing to the characteristics of terahertz wave, i.e., low energy, water sensitivity, and fingerprint spectrum, terahertz technology has a good application potential in the field of traumatic brain injury detection. For example, multi depth slice terahertz imaging technology can accurately identify the severity of traumatic brain injury (Fig. 15). Terahertz imaging technology based on machine learning can automatically recognize and classify different degrees of traumatic brain injury samples. Conclusions and Prospects Terahertz parametric and difference frequency sources based on optical nonlinear frequency conversion technology can generate terahertz wave with high output energy, wide frequency tuning range, and narrow linewidth. With the improvement of terahertz radiation source performance, terahertz technology will have greater applications in biomedical detection, nondestructive detection, safety inspection, terahertz radar, and so on.
