Fig. 1. Frequency range of terahertz wave and its position in electromagnetic spectrum

Fig. 2. Condition for non-collinear phase matching

Fig. 3. Terahertz parametric oscillator based on MgO∶SLN crystal (mole fraction of MgO is 1%)^[23]. (a) Experimental device; (b) crystal size and cut angle

Fig. 4. THz tuning curve of MgO∶SLN TPO^[23]

Fig. 5. Relationship between terahertz wave output energy of different crystals and pump energy^[23]

Fig. 6. Frequency tuning output curves of terahertz parametric oscillators based on different crystals^[24-25]. (a) KTP crystal; (b) KTA crystal

Fig. 7. Experimental device of terahertz parametric oscillator with vertical output on crystal surface based on Roland circle cavity structure^[27]

Fig. 8. Experimental device of high efficiency terahertz parametric oscillator based on circulating pump technology^[29]

Fig. 9. Relationship between terahertz wave energy and frequency of TPO with or without circulating pump, inset is terahertz light spot of TPO based on circulating pump technology at distance of 7 cm from crystal output surface^[29]

Fig. 10. Experimental device of pulsed seed injection terahertz parameter generator based on MgO∶SLN crystal (mole fraction of MgO is 1%)^[39]

Fig. 11. Terahertz wave output characteristic curves of ips-TPG and SLN-TPO under the same pumping conditions^[39]. (a) Output energy curves; (b) energy attenuation curves

Fig. 12. Experimental device for obtaining high frequency THz wave based on GaSe crystal difference frequency^[45]

Fig. 13. Experimental device of terahertz difference frequency radiation source based on DAST crystal^[48]

Fig. 14. Results of different degrees of brain trauma^[58]. (a) Magnetic resonance imaging; (b) photographs of the objects; (c) terahertz wave imaging

Fig. 15. Different types of rat brain 3D models^[58]. (a) Traumatic rat brain; (b) normal rat brain