Fig. 1. Band structures of active regions. (a) Bound-to-continuum structure^[41]; (b) resonant phonon structure^[16]; (c) hybrid structure^[42]; (d) extraction-controlled structure^[43]

Fig. 2. Waveguide structures, light field mode distributions, and beam profiles^[8]. (a) Semi-insulating substrate-surface plasma waveguide; (b) metal-metal waveguide

Fig. 3. Laser spectra. (a) High-frequency single-mode device^[64]; (b) wide-spectrum device^[15]; (c) tunable single-mode device^[14]

Fig. 4. Intensity-current-voltage relationship under different temperature conditions. (a) High-power single-mode device^[75]; (b) high-temperature device^[16]

Fig. 5. Schematics of QWP. (a) Band structure^[36]; (b) schematic of device with surface gratings^[79]; (c) schematic of device with 45° incident facet^[79]

Fig. 6. Microscope pictures and peak responsivity curve of THz QWPs^[85]. (a) Photo of device table; (b) picture of surface 2D grating; (c) peak responsivity versus grating size

Fig. 7. High-temperature THz QWP^[37]. (a) Schematic of metamaterial unit cell; (b) band structure; (c) micrograph of metamaterial; (d) schematic of resonator and coupling antenna; (e) photocurrent spectra at different temperatures

Fig. 8. Schematics of THz high-resolution spectroscopic methods and measured spectrum^[8]. (a) Schematic of direct-absorption spectroscopic method; (b) schematic of difference frequency detection method; (c) schematic of photoacoustic spectroscopic method; (d) oxygen atomic line detected by difference frequency method (4.7 THz)

Fig. 9. Schematic of spectroscopic method with QCL stabilized to frequency comb^[25]

Fig. 10. Pictures by THz imaging. (a)‒(c) Comparison of photographs and THz images^{[102,106,108]}; (d) THz QCL spot images taken by QWP-LED and Si CCD camera^[109]

Fig. 11. THz QCL-based ISAR imaging^[111]. (a) Schematic of ISAR imaging device; (b) photograph of scale model tank; (c) THz image of scale model tank with pixel resolution of 0.4 mm×0.6 mm

Fig. 12. Video transmission based on THz QCL and THz QWP^[38]. (a) Schematic of information transmission device; (b) modulating signal and transmitted signal; (c) video transmission result

Fig. 13. Mode locking technology based on graphene reflector^[127]. (a) Schematic of graphene-coupled THz QCL; (b) probe signal intensity versus delay time