Fig. 1. Conduction band diagrams of three types of THz QCL active regions and two kinds of waveguide structures^[7]. (a) Bound to continuum; (b) resonant phonon; (c) hybrid structure; (d) semi-insulating surface plasmon waveguide; (e) metal-metal waveguide

Fig. 2. Statistics of representative high-performance THz QCLs. (a) Relationship between representative THz QCL maximum operating temperature and frequency; (b) representative THz QCL maximum output power varies with year

Fig. 3. One-dimensional grating THz QCL. (a) Second-order DFB THz QCL^[25]; (b) graded photonic heterostructure THz QCL^[26]; (c) hybrid grating DFB THz QCL^[28]; (d) third-order DFB THz QCL^[29]; (e) antenna feedback plasmonic THz QCL^[30]; (f) wire DFB THz QCL<

Fig. 4. Two-dimensional photonic structure THz QCL. (a)(b) Ring DFB THz QCL and calculated TM_1,42 mode^[32]; (c)(d) concentric-circular-grating THz QCL and far-field^[33]; (e)(f) photonic-crystal THz QCL^[34]; (g)(h) grading photonic crystal THz QCL and far-field^[35]; (i)(j) photonic quasi-crystal T

Fig. 5. THz VECSEL based on metasurface. (a) Active metasurface structure^[38]; (b) schematic of THz VECSEL^[38]; (c) schematic of metal-metal waveguide ridge array for metasurface^[38]; (d) far field of THz VECSEL^[38]; (e) schematic of TM₀₃ metasurface^[39

Fig. 6. Effect of radio frequency modulation on THz QCL emission spectrum, where the gray spectrum line is emission spectrum in free running mode, and black spectrum line is emission spectrum with RF modulation. (a) Emission spectrum of THz QCL with semi-insulating surface plasmon waveguide^[42]; (b) emission spectrum of THz QCL with metal-metal waveguide^[24]

Fig. 7. Passive frequency comb based on graphene-coupled THz QCL. (a) Group velocity dispersion simulations for lasers with different ridge widths^[43]; (b) inter-mode beat note of three lasers with different ridge widths^[43]; (c) schematic of graphene-coupled THz QCL^[44]; (d) inter-mode beat note of FP-QCL, Si-QCL, and GiSAM-QCL^[

Fig. 8. On-chip dual-comb based on THz QCL^[45]

Fig. 9. Y-shape dual-comb based on THz QCL^[46]