Fig. 1. Structure of light emission using intersubband transitions in quantum wells. (a) Photon-assisted quantum tunneling in cascaded quantum wells to achieve optical amplification; (b) photon-assisted quantum tunneling structure in cascaded quantum wells

Fig. 2. Diagram of conduction band structure under the bias of QCL active region

Fig. 3. Growth structure of the QCL

Fig. 4. Band structure of QCL active designs. (a) Double-LO phonon resonance^[6]; (b) non-resonant extraction^[8]

Fig. 5. Conduction band diagram of coupled quantum wells active region responsible for laser action under an applied electric field. (a) In_0.53Ga_0.47As/In_0.52Al_0.48As material lattice matched to InP with ΔE_c=520 meV; (b) strain compensated In_1-xGa_xAs/In_1-yAl_yAs material on InP with ΔE_c>520 meV, where x<0.47, y>0.48

Fig. 6. Parameters of different material systems. (a) Lattice constant; (b) energy gap; (c) band offset

Fig. 7. Band structure and performance of long-infrared InAs-based QCL. (a) Conduction band diagram of InAs/AlSb QCL emitting at 20 μm. (b) voltage-current and light-current characteristics of a laser in pulsed mode, insert is emission spectrum of the laser

Fig. 8. Highest continuous-wave optical power for room temperature QCLs reported in the wavelength range from 3.5 μm to 10.7 μm^[32]

Fig. 9. Typical band structures of QCL. (a) Band diagram of an In_0.72Ga_0.28As/In_0.22Al_0.78As QCL structure based on non-resonant extraction design for light emission at 4.7 μm^[36]; (b) band diagram of an In_0.53Ga_0.47As/In_0.52Al_0.48As/In_0.69Ga_0.31As/In_0.36Al_0.64As/AlAs QCL structure based on shallow-well combined with tall ba

Fig. 10. Structure of an eight element buried ridge type QCL array by a tree-array multimode interferometer^[44]

Fig. 11. Band structure and performance of broad-gain spectrum QCL^[51]. (a) Active conduction band diagram based on dual-upper-state to multiple-lower-state design for center emission at 6.8 μm, the material is strain-balanced In_0.6Ga_0.4As/ In_0.44Al_0.56As; (b) EL spectra at room temperature QCL

Fig. 12. Low power consumption substrate emitting DFB-QCL. (a) Schematic of substrate emitting porous InP DFB-QCL; (b) light-current characteristics of a substrate emitting DFB-QCL laser in continuous wave mode at room temperature