Fig. 1. Schematic diagram of QCD energy band principle^[16]

Fig. 2. Simplified rate equation model of QCD under zero bias^[26]

Fig. 3. Simplified closed-loop three-level system of QCD^[26]

Fig. 4. The device structure diagram of QCD combining II-VI with III-V materials^[25]

Fig. 5. Three common optical coupling methods in QCD^[47]

Fig. 6. Nanophotonic structures. (a) SEM image of the fabricated PCS-QCD^[51]; (b) schematic illustration of the cross-section of the PCS-QCD; (c) SEM image of a 50 µm×50 µm mesa QCD device^[52]; (d) SEM image of the 8×8 PAR array co-integrated with a 50 Ω CPW; (e) enlarged view of the PAR array; (f) single patch antenna resonator containing the QCD active region and connected with Ti/Pt/Au wires of 200 nm width^[50]

Fig. 7. Structure and performances of QD-QCD^[55]. (a) Schematic of the device structure; (b) conduction band diagram of one period of the QD-QCD showing eigen-energies and wave functions; (c) normal incidence photoresponse of the QD-QCD measured at 77 K, 100 K, and 130 K under zero bias; (d) Johnson noise limited detectivity of the QD-QCD measured at 77 K, 100 K, and 130 K under zero bias

Fig. 8. Schematic conduction band diagram and moduli square of the relevant wavefunctions of the coupled quantum-well QCD^[21]

Fig. 9. Double-well coupled absorption QCD^[19]. (a) The energy band structure of double-well coupled absorption QCD; (b) responsivity and photocurrent data of QCD at 77 K and different biases, the inset is spectral responsivity of the QCD for applied bias of 0 V and 1.3 V

Fig. 10. The energy band structure of QCD with diagonal transition^[59]

Fig. 11. The energy band structure of QCD with miniband-diagonal transition^[61]

Fig. 12. Two-color quantum cascade detector^[64]. (a) Schematic structure of a two-color detector containing two independent GaAs/AlGaAs heterostructures in the active region; (b) energy band structure and wave functions of the active region containing two independent heterostructures; (c) energy band structure and wave functions of the active region with double upper excited state energy levels

Fig. 13. Single period QCD. (a) Energy band diagram of the active region of a single period QCD^[68]; (b) spectral responsivity curves at room temperature^[69]

Fig. 14. Co-working QCL-QCD. (a) Schematic diagram of the monolithically integrated QCL and QCD^[78]; (b) the experimental setup of the LWIR FSO transmission link^[82]; (c) schematic diagram of QCL and QCD single-chip photonic integrated sensing system^[85]; (d) CW light emission spectrum of a QCL and the photoresponse spectrum of the QCD measured at 293 K^[86]