Fig. 1. Energy band diagram of the first reported quantum cascade laser^［13］

Fig. 2. Design diagram of the active region of a high-power quantum cascade laser and schematic diagram of the device^［23-25］

Fig. 3. Maximum power output from room temperature continuous operation of QCLs grown using different crystal growth methods^{［30，36，37］}

Fig. 4. Example of single-mode quantum cascade laser^{［41，45］}

Fig. 5. Example of external cavity-quantum cascade laser^{［53，54，56］}

Fig. 6. The schematic diagram of monolithic tunable quantum cascade laser^{［57，59］}

Fig. 7. The schematic diagram of quantum cascade laser frequency comb^{［63，69，71］}

Fig. 8. Example of room temperature differential frequency terahertz light source based on quantum cascade laser^{［78，80］}

Fig. 9. Example of quantum cascade laser facing on-chip sensing^{［83，84，87］}