Fig. 1. Schematic diagram of the LWIR QCLs：（a） schematic diagram of the LWIR QCLs with AR and HR coatings applied to the front and back facets； scanning electron microscopy （SEM） images of coatings on （b） front and （c） back facet

Fig. 2. Reflectivity curves：experimental and theoretically simulated reflectivity curves of （a） Al₂O₃ and （b） Y₂O₃ AR coatings，the inset shows the lasing spectrum of the LWIR QCL at 298 K

Fig. 3. L-I-V and WPE curves of the QCLs at 298 K, (a) CW operation of uncoated and HR-AR (Al₂O₃ coating); (b) pulsed and (c) CW operation of uncoated, HR-only and HR-AR (Y₂O₃ coating); (d) measured lateral far-field profiles

Fig. 4. SEM image of the front facet of the laser coated with (a) Al₂O₃ and (b) Y₂O₃ AR coating after testing

Fig. 5. Predicted WPE and AR coating thickness as a function of front facet AR reflectivity，inset：the reflectivity curves for different Y₂O₃ AR coating thicknesses

Fig. 6. L-I-V and WPE curves of the QCL with optimized front facet Y₂O₃ AR coating reflectivity under (a) pulsed and (b) CW operation at 298 K