Fig. 1. (a) Monolithically integrated QCL device developed by MIT Lincoln Laboratory, part of the QCL waveguide is proton implanted to reduce the free-carrier and intersubband transition loss^[12]; (b) Schematic of the evanescently coupled monolithically integrated QCL device^[13]; (c) Monolithic integration of mid-infrared QCL with low-loss passive waveguides via butt-coupling^[14], the InGaAs passive waveguide layer is grown on the sample after most area of the wafer is removed by wet etching；(d) CW mode light-current-voltage (LIV) curve of the monolithically integrated QCL device schematically shown in Fig.1(c) ^[14]

Fig. 2. (a) Surface morphology of the QCL wafer grown on silicon substrate^[18]; (b) LIV characteristics of a QCL device grown on silicon substrate^[18]

Fig. 3. (a) Schematic of the QCL heterogeneously integrated on a silicon waveguide^[20]; (b) LIV characteristics of the heterogeneously integrated QCL in pulsed operation^[20]

Fig. 4. (a) Schematic of the III-V/germanium hybrid external cavity laser^[25]; (b) Superimposed lasing spectra obtained by tuning the DBR^[25]

Table 1. Comparison of different photonic integration approaches for mid-infrared QCLs