3.1.1. On-Chip Light Sources and Passive Waveguide Integration

Silicon, as a platform for integrated sensors, has been the most frequent choice due to the existing mature process technology and the inherent compatibility with silicon waveguides. However, due to the absence of efficient light sources based on silicon, the heterogeneous integration of light sources, based on III–V materials, has been the preferred choice. This involves attaching the laser chip to a separate chip, with pre-patterned waveguides and other optical components for sensing and manipulation purposes. Heterogeneously integrated devices have been reported both in the NIR and MIR, and this has been discussed in detail in recent reviews [149,150]. Recent works have also focused on InP integration platforms, demonstrated through QCL integration with InGaAs passive waveguides [151] (Figure 2a) and photonic crystal-based laser source integration with silicon waveguides [152]. As a rare example of monolithic integration, QCLs integrated with plasmonic waveguides were demonstrated by Schwarz et al. [153]. In addition, Consani et al. [154] demonstrated the integration of a waveguide sensor with a MIR emitter for CO2 sensing, showing that it is possible to abstain from using expensive laser sources and instead use a cheap thermal source (Figure 2b). In their case, the emitted light was broadband, requiring the use of filters on the detector side, but recent advances in narrowband filters and metamaterials emitters can eliminate the use of additional filters.

**Figure 2.** Integration schemes for waveguide-based IR absorption spectroscopy. (**a**) Integrated QCL source with a passive InGaAs waveguide (reproduced from [151] © 2021 Optical Society of America). (**b**) Integration of a waveguide sensor with a MIR thermal emitter (reproduced with permission from [154]). (**c**) Integration of a p-i-n-based GaInAsSb photodetector with a passive waveguide through evanescent and grating-assisted coupling (reproduced from [155] © 2021 Optical Society of America). (**d**) Monolithic integration of a PbTe detector, coupled with a spiral waveguide (reproduced with permission from [156]). (**e**) Prototype of a fully integrated silicon photonic sensor with laser, waveguide, and detector for sensing in the near-infrared (reproduced with permission from [157]). (**f**) Quantum cascade laser and detector, integrated with a dielectric-loaded plasmonic waveguide for sensing in the mid-infrared (copyright © 2021, the author(s) [153]).
