**2. Efforts toward Miniaturization**

#### *2.1. Light Sources*

#### 2.1.1. IR Absorption Spectroscopy

One of the most widely used IR absorption spectroscopic techniques is based on tunable laser absorption spectroscopy (TLAS), which relies on a narrow-band light source such as a single-mode laser, where the wavelength can be carefully tuned to overlap with an absorption peak of the target analyte. This strategy has been applied most particularly in high-end trace gas sensors in the traditional gas cell configurations and their associated derivations involving optical cavities (such as cavity ringdown spectroscopy, cavity-enhanced absorption spectroscopy, and noise-immune cavity-enhanced opticalheterodyne molecular spectroscopy). Advances in MIR photonics over the last two decades have brought about high-quality laser diode sources based on interband cascade lasers (ICLs) [7,8], quantum cascade lasers (QCLs) [9,10], vertical-external-cavity surface-emitting lasers (VECSEL) [11–13], and frequency comb lasers [14,15]. The possibility of integrating light sources into chip devices has made them particularly suited for use in waveguidebased spectroscopy devices [11,16,17]. Excellent stability, tunability and the narrow linewidth of these lasers have enabled IR laser absorption spectroscopy of unprecedented sensitivity and specificity, making TLAS gas sensors a powerful alternative to conventional FTIR and NDIR spectroscopy. While IR laser absorption spectroscopy has traditionally focused on single-species detection, extending the tunable wavelength range enables multispecies detection with a single laser source. Dual-wavelength distributed feedback (DFB) QCLs or lasers implementing Vernier-effect tuning are integrated light sources that have turned this into a reality, thus expanding the range and applicability of such lasers [18–20]. Further extension of the tunable range of QCLs has used a Fabry–Perot QCL chip in an external-cavity (EC) system, where the laser could be tuned across the whole gain curve. In addition, DFB QCL arrays have been used to extend the tuning range, making it possible to electrically switch between emission frequencies [21–23].

On the other hand, broadband-coherent sources, such as supercontinuum lasers [24,25], have also been used for MIR spectroscopy. The development of the waveguide-based generation of a supercontinuum [26–29] has provided the perfect impetus for the realization of compact on-chip light sources. The broadband nature of these supercontinuum sources allows the simultaneous probing of several analytes but this also translates to low selectivity; hence, complex post-processing algorithms are needed to demarcate between the overlapping absorption spectra across different analytes [30,31]. Often, configurations in conjunction with wavelength filters or spectrometers are needed in order to maintain selectivity. While these laser sources have good coherence and adequate power output, making them well-suited for detecting trace quantities of gas, they are generally very complex to fabricate and are hence quite expensive as well.

Some state-of-the-art light sources include nanolasers based on plasmonic structures [32] and metamaterials [33], which hold a lot of promise regarding the realization of compact light sources. Many of these metamaterial-based light sources offer an easy way to control the wavelength of the emission through scaling the unit cell design to longer or shorter wavelengths, as compared to other light sources [34,35]. In contrast to this, incoherent light sources, such as MIR light-emitting diodes (LEDs), have also received attention due to their small size and low power consumption [36–38]. Among the MIR LEDs, super-luminescent light-emitting diodes (sLEDs) offer a unique combination of high brightness, good beam directionality and broadband capability [39]. However, they have been limited to wavelengths smaller than 5 μm due to the poor efficiency of light emission at longer wavelengths [40]. In particular, on-chip LEDs on SOI that is fabricated through the heterogeneous integration of InP membranes help to couple the light efficiently to a single-mode waveguide and help to avoid high coupling losses and high packaging costs [41]. Another approach is the direct material integration of active emitters, such as quantum dots, within the waveguide itself. As demonstrated in a silicon nitride platform with embedded quantum dots, this represents an elegant solution for generating waveguide source light with high-mode coupling [42].

Thermal emitters have emerged as one of the latest and most promising means of generating MIR radiation. In this case, plasmonics and metamaterials principles are used to design nanostructures with high spectrally selective absorptivity. Kirchoff's law then requires the radiation from these structures to emit in the same spectrally selective region. Certain MEMS-based structures and micro hot plates have been demonstrated as offering a good light, suitable for analyte sensing, due to their energy efficiency, fast modulation capability, and CMOS-compatible processing steps [43–45].
