3.1.2. Passive Waveguide and Detector Integration

Many demonstrations of integrated systems combining a detector and a passive waveguide involve integration of the active detector element, either monolithically or by the hybrid bonding or transfer of the active detector material. In particular, p-i-n photodiodes and 2D materials have been used for the active detector element to achieve compactness. GalnAsSb-based p-i-n photodiodes interfaced with SOI waveguides, either through gratings or evanescent coupling, have been demonstrated at 2.29 μm [155,158] (Figure 2c). Similarly, an InAs0.91Sb-based p-i-n photodiode has been shown at 3 μm, integrated on the output grating couplers of a spectrometer [159]. In both cases, heterogeneous integration through adhesive bonding was used. Su et al. fabricated an on-chip waveguide integrated device with monolithically integrated PbTe detector film for the detection of methane at 3.3 μm [156] (Figure 2d). For longer wavelength regions, graphene photodetectors integrated with silicon waveguides have been demonstrated [160]. Yazici et al. showed the integration of a MEMS-based broadband infrared thermopile sensor attached through flip flop bonding with an SOI platform integrated with input and output

grating couplers [124]. In all these demonstrations, the active area of overlap of the optical mode with the detector element is still small, resulting in lower sensitivity. Highly sensitive detection needs to increase the detector's active area, which, unfortunately, also increases the dark current and thus decreases the signal-to-noise ratio. In order to counter this, ridge waveguide-based detection has been used, where the entire length of the detector element acts as the active material for enhanced detection capability and has particularly been used in context with ICDs and QCDs. In order to improve the performance further, distributed Bragg reflectors (DBR), high reflectivity coatings, or simply etched or cleaved facets that ensure multiple passes through the active region have been proposed for a waveguide-integrated ICD. Such a design simultaneously increases quantum efficiency, as well as reducing dark current [148].

### 3.1.3. Integration of All Three Components

Among very few works describing prototypes of the on-chip integration of all components, Zhang et al. demonstrated a complete integrated setup involving a fully integrated NIR photonic chip sensor, mounted on a PCB test card, with an on-chip laser, dual photodetectors, reference cell, and an evanescent field-based sensing waveguide on a single silicon substrate. With this device, the authors showed methane sensing with a sub-100 ppmv·Hz−1/2 sensitivity [157] (Figure 2e). While this sensitivity still lags behind the state of the art, the fully packaged nature of their demonstration is an important milestone and paves the way for fully integrated devices, particularly in a longer wavelength region. In the MIR, Benedikt Schwarz's work at TU Vienna and Ray Chen's group in UT Austin have shown great progress toward fully developed sensors integrated with QCL and QCD. Schwarz et al. [161,162] demonstrated the integration using QCL technology, relying on the bifunctional functionalities of the active region to work as both laser and detector. Coupled with a dielectric loaded plasmonic waveguide, they exhibited a complete system with liquid sensing capability [153] (Figure 2f). Even though a high-power emission was observed, the detector sensitivity was poor in these bifunctional structures. Later demonstrations separated the functions of the laser and detector, allowing their independent design and optimization [163]. In parallel, in order to extend the lasing wavelength to below 6 μm and simultaneously enable low power consumption, high sensitivity, and sufficient design flexibility, subsequent work focused on ICL technology and integrated setups have been demonstrated for 3.1 μm [164]. On the other front, Ray Chen's group has shown a sensor with QCL and QCD as sources, and detectors integrated with an InGaAs-InP monolithic platform, and gas sensing was demonstrated [165,166] making the pursuit of an on-chip integrated sensor for gas sensing close to reality.

Further advancements have been made through the integration of frequency comb MIR lasers with detectors, which showed ultrafast detection and up to two orders of magnitude lower power consumption, compared to QCLs [167]. In addition, mid-infrared dual-comb spectroscopy is an upcoming area of research, where, through the interference of two mutually coherent mode-locked frequency combs, the absorption spectrum signal can be converted from the optical domain to the radio frequency domain. Dual comb spectroscopy has a fast detection capability with higher resolution and accuracy, making it suitable for gas detection [168,169], and, due to the inherently large bandwidth of the frequency combs, covering even multiple species in parallel [29].

Another strategy to improve the compactness and robustness of the IR spectroscopy setup is to design the sensing functionality within the cavity of the laser, as demonstrated through intra-cavity laser absorption spectroscopy [148,170]. The in situ detection of chemical species within the laser cavity can be monitored directly through the laser's I-V characteristics, which can even eliminate the use of a separate detector [170].
