*3.2. Applications*

#### 3.2.1. Air-Clad

IR absorption spectroscopy on waveguide-based devices has been reported in recent years for various applications in environmental and industrial process monitoring, as well as in the biomedical sector [171–173]. In particular, air-clad waveguides have been proven to be useful for IR absorption spectroscopy, to identify and quantify common gases such as carbon dioxide, acetylene, ammonia and methane, some of them with major implications in global warming. However, most demonstrations are still proof-of-concept experiments, testing the capability of the novel integrated sensors. An overview of the air-clad waveguides used for spectroscopy, together with their principal characteristics, is provided in Table 1, while a more detailed description and a discussion of the achieved results are given in the next paragraphs.

**Table 1.** Overview of works using WIRAS for gas sensing.



**Table 1.** *Cont.*


**Table 1.** *Cont.*

The silicon-on-insulator (SOI) platform has been the most popular choice for integrated gas sensor applications in both NIR and MIR. Ranacher et al. [174] demonstrated detection of CO2 down to a concentration of 500 ppm with polysilicon strip waveguides on silicon dioxide at 4.26 μm. From the measurements, the confinement factor was estimated to be in the range of Γ = 14–16%, and losses down to 3.98 dB/cm were reported. Silicon strip waveguides were also used for the detection of acetylene and methane by Jin et al. [175]. The group fabricated a 1-cm long waveguide with a thickness of 1 μm, which presents a good compromise between coupling efficiency and evanescence field confinement. The losses were determined to be 1.74 dB/cm and the simulated evanescent field ratio (EFR) was around 13% (EFR does not take into account the group index of the mode; so, although related, this should not be taken as a synonym of the confinement factor). Although the limit of detection was not calculated and the lowest concentration measured was 25% for both gases, the experimental results indicate that a concentration down to 5% could be quantified. The SOI platform was also chosen by Tombez et al. [176], with methane gas as the target analyte. They successfully increased the confinement factor by using TM polarization with a simple strip waveguide to 25.5% and achieved losses near to 2 dB/cm when operating at 1650 μm. The results are shown in Figure 3a,b. Two years later, the group took a breakthrough step toward integration, as discussed in the previous section, being able to successfully integrate a 20 cm spiral waveguide with a 15% confinement factor to a source and a detector on a single chip.

**Figure 3.** Experimental demonstration of gas detection by WIRAS. (**a**) The spectrum of the methane R(4)2ν<sup>3</sup> line measured by the waveguide-based integrated spectrometer design by Zhang et al. [157]. The Voight spectral fit for 1.5% methane concentration is shown in red, together with the experimental data. (**b**) Upper plot: experimentally measured methane concentration before and after flushing the chamber with methane. Lower plot: correlation between the absorption measured with the waveguide device and a free-space reference beam, indicating Γ = 25.5% in the waveguide (reproduced with permission from reference [157]). (**c**) Comparison of experimental absorption spectra for 4% and 1% acetylene measured using a free-standing tantala waveguide and a free space beam of identical path length, reproduced from Vlk et al. [69]. (**d**) Correlation of the measured concentration to the reference concentration of data in (**c**). The slope gives Γ = 107% (reproduced with permission under a Creative Commons Attribution 4.0 International License. http://creativecommons.org/licenses/by/4.0/ assessed on 9th September 2021).

Due to the light absorption in silicon dioxide bottom cladding at wavelengths over 3.5 μm, silicon-on-nitride (SON) and silicon-on-sapphire (SOS) appeared as an alternative to the commonly used SOI [187,188]. SOS has a transparent window of up to 5.5 μm and a high refractive index contrast between the core and the cladding. Chen and collaborators compared experimentally the performance of photonic crystal waveguides (PCW), slot waveguides and strip waveguides on sapphire. Despite the theoretical 1- to 100-fold slowlight driven improvement in the confinement factor, PC waveguides exhibited only slightly higher light–analyte interaction compared to slot waveguides but they were significantly better than strip waveguides. The same group developed PC waveguides even further [183]. Three PCW designs were fabricated in silicon on sapphire: a regular line-defect PCW (socalled W1 waveguide), a holey PCW (HPCW) with smaller diameter holes etched within the light defect, and a slot PCW, wherein a rectangular slot is etched at the center of the PCW. The designs were optimized for 3.43 μm wavelength to quantify xylene and triethyl phosphate (TEP) vapors. In the case of the slot PCW, the authors simulated that the slow light effect, coupled with the high evanescent field confinement in the slot, should reduce the required absorption path length by a factor of 1000 compared to strip or rib waveguides. Although the authors observed a detectable signal change when the waveguide was exposed to 10 ppm TEP with an 800 μm long HPCW, the increase in sensitivity due to the slow light effect is difficult to quantify. The measurements only tracked the total power loss at one wavelength instead of spectrally resolved detection; therefore, the band diagram shift due to the refractive index or temperature variations may affect the signal in the sensitive slow-light regime as well.

As an alternative to CMOS-compatible materials, Charrier et al. [179] reported chalcogenide strip waveguides over silica and calcium fluoride. The strip waveguides presented by the group showed losses as low as 0.4 dB/cm at 1.55 μm but the waveguides were tested in solution and not for gas detection. In a similar work, Han and collaborators [178] fabricated a chalcogenide glass (Ge23Sb7S70) strip waveguide over silica. The 2-cm spiral waveguide showed an air confinement factor of 8%, losses of around 7 dB/cm, and a limit of detection of 2.5% for methane at 3.3 μm [143]. However, the comparably high detection limit is due to a broad-band laser source that cannot resolve the narrow methane lines; a better-suited single-mode continuous-wave laser, such as ICL, would enable much better performance. Finally, Agarwal's group used the chalcogenide platform to develop a monolithic-integrated on-chip MIR methane sensor [156]. They demonstrated a 5 mm long spiral chalcogenide strip waveguide (Ge23Sb7S70) capable of sensing methane at 10,000 ppm. The design has a similar configuration to the previous report and provides a 1 cm2 footprint sensor with both the waveguide and an integrated detector. The losses of the waveguide were measured to be 8 dB/cm, and the air confinement factor, 12.5%, according to simulations.

To further decrease the losses and increase the confinement factor, air-suspended waveguide structures were frequently used over the last few years. Lai et al. [182] proposed photonic crystal slot waveguides for methane detection in the NIR, capable of detecting methane absorption signatures down to several hundred ppm. Dicaire et al. [80] developed a 1.5 mm-long suspended GaInP photonic crystal waveguide and used acetylene to demonstrate its spectroscopic performance. The group indices in the waveguide were 1.5 to 6.7 for the TM and TE modes, while the experimentally obtained confinement factors were 100% and 31%, respectively. The fact that the interaction did not scale with the group index (i.e., the slow-down factor) was due to the considerably larger evanescent field ratio of the TM mode. Based on this result, the authors stress that not only a high group index but also a high evanescent field ratio must be addressed for strong light–analyte interaction, the latter being often neglected in works on photonic crystal waveguides for sensing. The waveguide design also included mode adapters on both end facets to gradually couple into the slow-light mode and thus reduce the Fabry–Perot oscillations. Chen's group [63] designed and fabricated fully suspended InGaAs waveguide devices with holey photonic crystal waveguides and sub-wavelength grating cladding waveguides for the mid-infrared sensing of ammonia at λ = 6.15 μm (Figure 1b,c). The propagation losses for the two waveguide types were 39.1 and 4.1 dB/cm, the light–analyte overlap was calculated to be 12% (TE) and 10% (TM), lengths, 1 and 3 mm, and the group indices, 39 and 15, respectively. Both waveguides were capable of detecting 5 ppm ammonia; nevertheless, no spectroscopy was performed during the measurement. Changes in power were tracked after flushing ammonia at a constant wavelength, leaving the results susceptible to interference from changes in the environment, including refractive index changes or temperature variations. Ranacher and collaborators [84] designed and fabricated a polysilicon waveguide on a silicon nitride membrane suspended over silica walls. They achieved a 19.5% confinement factor at 4.23 μm. The 1 cm-long waveguide was deployed for CO2 detection, with the lowest detected concentration down to 5000 ppm. However, as in the previous work, only the signal drop at one wavelength was recorded and no spectral scan across a CO2 absorption line was performed. Gylfason's group [73] also developed a silicon self-standing waveguide, operational at 4.2 μm wavelength, for CO2 gas sensing. Their waveguide was designed as a Si beam, partially suspended 3 μm above the Si handle substrate and supported by tapered SiO2 pillars (Figure 1d–f). The waveguide had a large confinement

factor of 44%, a total length of 0.5 cm, and losses of 2.9 dB/cm; the CO2 detection limit was estimated to be 350 ppm. Although the measurement sampled absorption across a spectral line, unfortunately, the spectroscopy data that was provided had very few points to justify the fitting, considering that fringes might be present. The same group was able to measure CO2 on ring resonators by dispersion spectroscopy [181]. As established by the Kramers–Kronig relationship, a strong variation in absorption implies a sharp change in the real part of the refractive index. The group proved that ring resonators based on suspended rib waveguide, with an air confinement factor of 50%, were able to quantify amounts as low as 1000 ppm of carbon dioxide. In 2021, Vlk et al. [69] reported a thin-film suspended tantala rib waveguide for acetylene detection in the MIR. They simulated and proved experimentally that the waveguide is able to achieve a confinement factor of 107%, and thus surpass free-space light–analyte interaction by the combining of a high evanescent field and a group index larger than unity (Figure 1g,h and Figure 3c,d). The authors also proved that the design can suppress the fringes from facet and defect reflections and improve coupling efficiency. The still relatively high losses of 6.8 dB/cm were mainly attributed to absorption into the tantala film.

To take advantage of the field inside the waveguide in addition to the evanescent field, a mesoporous waveguide for IR spectroscopy was reported by Datta and co-workers [180]. Titania rib waveguides over silica bottom cladding, with 52% field confinement in the waveguide core, were fabricated and tested, using acetylene as a calibration gas. They experimentally confirmed that the mesoporous material enables the gas to rapidly diffuse into the core of the waveguide while maintaining a rather low loss of 2 dB/cm. Nevertheless, the presence of O-H groups on the large surface area of the pore network impairs transparency over time. This problem has been partially overcome by annealing and functionalization [177,189].
