**1. Introduction**

Both IR absorption spectroscopy and Raman scattering spectroscopy are nowadays routine characterization techniques that are available in most organic and material processing labs, as well as in the industry. These techniques provide specific information about molecules or chemical functional groups in a fast, non-invasive, and reliable manner and have been used to identify compounds, follow reactions, and track absorption processes. Their wide variety of applications include environmental monitoring, (i.e., not only the monitoring of pollutants and greenhouse gases but also real-time monitoring of anesthetics and respiratory gases during surgery), explosives detection, medical diagnostics, and even the authentication of paintings, aside from their widespread use in research and industry [1–4].

Absorption and Raman spectroscopy, although providing similar information, are complementary techniques as the rotational-vibrational signal, silent in Raman scattering, can be highly noticeable in absorption experiments, and vice versa. It is therefore of no surprise that both techniques have been developed simultaneously for similar purposes. They exhibit different configurations according to the nature of the sample, i.e., for liquids, thin films, powders, or gases. Gas detection based on Raman and absorption spectroscopy relies heavily on the boost in sensitivity through the use of resonant cavities and multipass cells that increase the path length and, hence, the interactions of the beam with the analyte. Such cells and cavities have, almost exclusively, been realized by free-space beams and bulk optics; as a consequence, the standard high-end spectroscopy instruments still remain bulky, and samples often need to be collected and analyzed in the laboratory.

Recently, portable tunable laser absorption spectroscopy (TLAS) instruments for trace gas analysis have been developed by Aerodyne Research Inc., LI-COR, Picarro, IRsweep, and others. These, being typically packaged as 19-inch rack modules or of the size of a

**Citation:** Alberti, S.; Datta, A.; Jágerská, J. Integrated Nanophotonic Waveguide-Based Devices for IR and Raman Gas Spectroscopy. *Sensors* **2021**, *21*, 7224. https://doi.org/ 10.3390/s21217224

Academic Editors: Krzysztof M. Abramski and Piotr Jaworski

Received: 17 September 2021 Accepted: 26 October 2021 Published: 30 October 2021

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large suitcase, have been used for in situ monitoring in mobile vehicles, including airborne field campaigns. Shoebox-sized and only 3 kg in weight, instruments offered by Aeris Technologies followed, as well as a compact, 2.1 kg and battery-powered sensor developed by Empa for measurements aboard unmanned aerial vehicles (UAVs) [5]. Alternative portable devices include quartz-enhanced photoacoustic spectroscopy (QEPAS) [6], a variant of photoacoustic spectroscopy (PAS) in which the microphone is replaced by a quartz tuning fork. The instrumentation of Raman spectroscopy includes handheld Raman spectrometers, produced by Bruker, Thermo Fisher Scientific, and other companies.

Integrated on-chip devices appear to be the next logical steps to decreasing the size even further while keeping the advantages of molecular selectivity and sensitivity as offered by IR and Raman spectroscopy techniques. This will ultimately require the monolithic integration of a laser and a detector, with a photonic chip replacing a classical gas cell. In most cases, this photonic chip is constituted by a high-finesse photonic cavity, or a long, single-mode waveguide often curled into a spiral and tightly patterned on a photonic chip. Both these configurations enable long optical pathlengths for high sensitivity while keeping a minimal footprint, e.g., a chip size of the order of one square centimeter. In such waveguide-based sensors, the guided light penetrates the evanescent field outside the waveguide core and probes a sample close to the waveguide surface. Molecules present within the evanescent field will absorb light or generate Raman scattered light that will couple back into the waveguide modes. The respective interaction pathways give rise to analytical techniques that are also known as waveguide infrared absorption spectroscopy (WIRAS) and waveguide-enhanced Raman spectroscopy (WERS).

In this review, we will discuss the major advances made in waveguide-based absorption and Raman spectroscopy for gas detection. The first section will address the main components developed so far to achieve the miniaturization of integrated sensing devices: light sources, waveguides, cladding, and detectors. In particular, the materials and designs that are proposed to decrease losses while increasing light interaction with the surrounding environment will be described. Additionally, a brief description of cladding as a strategy to improve sensitivity will be provided. The second section will focus on the integrated Raman and IR absorption sensors reported so far. The main sensor configurations will be introduced, and the latest applications will be discussed, discriminating between airclad and functionalized/clad waveguides. Finally, a technology map will compare the performance of individual WIRAS and WERS sensors reported to date.
