*4.2. Applications*

## 4.2.1. Air-Clad

The first steps toward the use of waveguides for Raman spectroscopy came more than 40 years ago, with the use of planar waveguides to characterize polymeric thin films [213]. It was observed that the enhancement of Raman excitation in thin films resulted from maintaining a high excitation intensity over an increased scattering volume of the analyte [214]. Further studies revealed that high-index waveguides yielded the greatest field enhancement for Raman excitation at the surface of an optical waveguide [215]. To better quantify the waveguide performance for Raman sensing, Baets et al. defined the so-called conversion efficiency, a parameter that is dependent on both the waveguide geometry and the material. The group corroborated their finding experimentally with isobutanol at 785 nm [71].

Waveguide-based Raman spectroscopy demonstrations published to date rely on high refractive-index waveguide materials that are transparent in the visible–NIR range, such as silicon nitride and tantalum pentoxide. Similar to IR spectroscopy, the strip, rib and slot waveguides are the most popular designs [65,71,74,216]. Tantala rib waveguides were tested on isopropanol, methanol, and finally, with hemoglobin solutions at physiological concentrations as the first step toward future nanoscopy applications [52]. Silicon nitride strip waveguides were designed in a spiral pattern for WERS at 785 nm and were used to track isopropanol by Baets' group [71]. Soon after, the same group proposed a design based on slot waveguides, which allows for better light–analyte interaction in the slot and, due to lower optical field confinement in the waveguide material, it also mitigates the waveguide Raman background [212]. The introduction of slot waveguides resulted in a 6-fold improvement in performance for silicon nitride waveguides, compared with strip waveguides [51]. Furthermore, Baets and collaborators studied theoretically the influence of the refractive index and polarization on Raman conversion efficiency, concluding that the TE polarization in slot waveguides with a high refractive index contrast presents the highest value from the three designs discussed [217]. Despite the success of Si3N4 spiral waveguides, the Raman background and fluorescence signal in the material still pose a serious limitation to detection sensitivity. Therefore, the low density of molecules, such as those found in the gas phase, has not been detected in non-functionalized (or unclad) waveguides so far, and most reports have been on liquid samples.

The use of plasmonics has also been proposed and widely studied, in order to increase the Raman scattering yield. Interaction between nanoplasmonics antennas and waveguides was described both theoretically and experimentally by several authors [218–220] and the implications for sensing have been explored. Although great advances have been made in the field, including integrated plasmonic moieties to the waveguide [48,50], the performance of such devices in comparison to traditional WERS has not shown clear advantages. Plasmonics brings signal enhancement, but it simultaneously increases losses, thus limiting the propagation length to several micrometers in comparison to the centimeterlength scale achieved by common WERS. Therefore, the Raman conversion efficiency results in comparable values, being slightly superior for a 4-cm slot waveguide than 15-μm hybrid plasmonic waveguides. The main advantage of hybrid waveguides has been the reduced spurious Raman background generated from the core of the dielectric waveguide. Nevertheless, neither of these approaches proved sensitive enough for gas sensing.

Stimulated Raman scattering (SRS), as a strategy to increase the Raman signal, was tested experimentally on silicon nitride waveguides by Baets et al. [208]. The signal was enhanced but so was the noise, with a resulting signal-to-noise ratio only slightly improved compared to spontaneous Raman scattering. This mediocre result has been attributed to sub-optimized design and equipment, while more significant improvement with a better-optimized system was not excluded.
