Trends in vibrational spectroscopy on optical waveguides.

Vibrational spectroscopy, due to its inherent specificity in providing a spectral "fingerprint" of molecular composition, is an important tool to identify and quantify molecules in environmental, clinical, biotechnological, security, and manufacturing process applications, for example. Optical waveguide chips, offering mass production, robustness, sensitivity and the potential for widespread, low-cost deployment as sensors, are being exploited in two principal forms of vibrational spectroscopy, waveguide mid-infrared spectroscopy (WMIRS) and waveguide-enhanced Raman spectroscopy (WERS). For both, key strands of present research include reducing waveguide attenuation, increasing light/matter interaction strength, exploring methods to deal with complex samples such as using machine learning approaches and on-chip integration of photonic devices to enhance functionality and improve performance. The mid-infrared materials and devices for WMIRS are less well developed than the near-infrared materials and devices normally used for WERS, requiring focussed research into these aspects, including new sources and detectors, and into mitigating the water absorption which can dominate parts of the MIR spectrum. In the case of WERS, additional key strands of research include reducing and mitigating the background emission from waveguide materials, enhancing the Raman signal strength, and incorporating combined plasmonic nanometallic structures with WERS. Additionally, for WERS, the issue of sample complexity hindering molecular identification is being addressed by the inclusion of Raman reporters in assays. MWIRS has been successfully applied to gas sensing, detection of compounds in breath for cancer and infection diagnosis and to bacterial discrimination in aqueous samples, for example. WERS has recently been applied to the detection of antibiotics in plasma and, for example, to cardiac biomarkers. There is now a growing trend towards commercialisation of WERS in particular, which should lead to increased application to real samples and advances in the development of practical sensors. In this review, we focus on the trends in research in WMIRS and WERS in the past 2 years.