Raman spectroscopy is a promising tool for point-of-care diagnosis, but widespread clinical application is limited by its low signal strength, as well as complex and costly instrumentation. The growing field of waveguide-based Raman spectroscopy tries to solve these challenges by working toward fully integrated Raman sensors with increased interaction areas. Working with our collaborator, Dr. Balpreet Ahluwalia of the Artic University of Norway, we are developing highly sensitive waveguide-based Raman sensor by optimizing waveguide materials and design. We have recently demonstrated resonant Raman detection of hemoglobin in a tantalum pentoxide (Ta2O5) waveguide platform.
Tantalum pentoxide waveguide Raman spectroscopy platform
Blood analysis is an important diagnostic tool, as it provides a wealth of information about the patient's health. Raman spectroscopy is a promising tool for blood analysis, but widespread clinical application is limited by its low signal strength, as well as complex and costly instrumentation. The growing field of waveguide-based Raman spectroscopy tries to solve these challenges by working toward fully integrated Raman sensors with increased interaction areas. Raman spectroscopy using waveguides made of high refractive index contrast (HIC) materials is experiencing a renewed interest. Recent achievements include measurements of biological monolayers and trace gases. When light is guided in an optical waveguide, an evanescent field is generated outside the waveguide core, which can be used to probe a sample close to the waveguide surface. Most of the recent result on waveguide-based Raman spectroscopy has been performed using silicon nitride (Si3N4) nanowires and slot waveguides with 785 nm excitation light. The increased field intensity inside the slot can give a 5-fold or more increase in signal strength. Despite the success of Si3N4 spiral waveguides, getting sufficient Raman scattering without cooled detectors, which is relevant for a fully integrated solution, remains challenging.
>Resonance Raman occurs for certain samples at low wavelengths and can enhance the signal by several orders of magnitude. Using 532 nm excitation is particularly interesting due to resonance Raman effects in several clinically relevant samples, such as >hemoglobin and in cancerous brain and breast tissues. Taking advantage of resonance Raman effects can be key to really bring, e.g., Raman-based blood analysis to the market. The development of HIC waveguide platform for resonance Raman spectroscopy using 532 nm excitation is therefore beneficial. Evans et al. have recently demonstrated waveguide-based Raman spectroscopy using a TiO2 platform at 532 nm excitation [1]. Here, we explore another potential platform for shorter wavelength excitation with Ta2O5 [2]. Ta2O5 is a promising waveguide material for Raman spectroscopy due to its high refractive index contrast with silica cladding, providing strong optical mode confinement and thereby enabling the development of compact photonic circuits with tight bends and spirals (Fig. 1).
We have recently demonstrated resonance Raman measurements of hemoglobin, a crucial component of blood, at 532 nm excitation using the Ta2O5 waveguide platform. We have characterized the background signal from this waveguide material when excited at 532 nm. In addition, we demonstrate spontaneous Raman measurements of isopropanol and methanol using the same platform (Fig. 2). Our results suggest that Ta2O5 is a promising waveguide platform for resonance Raman spectroscopy at 532 nm and, in particular, for blood analysis.