Power Budget Analysis for Waveguide-Enhanced Raman Spectroscopy

Abstract

Waveguide-enhanced Raman spectroscopy (WERS) is emerging as an attractive alternative to plasmonic surface-enhanced Raman spectroscopy approaches as it can provide more reproducible quantitative spectra on a robust chip without the need for nanostructured plasmonic materials. Realizing portable WERS systems with high sensitivity using low-cost laser diodes and compact spectrometers requires a detailed analysis of the power budget from laser to spectrometer chip. In this paper, we describe theoretical optimization of planar waveguides for maximum Raman excitation efficiency, demonstrate WERS for toluene on a silicon process compatible high index contrast tantalum pentoxide waveguide, measure the absolute conversion efficiency from pump power to received power in an individual Raman line, and compare this with a power budget analysis of the complete system including collection with an optical fiber and interfacing to a compact spectrometer. Optimized 110 nm thick Ta2O5 waveguides on silica substrates excited at a wavelength of 637 nm are shown experimentally to yield overall system power conversion efficiency of ∼0.5 × 10−12 from the pump power in the waveguide to the collected Raman power in the 1002 cm−1 Raman line of toluene, in comparison with a calculated efficiency of 3.9 × 10−12. Collection efficiency is dictated by the numerical and physical apertures of the spectral detection system but may be improved by further engineering the spatial and angular Raman scattering distributions.