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.
© 2016 The Author(s)
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