  The unique ability of slot and sub-wavelength grating (SWG) waveguides to
confine light outside of the waveguide core material has attracted significant
interest in their application to chemical and biological sensing. However, high
sensitivity to sidewall roughness induced scattering loss in these structures
compared to strip waveguides casts doubt on their efficacy. In this article, we
seek to settle the controversy by quantitatively comparing the sensing
performance of various waveguide geometries through figures of merit that we
derive for each mode of sensing. These methods take into account both modal
confinement and roughness scattering loss, the latter of which is computed
using a volume-current (Green's-function) method with a first Born
approximation. For devices based on the standard 220 nm silicon-on-insulator
(SOI) platform whose propagation loss is predominantly limited by random
line-edge sidewall roughness scattering, our model predicts that properly
engineered TM-polarized strip waveguides claim the best performance for
refractometry and absorption spectroscopy, while optimized slot waveguides
demonstrate >5x performance enhancement over the other waveguide geometries for
waveguide-enhanced Raman spectroscopy.
