Are slot and sub-wavelength grating waveguides better than strip waveguides for sensing?

Abstract

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 for silicon-on- insulator (SOI) photonic devices by quantitatively comparing the sensing performance of various waveguide geometries through figures of merit that we derive for each mode of sensing. These methods (which may be readily applied to other material systems) 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 SOI platform at telecommu- nication wavelengths ( λ = 1550 nm) 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 refractome- try and absorption spectroscopy, while optimized slot waveguides demonstrate > 5 × performance enhancement over the other waveguide geometries for waveguide-enhanced Raman spectroscopy.