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\nconfine light outside of the waveguide core material has attracted significant\ninterest in their application to chemical and biological sensing. However, high\nsensitivity to sidewall roughness induced scattering loss in these structures\ncompared to strip waveguides casts doubt on their efficacy. In this article, we\nseek to settle the controversy by quantitatively comparing the sensing\nperformance of various waveguide geometries through figures of merit that we\nderive for each mode of sensing. These methods take into account both modal\nconfinement and roughness scattering loss, the latter of which is computed\nusing a volume-current (Green's-function) method with a first Born\napproximation. For devices based on the standard 220 nm silicon-on-insulator\n(SOI) platform whose propagation loss is predominantly limited by random\nline-edge sidewall roughness scattering, our model predicts that properly\nengineered TM-polarized strip waveguides claim the best performance for\nrefractometry and absorption spectroscopy, while optimized slot waveguides\ndemonstrate >5x performance enhancement over the other waveguide geometries for\nwaveguide-enhanced Raman spectroscopy.\n