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

TL;DR

This paper compares slot, sub-wavelength grating, and strip waveguides for sensing, finding that while strip waveguides excel in refractometry and absorption, optimized slot waveguides significantly outperform others in Raman spectroscopy due to their light confinement.

Contribution

It provides a quantitative comparison of different waveguide geometries for sensing, incorporating modal confinement and roughness scattering loss into a unified performance metric.

Findings

Strip waveguides perform best in refractometry and absorption sensing.

Optimized slot waveguides outperform others in waveguide-enhanced Raman spectroscopy.

Model predicts performance differences based on waveguide geometry and mode polarization.

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 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.