Design of Slow-Light Subwavelength Grating Waveguides for Enhanced On-Chip Methane Sensing by Absorption Spectroscopy

The performance of on-chip gas sensors using absorption spectroscopy are currently limited by the small overlap and reduced interaction length between the light and the analyte. Here, the use of slow-light in subwavelength grating (SWG) waveguide integrated on a silicon photonic chip is proposed to improve methane sensing by tunable diode laser absorption spectroscopy in the near infrared. Such SWG waveguide increases the interaction by two means. First, close to the photonic bandgap edge, a SWG waveguide no longer acts as a metamaterial with a homogeneous index, but rather as a 1D photonic crystal in which slow-light effect enhances the light-analyte interaction. Second, the subwavelength segmentation of the waveguide increases the modal overlap with the air. These two enhancement mechanisms results in a six-fold improvement of the interaction with respect to strip waveguides. In this paper, we discuss how to engineer the group index of SWG waveguides to exploit slow-light effect for the first time. Design guidelines for minimizing propagation loss and disorder effect are discussed considering limitations of typical fabrication processes. SWG waveguides could improve the sensitivity and the limit of detection of on-chip trace-gas sensors that provide a compact, fabrication tolerant, inexpensive, and selective sensing technology.