Design of a mid-infrared suspended chalcogenide/silica-on-silicon slot-waveguide spectroscopic gas sensor with enhanced light-gas interaction effect

Abstract

The reported chalcogenide (ChG) waveguide sensor mainly focuses on rectangular waveguide with a small evanescent field for light-gas interaction. In order to improve the sensing performance in the mid-infrared spectral range and ease wavguide fabrication, a suspended ChG/silica-on-silicon slot-waveguide gas sensor was proposed, where ChG was used as the core layer, silica was adopted as the lower buffer as well as a support layer with the silica under the waveguide core removed. The new sensor structure resulted in an increased light-gas interaction, a decreased waveguide loss in the mid-infrared and a small depth-to-width ratio for feasible slot fabrication. The optimized suspended slot waveguide reveals a large power confinement factor (PCF) of 85.77% at 3291 nm. CH4 was adopted as the target gas for performance evaluation of the ChG/silica-on-silicon slot-waveguide sensor at the absorption line of 3038.5 cm−1. An optimal waveguide length of 1.45 cm was determined with a waveguide loss of 3 dB/cm for maximizing the sensitivity. The response time is as short as <3 μs due to the exposure of the waveguide sensing area to the atmosphere. The limit of detection (LoD) was decided to be 1.70 parts-per-million (ppm) for a minimum detectable signal-to-noise ratio of 10. Effects of ambient temperature/pressure change and fabrication error on the sensor performance were discussed. The proposed suspended ChG/silica-on-silicon slot-waveguide sensor possesses a ∼ 10 times larger PCF than other reported ChG rectangular waveguide sensors in the mid-infrared and reveals an improved performance for potential environmental monitoring and wearable gas sensing applications.