# Suspended waveguide-enhanced near-infrared photothermal spectroscopy for ppb-level molecular gas sensing on a chalcogenide chip

## TL;DR

A new on-chip sensor uses suspended waveguides to detect gases at extremely low concentrations, improving sensitivity and speed for photonic sensing.

## Contribution

A suspended chalcogenide glass waveguide platform is introduced for ppb-level gas sensing using enhanced photothermal spectroscopy.

## Key findings

- A suspended chalcogenide glass waveguide achieved a 45-fold enhancement in photothermal phase modulation efficiency.
- Acetylene detection limit reached 330 ppb with a dynamic range near 6 orders of magnitude and sub-second response time.
- The system achieved a noise-equivalent absorption coefficient of 3.8×10−7 cm−1, setting a new benchmark for waveguide gas sensors.

## Abstract

On-chip waveguide sensors have attracted significant attention recently due to their potential for high-level integration. However, so far, on-chip gas sensing based on traditional laser absorption spectroscopy has demonstrated low detection sensitivity, due to weak light-gas interaction over a limited interaction distance. On-chip photothermal spectroscopy (PTS) appears to be a powerful technique to achieve higher sensitivity; its performance is yet constrained to parts-per-million (ppm)-level due to a small fraction of evanescent field in the light-gas interaction zone and fast thermal dissipation through the solid substrate. Herein, we demonstrated suspended chalcogenide glass waveguide (ChGW)-enhanced PTS that overcomes these limitations, enabling highly sensitive parts-per-billion (ppb)-level molecular gas sensing. We fabricated a nanoscale suspended ChGW with a low loss of 2.6 dB/cm using a CMOS-compatible two-step patterning process. By establishing an equivalent PTS model to guide the optimization of the ChGW geometry, we achieved a 4-fold increase in the absorption-induced heat source power and a 10.6-fold decrease in the equivalent heat conductivity, resulting in a 45-fold enhancement in photothermal phase modulation efficiency over the non-suspended waveguides. Combining with a high-contrast waveguide facet-formed Fabry-Perot interferometer, we achieved an unprecedented acetylene detection limit of 330 ppb, a large dynamic range close to 6 orders of magnitude, and a fast response of less than 1 s. The overall system exhibits a noise-equivalent absorption coefficient of 3.8×10−7 cm−1, setting a new benchmark for photonic waveguide gas sensors to the best of our knowledge. This work provides a key advancement towards prototyping an integrated sensor-on-a-chip for highly sensitive and background-free photonic sensing applications.

This work reports a novel nanophotonic platform based on a suspended chalcogenide glass waveguide for on-chip photothermal spectroscopy, enabling highly sensitive near-infrared molecular gas sensing at the parts-per-billion level.