Waveguide-Based On-Chip Photothermal Spectroscopy for Gas Sensing

On-chip waveguide spectroscopic sensors have attracted considerable attention due to its potential for large-scale integration. However, existing waveguide gas sensors based on direct absorption spectroscopy (DAS) suffer from limited sensitivity and measurement range. Here waveguide-based on-chip photothermal spectroscopy (PTS) is demonstrated for gas detection with high sensitivity and large dynamic range. On-chip photothermal field due to non-radiation relaxation of gas molecules and the resulted photothermal phase modulation are analyzed. By selecting chalcogenide glass (ChG) as the core-layer material and fabricating thermally-isolated ChG-on-SU8 waveguide for thermal field accumulation, a twofold increase in photothermal phase modulation is achieved as compared to ChG-on-SiO₂ waveguides. Different from the major concern of multi-path etalon noise in DAS, piezoelectric transducer noise in the interferometer is identified as the main source in this PTS. For a fair comparison, acetylene (C₂H₂) detection experiments are conducted using PTS and DAS with a 2 cm-long ChG-on-SU8 waveguide. A remarkable sensitivity of 4 parts-per-million (ppm) is achieved, which is 16 times better than that of DAS. The dynamic range extends over five orders of magnitude for PTS, ≈3 orders of magnitude larger than that of DAS. Such high performance opens the possibility of fully-integrated chip-level sensors for low-power, light-weight applications.