Silicon Photonic MEMS Chip for Photothermal Spectroscopy Gas Sensing

Photothermal spectroscopy (PTS) [1]–[4] emerges as an excellent alterative to direct absorption spectroscopy and to photoacoustic spectroscopy, especially for high sensitivity gas sensing, with demonstrated lower limits of detection (LLOD) from ppm down to sub-ppb gas concentration levels [1]–[2]. A Fabry-Pérot Interferometer (FPI) can be used as the core sensing element for PTS. A first infrared laser beam (excitation) is injected to the FPI cavity to induce gas absorption. Its wavelength is chosen at a specific absorption line of the target gas. Light absorption by the gas causes local heating within the cavity, which translates into a variation of refractive index and hence, a shift of the FPI resonant wavelengths. This shift is detected by operating the FPI at fixed wavelength near resonance, using another near-infrared (probe) laser beam. This shift causes a variation of intensity of the transmitted and reflected probe light. In this operation scheme, the FPI requirements include both high finesse and high throughput, which are not trivial to attain, when targeting extreme miniaturization [4] at low cost, which would benefit for large-scale deployment of PTS that includes optical fiber communication networks [3].