Trace Gas Spectroscopy Using Cavity Optomechanics

In this paper, we present a new concept of spectroscopic sensor based on an optomechanical transduction. This novel trace gas detection technique exploits the so-called “membrane-in-the-middle” (MIM) setup, composed of a mechanical membrane in a middle of a high finesse Fabry-Perot cavity, filled by an absorbing gas. Preliminary thermomechanical noise measurements of a commercial low stress silicon nitride (SiN) membrane in air at ambient conditions with a homemade external fibered MIM cavity in the near infrared (NIR) region are presented as a proof of concept. The sensor in the mid-infrared (MIR) region is based on shifts of the mechanical resonance frequency induced by gas absorption using the optical spring effect. Typical limit of detection (LOD) for carbon dioxide (CO 2 ) at 4.232 µm is 45 ppb with a stabilized Quantum Cascade Laser (QCL), with a responsivity of 2.61 kHz/ppb of frequency shifts, for a circular stoichiometric silicon nitride (Si3N4) membrane of 50 µm diameter, 200 nm thick, quality factor of 50, and in a 10 ⁴ finesse air-filled optical cavity, corresponding to silicon-integrated devices currently in fabrication.