Elastic Fabry-Pérot microcavity for high-sensitivity photoacoustic spectroscopy

We report an elastic Fabry-Pérot microcavity (μFP) acoustic transducer that combines mechanical compliance with resonantly enhanced optical interrogation, boosting the conversion of pressure-induced diaphragm displacement into an optical signal. The transducer comprises a high-reflectivity concave mirror fabricated on a fiber tip and a micro-electro-mechanical systems (MEMS) diaphragm, forming a microcavity of 80.7 μm length. When interrogated at a probe wavelength near 1.55 μm, the μFP exhibits an optical quality factor of 7.55 × 10⁴ and a mechanical resonance at 27 kHz. As a proof-of-concept demonstration, we integrate the μFP transducer into a quantum cascade laser-based photoacoustic system operating near 10.5 μm to analyze SF₆ trace gas samples. The μFP-based photoacoustic spectrometer achieves a 46 ppt minimum detection limit and a normalized noise-equivalent absorption (NNEA) coefficient of 1.6 × 10^-9cm^-1⋅W⋅Hz^-1/2. The results establish resonant optomechanical microcavities as a new route to sensitive photoacoustic spectroscopy.