A cavity-enhanced MEMS-based photoacoustic sensor for ppt-level trace-gas detection

Part-per-trillion level trace-molecule detection is becoming increasingly crucial for a variety of fields in our modern society, from climate change monitoring and mitigation to health studies, from industrial processes control to safety and security. The race towards more performing sensors is witnessing a rapid evolution of photoacoustic systems, whose high degree of flexibility allows them to merge their robustness and compactness to cavity-enhanced configurations, boosting their ultimate sensitivity. This work proposes an advanced configuration of a cavity-enhanced cantilever-based photo-acoustic sensor. The developed setup exploits the advantages of mid-IR detection and introduces significant novelties in the key components, namely a non-conventional silicon “racket-shaped” cantilever, a combination of a dual-tube acoustic resonator and optical cavity to enhance the photoacoustic signal, and an improved optical readout system consisting in a stabilized balanced Michelson interferometer. With a final detection sensitivity of dry N${}_2$O down to 17 parts-per-trillion for 20 s of integration time, corresponding to a Normalized Noise Equivalent Absorption coefficient equal to 5.98 $\times 10$^-11cm ^-1 WHz^-1/2, the achieved performance is in line with the best results obtained with PAS-based sensing addressing the same target molecule. This demonstrates the wide range of yet unexplored configurations of photoacoustic systems that can be exploited towards real-time sub-ppt sensors for practical detection of trace chemicals in the air.