Trace-Molecule Detection below the Part-Per-Trillion Level with Doubly-Resonant Cantilever-Enhanced Photoacoustic Spectroscopy

The race towards compact and robust sensors able to detect extremely low concentrations of molecules in the air plays an important role in our modern society, impacting sectors such as energy production, environmental monitoring, transportation, agriculture, safety, and security. During the last decade, optical detection with ultra-high sensitivity, down to the part-per-quadrillion level, was demonstrated with cavity-ring down techniques [1], enabling laser sensors to enter areas such as archaeology (radiocarbon dating), climate change monitoring, bio-fuel control, contaminant assessment for the semiconductor industry and so on. More recently, photoacoustic sensors based on quartz tuning forks and silicon cantilevers have shown great potential in achieving a sensitivity at the level of the techniques mentioned above, especially when combined with narrow-linewidth mid-infrared lasers and high-finesse optical cavities [2]–[5]. Besides the extremely high sensitivity, such sensors have unique characteristics of robustness, record dynamic range, and compact size, which make them particularly attractive for in-field applications.