Exploring the frontier of trace gas sensing, around the classical limit and beyond

Laser spectroscopy of trace molecular gases has already hit his record with the Saturated Absorption Cavity Ringdown-SCAR technique, detecting radiocarbon dioxide (14CO2) with a precision of a few parts in 10-15, or parts-per-quadrillion. Such results have become possible certainly due to the invention and effective implementation of the SCAR technique, but also due to the ultra-low noise operation of quantum cascade lasers used both as a reference and to excite the target molecular transition, around 4.5 micron wavelength. Recently, we explored further miniaturization using cantilever-enhanced photoacoustic spectroscopy, which has already proven to achieve sub-ppt sensitivity in an ultra-compact set-up. We are now studying the noise spectral density of the QCLs used for these experiments, the influence of current drivers, and the overall stability of the whole set-ups, to explore the ultimate sensitivity achievable, in the classical regime or, possibly, beyond the shot-noise limit.