Spatially engineered optical-acoustic matching in quartz-enhanced photoacoustic spectroscopy.

Quartz-enhanced photoacoustic spectroscopy (QEPAS) offers high sensitivity for trace-gas detection, but its performance is often limited by a spatial mismatch between distributed photoacoustic excitation and the intrinsic sensitivity region of the quartz tuning fork (QTF). In many multi-pass QEPAS configurations, extending the optical path length alone does not ensure efficient signal enhancement. Here, we present a spatially engineered optical-acoustic matching strategy for QEPAS. A confocal-like multi-pass cell folds the excitation beam multiple times while spatially confining optical absorption within the intrinsic high-sensitivity region of the QTF. In parallel, non-resonant conical acoustic collectors (CACs) geometrically match and efficiently collect the resulting distributed photoacoustic waves. Experimental validation using water vapor detection demonstrates an approximately 42-fold signal enhancement compared with a conventional single-pass QEPAS configuration under identical conditions. The enhancement is achieved without relying on narrowband acoustic resonance or stringent optical alignment, establishing spatial engineering as a robust and general framework for improving QEPAS performance.