Ultrasensitive detections in atomic and molecular physics: demonstration in molecular overtone spectroscopy

We consider several highly sensitive techniques commonly used in detection of atomic and molecular absorptions. Their basic operating principles and corresponding performances are summarized and compared. We then present our latest results on the ultrasensitive detection of molecular overtone transitions to illustrate the principle and application of the cavity-enhanced frequency-modulation (FM) spectroscopy. An external cavity is used to enhance the molecular response to the light field, and an FM technique is applied for shot-noise-limited signal recovery. A perfect match between the FM sideband frequency and the cavity free spectral range makes the detection process insensitive to the laser-frequency noise relative to the cavity, and, at the same time, overcomes the cavity bandwidth limit. Working with a 1.064-μm Nd:YAG laser, we obtained sub-Doppler overtone resonances of C2HD,C2H2, and CO2 molecules. A detection sensitivity of 5×10-13 of integrated absorption (1×10-14/cm) over 1-s averaging time has been achieved.