Advances in cavity-enhanced methods for high precision molecular spectroscopy and test of fundamental physics

Cavity-enhanced spectroscopic techniques are highly sensitive laser-based methods for interrogating the atomic and molecular constituents of any gaseous medium that is confined into an optical resonator. A first advantage over conventional absorption spectroscopy comes from the extremely long path length of the laser radiation inside the stable, high-finesse, optical cavity, which allows the sample to be probed over several tens of kilometers. After more than 30 years of research and development, techniques like cavity ring-down spectroscopy, cavity-enhanced absorption spectroscopy, and noise-immune cavity-enhanced optical-heterodyne molecular spectroscopy, have reached extraordinary levels of detection sensitivity, such that it is possible to measure light absorption from molecules in trace amounts or extremely weak spectral lines of more abundant species. A second advantage of the use of high-finesse cavities lies in the power amplification achieved inside the optical resonator, making it possible to saturate even weak transitions, thus reducing the width of spectral lines by some three orders of magnitude. Combining these methods with frequency comb technologies has further enhanced their capabilities, adding metrology-grade qualities to spectroscopic determinations such as transition frequencies of molecular resonances, which can be measured with sub-kHz accuracy. In this review article, we discuss the current status of highly precise and highly sensitive laser spectroscopy for fundamental tests and measurements. We describe state-of-the-art molecular spectroscopy methods and their application to a few selected molecules of fundamental importance in understanding quantum chemistry theories or testing quantum electrodynamics.