CLEO® /Europe-EQEC 2023 Cavity-Enhanced Precision Spectroscopy of Molecules

Molecules have rich vibrational, rotational, and spin degrees of freedom, making the precision spectroscopy of molecules attractive in various studies, such as searching the variation of fundamental physical constants [1], the parity-violation effects in chiral molecules [2], and the time-reversal symmetry violation [3], and also applications in quantum information [4]. They are also ideal optical frequency standards [5] for their narrow natural widths and spreading in a wide spectral region. Ro-vibrational transitions of molecules are usually much weaker than electronic transitions, which presents an obstacle in many applications. Using high-finesse optical cavities can effectively increase the molecule-light interaction path length and improve the detection sensitivity. Cavity-enhanced spectroscopy methods, such as cavity ring-down spectroscopy (CRDS), and noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS), have been widely applied to probe weak molecular transitions with high sensitivity and high precision. The resonant cavity also increases the laser power inside the cavity by 3–4 orders of magnitude, allowing for saturation spectroscopy and two-photon spectroscopy measurements [6], [7]. Combined with the optical comb techniques, the frequency accuracy has been improved to kHz or even sub-kHz [5], [8]. Precision measurements of extremely weak transitions, with Einstein A coefficients at the
$10^{-5}\ \mathrm{s}^{-1}$
level, have been demonstrated [9], [10]. Cavity-enhanced absorption spectroscopy with high precision in the vertical axis (absorbance) is also very competitive in applications measuring the number density of molecules. SI-traceable measurements have shown great potential in metrological applications [11]. In this talk, we will review the progress in cavity-enhanced spectroscopy methods, application examples, and perspectives.