Abstract

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 10⁻⁵s ⁻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.

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