Continuous-Wave Cavity-Enhanced Polarimetry for Optical Rotation Measurement of Chiral Molecules

Precise optical rotation measurements play an important role in the analysis of chiral molecules in various fields, especially in biological chemistry and pharmacology. In this paper, we demonstrate a new variant of continuous-wave cavity-enhanced polarimetry for detecting the optical activity of two enantiomers of a chiral molecule at 730 nm. It is based on a signal-reversing technique for which the chiral specific rotation is directly determined by the cavity ring-down signal from two counter-propagating beams in a bow-tie cavity. In particular, we ensure reproducible excitation of both modes by broadening the linewidth of a diode laser source by application of a radio frequency perturbation to its injection current. The performance of the polarimeter is demonstrated for the specific rotation of (+)- and (-)-α-pinene in different environments, including the pure vapor, open air, and the liquid phase; the detection precision ranges between 10^-5 and 10^-4 degrees per cavity pass depending on the environment. The apparatus is a robust and practical tool for quantifying chirality and can be developed for the entire visible and near-infrared spectral regions.