Continuous-wave cavity ringdown for high-sensitivity polarimetry and magnetometry measurements.

We report the development of a novel variant of cavity ringdown polarimetry using a continuous-wave laser operating at 532 nm for highly precise chiroptical activity and magnetometry measurements. The key methodology of the apparatus relies upon the external modulation of the laser frequency at the frequency splitting between non-degenerate left- and right-circularly polarized cavity modes. The method is demonstrated by the evaluation of the Verdet constants of crystalline CeF3 and fused silica, in addition to the observation of gas- and solution-phase optical rotations of selected chiral molecules. Specifically, optical rotations of (i) vapors of α-pinene and R-(+)-limonene, (ii) mutarotating D-glucose in water, and (iii) acidified L-histidine solutions are determined. The detection sensitivities for the gas- and solution-phase chiral activity measurements are ∼30 and ∼120µdeg over a 30 s detection period per cavity round trip pass, respectively. Furthermore, the measured optical rotations for R-(+)-limonene are compared with computations performed using the TURBOMOLE quantum chemistry package. The experimentally observed optically rotatory dispersion of this cyclic monoterpene was thus rationalized via a consideration of its room temperature conformer distribution as determined by the aforementioned single-point energy calculations.

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.

High performance continuous-wave laser cavity enhanced polarimetry using RF-induced linewidth broadening.

We present precise optical rotation measurements of gaseous chiral samples using near-IR continuous-wave cavity-enhanced polarimetry. Optical rotation is determined by comparing cavity ring-down signals for two counter-propagating beams of orthogonal polarisation which are subject to polarisation rotation by the presence of both an optically active sample and a magneto-optic crystal. A broadband RF noise source applied to the laser drive current is used to tune the laser linewidth and optimise the polarimeter, and this noise-induced laser linewidth is quantified using self-heterodyne beat-note detection. We demonstrate the optical rotation measurement of gas phase samples of enantiomers of α-pinene and limonene with an optimum detection precision of 10 µdeg per cavity pass and an uncertainty in the specific rotation of ∼0.1 deg dm-1 (g/ml)-1 and determine the specific rotation parameters at 730 nm, for (+)- and (-)-α-pinene to be 32.10 ± 0.13 and -32.21 ± 0.11 deg dm-1 (g/ml)-1, respectively. Measurements of both a pure R-(+)-limonene sample and a non-racemic mixture of limonene of unknown enantiomeric excess are also presented, illustrating the utility of the technique.