Raman optical activity (ROA) is a well-known chiroptical vibrational spectroscopy that is sensitive to molecular chirality and facilitates the determination of absolute configuration. It is a powerful albeit time-consuming technique since standard ROA measurements usually take from several tens of minutes up to a few hours. The low signal strength is enhanced by many orders of magnitude in ROA’s nonlinear version: coherent anti-Stokes Raman scattering–Raman optical activity (CARS-ROA) spectroscopy. Until now, CARS-ROA spectroscopy has been reported for neat liquid (−)-β-pinene as the ROA gold standard with an acquisition time as short as 1 min. In this work, an experimental setup for rapid CARS-ROA spectroscopy in the millisecond regime is presented. The neat liquid (−)-β-pinene, equivalent to a molar concentration of ∼6.4 M, is detected in only 1 ms, which is 60 000 times faster than achievable so far. The high sensitivity of our setup additionally allowed us to detect high millimolar solutions of this chiral liquid in diethyl ether as an achiral organic solvent—down to ∼128 mM or 2-vol%; this is experimentally confirming the theoretically predicted orders of magnitude higher signal strength for heterodyne-detected CARS-ROA compared to spontaneous-ROA. Moreover, the CARS-ROA signal dependence on various parameters is systematically characterized for the first time, including pump power, Stokes power, concentration of the chiral molecule, and the strength of achiral CARS serving as the local oscillator. This work paves the way toward real-time monitoring of chirality-related processes by rapid chiroptical coherent Raman spectroscopy.
Kumar et al. (Mon,) studied this question.