Raman Optical Activity (ROA) spectroscopy is an essential analytical tool that has proved indispensable in, among other fields, biochemistry, where the majority of the studied objects are chiral molecules. However, the extremely weak signal, due to the generally off-resonant nature of spontaneous Raman process, makes this tool a challenge to use. Moreover, the difference between Raman signals of right- and left-circularly polarized beams is three orders of magnitude smaller than the Raman signal itself. At the same time, Coherent Anti-Stokes Raman Scattering (CARS), that probes the same vibrational signature of molecules, allows for a much stronger signal because of its coherent and resonant nature. CARS is a third-order nonlinear optical process that utilizes two incident optical waves with frequency difference νpump
equal to the frequency of a specific nuclear vibration, stimulating active scattering of the incident photons at the anti-Stokes frequency νAS
. Combining ROA with CARS has the advantage over the traditional ROA spectroscopy of a much higher contrast of the differential chiral signal to the achiral background. However, the signal-to-noise ratio (SNR) of ROA-CARS spectroscopy, utilizing the near infrared excitation at 1064 nm, has so far been worse than that of traditional ROA.
In the recent Optics Letters article, Hiramatsu et al. report on developing a CARS-ROA spectrometer with a visible CARS pump at 532 nm, which allows for an increased SNR and much higher contrast in comparison with NIR excitation. The authors employ a dual pumping scheme for generating a supercontinuum in a broad range around 532 nm, allowing them to cover Raman shifts (vibrational frequencies) up to 2000 cm-1
for multiplexed spectroscopy. With their new technique, they were able to obtain the ROA spectra of (+)- and (-)-β-pinene with excellent SNR and contrast. This advance paves the road for time-resolved ROA-CARS spectroscopy, with the immediate application to studying conformational dynamics and interactions of biomolecules.
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