Coherent anti-Stokes hyper-Raman Spectroscopy

Coherent Raman scattering spectroscopies have been established as a powerful tool for investigating molecular systems with high chemical specificity. The existing coherent Raman scattering techniques detect only Raman active modes, which are a part of the whole molecular vibrations. Here, we report the first observation of coherent anti-Stokes hyper-Raman scattering (CAHRS) spectroscopy, which allows measuring hyper-Raman active vibrations at high speed. The CAHRS process relies on a fifth-order nonlinear process that combines hyper-Raman scattering with coherent Raman scattering. Observed signals are proven to come from the CAHRS process through various experiments concerning the dependences of the signals on incident laser powers, time-delay, polarizations, and selection rules of molecular vibrations. Comparisons of CAHRS signals with spontaneous hyper-Raman signals from para-nitroaniline solutions and benzene liquid manifest much higher signal-to-noise ratios of CAHRS signals than spontaneous hyper-Raman signals. This study illustrates that CAHRS spectroscopy can offer new information on molecular vibrations unobtainable from the present coherent Raman techniques at a much higher speed than spontaneous hyper-Raman spectroscopy.