Stimulated Raman scattering amplification under ultrasound

We observe that ultrasound applied to water surprisingly results in a substantial enhancement of the stimulated Raman scattering (SRS) intensity in both forward and backward directions, and returns to the initial level after the ultrasound is stopped. Quantitatively, the first Stokes component of SRS increases by approximately 4 and 2.5–3 times in the forward and backward directions, respectively, while the second Stokes component exhibits a 5–6-fold increase. This striking SRS enhancement is accompanied by a simultaneous decrease in the elastic scattering intensity, which suggests an energy redistribution between scattering mechanisms. The effect is observed only at picosecond laser pumping (30 ps, 10 mJ, 10 Hz): no increase is observed when nanosecond laser pulses are used. This points out to randomly distributed feedback as an underlying physical mechanism. The SRS intensity enhancement was also observed in ethanol and acetone. The mechanism certainly requires further careful and detailed study.