Experimental observation of spectral super broadening of backward stimulated Raman scattering in liquids
The spectral super broadening of backward stimulated Raman scattering (SRS) in four liquid samples (CCl4, dimethyl sulfoxide, acetone, and CH2Cl2) have been observed under the pump conditions of using 532 nm and ~11 ns pulsed laser beams from a frequency doubled and multilongitudinal mode Pockels Q switched Nd:YAG laser device. Under the same pump conditions, in acetone the observed maximum broadening range was =450cm 1 for the backward SRS, while it was =45cm 1 for the forward SRS. The physical origin of this observed effect is essentially related to the detailed temporal structure of the 532 nm pump pulse that consists of a series of subpulse of ~50ps width, owing to the randomly beating effect among a large number of longitudinal modes of the Pockels Q switched laser source. Each backward SRS subpulse undergoes multiple cross phase modulation (XPM) by interacting with a certain number of forward pump subpulses, and consequently manifests a super broadened spectral distribution. In contrast, each forward SRS subpulse interacts only with a single pump subpulse and therefore experiences a much limited XPM influence, resulting in a much smaller spectral broadening. Furthermore, when the 532 nm pump beam was from a single longitudinal mode seeded Nd:YAG laser, there was no beating effect induced subpulse structure of the pump pulse, and thereby no noticeable spectral broadening for both backward and forward SRS could be observed.