Pumping of stimulated Raman scattering by stimulated Brillouin scattering within a single liquid droplet: input laser linewidth effects

The intensity threshold for stimulated Raman scattering (SRS) with a single-mode laser beam is noted to be ≈3 times lower than that with a multimode beam. The intensity threshold for stimulated Brillouin scattering (SBS) from droplets is lower than that for SRS. The temporal profiles of the laser pulse, SRS, and SBS are simultaneously measured with a streak camera (100-psec resolution). The first SBS pulse always occurs earlier than the first SRS pulse. In addition, the subsequent series of SBS and SRS pulses is temporally correlated; i.e., the minimum of the (n + 1)th SBS pulse occurs when the nth SRS pulse reaches a maximum. The second-harmonic beam of a single-mode or multimode Q-switched Nd:YAG laser is tightly focused at the center of the droplet’s illuminated face in order to avoid excitation of any morphology-dependent resonances of a droplet. We conclude that, for single-mode laser excitation of droplets, the internal SBS pumps the SRS.