Growth of Optical Plane Waves in Stimulated Brillouin Scattering

Abstract

Several classical plane-wave theories have been proposed recently to explain the observations of stimulated Brillouin scattering in solids and liquids. While previous experiments have been successful in demonstrating the effect, the use of focused excitation has prevented a critical test of theory. In the present work, the collimated beam of a giant pulse laser source excites a liquid, $n$-hexane, external to a cavity, and allows the Brillouin scattering intensity to be measured as a function of optical path length. The type of transient solution considered by Kroll comes closest to describing the observed gain curve, which includes a large spatial amplification and no apparent threshold condition. The results lead to values for the Pockel's photoelastic constant and the hypersonic acoustic lifetime. With collimated excitation, a striking "acoustic burst" whose intensity parallels the path-length dependence of the Brillouin scattering is noticed. This suggests that the acoustic wave generated in the Brillouin process initiates a shock wave which could lead to material fracture or cavitation. With focused excitation, an additional mechanism involving plasma formation appears to operate.