Mechanism of low-threshold hypersonic cavitation stimulated by broadband laser pump

A low threshold acoustic cavitation regime was observed for the excitation of hypersonic waves due to a stimulated Brillouin scattering (SBS) mechanis m, when the optical pump lies within the uv frequency range. Cavitation occ urs if the optical pump bandwidth Delta(+) much greater than Omega(0), wher e Omega(0) is the Stokes frequency shift (the hypersonic frequency). In the opposite case (Delta(+) much less than Omega(0)), cavitation does not occu r despite the fact that the hypersonic wave intensity is much higher. The e ffect is associated with the stimulation of a broad frequency spectrum of h ypersonic pressure in a field provided by the broadband optical pump. In co ntrast, for a monochromatic optical primp, the hypersonic wave is of single -frequency character. Induction of cavitation at the low intensities of aco ustic pressure is attributed to nanobubbles of fixed size that occur in the liquid. The resonant frequency of the nanobubbles coincides with the frequ ency of some spectral component of hypersound present in the: broadband SBS process. That conclusion is reinforced by the further observation that at the same intensity of broadband pumping the cavitation vanishes after degas sing the liquid. In parallel experiments on four-photon polarization Ray le igh wing spectroscopy, it was also demonstrated that spectral lines exist i n ordinary (not degassed) water, which can be ascribed to resonances of rad ial vibrations of nanobubbles. Those lines are absent in the degassed water spectrum. [S1063-651X(99)06107-3].