Absorption-induced stimulated scattering of second sound in a superfluid

Abstract

A new type of wave interaction for hydrodynamics—absorption-induced stimulated scattering—is considered. The present treatment is particularized to the case of stimulated scattering of second sound in a superfluid. If two waves—an intense pump wave and a signal wave—are propagating in a medium, the power released in the medium, which is proportional to the square modulus of the total field, contains an interference term at the difference frequency of the pump and signal waves. Since the properties of the medium depend on temperature, the temperature inhomogeneities arising at the difference frequency cause scattering of the pump wave, resulting in a field which is resonant for the signal wave. That field is proportional to the absorption coefficient and the intensity of the pump wave. At low intensities of the pump wave such scattering leads to partial compensation of the dissipation of the signal wave, and with increasing intensity of the pump wave it can lead to amplification of the signal wave. This means that stimulated scattering can arise. Analysis of the experimental data suggests that the threshold intensity of this stimulated scattering can be a real experimental quantity.