Parametric Self-Enhancement of the Spontaneous Decay of Sound in Superfluid Helium

Abstract

The spontaneous decay of coherent monochromatic sound is dramatically self-enhanced by parametric amplification. We observed this effect for the first time with 3.25-GHz sound in superfluid helium at 85 mK. Starting with a typical intensity of 10 W/${\mathrm{m}}^2$ the sound beam is depleted by more than 99.9% over just 1 mm of propagation distance. In addition we observed, in contrast to Landau-Rumer processes, an exponential decay coefficient proportional to the square root of the initial intensity.