Two-phonon absorption by the real squashing mode in superfluid B3

We have observed in superfluid B3 two-phonon absorption, which is one of the nonlinear effects recently predicted by McKenzie and Sauls. The J=2+ real squashing mode was excited with two coincident sound pulses at the temperature T+, fsignal+fpump=frsq(T+), under 0, 2.7, and 3.5 bars pressure. The heights of the attenuation peaks scale almost linearly with the energy of the higher-intensity wave, in qualitative agreement with the theory. Furthermore, a satellite peak appears even in zero magnetic field, which originates from the reflected wave.