Nonlinear acoustics in superfluid ³He‐B

The nonlinear interaction of zero sound with the order parameter collective modes in superfluid ³He‐B is considered within perturbation theory in the amplitude of the sound field. Selection rules for nonlinear excitation of the order parameter modes are determined by the approximate particle‐hole symmetry of the ³He Fermi liquid. A diagrammatic algorithm, based on the quasiclassical theory of superfluid ³He, is used to calculate nonlinear coupling constants. These nonlinearities are sufficiently large that it should be possible to observe two phonon absorption and stimulated Raman scattering of zero sound by the real squashing (J=2⁺) mode. Finally, we discuss the possibility of using these nonlinearities to produce zero sound with ‘squeezed’ noise.