Properties of nonlinear elastic waves propagating along acoustic axes

Amplitudes and polarization of nonlinear elastic harmonics exited along an arbitrary acoustic axis of the conic type have been studied as functions of the rotation angle of the polarization vector of the degenerate reference wave. When the reference polarization is rotated by an angle of π, the resulting rotation angle for the polarization vector of the second harmonic turns out to be equal to either zero or 2π or ?2π. The actual type of the behavior of the second harmonic propagating along the given acoustic axis is determined by an algebraical criterion for elastic moduli of the second and the third order for the medium. For polarization of the reference wave, we determined the orientations corresponding to the maximum and minimum for the amplitude of the second harmonic. Similar results were obtained for different cases of collinear interactions between degenerate waves, in particular, for the excitation of longitudinal harmonics. The possible control of nonlinear interactions with the use of degenerate-wave polarization in optics and acoustooptics is also discussed.