Transformation of Incoherent Phonon Flux into a Coherent Acoustical Signal as the Result of the Nonlinear Amplification of Acoustical Phonons in Semiconductors

A theory of nonlinear interaction of sound waves in a noise beam is developed. The case is considered where the intensity of the beam grows along a semiconducting crystal in a do electric field. It is demonstrated that if the nonlinear contribution to the amplification constant is negative the transformation of a noise beam into a coherent sound signal with fixed amplitude is possible during a single transit through the crystal. This occurs if the linear amplification constant has a sufficiently sharp maximum while the nonlinear attenuation constant is a smooth function of the wave vector.