Low Frequency Emission by Nonlinear Interaction of Phase Conjugate Ultrasound Waves

Low frequency emission by means of the nonlinear interaction of phase conjugate ultrasound beams is studied experimentally and theoretically. The phase conjugation effect is used for the automatic overlap of interacting focused beams of frequencies omega 1/2 pi = 11 MHz and omega 2/2 pi = 10 MHz. The interaction of beams propagating in a homogeneous liquid and beams scattered by an object immersed in a liquid are considered. While variation in the object position for resonance interaction of co-directional waves does not affect the phase of the emitted wave of low frequency Omega - = Delta omega = omega 1 - omega 2, the anomalous phase shift proportional to Omega + ≊ ( omega 1 + omega 2) >> Delta omega is obtained for non-resonance interaction of contra-propagating waves. The application of anomalous phase shift for 'super resolution' in object positioning by means of nonlinear low frequency emission is discussed.