New nonlinear acoustic effects in pressurized superfluid 4 He and acoustic microscopy

The power dependence of the propagation of coherent focused acoustic pulses in superfluid ⁴He was measured under different pressures. In the lower pressure region where the dispersion relation is anomalous, simple saturation and weak depletion were observed. In the higher pressure region where the dispersion relation becomes normal, new nonlinear phenomena have been observed; that is when the input power increases, the received power increases monotonically, saturates, and rapidly depletes. As the input power is increased, the received power increases and depletes again. A resolution improvement beyond the diffraction limit via the nonlinear effect in the superfluid helium acoustic microscope has been observed