Acoustic phase conjugation in superfluid helium

We present the first experimental observation of phase conjugate (PC) secon d sound (SS) waves superfluid helium. The main feature of a PC wave is that its phase is complex conjugate to the Incident wave phase. It is generated and amplified as a result of nonlinear interaction between an incident SS wave and a first sound (FS) pumping wave. At FS amplitudes larger than the threshold value, a parametric instability, i.e., spontaneous decay of a FS wave into two SS waves, takes place. Three main, theoretically predicted, f eatures of phase conjugate waves were verified experimentally.