Obtaining a Spiral Standing Wave Using Wave Phase Conjugation of a Vortex Ultrasonic Beam

Wave phase conjugation has been experimentally implemented for a vortex ultrasonic beam. An acoustic vortex was formed in water using a spherically focused emitter and a disk, introducing an angular phase shift. The opposite vortex was generated due to the parametric wave phase conjugation in a magnetostrictive material under electromagnetic pumping at the double frequency. A hydrophone was used to measure the longitudinal and transverse distributions of the amplitude and phase of the sound pressure fields of the incident and conjugate waves near the emitter focus; their mutual correspondence was demonstrated. It was shown that both beams are indeed vortex ones; the conjugate beam has the same chirality as the incident beam. Thus, the beams in the observation region are completely counter-propagating (in the sense of wave vector orientation). A standing wave with a nodal surface in the form of a helicoid was obtained as a result of the interference of the incident and the conjugate vortex beams near the focus.