Sub-Diffractive Acoustic Vortices by Nonlinear Mixing

Abstract

[EN] Vortex beams, characterized by a collimated wavefront with a phase dislocation at their principal axis, have found practical applications to construct acoustic tweezers for particle trapping and manipulation, or underwater communications. However, the natural diffraction of the wavefront limits the size of the vortex. This result in vortices whose bright core is larger than the wavelength, limiting their use for practical applications such as long-range underwater communications. In this work, we synthesize a vortex beam of sub-wavelength size at a distance beyond Rayleigh diffraction length using the nonlinear mixing of two confocal, high-frequency and detuned vortex beams of different topological charges. By using the nonlinear mixing of two confocal vortices, it was generated a low-frequency (1 kHz) focused vortex beam of integer topological charge whose distance between magnitude maxima is about 18 times smaller than its wavelength at a distance about 3 times the Rayleigh diffraction length. Sub-wavelength vortices emerge as a result of the spatiotemporal interference of two primary vortex beams due to the conservation of angular momentum during nonlinear wave-mixing. This mechanism opens new paths to design directive parametric antennas for vortex transceivers or particle manipulation systems at scales well below the diffraction limit.