Observation of Polarization Singularities and Topological Textures in Sound Waves.

Polarization singularities and topological polarization structures are generic features of inhomogeneous vector wave fields of any nature. However, their experimental studies mostly remain restricted to optical waves. Here, we report the observation of polarization singularities, topological Möbius-strip structures, and skyrmionic textures in 3D polarization fields of inhomogeneous sound waves. Our experiments are made in the ultrasonic domain using nonparaxial propagating fields generated by space-variant 2D acoustic sources. We also retrieve distributions of the 3D spin density in these fields. Our results open the avenue to investigations and applications of topological features and nontrivial 3D vector properties of structured sound waves.

Acoustic analysis of a broadband spiral source for the simultaneous generation of multiple Bessel vortices in air.

A detailed characterization and analysis of the acoustic field radiated by an active diffraction grating is presented. This is a spiral-shaped source of constant width of the spiral paths and spacing among them, built with flexible ferroelectrets, which is suitable to operate within a broad range of ultrasonic frequencies. The simultaneous airborne generation of multiple Bessel vortex beams with different topological charges along the propagation axis is thoroughly discussed in terms of the structure of the spiral source and the driving frequency, along with some technical details of the prototype. Finally, the quality of the experimentally generated Bessel vortices is discussed, including the phase structure and the amplitude of the acoustic pressure.