Electro-active diffraction gratings for the generation of acoustic vortex beams

In this work we present a powerful and efficient technique to generate structured acoustic beams, e.g., vortex beams, by means of active-spiral diffraction gratings that are driven by electrical signals and operated as a single radiator. The main feature of this technique is the capacity to work over a broad range of frequencies, i.e., from 70 kHz to beyond 300 kHz, in air. By varying the excitation frequency, a fine and continuous tuning of the focal length of the acoustic vortex is achieved, and as a result, the vortex beams generated can be axially steered while their spatial distribution is preserved. To experimentally demonstrate these features, two prototypes are presented: the Archimedean spiral, and the Spiral Fresnel Zone Plate. The former can generate simultaneous higher order Bessel beams with different topological charge along the propagation axis. The latter allows us to create sharp focused vortex beams. The experiments show a good agreement in comparison with simulations. The versatility and simplicity of the device make it highly suitable and with a potential in emerging applications such as particle manipulation, acoustic imaging, transfer of angular momentum to matter, acoustic communication, and biomedical applications.