Focusing airborne acoustic vortex beams by means of active diffraction gratings

Abstract

New technologies to generate acoustic vortex beams (VB) have received increasing attention due to their interesting capabilities to transfer angular momentum to matter and to attract particles to the propagation axis. One of the recently reported methods to generate acoustic VB is the fabrication of active diffraction gratings, i.e., acoustic sources whose geometry and vibration mode emulate the field obtained after passing a plane wave through a given passive grating [1]. In this work, we extend this approach by using an active spiral zone plate to produce a beam with a helical and focused structure. This is obtained by gluing an electroactive ferroelectret film on top of a lower electrode structured on a printed circuit board, which in turn results easy and reliable. The broadband feature of the film makes it possible to generate VB throughout the range of interest for most applications in air. Experimental results are compared with numerical simulations. The proposed ultrasonic source opens up new possibilities for particle manipulation and rotation control of small objects. [1] R. D. Muelas H, J. F. Pazos-Ospina, and J. L. Ealo, “Active diffracting gratings for vortex beam generation in air,” J. Acoust. Soc. Am. 141(5), 3569–3569, 2017.