Doing high power airborne vortex beams with ultrasonic vibrations

Abstract

Vortex beams (VBs) are able to transfer angular momentum to matter and exhibit self-reconstruction capacity and a great potential in engineering applications such as particle manipulation, communications, imaging, among others. Several VB generation methods have been put forward, e.g., physically helical sources, phase-driven multi emitters, passive metastructured devices, and recently, electroactive spiral gratings. Even though airborne ultrasound technologies offer interesting benefits, such as the free access to the manipulated specimen/particle, most of the reported methods are intended for water applications. Here, a sonotrode-based approach is introduced, in which ultrasonic vibrations are induced to produce a high quality, high power VB in air. We designed, fabricated, and characterized two different prototypes with radiant structures that operate in length-extension and transverse vibration modes at 27.6 kHz and 40.7 kHz, respectively. Numerical simulation models were implemented at the design stage, which allowed us a proper selection of the materials and geometry of the assembly. The prototypes allow for intensities over 0.5 W/cm2 in open-loop operation. A discussion on the electro-acoustic performance of the devices is included. New design configurations are possible for higher order VB generation with amplified acoustic output. This work opens up new manipulation possibilities for engineering applications in air and also in water.