Rotating Acoustic Drills by the Interference of Detuned Vortices

We present acoustic drill beams, structured sound beams exhibiting a dynamic intensity distribution that matches the shape of a helix. The intensity distribution rotates along the beam axis with controlled direction and angular frequency, resembling in this way the shape of a mechanical drill bit. Acoustic drill beams emerge as the spatiotemporal interference of two confocal, detuned vortex beams. The beam parameters are fully tuneable. The detuned frequency, the detuned wavenumber and the detuned topological charge of the composing beams can be tuned to control the drill parameters such as the number of drill arms, the winding period, the rotational speed, and their direction. We show that elongated drill beams are obtained using two confocal high-order Bessel beams, allowing analytical solutions for optimal overlap. In addition, drill beams can also be synthesised using focused ultrasound vortices. Analytical, numerical, and experimental results are shown in the ultrasound regime using a low-cost device based on two 1-MHz confocal piezoelectric transducers and 3D printed acoustic holograms. This exotic structured wavefront opens new avenues for wave-matter interaction, such as contactless particle manipulation, matter processing or biomedical applications.