Laser-generation of focused acoustic vortex with Fresnel spiral optoacoustic surfaces

We report a laser-based method to generate high central-frequency focused ultrasound vortices by introducing the Fresnel spiral for optoacoustic conversion. The formed broadband optoacoustic waves are coherently superimposed with each other, giving rise to acoustic vortices at different positions along the propagating axis according to their corresponding frequencies, while still maintaining the generation of focused vortex at the predetermined focal length. The laser-generated vortices show the typical vortex features including a helical wavefront and a central null pressure. A semi-parabolic focusing detection system is designed and constructed to scan the generated ultrasound vortices with high spatial resolution. The resulting acoustic vortices with the central frequency of about 3 MHz are presented in three dimensions, and agree well with the predictions. The flexibility of this scheme and its scalability to higher frequencies open tremendous perspectives in particle manipulation, ultrasonic therapy and so on.