Structuring acoustic fields for particle manipulation

We present a powerful and efficient technique to structure acoustic fields, based on the use of active diffraction elements, which can be designed in practically arbitrary shapes and can be operated within a broad spectral range of ultrasonic frequencies in air. Two kinds of structured fields are demonstrated: focused acoustic vortices and the simultaneous generation of multiple acoustic Bessel beams of different topological charges, well separated among each other along the propagation axis. In both cases, the main parameters of the field can be finely and continuously tuned by setting the operation frequency. Experimental results show very good agreement with theoretical analysis and numerical simulations.