Active-spiral Fresnel zone plate with tunable focal length for airborne generation of focused acoustic vortices

We present a simple and efficient method for generating focused acoustic vortices in air over a wide range of ultrasonic frequencies by means of an Active-Spiral Fresnel Zone Plate. An important advantage of this device is that the focal length can be finely and continuously tuned by setting the operation frequency. The role of the different design parameters is analyzed in terms of the minimum beam width and the focal depth of the resulting field, allowing an optimized device according to the application. Experimental results show very good agreement with numerical simulations.