Focused ultrasound transducer with electrically controllable focal length

This paper reports a novel acoustic transducer capable of delivering focused acoustic beam with electrically tunable focal length over 7 mm. Constructed on a 1.02 mm thick lead zirconate titanate (PZT) substrate, the transducer uses a collection of equal-width-equal-spacing concentric ring electrodes (and a circular electrode at the center) on one side of the substrate. With each electrode individually addressable, a desired focal length is mapped to a set of the electrodes generating the acoustic waves that arrive at the focal point in-phase for constructive interference. We have demonstrated a device capable of electrically tuning the focal length (of a focal spot of sub-mm in diameter) from 5 to 12 mm, with the electrical tunability confirmed through droplet ejection from liquid surface (that is at the focal plane), as the liquid level is varied.