Optoacoustic generation of a helicoidal ultrasonic beam

The optoacoustic generation of a helicoidal ultrasonic beam is demonstrated. Such an ultrasonic “doughnut” beam has a pressure amplitude minimum in the center along its entire longitudinal extension, and it carries orbital angular momentum. It is produced by illuminating a specially structured absorbing surface in a water tank with pulsed laser light. The absorbing surface has a profile with a screw dislocation, similar to the transverse cross-sectional surface of a helix. Upon illumination with modulated light, a correspondingly prepared absorber generates an ultrasonic wave with the desired phase discontinuity in its wave front, which propagates through the water tank and is detected with spatial resolution using a scanning needle hydrophone. This situation can be viewed as the optoacoustic realization of a diffractive acoustical element. The method can be extended to tailor optoacoustically generated ultrasonic waves in a customized way.