Ultrasound with a twist

Faced with danger, The Doctor, protagonist of the science fiction television series Doctor Who, often availed himself of a sonic screwdriver, an all-in-one lock pick, remote control, and alien-detection device. The Doctor’s tool was fictional, of course, but now researchers at the University of Dundee in the UK, in collaboration with Gabriel Spalding of Illinois Wesleyan University, have developed a real-life sonic screwdriver—an ultrasound transducer array capable of generating high-angular-momentum acoustic vortices (see the figure). With it, they’ve obtained an elusive measurement of the ratio of orbital angular momentum L to energy E in a vortex beam. Theory predicts that L/E in any vortex beam should be equal to the beam’s topological charge—essentially, the number of intertwined helical wavefronts in the beam—divided by the wave frequency. The Dundee experiment, however, was the first real test of that prediction. The researchers used the sonic screwdriver to lob acoustic waves of varied topological charge at a sound-absorbing disk submersed in a water chamber. A rising vortex lifts the disk by a distance relatable to E and spins it at a rate relatable to L. Indeed, the measured ratios of the two quantities agreed comfortably with the theory. With one case now closed, the researchers look to use the sonic screwdriver to design nondiffracting Bessel beams and other complex beam shapes that could prove useful for ultrasound surgery. (C. E. M. Demore et al., Phys. Rev. Lett., 108, 194301, 2012.)—Ashley G. Smart