Helicity sensitive processing and imaging of a first order vortex beam scattered by a sphere

Abstract

A vortex beam’s wavefield has a null on the axis of propagation and an angular phase ramp proportional to the order of the beam. The rapid phase ramp at the null leads to the possibility of high-resolution imaging and precise alignment of the system. Using a modified four-panel transducer, a first order vortex beam was generated by driving each panel with an appropriate phase shift [V. Bollen et al., Proc. Meet. Acoust. 19, 070075 (2013)]. Utilizing this transducer, a solid sphere was insonified and the backscattering measured. Recording the backscattering on each panel separately allowed selection between helicity neutral and helicity sensitive detection modes, without changing the experimental setup, by introducing individual phase shifts in post-processing [V. Bollen, et al., J. Acoust. Soc. Am. 137, 2439 (2015)]. Using time delay-and-sum imaging algorithms, we created high-resolution three-dimensional profiles of the beam, relating the sphere location to the beam pattern. With cross-correlation involv...