Helix-shaped tractor and repulsor beams enabling bidirectional optical transport of particles en masse

Abstract

Three-dimensional programmable transport of micro/nano-particles can be straightforwardly achieved by using optical forces arising from intensity and phase gradients of a structured laser beam. Repulsor and tractor beams based on such forces and shaped in the form of a curved trajectory allow for downstream and upstream (against light propagation) transportation of particles along the beams, respectively. By using both types of beams, bidirectional transport has been demonstrated on the example of a circular helix beam just by tuning its phase gradient. Specifically, the transport of a single particle along a loop of the helix has been reported. However, the design and generation of helix-shaped beams is a complex problem that has not been completely addressed, which makes their practical application challenging. Moreover, there is no evidence of simultaneous transport of multiple particles along the helix trajectory, which is a crucial requisite in practice. Here, we address these challenges by introducing a theoretical background for designing helix beams of any axial extension, shape, and phase gradient that takes into account the experimental limitations of the optical system required for their generation. We have found that only certain phase gradients prescribed along the helix beam are possible. Based on these findings, we have experimentally demonstrated, for the first time, helix-shaped repulsor and tractor beams enabling programmable bidirectional optical transport of particles en masse. This is direct evidence of the essential functional robustness of helix beams arising from their self-reconstructing character. These achievements provide new insight into the behavior of helix-shaped beams, and the proven technique makes their implementation easier for optical transport of particles as well as for other light-matter interaction applications.