Manipulating two-dimensional colloidal crystals using Laguerre-Gaussian beams.

Optical tweezing has firmly established itself as a versatile tool in fields ranging from biology to physics. In colloidal systems, optical tweezers are used to e.g. measure small forces or to generate optical potential energy landscapes. More complex and dynamic optical tweezing can be readily achieved using holographic wavefront engineering, though this is typically applied to only a handful of particles. Here, we discuss how we use holographically generated Laguerre-Gaussian (LG) beams to manipulate hundreds to thousands of particles in a two-dimensional colloidal crystal to form well-defined loop-shaped grain boundaries. We characterise the axial intensity distribution of the LG beam and show the effect of applying LG beams at different distances from the focal point to the colloidal crystals. In addition, we demonstrate how LG beams with different vortex numbers can be used to generate grain boundary loops with different radii, and finally we briefly compare and contrast manipulating 2D colloidal crystals with a single defocussed LG beam versus using a focussed LG beam combined with a defocussed Gaussian beam.