Simulations of the dynamics of colloidal sphere clusters in Laguerre-Gaussian beams

The orbital and spin angular momentum carried by light in elliptically-polarized Laguerre-Gaussian (LG) beams can couple to particles trapped in them, resulting in optically-driven particle motion. We discuss recent Brownian dynamics simulations of the motion of wavelength-sized sphere clusters in LG beams. We use T-matrix light scattering computations to efficiently model the optical forces acting on the clusters even when the clusters are far from positional or orientational trapping equilibria. We also incorporate the full diffusion tensor for the clusters to model the hydrodynamic resistance and thermal fluctuations they experience. For clusters of two identical spheres, we show that we can control the spin of the clusters about their long axis via the the beam polarization and can control the clusters’ orbital motion around the beam axis via the LG mode index. We perform additional simulations to show that the full three-dimensional dynamics of non-axisymmetric clusters can be measured through an interferometric technique, digital holographic microscopy. Our results may lead to new insights into the optical manipulation of colloidal particles for studying self-assembly or performing microrheology.