Laser trapping mechanisms in nematic liquid crystal colloids

We describe and analyze laser trapping of small colloidal particles in a nematic liquid crystal, where the index of refraction of colloids is smaller compared to the indices of the liquid crystal. Two mechanisms are identified that are responsible for this anomalous trapping: (i) surface-induced distortion of the birefringent media around the particle, creating a high-index ”cloud” around the colloid, and (ii) laser induced distortion due to the local Freedericksz transition of a nematic, creating a ”ghost” colloid. In majority of the experiments, the trapping above the Freedericksz transition is highly anisotropic. A very good agreement is found with a numerical analysis, considering nematic director elastic distortion, dielectric director-light field coupling and optical repulsion due to low index colloid in a high index surroundings.