Laser trapping of small colloidal particles in a nematic liquid crystal: Clouds and ghosts

We show that, contrary to intuition, small (less than or equal to 1mum) transparent particles can be trapped and manipulated in a nematic liquid crystal using an intense laser beam, although their index of refraction is lower than both refractive indices of the surrounding birefringent fluid. 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 or (partial) melting of a nematic, creating a ghost colloid.