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Laser trapping of low refractive index colloidal particles in a nematic liquid crystal

We describe and analyze laser trapping of small colloidal particles in a nematic liquid crystal, where the index of refraction of colloidal particles is smaller compared to the indices of the liquid crystal. Two mechanisms are identified that are responsible for this anomalous trapping: (i) below the optical Freedericksz transition, the trapping is due to the anisotropic dielectric interaction of the polarized light with the inhomogeneous director field around the colloidal particle, (ii) above the optical Freedericksz transition, the optical trapping is accompanied by the elasticity-mediated interaction between the optically distorted region of a liquid crystal and the particle. In majority of the experiments, the trapping above the Freedericksz transition is highly anisotropic. Qualitative agreement is found with a numerical analysis, considering nematic director elastic distortion, dielectric director-light field coupling and optical repulsion due to low refraction index colloid in a high index surroundings.