Optical trapping in anisotropic fluids such as liquid crystals shows inherently different behavior compared to that in isotropic media. Anisotropic optical and visco-elastic properties of these materials result in direction-sensitive and polarization-dependent interaction of the focused laser beam with colloidal inclusions, defects and structures of long-range molecular order, providing new means of non-contact optical control. Optical trapping properties are further enriched by laser-induced realignment of the optical axis that can be observed in these liquid crystalline materials at relatively low trapping laser powers. Optical manipulation of particles and defects in these anisotropic fluids is of immense importance for their fundamental study and from the standpoint of technological applications such as light-directed colloidal self-assembly and generation of tunable photonic architectures in liquid crystals. We review the basic physical mechanisms related to optical trapping in anisotropic liquid crystalline fluids and demonstrate how it can be employed in quantitative studies of colloidal interactions and both topological and mechanical properties of defects.

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We use nanowires with faceted sidewalls for mapping of the patterns of three-dimensional orientational order and defect structures. In chiral nematics, the nanowires follow the local average orientation of rod-shaped molecules. When spatially translated by use of holographic optical tweezers in three dimensions, they mediate direct nondestructive visualization of the helicoidal ground-state structures, edge and screw dislocations, and kinks, as well as enable non-contact manipulation of these defects. We probe interactions of faceted nanowires with different defects and demonstrate their spontaneous selfalignment along the cores of singular defect lines.