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Focused laser beams and liquid crystals: Three-dimensional imaging and control of topological defects and measurements of colloidal interactions – Oleg Lavrentovich

Date: Mon. April 11th, 2005, 12:30 pm-1:30 pm
Location: Rockefeller 221

Orientational order is a universal feature of numerous soft-matter systems, most notably liquid crystals. These systems are extremely flexible, producing a rich variety of complex 3D patterns of order parameter. Non-destructive techniques to study and control these patterns are in a great demand. This talk discusses how a tightly focused laser beam can serve as a tool to image complex patterns of the director and to manipulate them. (1) In the fluorescence confocal polarizing microscopy (FCPM), the focused laser beam allows one to image 3D patterns of orientational order. We employ the property of anisotropic media to align fluorescent dye molecules. In polarized light, the measured fluorescence signal is determined by the spatial orientation of the molecules rather than by concentration gradients of dyes. We apply the technique to a variety of complex 3D director structures in liquid crystals, such as gliding and sliding dislocations, undulations, formation of colloidal crystals at the nematic surface, etc. (2) In a medium with a spatially varying refractive indices, a focused beam can serve as an optical tweezer. We demonstrate that the optical tweezers can manipulate colloidal particles and topological defects in liquid crystals and to determine colloidal forces in the anisotropic medium.

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