Focused laser beams and liquid crystals: fast three-dimensional imaging of structures and topological defects

We show how a tightly focused laser beam can serve as a tool to image complex patterns of the director using the technique of fluorescence confocal polarizing microscopy (FCPM). We expand the capabilities of FCPM into the domain of real-time scanning in order to study the dynamic processes at the time scale of about 1ms. In this approach which we call Fast FCPM, confocal imaging is performed using a modified Nipkow-disc scanning confocal microscope. In the Fast FCPM set up, we use a twisted nematic cell as a fast achromatic polarization rotator to change the polarization of probing light by 90. The achromatic polarization rotator switches between two orthogonal polarization states when a sufficiently strong electric field is applied to reorient the director structure from the twisted to the homeotropic state. Both FCPM and Fast FCPM employ the property of anisotropic media to align fluorescent dye molecules. When observation is performed in polarized light, the measured fluorescence signal is determined by orientation of the dye molecules. As the dye molecules are aligned by the liquid crystal, the detected fluorescence signal visualizes the spatial patterns of the director rather than concentration gradients of dyes. Finally, we present 3D patterns of director associated with both static and dynamic processes in liquid crystals, anisotropic emulsions, and colloidal suspensions.