Liquid crystal (LC) defects have recently attracted a great deal of interest due to their ability of pre-defining the symmetry of colloidal interactions in nematics and enabling the existence of distinct thermodynamic phases such as the twist grain boundary and blue phases. In confined nematic and cholesteric fluids, defects appear as a result of temperature quenching, symmetry-breaking phase transitions, mechanical stresses, etc. These defects commonly annihilate to minimize the free energy and are hard to be controlled or utilized for applications in a reliable way.
This lecture will discuss the facile optical creation and multistable optical and electrical switching of 3D localized defect configurations in confined chiral nematic LCs. By use of focused Laguerre-Gaussian vortex laser beams with different optical phase singularities, we generate topological LC defect architectures containing both point and line singularities bound to each other by the director twist and forming stable or metastable configurations. While being generated, the defect architectures are probed in 3D by multimodal labeling-free nonlinear optical imaging that incorporates simultaneous study with three-photon and two-photon excitation self-fluorescence microscopy, second harmonic generation microscopy, and broadband coherent anti-Stokes Raman scattering polarizing microscopy, in addition to the conventional fluorescence confocal polarizing microscopy. In chiral nematic LCs confined into homeotropic cells, the laser-generated topological defects embed the localized 3D twist into the uniform background of confinement-untwisted director field, forming localized chiro-elastic particle-like excitations - dubbed “torons” [1] - that interact with each other via short-range repulsive elasticity-mediated interactions. In the field-controlled cells with the in-plane director field, one observes formation of dipolar structures of twist-bound defects composed of the torons and umbilics. Similar to the elastic dipoles formed by colloidal particles accompanied by hyperbolic point defects, the toron-umbilical pairs interact with each other via long-range elastic interactions and form dipolar chains. At high densities of the toron-umbilical pairs, these chains self-organize into super-lattices with toron-umbilical pairs bound into antiferroelectric-like two-dimensional crystals of dipolar colloidal chains. We demonstrate that the periodic lattices of twist-bound defects can be used as optically- and electrically- controlled diffraction gratings, for optical data storage, as well as in the design of all-optical information displays.
This work was supported by the Renewable and Sustainable Energy Initiative and Innovation Initiative Seed Grant Programs of University of Colorado, International Institute for Complex Adaptive Matter, and by the NSF grants DMR0645461, DMR0820579, and DMR0847782.

[1] I. I. Smalyukh, Y. Lansac, N. Clark, and R. P. Trivedi, “Three-dimensional structure and multistable optical switching of Triple Twist Toron quasiparticles in anisotropic fluids” Nature Materials 9, 139-145 (2010).