Liquid Crystals (LCs) have proven to be important for electro-optic device applications such as displays, spatial light modulators, non-mechanical beam-steerers, etc. Owing to their unique mechanical, electrical, and optical properties, they are also being explored for wide array of advanced technological applications such as biosensors, tunable lenses, distributed feedback lasers, muscle-like actuators, etc. The thesis explores LC media from the standpoint of controlling their elastic and optical properties by generating and manipulating assemblies of defects and colloidal particles. To achieve the goal of optically manipulating these configurations comprising defects and particles at microscale with an unprecedented control, and then to visualize the resultant molecular director patterns, requires development of powerful optical system. The thesis discusses design and implementation of such an integrated system capable of 3D holographic optical manipulation and multi-modal 3D imaging (in nonlinear optical modes like multiphoton fluorescence, coherent anti-Stokes Raman scattering, etc.) and how they are used to extensively study a vast number of LC based systems.;Understanding of LCs and topological defects go hand in hand. Appreciation of defects leads to their precise control, which in turn can lead to applications. The thesis describes discovery of optically generated stable, quasiparticle-like, localized defect structures in a LC cell, that we call "Torons". Torons enable twist of molecules in three dimensions and resemble both Skyrmion-like and Hopf fibration features. Under different conditions of generation, we optically realize an intriguing variety of novel solitonic defect structures comprising rather complicated configurations of point and line topological defects.;Introducing colloidal particles to LC systems imparts to these hybrid material system a fascinating degree of richness of properties on account of colloidal assemblies supported by networks of LC defects as well as variety of localized defects supported by colloidal particles. To fully understand and exploit the resultant interactions involving colloids and defects in LC systems to achieve the full potential of their practical applications, it is required that these be explored on the level of individual particles and defects. We explore a multitude of interactions mediated by defects over different length scales and demonstrate for the first time, creation of several types of colloidal assemblies such as sparse colloidal structures and three-dimensional defect-bound colloidal structures.
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