| Liquid crystals are very interesting and unique optical materials which have been thoroughly studied for decades. In recent years, nonlinear optical responses of liquid crystalline materials and their applications have created another revolution in scientific research worldwide. This nonlinear response of liquid crystal is dramatically enhanced by adding certain impurities or dopants. In fact, the discovery and research into photorefractive-like phenomena within nematic liquid crystals containing photo-conductive dopants has opened up new directions of research and applications. In addition, azobenzene and its derivatives doped nematic liquid crystal possesses the most nonlinear and photo-sensitive effects among all the optical materials. Doping liquid crystals with these particular dopants greatly increases their light sensitivities and lowers the intensity thresholds for molecular reorientation.; However, in spite of many experimental studies of light-induced reorientation processes in doped liquid crystals, so far there is no theoretical model that would provide a complete description of the physical mechanisms involved. So far, it is believed that there are many mechanisms involved in reorientation of liquid crystal molecules such as optical and dye-induced torques, a photorefractive space-charge field, and photoisomerization. In this thesis, I present a comprehensive study of nonlinear optical phenomena of dopant enhanced nematic liquid crystals including a complete elucidation of the basic mechanisms and some potential applications.; Apart from the study of photorefractive-like doped liquid crystals, I performed a series of experimental measurements and quantitative analyses on azobenzene derivative doped nematic liquid crystals. By comparing three azobenzene derivative dopants that possess different rates of photoismerization, I was able to determine the mechanisms of the undergoing phenomena. In doped liquid crystals, the enhanced light-induced reorientation of the molecules is found to lead to a strong modulation of the refractive index. Both reorientation and order-parameter change are shown to contribute significantly and equally to the resultant index change, originating from certain intermolecular torques. For methyl red doped materials in particular, the coexistence of photoisomerization and photorefractive-like effects induce the best nonlinearity. Hence, based on the understanding of the large optical nonlinearity of these materials, I also investigated and demonstrated several promising applications. This high photo-sensitivities of these dopant enhanced nematic liquid crystals, to either polarization or intensity of applied optical beams, prove to have great potentials of practical applications for sensor protection and image processing such as wavelength converter, intensity inversion, edge enhancement, and image addition/subtraction. |
