Nonlinear light beam propagation in reorientational nematic liquid crystals

Abstract

Liquid crystals (LC) are excellent media for the exploration of nonlinear phenomena, particularly due to their high and nonlocal response to electric and magnetic fields. Besides the fundamental interest, the field-induced reorientation of LC molecules is widely used in display applications and is promising for optical signals processing such as attenuators, tunable lens and smart-windows. Recently, a great deal of attention has been dedicated to spatial (2+1)D optical solitons in nematic liquid crystals, the so called nematicons. Nematicons are diffractionless self-confined light beams, that is, guided by their own waveguide in the dielectric. Specifically, diffraction is balanced out by self-focusing through the reorientational nonlinearity. Moreover, owing to the highly nonlocal character of the response, nematicons are stable and robust to external perturbations. Being self-induced graded-index guides for co-polarized signals, nematicons are an avenue to a wide range of all-optical devices and reconfigurable routers. When the electric field and the average alignment of molecules are normal to each other, the reorientational response presents a threshold in power before the material properties get modified by the impinging light. This dissertation deals with the investigation of beam self-trapping near such threshold in nematic liquid crystals, a configuration scarcely explored in the existing literature. The first chapter introduces the main features of liquid crystals and of nematicons in threshold-free geometries, including self-focusing, self-trapping and self-steering. Tthe second chapter discusses how self-focusing in the presence of threshold provides optical bistability and hysteresis in beam width versus power, due to step-like nonlinear response with excitation. This is indeed the first report on bistability with propagating beams and the first experimental demonstration of soliton bistability. The third chapter studies nematicon propagation near the threshold, in bias-free cells allowing the direct observation of beam-walk-off: combining self-steering and the inherent anisotropy of liquid crystals, the system is capable of switching from positive to negative refraction as power changes. This is the first demonstration of such a power controlled transition in refraction, which, besides the other, allows optimizing all-optical routers based on nematicons, as optical self-steering is maximized.