The Physics of Spatially Twisted Nematic Liquid Crystals

Abstract

When nematic liquid crystals are placed between parallel glass plates with differing alignment directions, the bulk will twist in order to match the boundary conditions. This phenomenon of a twisted cell has been used extensively for the development of everyday liquid-crystal displays. However, there has been limited study of the twisted cell beyond the 90-degree twist case. In this thesis, I explore the behaviour of inhomogeneous liquid-crystal devices where the front and back alignment layers are uniquely and spatially patterned. This creates a non-symmetric device which can act on light differently depending on the orientation of the device and an externally applied voltage. The effect on the polarization of light is theoretically modelled using Jones matrices, and elastic continuum theory is employed to fully understand how the twist and tilt distributions of the liquid crystals change with field strength. Different pattern configurations were fabricated, tested, and characterized, revealing the complex behaviour that occurs with an applied electric field. Liquid-crystal devices provide a bespoke way of tailoring the spatial distribution of light and photons. A set of quantum key distribution experiments through underwater channels, leveraging these devices to encode information on structured photons, is also presented.