Control of liquid crystal alignment using surface-localized low-density polymer networks and its applications to electro-optical devices

We developed an alignment method for control of the polar pretilt angle at the boundaries of a liquid crystal (LC) layer in a continuous range 2-90° by introducing a low-density polymer network. The network was formed as a result of the electric field-induced segregation of a photo-reactive monomer added in concentrations of 0.5-1% to the LC host and subsequent in-situ UV-light-induced polymerization. LC director was “frozen” within a less than a micron-thick layer in the vicinity of the cell substrates at high average tilt angles anchoring the LC in the bulk similarly to a high-pretilt alignment layer. The resultant pretilt angle was determined by the voltage applied before and during polymerization and the duration of voltage application before UV-light exposure. The desired pretilt angle could be set over a small area of a sample which allowed for the fabrication of LC devices with spatially variable optical retardation. Using this method, we fabricated high-pretilt splay and bend LC cells, optically recorded converging lenses and bi-prisms with electrically controlled optical characteristics, and phase diffraction gratings with resolution better than 50 lines/mm.