In situ control of surface molecular order in liquid crystals using a localised polymer network and its application to electro-optical devices

Abstract

A liquid crystal (LC) alignment technique has been developed that allows local control of the polar pretilt angle over the range of 0–90°. This was achieved through the formation of a polymer network localised in the vicinity of the LC cell substrates. The network was formed as a result of in situ UV-induced polymerisation of a photo-reactive monomer added at concentrations of 0.5–1%. Localisation of the polymer network at the LC–substrate boundary was achieved by the application of a high voltage before polymerisation. The resultant pretilt angle was determined by the voltage applied during the polymerisation and/or the duration of the voltage application before the polymerisation step. The desired pretilt angle could be set over a small area of the sample, which allows the fabrication of LC devices with spatially variable optical retardation. Using this method we fabricated a converging lens, a bi-prism, and a phase diffraction grating with resolution greater than 50 lines mm⁻¹.

Keywords:

• liquid crystals
• pretilt angle
• photo-polymerisation
• phase separation
• liquid crystal devices

Acknowledgements

This research was supported by the Air Force Office of Scientific Research, sub-award ‘Supervision’ # FA7014/07/C/0013, by the CSUS Foundation, the Hu Foundation, and by the National Science Foundation through REU Program (PHY–0851594). Special thanks are due to Professors John Noe and Harold Metcalf at Laser Teaching Center, Stony Brook University, Stony Brook, NY, both for valuable discussions and for providing the laser for recording diffraction gratings.