Optical studies of grating alignment of liquid crystals

Brief Report on EPSRC research grant: GR/H97406
Principal Investigator Professor J Roy Sambles, School of Physics, University of Exeter.

In this three year, EPSRC funded, research project the primary aim was to explore the use of gratings in the alignment of liquid crystals. The gratings are fabricated at Exeter using interferography and characterised when metallised using surface plasmon excitation[1]. This yields their pitch and the first three or four Fourier coefficients of their amplitude spectrum. With such gratings, well aligned liquid crystal cells were fabricated in the nematic, S_A and S_c* phases. The primary characterisation techniques for exploring director structure in these cells were optical microscopy and guided wave transmission and reflection spetroscopy. Principal achievements of the work are as follows:

1. Confirmation of Berreman's theory for the strength of surface anchoring by gratings proving it to be proportional to amplitude squared and inversely proportional to the cube of the pitch[2]. Much shorter pitch gratings (<200nm) are now being explored to give strong surface anchoring with no diffraction.
2. Development and experimental verification of entirely new optical modelling codes for liquid crystals contained within diffractive structures[3,4]. This allows for detailed evaluation of twisted (conical) diffractive structures with voltage controllable liquid crystal layers. These programs which are unique to Exeter provide a very powerful resource for exploring new device structures.

3. The first detailed exploration of the grating alignment of smectic phases - as yet the near-grating structure adopted by these phases is not fully characterised - work continues to try to understand the details of grating alignment of these phases.

4. Exploration of an optical switch using the electroclinic effect in the S_A phase and grating coupling to guided modes. This compact goemetry has potential for high contrast, fast, optical switches.

Four papers have so far been published from this work, two more are submitted and two are in preparation. In addition, our collaborators at DERA Malvern (joint funders) have taken out patents on grating aligned bistable nematic structures.

This study is the first to place the grating alignment of liquid crystals on to a firm scientific foundation, providing a test of the underlying theory of grating alignment and also producing a unique set of computer codes for modelling the optical response of a large range of new structures. In addition, we have begun work on characterising grating alignment of smectic phases as well as exploring voltage controlled switching processes in new cell geometries.

References
[1] Wood E L, Sambles J R, Cotter N P and Kitson S C (1995) J Mod Opt 42, 1343
[2] Wood E L, Bradberry G W, Cann P S and Sambles J R (1997) J Appl Phys (accepted)
[3] Harris J B, Wood E L, Preist T W and Sambles J R (1996) J Mod Opt 43, 1377
[4] Harris J B, Preist T W, Wood E L and Sambles J R (1996) J Opt Soc Am, A13, 803 2