Optically probing surface anchoring of liquid crystals

Summary of EPSRC Grant GR/L41004
Principal Investigator Professor J Roy Sambles.

In this three year research project the primary aim has been to explore the surface alignment of liquid crystals. This required the development of the optical waveguide technique to enable the exploration of director profiles in a wide range of cells. To this end we established the fully leaky waveguide technique to explore standard liquid crystal cells and also initiated experiments with a focussed beam technique to allow the exploration of the cell-thickness dependent effects. Silicon oxide, rubbed polyimide and grating aligning surfaces have all been characterised. We have also shown that many of the earlier studies which use a Jones matrix approach to quantify surface anchoring through optical retardation measurements are seriously flawed.

Principal achievements of the work are as follows:

Development of the fully leaky guided wave technique (1-6) has provided a major breakthrough in the study of the director profile in liquid crystal cells. This is now allowing the study of standard commercial cells from two international companies. Accurate characterisation of both homogeneous (1,3-8) and homeotropic (2) aligning surfaces. Quantification of both azimuthal (4,6-8) and zenithal (1,2) anchoring strengths. Characterisation of the variation of both surface and bulk order parameters with temperature (7). Characterisation of grating alignment (9,10) which has largely confirmed model theory thereby underpinning work on the bistable nematic devices based on grating alignment. Exploration of the thickness dependence (11) of surface anchoring which has shown how longer range forces may influence the strength of surface anchoring and thereby device behaviour. Demonstration that previous Jones matrix based determinations of surface anchoring strengths are suspect (12).

Nine papers have so far been published from this work, six more are accepted and three others have been submitted.

References

[1] Yang Fuzi, Sambles J R, Dong Youmei and Gao Hongjin (2000) J Appl Phys, 87, pp 2726-2735: Fully-leaky guided wave determination of the polar anchoring energy of a homogeneously aligned nematic liquid crystal.
[2] Yang Fuzi, Ruan Lizhen and Sambles J R (2000) J Appl Phys (submitted) Homeotropic polar anchoring energy of a nematic liquid crystal using the fully-leaky waveguide technique.
[3] Yang Fuzi and Sambles J R. (1999) J Opt Soc Am B, 16, pp 488 497 Optical fully-leaky mode characterisation of a standard liquid crystal cell.
[4] Hallam B T, Yang Fuzi and Sambles J R (1999) Liq Cryst, 26, pp 657 662 Quantification of the azimuthal anchoring of a homogeneously aligned nematic liquid crystal using fully- leaky guided modes.
[5] Hallam B T, Brown C V and Sambles J R (1999) J Appl Phys, 86, 6682-6689: Quantification of the surface and bulk order parameters of a homogeneously aligned nematic liquid crystal using fully-leaky guided modes.
[6] Hallam B T and Sambles J R (2000) Mol Cryst Liq Cryst: (accepted) Quantifying azimuthal anchoring of nematics using in-plane fields and fully-leaky guided modes.
[7] Yang Fuzi, Sambles J R and Bradberry G W (1999) J Appl Phys, 85, pp 728 733 Half leaky guided wave determination of azimuthal anchoring energy and twist elastic constant of a homogeneously aligned nematic liquid crystal.
[8] Yang F Z, Cheng H F, Gao H J and Sambles J R (2000) Liquid Crystals (accepted) Determination of the Torsional Anchoring of a Twisted Nematic Liquid Crystal Using the Half-Leaky Guided Mode Technique.
[9] Hallam B T and Sambles J R (2000) Phys Rev E (accepted) An optical guided mode study of nematic liquid crystal alignment on a zero-order grating.
[10] Hallam B T and Sambles J R (2000) Liq Cryst (accepted) Groove depth dependence of the anchoring strength of a zero-order grating aligned liquid crystal.
[11] Ruan L Z and Sambles J R (2000) Phys Rev E (submitted) Thickness dependent liquid crystal anchoring energy, determined by a convergent beam waveguide technique.
[12] Yang Fuzi and Sambles J R. (1998) Jap J Appl Phys, 37, pp 3998-4007 The influence of surface reflectivities on measurements of the torsional anchoring strength of nematic liquid crystals.
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