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Deconvoluting the Kerr constant and electrostriction in liquid crystal blue phases. L. Tian, H. Gleeson School of Physics and Astronomy, University of Manchester, Manchester, M13 9PL, U.K. The optical and electro-optical behaviour of liquid crystalline blue phases has been investigated via a detailed analysis of the reflection spectrum from thin, vertical field (VF) cells. Two effects are expected in low electric field; electrostriction (which gives a significant colour shift to the system) and the Kerr effect (field-induced birefringence). In thin cells, the field-induced change in lattice constant is constrained in the same way as occurs for ‘pitch jumps’ in chiral nematic systems. The electrostriction in the blue phase materials is therefore observed together with pitch jump phenomena in thin cells, and the field-dependent blue phase cubic structures are shown to be modified by the boundary conditions. In addition, when fields are applied there can also be a large Kerr effect in some blue phase materials, which will affect the accuracy of the electrostriction measurement. Subtraction of the Kerr effect will be important in improving the experimental accuracy. One measurement method for the Kerr constant in blue phases has been suggested by Singh [1] who used an, in-plane-switching geometry. However, because of the non-uniform electric field properties of in-plane-switching cells, significant errors can occur. In our work, the Kerr effect has been studied by computer fitting to the reflection spectra from VR liquid crystal devices, a method based on the 4x4 matrix technique[2]. The small change in cell thickness that also occurs on application of the external electric field has been determined. We report the influence of the Kerr effect on electrostriction phenomenon in two different blue phase mixtures, one in which the Kerr effect is substantial and the other where it is negligible. The study of the two systems allows a direct comparison of each field-induced phenomenon for blue phase systems [3]. References [1]Singh, U. and P.H. Keyes, Measurement of the Kerr effect in Cholesteric blue phases. Liquid Crystals, 1990. 8(6): p. 851-860. [2]Berreman, D.W., Optics in stratified and anisotropic media - 4x4-matrix formulation. Journal of the Optical Society of America, 1972. 62(4): p. 502-&. [3] Linan Tian, John W. Goodby, Verena Görtz & Helen F. Gleeson, The magnitude and temperature dependence of the Kerr constant in liquid crystal blue phases and the dark conglomerate phase. Liquid Crystals, 2013. 40(11): p.1446-1454 _____________________________________________ * presenting author; E-mail: [email protected]
