Rapid switching in a dual-frequency hybrid aligned nematic liquid crystal cell

Copyright © 2005 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Applied Physics Letters 87 (2005) and may be found at http://link.aip.org/link/?APPLAB/87/021106/1We report the optical characterization of a dual-frequency hybrid aligned nematic (HAN) liquid crystal cell driven between two states using a multiple-frequency sinusoidal pulse. The complex dynamic director structure formed during the fast switching process is resolved in unprecedented detail on a submillisecond time scale. The results reveal backflow effects and a total switching time that is substantially faster than that achievable with conventional HAN cells

Analysis of the sign-dependent switching observed in a hybrid aligned nematic cell

Copyright © 2009 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft. This is the published version of an article published in New Journal of Physics Vol. 11, article 013045. DOI: 10.1088/1367-2630/11/1/013045An optical waveguide experiment was used to study the influence of dc electric fields on a hybrid aligned nematic liquid crystal cell. This dc switching differed from ac switching in two ways: first, the equilibrium states depended on the sign of the applied voltage, and second, there was transient activity over long (~100 ms) timescales. To understand both of these, a numerical model of the cell's dynamics, which included both the Ericksen–Leslie theory and a drift-diffusion model of mobile ions, has been developed. Comparing modelling with observations, we find that the transients are caused by the motion of tiny concentrations of ionic impurities, and that the sign dependence is caused by an asymmetric distribution of surface charge, rather than the flexoelectric effect

Time-resolved sign-dependent switching in a hybrid aligned nematic liquid crystal cell

Copyright © 2008 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft. This is the published version of an article published in New Journal of Physics Vol. 10, article 083045. DOI: 10.1088/1367-2630/10/8/083045An optical waveguide technique is used to determine the director tilt profile across a hybrid aligned nematic (HAN) liquid crystal cell, in which the optical response is dependent on the sign of the applied voltage. Two physical models are shown that fit the equilibrium experimental data, but with alternative explanations for this sign dependence. Models with either a flexoelectric coefficient of 2.25×10−11 C m−1 or a bound surface charge of 12.2 μC m−2 are shown that fit this equilibrium data. In an attempt to resolve this degeneracy sign-dependent switching data are analysed. However, neither model can explain these switching data, which are affected by slow transients of ~100 ms which are believed to be due to the motion of free ions in the liquid crystal. From the form of these slow transients, it is suggested that the equilibrium position of the ions is next to a cell substrate

Determination of the director profile in a nematic cell from guided wave data: an inverse problem

Copyright © 2007 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft. This is the published version of an article published in New Journal of Physics Vol. 9, article 166. DOI: 10.1088/1367-2630/9/6/166We consider an inverse problem: the estimation of the nematic director profile from experimental fully leaky guided mode data. This inverse problem is ill-posed in that small changes in the data may lead to large changes in the estimates of the director profile. The continuum equations for a nematic are exploited to stabilize the problem. We use experimental data drawn from a study of the dynamics of a hybrid-aligned nematic cell as an example