The pi cell is a fast-switching nematic liquid crystal device that uses parallel pretilt [1]. This surface alignment causes a splayed ground state to form when there is no applied voltage. Above ~3Vrms a bend state known as the V state forms via a nucleation process and it is this state that is used for pi cell operation. This nucleation process means that it is difficult to retain the V state throughout the device, with inter-pixel gaps being particularly problematic.

It has been demonstrated that when a voltage is applied suddenly to the splayed ground state, the symmetric H (Hs) state forms [2]. This state is of particular interest since it has a fast relaxation time of under 1 ms. We present a detailed examination of the relaxation of the Hs state. Experimental evidence indicates that this state does not experience optical bounce during relaxation. A dynamic model based on Leslie-Eriksen-Parodi theory confirms that there is no kick-back of the director during this relaxation process, and that the flow profile is similar to that present during relaxation of the V state. This flow profile is known to enhance the switching time of the V state [3], and the same effect is seen in the Hs state leading to a faster relaxation than might be expected. The influence of the different viscosity parameters is examined in detail, and a comparison between the experimental and simulated results is given.

The Hs state is not the minimum energy director profile, and so over time it decays into the asymmetric H (Ha) states. This finite Hs state lifetime limits the usefulness of this state in real applications. We present experimental and simulated results showing the dynamic device behaviour of the device during sudden voltage application and show that the lifetime of the Hs state is strongly dependent on the liquid crystal material parameters and the applied voltage. Significant extension of the Hs state lifetime is demonstrated through drive and material parameter optimisation. The decay of the Hs state to the Ha states is also shown to occur via domain growth: isolated regions of the device form into the Ha state, and these domains then grow into areas of the Hs state.

References:

[1] P. J. Bos, and K. R. Koehler/Beran, , 'The pi-cell: a fast liquid-crystal optical switching device', Mol. Cryst. Liq. Cryst., 113, 329 (1984).

[2] M. J. Towler, and E. P. Raynes, ‘A 1 millisecond response time liquid crystal device', Proceedings of the 2002 International Display Research Conference, 2, 877 (2002).

[3] H. G. Walton and M. J. Towler, ‘On the response speed of pi cells', Liq. Cryst., 27, 1329 (2000).

Acknowledgements

The authors would like to thank Merck UK, the EPSRC and the COMIT Faraday Partnership for funding this project.