Influence of Device and Material Parameters

A twist angle 0 9O° can be achieved by doping the nematic liquid crystal mixture with a cholesteric liquid crystal. For a cell with zero pretilt the twist angle P depends on the ratio of the cell-thickness d to the pitch p of the doped liquid-crystal mixture  

The reduced threshold voltage V th for a cell with arbitrary twist angle 0and zero pretilt is given by [40] 

The influence of different values of the pretilt angle, the d/p ratio, the ratios of the elastic constants and. 22( 11 

From Table 2 it follows that a variation of twist angle 0,K IKn, K22IK1, and Ae/fj towards a steeper electrodistortional curve will result in a destabilization of the Freedericksz configuration. A simultaneous increase of the slope and the stability of the Freedericksz configuration can only be achieved by an increase of the pretilt angle [45] and a decrease of the d/p ratio. Decreasing the d/p ratio below a certain value however results in the formation of a twisted structure with a twist angle 0-180°. 

In the following typical values for the different parameters will be given  

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