Linear and Non-Linear Polar Effects in Liquid Crystals
Artikel i vetenskaplig tidskrift, 2011
In cholesterics and chiral smectics there are linear electro-optic modes of considerable interest. As they are fast-switching and have a natural gray scale we will discuss their future potential, in general for active matrix addressing. In spite of differences in physical origin they are all dielectric effects (even in the smectic C* phase) and are generically related by symmetry. This leads to the common feature that there is only one time constant involved (not one oONo and another oOFFo as for effects related to dielectric anisotropy). Furthermore this time constant is independent on the applied electric field. We will also see that cholesterics and chiral smectics have more in common - but seldom pointed out - than just being chiral. In surface-stabilized tilted chiral smectics there are instead strongly non-linear effects one of which may achieve a more or less perfect memory (bistability). If, in addition, the liquid crystal is polymer-stabilized, the cell can be made monostable with a continuous gray scale. As the effects are very fast they were originally successfully developed for passive matrix addressing. Industrial interest in these modes (ferroelectric and antiferroelectric) declined, nevertheless, in the late 1990s. This was not because other types of liquid crystals had been discovered but rather because a cost-effective thin-film-transistor (TFT) technology had been developed. With a TFT matrix every liquid crystal pixel is addressed individually and needs neither to have memory nor to switch at higher speed than can be achieved with nematics. As, however, active-matrix addressing is now commonplace there is naturally a renewed interest in these modes, especially for the future transfer from displays with color filters to displays in which the full color is generated by modulating the red, green and blue LEDs in the time domain.