Twisted nematic mode characteristics

Prior to the development of thin film transistors (TFT) and active matrix technology for liquid crystal displays, the maximum number of hnes or rows in any display with an acceptable contrast was severely limited by the shallow voltage-transmission characteristics of, for instance, the twisted nematic mode. This inspired work to develop a liquid crystal device that could remain in either of two states (ideally black and white) after the removal of the electric field used to switch the liquid crystal into the selected state. With a memory in the liquid crystal an unlimited number of hnes can be displayed using a simple passive matrix and the constraints are instead in the refreshment requirements. 

Several exciting phenomena described for non-chiral nematic systems were also reported for nanoparticle-doped chiral nematic liquid crystals. We mentioned the work of Kobayashi et al., who, most notably, demonstrated a frequency modulation twisted nematic (FM-TN) mode and fast switching characteristics using metal nanoparticles as dopants [301-307, 313, 314],... 

In N and S liquid crystals, as well as in the twisted nematic cell, the formalism can be simplified, since the Jones matrix is independent of z, dcp/dz and being both constants in these cases. By setting k(0)=const=k2, the characteristic equation for the wave vectors of the forward propagating modes is simply... 

High contrast and uniformity of transmission characteristics at oblique incidence are also beneficial features of the SBE mode which considerable improve the legibility of supertwist displays. Better viewing angles than that for the 90° twist structure seem to be a peculiar feature of highly twisted chiral nematics. Figure 4.23 demonstrates this for a 200° supertwist cell in comparison with the usual 90° twist cell [127]. 

In display applications, the twist cell is placed between two polarizers. They can either be oriented parallel or perpendicular to each other, the switching modes of these two configurations being complementary. The electro-optic characteristics of nematic twist cells have been studied by several authors [56—59]. Applications will be treated in Part III of this book. 

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