Page 271 - Electrical Properties of Materials

Liquid crystals 253

Fig. 10.26
Logs drifting in a Canadian river.

liquid crystals. Mind you, the analogy is not exact. In contrast with logs liquid
crystals have in addition strong electric dipole moments and easily polarizable
chemical groups, so there is a voltage-dependent anisotropy.
There are three types of liquid crystal structures, nematic, cholesteric, and
smectic. The one most often used for display purposes belongs to the nematic
type and is known as the twisted nematic display for reasons which will
become obvious in a moment.
The liquid crystal is held between two glass plates. If the glass plate is suit-
ably treated, then the molecules next to the surface will align in any desired
direction. We can thus achieve that the molecules on the two opposite surfaces
will lie at right angles to each other, and those in between change gradually
from one orientation to the other [Fig. 10.27(a)]. It is not too difficult to ima-
gine now that light incident with a polarization perpendicular to the direction
of the molecules will be able to follow the twist and will emerge with a po-
◦
larization twisted by 90 . Thus, in the configuration of Fig. 10.27(a), with the
two polarizers in place, light will be easily transmitted. When a voltage is ap-
plied, the molecules line up in parallel with the electric field [Fig. 10.27(b)],
the incident light no longer twists its polarization, and consequently no light
transmission takes place.
You may legitimately ask at this stage, why is this a display device? The
answer is that this is only a valve, but it can be turned into a display device by
placing a mirror behind it. With the voltage off the display is bright because it
reflects ambient light. With the voltage on there is no light reflection.
Other types of liquid crystal displays also exist. A colour response can be
obtained by the so-called guest–host effect, which relies on an anisotropic dye
aligning with the liquid crystal molecules.
From a technical point of view, the biggest advantage of liquid crystal
displays is that the voltages they need are compatible with those used for
semiconductor devices. From the customer’s point of view, they have the ma-
jor advantage of providing a flat display. In the last five years they have been