Introduction to Liquid Crystals

LCs | Twisted Nematic | FLCs | SSFLCs

Twisted Nematic Devices

The twisted nematic, invented by Schadt and Helfrich and first demonstrated by Fergason in 1971, represents the first successful application of liquid crystals. In this archetypal device, the liquid crystal is confined between crossed polarizers and its birefringence controlled electrically. This basic principle is explained in more detail below.

How Polarizers Work

When unpolarized light passes through a polarizing filter, only one plane of polarization is transmitted. Two polarizing filters used together transmit light differently depending on their relative orientation.

 
Figure 1. Polarizing filters in an isotropic medium (such as air). The system's optical throughput depends on the relative orientation of the polarizer and analyzer.
 

For example, when the polarizers are arranged so that their planes of polarization are perpendicular to each other, the light is blocked (Fig. 1a). When the second filter (called the analyzer) is parallel to the first, all of the light passed by the first filter is also transmitted by the second (Fig. 1b).

Twisted Nematic Liquid Crystal Cells

A Twisted Nematic (TN) cell is made up of

  • two bounding plates (usually glass slides), each with a transparent conductive coating (such as indium tin oxide) that acts as an electrode;
  • spacers to control the cell gap precisely;
  • two crossed polarizers (the polarizer and the analyzer);
  • the nematic liquid crystal material.

  Figure 2. Twisted nematic device geometry. The polarizer and analyzer, which are arranged parallel to the director orientation at their adjacent glass plates, are oriented at 90 degrees to each other.  

The surfaces of the transparent electrodes in contact with the LC are coated with a thin layer of polymer, which has been rubbed or brushed in one direction. The nematic LC molecules tend to orient with their long axes parallel to this direction. The glass plates are arranged so the molecules adjacent to the top electrode are oriented at a right angle to those at the bottom (Fig. 2a). Each polarizer is oriented with its easy axis parallel to the rubbing direction of the adjacent electrode (so the polarizer and analyzer are crossed).

In the absence of an electric field, the nematic director undergoes a smooth 90 degree twist within the cell (hence the name "twisted" nematic liquid crystal). Unpolarized light enters the first polarizing filter and emerges polarized in the same plane as the local orientation of the LC molecules. The twisted arrangement of the LC molecules within the cell then acts as an optical wave guide and rotates the plane of polarization by a quarter turn (90 degrees) so that the light which reaches the second polarizer can pass through it. In this state the LC cell is transparent.

When a voltage is applied to the electrodes, the liquid crystal molecules tend to align with the resulting electric field E (Fig. 2b) and the optical wave guiding property of the cell is lost. The cell is now dark, as it would be without the LC present (as in Fig. 1a). When the electric field is turned off, the molecules relax back to their twisted state and the cell becomes transparent again.

For applications such as digital watches and calculators, a mirror is used under the bottom polarizer. With no voltage applied, ambient light passes through the cell, reflects off the mirror, reverses its path, and re-emerges from the top of the cell, giving it a silvery appearance. When the electric field is on, the aligned LC molecules do not affect the polarization of the light. The analyzer prevents the incident light from reaching the mirror and no light is reflected, causing the cell to be dark. When the electrodes are shaped in the form of segments of numbers and letters they can be turned on and off to form an alpha-numeric display. Passive displays such as these can function solely using ambient lighting, which makes them ideal for battery-operated devices.

Text and images contributed by Chris Conery.