Polarized Light in Liquid Crystals and Polymers



OPTICAL PROPERTIES OF LIQUID CRYSTALS

EFFECT OF LIQUID CRYSTALS ON POLARIZED LIGHT

During the last decades of the 19th century, pioneering investigators of liquid crystals, such as the German physicist Otto Lehmann and the Austrian botanist Friedrich Reinitzer, equipped ordinary microscopes with pairs of polarizing filters to obtain images of nematic and smectic phases. Spatial variation in the alignment of the nematic director causes spatial variation in light intensity. Since the nematic is defined by having all directors nearly parallel to one another, the images arise from defects in the nematic structure. The resulting threadlike images inspired the name nematic, which is based on the Greek word for thread. The layered smectie structure causes layering of defects.

When optically active materials, such as twisted nematic (TN) liquid crystals, are placed between polarizing filters arranged perpendicular some light may get through, because the intervening material changes the polarization of the light. As light strikes the first filter, it is polarized. The molecules in each layer then guide the light they receive to the next layer. As the light passes through the liquid crystal layers, the molecules also change the light’s plane of vibration to match their own angle. When the light reaches the far side of the liquid crystal substance, it vibrates at the same angle as the final layer of molecules. If the final layer is matched up with the second polarized glass filter, then the light will pass through. If we apply an electric charge to liquid crystal molecules, they untwist. When they straighten out, they change the angle of the light passing through them so that it no longer matches the angle of the top polarizing filter. Consequently, no light can pass through that area of the liquid crystal display, which makes that area darker than the surrounding are as. The amount of light passing through is largest when the nematic director is positioned at a 45° angle from both filters. The light is fully blocked when the director lies parallel to one filter or the other.

Optical behaviour and orienting fields underlie the important contemporary use of liquid crystals as opto-electronic displays. Consider, for example, the twisted-nematic cell shown in Figure 8. The two polarizers are crossed, forcing the nematic to rotate between them. The rotation is slow and smooth, assuming a 90° twist across the cell. Light passing through the first polarizcr is aligned with the bottom of the nematic layer. As the nematic twists, it rotates the polarization of the light so that, as the light leaves the top of the nematic layer, its polarization is rotated by 90° from that at the bottom. The new polarization is just right for passing through the top filter, and so light travels unhindered through the assembly.

If an electric field is applied in the direction of light propagation. the liquid crystal directors align with the orienting field, so they are no longer parallel to the light passing though the bottom polarizer (Figure 8). They are no longer capable of rotating this polarization through the 90° needed to allow the light to emerge from the top polarizer. Although this assembly is transparent when no field is applied, it becomes opaque when the field is present. A grid of such assemblies placed side by side may be used to display images. If one turns on the electric field attached to the parts of the grid that lie where the image is to appear, these points will turn black while the remaining points of the grid stay white. The resulting patchwork of dark and light creates the image on the display. In a wristwatch, calculator, or computer these may be simply numbers or letters, and in a television the images may be detailed pictures. Switching the electric fields on or off will cause the picture to move. just as ordinary television pictures display an ever-changing stream of electrically encoded images.

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Fig.7.10. Liquid crystal display assembly

In displays, the liquid crystal cell design usually begins with a thin film of a room-temperature liquid crystal sandwiched between two transparent electrodes (glass coated with a metal or metal oxide film). The thickness of the liquid crystal film is 6- 25 inn and is controlled by a spacer which is chemically inert. The cell is hermetically sealed to eliminate oxygen and moisture, both of which may chemically attack the liquid crystalline material.

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