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[Cholesteric Liquid Crystal Reflective mode]

Cholesteric liquid crystals (ChLC), also known as chiral nematic LC, either consist of chiral nematic LC or nematic LC with addition of chiral agent. The ChLC display utilizes the brag reflection as the reflection state, and the focal conic domain as the non-reflection state.

Figure 1. A simple structure of Cholesteric Liquid Crystal Display.

When the helix of the twist aligns along the normal of the substrate, a planar or Grandjean texture is obtained. This director configuration has the property of reflecting a component of the incoming light. When the component of the circularly polarized light which has the same sense of the helical twist, it will be reflected, and the other opposite component is transmitted. The center wavelength λ0 of the reflected light can be described:

where the band width of reflection is:

A typical reflectivity versus wavelength for a planar cholesteric cell is shown below:

Figure 2. Reflection band in a planar cholesteric cell.

When a voltage is applied on the planar texture, a focal conic domain configuration is obtained. In this state, the reflection is mostly eliminated, and light can pass through the cell, and absorbed if an absorption layer is coated on the exit. Both the planar and focal conic domain are stable states, no field is required to maintain it, therefore bistability can be achieved.

ChLC display is now widely used from reflective display, signage to electronic paper, due to its virtually no power consumption, sun-readable and excellent viewing angles advantages. Many modifications were added to improvement its performance, like polymer stabilized cholesteric texture, full-color ChLC LCD etc.


Further Readings and References:

Deng-Ke Yang, Xiao-Yang Huang, and Yang-Ming Zhu, "Bistable Cholesteric Reflective Displays: Materials and Drive Schemes", Annu. Rev. Mater. Sci.  27:117C46, (1997)

 

Last update: April, 2006
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