Thermal tuning band gap in cholesteric liquid crystals

Abstract

The phase of a liquid crystal (LC) changing from a nematic phase to a cholesteric (Ch) mesophase is achieved by adding different ratios of chiral dopants S811. By studying the transmission spectrum, we are able to measure the helical pitch in cholesteric phase. The pitch in the mixtures of nematic E7 and chiral dopants S811 as a function of the concentration of the dopant and temperature is investigated. The sensitivity of the selective reflection notch of the cholesteric phase to the thermal tuning depends strongly on the ratios of the chiral dopants. It reveals that the influence of temperature is more profound for those cholesteric liquid crystals (CLCs) which exhibit smectic A (SmA) at lower temperatures. When fitted using Keating's formula, the helical pitch calculated from our experimental results lies on the predicted curve. Optimised ratios of the mixture CLCs for the optimised reflection band with the specified wavelength ranging from 467 nm to 2123 nm are suggested.

Keywords:

• liquid crystal
• cholesteric liquid crystal
• smectic A
• nematic
• transition temperature
• pitch

Acknowledgements

The authors wish to thank Professors J.J. Wu and Y.J. Jen for fruitful discussions. The authors are indebted to Professor T.J. Wang for assisting in establishing the thermal stage and C.F. Tsai for operating differential scanning calorimetry. This work was supported partly by National Science Council of the Republic of China under contract No. NSC 96-2112-M-027-001-MY3.