Liquid Crystal Technology Overview

1. Liquid Crystal Technology Overview ECE-E443 Kai Chung Wong

2. Introduction • The study of liquid crystals began in 1888 when an Austrian botanist named Friedrich Reinitzer observed that a material known as cholesteryl benzoate had two distinct melting points. • He increased the temperature of a solid sample and watched the crystal change into a hazy liquid

3. Liquid Crystal • Characteristic • Organic substance in the state between solid and Liquid state • Temperature between Tm and Tc • Thin and Elongated structure • Strong dipoles or easily polarizable substituents

4. Dielectric anisotropy Property • Positive dielectric anisotropy • Molecules will align parallel with electric field • Negative dielectric anisotropy • Molecules will align perpendicular with electric field • The index of refraction is larger along with the long axis of the molecules • Anisotropies of LC enable the index of refraction to be controlled electrically

5. Anisotropy: Dielectric Constant E ++ e + ++ positive - - - - - e De = e - e > 0 E E + + + + - - - - negative all angles in the plane  to E are possible for the -De materials De = e - e < 0

6. Twisted Nematic LC • Twisted nematic liquid crystal acts as a polarization rotator, with polarization plane rotating in alignment with the molecular twist.

8. Twisted Nematic LC Display • liquid crystal place between two polarizer • Alignment layer force molecules orientate in the direction of the polarization plane • Light follows the helix shaped molecules

9. Passive matrix LCD

10. Super Twisted Nematic Displays

12. TFT Active Matrix LCD

13. Applications • LC Display • Sensor • Temperature • Pressure • Chemical • WDM

14. Questions?

15. Order Parameter