Liquid Crystal Technology Overview

ECE-E443

Kai Chung Wong

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

Liquid CrystalCharacteristic •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

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

Anisotropy: Dielectric Constant

+++++

- -- --

E

E

++++

----

positive

negative

all angles inthe plane to E arepossible for the- materials

E

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

 

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

Passive matrix LCD

Super Twisted Nematic Displays

TFT Active Matrix LCD

Applications

LC Display Sensor

TemperaturePressureChemical

WDM

Questions?

Order Parameter