Optoelectronics: Photonic Materials and Devices
DT086/DT085
Operation Principle of a Twist-Nematic Liquid Crystal Display
Lab #2
Objective
To study the operation of a Twist-Nematic (TN) liquid crystal cell (one display pixel) and to
measure the threshold voltage of the twist effect in the nematic liquid crystal.
Equipment
Optical breadboard, He-Ne Laser diode ( = 633 nm), Twist-Nematic liquid crystal cell with
electrodes, Polarizer, Photodetector, Digital multimeter, Signal generator.
Theory
There are many types of liquid crystal displays, each with unique properties. The most
common LCD that is used for everyday items like watches and calculators is called the
twisted nematic (TN) display. This device consists of a nematic liquid crystal sandwiched
between two plates of glass. A special surface treatment is given to the glass such that the
molecules at the top glass surface are aligned perpendicularly to those at the surface of bottom
glass. This configuration sets up a 90 degree twist into the bulk of the liquid crystal, hence the
name of the display (Fig.1).
Fig. 1. Molecules inside the sample create a “twist”- structure.
The underlying principle in a TN display is the manipulation of polarized light. When light
enters the TN cell, the polarization state twists with the director of the liquid crystal material.
For example, consider light polarized parallel to the director at the bottom of the sample. As
is travels through the cell, its polarization rotates with the molecules. When the light emerges,
it's polarization has rotated 90 degrees from when it entered.
A schematic of a TN cell is shown in the following figure (Fig. 2). The black lines represent
crossed polarizers that are attached to the top and bottom of the display. As light enters the
cell, its polarization rotates with the molecules. When the light reaches the top of the cell, its
polarization vector has rotated by 90 degrees, and now can pass through the second polarizer.
In a reflecting TN display, a mirror is placed at the bottom of the cell to reflect the transmitted
light. Once again the polarization twists as the light traverses the sample, and is able to
emerge from the top of the cell.
When an electric field of sufficient magnitude is applied to a sample, the molecules undergo a
Freederickzs transition (Fig. 2, right). Note that in this state, the twist is destroyed. The
director of the bulk liquid crystal is parallel to the field and no longer twisted. When polarized
light enters a cell in such a configuration, it is not twisted, and is canceled by the second
polarizer. Regions where an electric field is applied appear dark against a bright background.
Fig. 2. Schematic diagram illustrating operating principle of the twisted nematic LCD.
Depending on the field strength, twisted nematic displays can switch between light and dark
states, or somewhere in between (grayscale.) How the molecules respond to a voltage is the
important characteristic of this type of display. The response of a typical twisted nematic cell
to an applied voltage is shown in the following diagram (called an electro-distortional curve).
The tilt of the molecules out of the plane of the glass slides is measured as a function of the
applied voltage.
Fig. 3. Dependence of the molecular tilt versus applied voltage in the middle of the liquid
crystal layer.
In the TN display, the electro-distortional response determines the transmission of light
through the cell. Percent transmission as a function of voltage is shown in the following
diagram. Keep in mind that maximum transmission for a reflective TN device is only 50
percent because polarized light must be used.
Fig. 4. Electro-distortional response of a TN LCD pixel.
The vertical lines represent the voltages at which the cell is OFF or ON. In order to address
many pixels with a multiplexing scheme, the differences in the OFF and ON voltages must be
very small. This was difficult to achieve with the traditional TN structure. This problem was
solved with the invention of the super-twisted nematic (STN) display.
Procedure
To measure the electro-distortional response of the twist-nematic liquid crystal cell will use
the experimental setup shown in Fig. 5.
He-Ne
Laser diode
Photodetector
LC cell
Horizontal
Polarizer
Digital
Multimeter
Signal
Generator
Fig. 5. Experimental setup
1. Turn on the laser diode and using the readings of the digital multi-meter adjust the
components of the system to achieve a maximum transmittance in the absence of the
voltage applied to the LC cell.
2. Turn on the signal generator. Apply a square wave signal of frequency 1 kHz
increasing the peak-to-peak amplitude from 0 to 10 V in small steps. For each applied
voltage record the readings of the digital multimeter.
3. Plot the dependence of the normalized transmittance versus applied voltage (similar
to the one shown in Fig. 4). Note, that maximum transmitted intensity corresponds to
a unity absolute transmittance (or 0.5 for unpolarized light).
4. From the obtained plot find the values of OFF and ON voltages (0.9 and 0.1
transmittance correspondingly).
5. Compare your results with typical literature reference values.