In this example, we will model a Twisted Nematic LCD (TN-LCD). For a simple uniaxial LCD, see Simple LCD.
When no voltage is applied across the TN-LCD, the liquid crystals are oriented in a twisted configuration, and the LC director makes a homogeneous turn of 90 degrees from the bottom surface of the LC layer to the top. As the light enters the LC with a polarization parallel the bottom director (x axis in this example), the linear polarization of the light will roughly follow the rotation of the director, and as a result the light transmitted will be polarized in the y direction. When a voltage is applied, the external electric field forces the LC director to become homogeneous and parallel to the propagation direction, and there will be no change in the polarization state of the light.
To model the 90 degree twist in the LC orientation in FDTD, we will use an anisotropic material combined with a LC import grid attribute. The script LCD_twist.lsf sets up the LC orientation in the helical twist configuration, as can be seen in the file LCD_twist.fsp. We start with a plane wave source that has the same polarization as the bottom LC director (along the x axis), and use a 2D DFT monitor to track the change in polarization of the light as it propagates through the TN-LCD .
Here, we can define the polarization (at each point along z) by the TE fraction:
The script LCD_twist_analysis.lsf plots the „TE fraction vs z” in the TN-LCD layer:
When operating in a real situation, the TN-LCD layer will be sandwiched between two polarizers (with a 90 degree polarization difference). Without an external field, the light will go through a 90 degree polarization change in the LCD layer and will therefore be allowed to pass through the top polarizer (hence the “ON” state). When an external field is applied, the crystal will become aligned with the field, and as a result, there will be no change in the polarization of the incident light, and the top polarizer will not allow any light through (hence the “OFF” state).