When left and right circularly polarized beams of light from a pump laser interfere in a nematic liquid crystal doped with
azo dye, a polarization twisted nematic (PTN) grating is formed in the sample. The same is not true when linearly
polarized light interferes, regardless of the polarization. For circularly polarized light, the easy axis is rotated toward the
polarization direction of interfering beams. The irradiance is uniform so there is less contribution to refractive index
variations. In the latter case the diffraction grating arises from variation in refractive index. Gratings written with
Disperse Orange 3 (DO3) as the dopant disappear after removal of the pump beams, whereas grating written with Methyl
Red (MR) as the dopant tend to be semi-permanent.