Photorefractive effect and local heating to fix holographic gratings in dyed PDLC

A detailed investigation of the photorefractive origin of permanent gratings in PDLC has been performed using wave-mixing characterisation by two beam coupling (TBC) experiments. The photorefractive origin of the effect has been proved by experimental results. Nevertheless, some peculiarities show the presence of other mechanisms that combine with the photorefractivity to give the observed storage effect. The long time stability and some characteristic of the recorded structure have been explained as a thermal fixing of the grating. During the writing process, due to the strong light absorption, the sample is locally heated; in this condition the space charge field effect can modify the droplets interfaces and consequently the orientation of liquid crystal inside the droplets. After removing the two writing beams the new configuration is frozen. The photorefractive effect is commonly known as due to a redistribution of the photogenerated charge carriers that produce a non uniform space charge field; generally this field induce a modulation of the refractive index of the material through an electro-optic effect.