Polymer dispersed liquid crystals: effects of photorefractivity and local heating on holographic recording

We report a detailed investigation of the photorefractive origin of permanent orientational gratings recorded by holographic technique in dye-doped polymer-dispersed liquid crystal (PDLC) films. This investigation was performed using wave-mixing characterization by two beam coupling (TBC) experiments. We determined the TBC gain by means of asymmetric energy transfer measurements and the phase shift by TBC translation technique measurements. The photorefractive origin of the effect were proved by the experimental results. Nevertheless, some peculiarities showed the presence of other mechanisms that combined 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 was locally heated; under these conditions, 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 was frozen.