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

We report a detailed investigation of the photorefractive origin of permane nt orientational gratings recorded by holographic technique in dye-doped po lymer-dispersed liquid crystal (PDLC) films. This investigation was perform ed using wave-mixing characterization by two beam coupling (TBC) experiment s. We determined the TBC gain by means of asymmetric energy transfer measur ements and the phase shift by TBC translation technique measurements. The p hotorefractive 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 effec t. The long time stability and some characteristic of the recorded structur e have been explained as a thermal fixing of the grating. During the writin g process, due to the strong light absorption, the sample was locally heate d; under these conditions, the space charge field effect can modify the dro plets interfaces and consequently the orientation of liquid crystal inside the droplets. After removing the two writing beams, the new configuration w as frozen. (C) 1999 Elsevier Science B.V. All rights reserved.