Exploring Dynamics in Photorefractive Polymer-Dispersed Liquid Crystals Using Near-Field Scanning Optical Microscopy

Abstract

Dynamic near-field scanning optical microscopy (NSOM) imaging methods are employed to study liquid crystal dynamics in photorefractive polymer dispersed liquid crystal (PDLC) films. Micrometer-sized nematic droplets dopedwith photoexcitable electron donor and acceptor dyes and encapsulated within a thin polymer film are studied. Liquid crystal reorientation dynamics and ion migration dynamics are induced by applying a modulated electric field between the metalized NSOM probe and the electrically conductive, optically transparent substrate upon which the sample is supported. The induced dynamics are detected by monitoring the intensity of 633 nm light transmitted through the sample under crossed polarization conditions. Comparison of dynamics images taken before, during, and after photogeneration of ions using 488 nm light shows the influence of these ions on the liquid crystal dynamics. Computer simulations of the ion and liquid crystal dynamics are employed to better interpret the results. Taken together, these results indicate that differences observed between images recorded before (or after) and during ion generation result from changes in the liquid crystal reorientation dynamics deep within central droplet regions. In contrast, interfacial liquid crystal remains strongly aligned by the local space charge fields that develop. It is concluded that photorefractivity in PDLC films arises primarily from ion-induced relaxation of the liquid crystal in central droplet regions.