Simultaneous near-field optical birefringence and fluorescence contrast applied to the study of dye-doped polymer-dispersed liquid crystals

The local optical and electrooptical properties of dye-doped polymer-disper sed Liquid crystal (PDLC) thin films are explored by near-field scanning op tical microscopy (NSOM). Detailed information on the mechanisms for dye ali gnment and electric-field-induced- dye reorientation in localized sample re gions is sought. Dye reorientation is predicted to occur by two distinct pr ocesses: (i) via its interactions with the Liquid crystal host and (ii) via the direct interactions of its permanent and induced dipole moments with t he,applied field. The liquid crystal is doped with a nearly isotropically a ligned BODIPY dye so that contributions of both reorientation mechanisms ca n be explored. An order parameter of 0.10 +/- 0.02 is measured for BODIPY i n bulk aligned samples, suggesting that host-guest interactions are very we ak. Simultaneously recorded birefringence (transmission) and fluorescence N SOM images confirm that the dye is apparently not well aligned, even on sub micrometer length scales. Surprisingly, NSOM studies of dye reorientation p rove that host-guest interactions still play a substantial role in controll ing the dye reorientation process. The field dependence of dye reorientatio n is found to be correlated with liquid crystal reorientation, varying spat ially across individual droplets. Analysis of the simultaneously recorded b irefringence and fluorescence data shows that the dye is strongly reoriente d via its interactions with the reorienting liquid-crystal host and also vi a its independent interactions with the applied field. The former mechanism is manifested as a rotation of the dye alignment axis while the latter lea ds to a field-dependent increase in the local dye order parameter. "Bistabl e" alignment of the dye in the nematic host is proposed as a possible expla nation for the apparent coupling of the two reorientation mechanisms.