Nanometer-scale resolution and depth discrimination in near-field optical microscopy studies of electric-field-induced molecular reorientation dynamics

Electric-field-induced molecular reorientation dynamics in polymer-disperse d liquid crystal (PDLC) films are characterized in detail using near-field scanning optical microscopy (NSOM) methods developed previously [Mei and Hi ggins, J. Phys. Chem. A 102, 7558 (1998)]. In these experiments, a modulate d electric field is applied between the aluminum-coated NSOM probe and an i ndium-tin-oxide (ITO) substrate. The field causes reorientation of the liqu id crystal within the ITO-supported PDLC film. The reorientation process is observed by near-field optical means. In this paper, it is conclusively sh own that under appropriate conditions the dynamics observed occur in extrem ely small volumes, and are substantially confined within the near-field opt ical regime. The volume in which the dynamics are probed may be controlled by varying the experimental parameters (i.e., field strength and modulation frequency) employed. Conclusive evidence for confinement is obtained from both theoretical arguments and experimental results. Calculations of the el ectric fields in a model dielectric medium show that the largest fields occ ur very near the NSOM probe. Experimental observation of spatial variations in the threshold (i.e., the "Frederiks transition") for liquid crystal reo rientation provide further evidence. The most direct evidence is provided b y the observation of sub-diffraction-limited resolution in dynamics images of approximate to 1 mu m thick samples. Spatial variations in the observed dynamics are interpreted to reflect the energetics of local liquid crystal organization, the details of the reorientation process, and also polymer/li quid-crystal interfacial interactions. Finally, important information on th e local rotational viscosity and elastic force constants within individual liquid-crystal droplets is obtained. (C) 2000 American Institute of Physics . [S0021-9606(00)70816-6].