MECHANISMS OF 2ND-HARMONIC GENERATION EFFICIENCY RELAXATION IN POLED GUEST-HOST POLYMERS

We have investigated the second harmonic generation (SHG) signals of a guest/host system based on 4-(dimethylamino)-4'-nitrostilbene dissolv ed at 1 wt % in poly(isobutyl methacrylate) and poled in a sandwich co nfiguration of ITO electrodes. By blocking the injection from the ITO surfaces with SiO coatings, a significant impact of charge carriers on the SHG decay pattern can be excluded. Using chloroform as a solvent, an additional contribution to the SHG decay kinetics is observed whic h is also active in the presence of the external electric poling field . Separating this effect from the pure orientational relaxation of the dopants leads to a highly improved reproducibility. Due to its weak t emperature dependence, we tentatively ascribe this solvent-specific ef fect to the diffusion of dipolar impurities which may affect the local field factors and hyperpolarizability tensors. The resulting correcte d SHG time constants related to the pure orientational relaxation stil l appear to be strongly decoupled from the alpha-process of the matrix as inferred from dielectric relaxation data of the bulk polymer. This basic discrepancy between SHG decay and alpha-process is found not to depend on film preparation, sample conductivity, sample geometry, and residual solvent content.