NONLINEAR-OPTICAL SUSCEPTIBILITY OF A MODEL GUEST-HOST POLYMERIC SYSTEM AS INVESTIGATED BY ELECTROOPTICS AND 2ND-HARMONIC GENERATION

The magnitude and stability of the induced dipolar orientation of a nonlinear optical (NLO) chromophore doped in an amorphous polymeric matrix is investigated. The chromophores are aligned using electric field poling. The linear electrooptic (EO) Pockels effect and second-harmonic generation (SHG) techniques are used to probe the electric field-induced alignment. Both the Pockets coefficient and the second-order susceptibility show a nonlinear relationship with the NLO chromophore concentration. Using a relationship based on the two level model, the Pockels coefficient is compared with the second-order nonlinear susceptibility measured by SHG. The temperature dependence of the decay time constant of the SHG signal is found to follow an empirical relationship such as the Vogel-Tammen-Fulcher (VFT) equation. The decay time constant of the SHG signal is also found to be increasing with increasing chromophore concentration. This concentration dependence is interpreted as due to orientational pair correlation between chromophores and between chromophores and polymer chains.