Ellipsometric study of polymer thin films: Nonlinear optical guest-host system

We measured the pseudo-dielectric functions of several nonlinear optical (NLO) polymeric systems using spectroscopic ellipsometry in the spectral range between 0.8 and 4.7 eV. We fabricated the thin films after doping the nonlinear optical active chromophores into amorphous polycarbonate to show good optical transparency. We measured the ultraviolet-visible absorption spectra of the NLO chromophores in the film state. In order to determine the dielectric functions, we employed the Bruggeman effective medium approximation, assuming a mixture of two phases: host and guest. We used the Cauchy model for the dielectric function of the host polymer and a parametric optical constant model for that of the guest chromophore. We discussed the variation of the dielectric functions and the transition energies for various host-guest systems. In the gradient donor structural chromophores, we observed a decrease of the highest occupied molecular orbital (HOMO) lowest-unoccupied-molecular-orbital (LUMO) transition energy, and found a transition at about 3 eV. The decrease of HOMO-LUMO transition energy is attributed to the increase of electron donability and conjugative effect. We could fit the volume fractions of host and guest using spectroscopic ellipsometry, and compared them with the values estimated from the chemical structures of the molecules.