Recent progress in nonlinear spectroscopy of conjugated polymers

An ellipsometric method enabling nonlinear spectroscopy studies in thin films through wave dispersed quadratic electrooptic coefficient measurements is described. The technique is based on the interference between electric field induced modulation of transverse electric (TE) and transfer magnetic (TM) fields propagating through a stratified medium consisting on a substrate, semitransparent electrode and the measured thin film. It allows to determine both the real and the imaginary parts of quadratic electrooptic coefficient as well as its dispersion. The use of thin film enables the electrooptic coefficient measurements even in its absorption band. The technique is applied to a functionalized polymer thin film and the results are interpreted within a two level model. This allows a determination of microscopic parameters such as: excited and fundamental state permanent dipole moment difference and transition moment between fundamental and first excited state. The influence of poling on quadratic electrooptic coefficient is also studied and discussed.