Linear and non-linear optical properties of substituted polyphenylacetylene thin films

Optical third-harmonic generation (THG) at 1064 nm and wavelength dispersed degenerate four-wave mixing (DFWM) have been performed on substituted polyphenylacetylenes (PPAS). Strong changes in the linear and nonlinear optical properties can be achieved by varying the substituent. Resonant values of chi (3) (-3 omega ; omega , omega , omega ) up to 10 -11 esu and chi (3) (- omega ; omega , - omega , omega ) of more than 10 -9 esu with ultra-fast response times were observed. A linear relationship between chi (3) (- omega ; omega , - omega , omega ) and the absorption coefficient alpha was found. This is compatible with an inhomogeneous broadening of the absorption band or with phase-space filling by one-dimensional excitons. Model calculations are presented that relate the modulus and phase of chi (3) (-3 omega , omega , omega , omega ) to the linear optical properties of PPA and indicates a direct two-photon interaction with the absorption band. This may be due to a non-centrosymmetric electronic structure of the PPA backbone.