Nonlinear optical response of a novel polydiacetylene poly(1,4 bis(3-quinolyl)-1,3 butadyne): Role of exciton-phonon interaction

A model incorporating the exciton-phonon interaction for a quantitative des cription of the third-order nonlinear optical response of a one-dimensional conjugated polymer chain in the vicinity of a one-photon resonance is prop osed. This model has been used to describe our observations on the dispersi on of the second molecular hyperpolarizability, gamma (-omega; omega,omega, -omega), of poly(1,4 bis(3-quinolyl))-1,3 butadyne in the spectral range 7 20-810 nm satisfactorily. The large observed precipitous enhancement close to the absorption tail could not be explained by the essential states model s. It is found that the phonon-mediated shift of the exciton resonance is r esponsible for the large monotonic variation of gamma, whilst the phase spa ce filling effect renders gamma its self-defocusing character.