Phonon-Mediated Optical Nonlinearity in Polydiacetylene

Abstract

We have studied the nonlinear optical response of the quasi-1D semiconductor polydiacetylene-toluene sulfonate under ultrashort optical pulse excitation. The effect of the excitation is to "burn a hole" in the exciton line. The line shape of the hole depends sensitively on the pump frequency, ${\ensuremath{\omega}}_{p}$, and its magnitude peaks when ${\ensuremath{\omega}}_{p}={\ensuremath{\omega}}_{x}\ensuremath{-}{\ensuremath{\omega}}_{o}$, where ${\ensuremath{\omega}}_{o}$ and ${\ensuremath{\omega}}_{x}$ are the phonon and exciton frequencies. We discuss these results with a model in which optical nonlinearity results from phonon-mediated interactions between virtual excitons.