Phonon-Mediated Optical Nonlinearity in Polydiacetylene

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, ${\omega }_p$, and its magnitude peaks when ${\omega }_p={\omega }_x-{\omega }_o$, where ${\omega }_o$ and ${\omega }_x$ 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.