Optical nutation in polymers irradiated by ultrashort laser pulses.

The transient behavior of the optical susceptibility of polydiacetylene induced by an ultrafast pump field is investigated. Within a two-level model which includes phonon effects phenomenologically, an analytical expression for the nonlinear susceptibility is obtained. In addition to spectral hole burning, the novel phenomenon of optical nutation is found. Both this nutation and the shape of the hole depend sensitively on the detuning between the exciton frequency and the sum of the pump field and the phonon mode frequencies. The electronic state and phonon-mediated optical Stark blue shift are also found in this model. The results are in qualitative agreement with experiments and indicate that the steady-state approximation is reliable only when the pulse of the pump field is longer than several exciton