We present femtosecond time-resolved transient reflectivity measurements on polydiacetylene performed at the peak of the exciton resonance. The magnitude of the nonlinear refractive index at 1.97 eV was found to be 3.0×${10}^{\mathrm{-}8}$ ${\mathrm{cm}}^2$/W. We discuss a model which accounts quantitatively for the large resonant and nonresonant optical nonlinearity observed in polydiacetylene in terms of phase-space filling by one-dimensional excitons.

• Received 17 September 1986

DOI:https://doi.org/10.1103/PhysRevLett.58.2750