Time-resolved waveguide modulation of a conjugated polymer

Abstract
We present a new pump and probe technique for measuring intensity‐dependent refractive indices (n2) of waveguide quality thin films, and we apply it to films of polydiacetylene‐(CH2)4OCONHOCOC4H9 (PDA‐4BCMU). Just below the exciton absorption band, the real part of n2 is negative with magnitude ≊107(MW/cm2)1. The initial fast response of n2 is followed by a slower (∼2.5 ps) decay, in close agreement with the decay of the bleaching of the exciton absorption following resonant excitation. These results are consistent with phase space filling by excitons as the mechanism for the nonlinear index.