Picosecond waveguide modulation in conjugated polymers

Abstract

The technique of time-resolved waveguide modulation is applied to optical waveguides of the conjugated polymers, polydiacetylene-(CH 2) 4OCONHOCOC 4H 9 (poly-4BCMU) and poly(3-hexylthiophene (P3HT). This method yields values of the sign and magnitude of the complex intensity-dependent refractive index, n 2 , for guided be agt 630 nm ( poly-4 BCMU) and at 1.06 μm (P3HT), upon pumping near the principal absorption band. In poly-4BCMU, n 2 is real and negative, with a magnitude in the order of −10 −7 (MW/cm 2) −1. The decay of 2.5 ps closely matches the decay of the bleaching of the exiton absorption following resonant excitation. These results are consistent with phase-space filling by excitons as the principal nonlinear mechanism. In P3HT, fn 2 has a negative real part and a positive imaginary part, with |n 2|≌1×10 −4 ( MW/cm 2) −1 , for a pump photon energy of 2.06 eV. The positive imaginary part correllates with a photoinduced absorption at 1.06 μm (1.17 eV), indicating a shift of oscillator strength from the interband transition to localized absorptions in the infrared. The negative real part implies that the observed photoinduced absorption peaks at an energy below 1.17 eV.