Abstract
We report here for the first time on the effect of photogeneration of gap states on the non-resonance cubic susceptibility in undoped polythiophene thin films. Photoexcitation of polythiophene, which possesses a non degenerate ground state, yields a major shift in the oscillator strength due to the formation of nonlinear excitations (polarons, bipolarons) in the gap. Using the degenerate four-wave mixing (DFWM) configuration with incident photons at 1.17 eV we have observed an enhancement of the DFWM signal upon illumination with photons having an energy (2.33 eV) larger than the band gap. Resonance contribution due to intrachain polarons is proposed as the possible mechanism responsible for the observed enhancement.
Published Version
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