Ultrafast Nonlinear Optical Properties of Charge-Transfer Polymers

Charge-transfer polymers are a new class of nonlinear optical materials which can be used for generating femtosecond holographic gratings. Using semiconducting polymers sensitized with varying concentrations of C60, holographic gratings were recorded by individual ultrafast laser pulses; the diffraction efficiency and time decay of the gratings were measured using nondegenerate four-wave mixing. Using a figure of merit for dynamic data processing, the temporal diffraction efficiency, this new class of materials exhibits between two and 12 orders of magnitude higher response than previous reports. The charge-transfer range at polymer/C60 interfaces was further studied using transient absorption spectroscopy. The fact that charge transfer occurs in the picosecond-time scale in bilayer structures (thickness 200 A) implies that diffusion of localized excitations to the interface is not the dominant mechanism; the charge-transfer range is a significant fraction of the film thickness. From analysis of the excited state decay curves, we estimate the charge-transfer range to be 80 A and interpret that range as resulting from quantum delocalization of the photoexcitations.