Ultrafast spectroscopy of conducting polymers

Abstract

We have used various experimental techniques in the femtosecond and picosecond time domains to study the ultrafast time response, the optical nonlinearities and the character of the fundamental photoexcitations in conducting polymers with (polyacetylene) and without (polythiophene, polydiacetylene) degenerate ground states. Using the technique of photoinduced absorption (PA) we found that photoexcitations in conducting polymers become self trapped in about 300 fs. They form charged and neutral solitons in polyacetylene and polarons and singlet or triplet excitons in other polymers. Due to the existence of disorder and inhomogeneity in conducting polymers thin films, the dynamics of the PA signal, which is due to recombination, is in the form of a power law t −α (α < 1) or stretched exponential exp [−( t/τ) β] (β < 1). The decays are sensitive to high pressure showing that the recombination is dominated by 3D defects in the chains, which act as recombination centers. We have also measured the magnitude and dynamical response of the third order nonlinear optical coefficient χ( 3) in conducting polymers thin films,using the femtosecond degenerate four wave mixing (DFWM) technique. χ( 3) was in the range of 10 −11 to 10 −8 esu depending on the resonance excitation conditions. Also the dynamical response of χ( 3) for resonant and non-resonant conditions is categorically different. We found, however, that the dephasing time of the photoexcitations (< 50 fs) does not depend on resonance excitation conditions.