Two-photon excited states and optical nonlinearity in conjugated polymers

Summary form only given. The nonlinear two-photon absorption spectrum of conjugated polymers is theoretically studied in the Pariser-Parr-Pople model by taking into account conhiguration interaction among all the single and double excitations on the Hartree-Fock basis using the cyclic boundary condition. It is shown that, for relatively long (-40 sites) chains and moderate electron-electron interactions, the spectrum consists of two absorption bands (here a band means a group of peaks): a low-energy band involving mainly single electron-hole excitations, and a high energy band with contributions mainly from double excited states. The intensity of the latter band is at least one order of magnitude greater than that of the former. These results are in contrast to the previously studied case of small chains with very strong interactions [1], where the single and double excitations are fairly mixed and the distinction between the two bands is lost In increasing either the interaction strength or the bond alternation, the magnitude of optical nonlinearity decreases rapidly, implying that conjugated polymers with weak electron-electron and electron-phonon interactions are better candidates for optically nonlinear systems