Generalized few-state models for two-photon absorption of conjugated molecules

We present three- and four-state models for two-photon absorption (TPA) cross-sections generalized to include dipolar direction information and polarization effects of the laser beam. By introducing an effective dipole moment which incorporates the directions of the transition dipole moments and the influence of additionally strongly coupled excited states we can distinguish between different effects leading to enhanced two-photon absorption.