## Abstract

We report experimental investigations of two-photon and excited-state absorption in $D\text{-}\pi \text{-}A$ chromophores and propose a model to explain nonlinear absorption in the nanosecond regime. The key parameter is an effective three-photon absorption coefficient that depends on the two-photon absorption cross section and excited-state photophysical properties. We obtain all these parameters from independent measurements and then compare the model with nanosecond nonlinear transmission measurements. We find good agreement with the data, using no free parameters, for specific values of some published two-photon absorption cross sections. We conclude that excited singlet–singlet and triplet–triplet absorption are the dominant sources of nonlinear transmittance loss and that the chief role of two-photon absorption is to populate these states.

© 2005 Optical Society of America

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