Nonlinear saturation and determination of the two-photon absorption cross section of green fluorescent protein

Two-photon absorption is a nonlinear phenomenon that is observed in materia ls irradiated with the combined energy of two photons, which matches the tr ansition energy between the ground state and the excited state. Molecules e xhibiting strong two-photon absorption have great potential in a wide range of applications such as three-dimensional fluorescence imaging, optical da ta storage, and microfabrication.(9.6) Green fluorescent protein (GFP) can be excited by a two-photon mechanism and exhibits a nonlinear behavior of t he effective two-photon absorption coefficient. Here we present the quantit ative determination of the two-photon absorption saturation of GFP in both a homogeneously broadened and inhomogeneously broadened system. We further show that, dominated by strong nonlinear saturation, GFP has a large intrin sic two-photon absorption cross section (sigma (TPA)) value that is several orders of magnitude larger than that of chemically synthesized compounds.