Determination of absolute two-photon excitation cross sections by in situ second-order autocorrelation

It is shown that one can obtain absolute two-photon f luorescence excitation (TPE) cross sections without prior knowledge of the temporal profile of the excitation pulse. It is sufficient to record the f luorescence generated by two spatially superimposed but temporally shifted halves of the excitation beam as a function of their relative delay. Measurements employed a Michelson interferometer with a mode-locked Ti:sapphire laser as the excitation source. The TPE cross section for f luorescein [(46 6 10) 3 10258 m4 syphoton] obtained at 782 nm with this technique agrees with single-mode cw excitation data. We find no indication of a deviation from the power-squared dependence of TPE.