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

Abstract

It is shown that one can obtain absolute two-photon fluorescence excitation (TPE) cross sections without prior knowledge of the temporal profile of the excitation pulse. It is sufficient to record the fluorescence 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 fluorescein [(46 ± 10) × 10−58 m4 s/photon] 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.