Dependence of the two-photon photoluminescence yield of gold nanostructures on the laser pulse duration

Two-photon photoluminescence (TPPL) from gold nanostructures is becoming one of the most relevant tools for plasmon-assisted biological imaging and photothermal therapy as well as for the investigation of plasmonic devices. Here we study the yield of TPPL as a function of the temporal width delta of the excitation laser pulses for a fixed average power. In the delta > 1 ps regime, the TPPL yield decreases as delta is increased, while for shorter pulse widths it becomes independent of delta and, consequently, of the laser-pulse peak power. This peculiar dynamics is understood and modeled by considering that two-photon absorption in Au is a two-step process governed by the lifetime of the metastable state populated by the first photon absorption.