Gold nanorods (Au NRs) and coupled gold nanospheres (Au NSs) are known to display strong two-photon photoluminescence (TPPL). Here two-pulse emission modulation (TPEM) and pump-probe measurements were performed on Au NRs and coupled Au NSs to understand their excitation mechanisms. The TPEM cross contributions of Au NRs and coupled Au NSs showed much slower decay compared with a two-photon absorption organic dye. Their decay time constants (4.0 ps for Au NRs and 3.1 ps for coupled Au NSs) match well with the lifetimes of intermediate states measured from pump-probe experiments. These results support the fact that strong TPPL in Au NRs and coupled Au NSs arises from two sequential one-photon absorption steps involving real intermediate states instead of coherent two-photon absorption. These results give direct evidence that previously observed aggregation-enhanced TPPL arises from enhanced two-photon excitation efficiency, which was facilitated by two sequential plasmon-coupling-enhanced one-photon absorption steps via real intermediate states.
Keywords: gold nanoparticles; photoluminescence; plasmon coupling; two-photon excitation; ultrafast spectroscopy.