Femtosecond excitation dynamics in gold nanospheres and nanorods

Femtosecond visible photoluminescence is detected from gold nanoparticles using time-resolved fluorescence upconversion spectroscopy. We directly compared this fast luminescence from gold nanospheres $(\ensuremath{\emptyset}25\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ with that obtained from nanorods ($\ensuremath{\emptyset}15\ifmmode\times\else\texttimes\fi{}40\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, $\ensuremath{\emptyset}15\ifmmode\times\else\texttimes\fi{}27\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$) under vis $(3.02\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ and $\mathrm{UV}(4.65\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ excitation. A fast $(\ensuremath{\sim}50\phantom{\rule{0.3em}{0ex}}\mathrm{fs})$ decay was obtained for the nanoparticles and the emission was depolarized. Degenerate femtosecond pump-probe experiments in the low excitation intensity regime demonstrated much slower electron thermalization and/or equilibration dynamics on the time scale of a few hundred femtoseconds. These features strongly indicate a $d$-hole-conduction electron recombination process as the origin of this photoluminescence. A direct comparison of the fast emission spectra from nanorods and nanospheres is used to discuss the emission enhancement mechanism. These results suggest that the classical local field enhancement theory describes quantitatively well many of the emission features of nanorods with respect to those for nanospheres without invoking more complex models.