Femtosecond visible photoluminescence is detected from gold nanoparticles
using time-resolved fluorescence upconversion spectroscopy. We directly
compared this fast luminescence from gold nanospheres (subset of
25 nm) with that obtained from nanorods (subset of 15x40 nm, subset
of 15x27 nm) under vis (3.02 eV) and UV(4.65 eV) excitation. A fast
(similar to 50 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.
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