  Two-pulse correlation is employed to investigate the temporal dynamics of
both two-photon photoluminescence (2PPL) and four-photon photoluminescence
(4PPL) in resonant and nonresonant nanoantennas excited at a wavelength of 800
nm. Our data are consistent with the same two-step model being the cause of
both 4PPL and 2PPL, implying that the first excitation step in 4PPL is a
three-photon sp->sp direct interband transition. Considering energy and parity
conservation, we also explain why 4PPL behavior is favored over three-and
five-photon photoluminescence in the power range below the damage threshold of
our antennas. Since sizeable 4PPL requires larger peak intensities of the local
field, we are able to select either 2PPL or 4PPL in the same gold nanoantennas
by choosing a suitable laser pulse duration. We thus provide a first consistent
model for the understanding of multiphoton photoluminescence generation in gold
nanoantennas, opening new perspectives for applications ranging from the
characterization of plasmonic resonances to biomedical imaging.
