  The interaction between light and metal nanoparticles enables investigations
of microscopic phenomena on nanometer length and ultrashort time scales,
benefiting from strong confinement and enhancement of the optical field.
However, the ultrafast dynamics of these nanoparticles are primarily
investigated by multiphoton photoluminescence on picoseconds or photoemission
on femtoseconds independently. Here, we presented two-photon photoluminescence
(TPPL) measurements on individual Au nanobipyramids (AuNP) to reveal their
ultrafast dynamics by two-pulse excitation on a global time scale ranging from
sub-femtosecond to tens of picoseconds. Two-orders-of-magnitude
photoluminescence enhancement, namely super interference fringes, has been
demonstrated on tens of femtoseconds. Power-dependent measurements uncovered
the transform of the nonlinearity from 1 to 2 when the interpulse delay varied
from tens of femtoseconds to tens of picoseconds. We proved that the real
intermediate state plays a critical role in the observed phenomena, supported
by numerical simulations with a three eigenstates model and further experiments
on Au nanospheres with different diameters. The crucial parameters, including
the dephasing time, the radiative rate, and the coupling between different
states, have been estimated using numerical simulations. Our results provide
insight into the role of intermediate states in the ultrafast dynamics of noble
metal nanoparticles. The giant photoluminescence in super interference fringes
enables potential practical applications in imaging, sensing, and
nanophotonics.
