  Contemporary quantum plasmonics capture subtle corrections to the properties
of plasmonic nano-objects in equilibrium. Here, we demonstrate nonequilibrium
spill-out redistribution of the electronic density at the ultrafast time scale.
As revealed by time-resolved 2D spectroscopy of nanoplasmonic Fe/Au bilayers,
an injection of the laser-excited non-thermal electrons induces transient
electron spill-out thus changing the plasma frequency. The response of the
local electronic density switches the electronic density behavior from spill-in
to strong (an order of magnitude larger) spill-out at the femtosecond time
scale. The superdiffusive transport of hot electrons and the lack of a direct
laser heating indicate significantly non-thermal origin of the underlying
physics. Our results demonstrate an ultrafast and non-thermal way to control
surface plasmon dispersion through transient variations of the spatial electron
distribution at the nanoscale. These findings expand quantum plasmonics into
previously unexplored directions by introducing ultrashort time scales in the
non-equilibrium electronic systems.
