  We have demonstrated for the first time that an array of nanoantennas
(central nanotips inside sub-micron pits) on an aluminum surface, fabricated
using a specific double-pulse femtosecond laser irradiation scheme, results in
a 28-fold enhancement of the non-linear (three-photon) electron photoemission
yield, driven by a third intense IR femtosecond laser pulse. The supporting
numerical electrodynamic modeling indicates that the electron emission is
increased not owing to a larger effective aluminum surface, but due to instant
local electromagnetic field enhancement near the nanoantenna, contributed by
both the tip's lightning rod effect and the focusing effect of the pit as a
microreflector and annular edge as a plasmonic lens.
