  The strong-field control of plasmonic nanosystems opens up new perspectives
for nonlinear plasmonic spectroscopy and petahertz electronics. Questions,
however, remain regarding the nature of nonlinear light-matter interactions at
sub-wavelength spatial and ultrafast temporal scales. Addressing this
challenge, we investigated the strong-field control of the plasmonic response
of Au nanoshells with a SiO$_2$ core to an intense laser pulse. We show that
the photoelectron energy spectrum from these core-shell nanoparticles displays
a striking transition between the weak and strong-field regime. This observed
transition agrees with the prediction of our modified Mie-theory simulation
that incorporates the nonlinear dielectric nanoshell response. The demonstrated
intensity-dependent optical control of the plasmonic response in prototypical
core-shell nanoparticles paves the way towards ultrafast switching and
opto-electronic signal modulation with more complex nanostructures.
