  We report for the first time on the anticorrelated emission of high-order
harmonics and energetic electron beams from a solid-density plasma with a sharp
vacuum interface$-$plasma mirror$-$driven by an intense ultrashort laser pulse.
We highlight the key role played by the nanoscale structure of the plasma
surface during the interaction by measuring the spatial and spectral properties
of harmonics and electron beams emitted by a plasma mirror. We show that the
nanoscale behavior of the plasma mirror can be controlled by tuning the scale
length of the electronic density gradient, which is measured in-situ using
spatial-domain interferometry.
