  Terahertz near fields of gold metamaterials resonant at a frequency of
$0.88\,\rm THz$ allow us to enter an extreme limit of non-perturbative
ultrafast THz electronics: Fields reaching a ponderomotive energy in the keV
range are exploited to drive nondestructive, quasi-static interband tunneling
and impact ionization in undoped bulk GaAs, injecting electron-hole plasmas
with densities in excess of $10^{19}\,\rm cm^{-3}$. This process causes bright
luminescence at energies up to $0.5\,\rm eV$ above the band gap and induces a
complete switch-off of the metamaterial resonance accompanied by self-amplitude
modulation of transmitted few-cycle THz transients. Our results pave the way
towards highly nonlinear THz optics and optoelectronic nanocircuitry with
sub-picosecond switching times.
