Extremely Nonperturbative Nonlinearities in GaAs Driven by Atomically   Strong Terahertz Fields in Gold Metamaterials
  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. 
C. Lange, T. Maag, M. Hohenleutner, S. Baierl, O. Schubert, E.   Edwards, D. Bougeard, G. Woltersdorf, and R. Huber
physics.optics