Strong-Field nonperturbative effects in solids

We present recent results of nonperturbative high harmonic generation\footnote{ S. Ghimire, A. D. DiChiara, E. Sistrunk, P. Agostini, L. F. DiMauro, and D. A. Reis. Observation of high-order harmonic generation in a bulk crystal. \textit{Nat Phys}, 7(2):138-141, 2011.} and below gap absorption\footnote{S. Ghimire, A. D. DiChiara, E. Sistrunk, P. Agostini, L. F. DiMauro, and D. A. Reis, Strong-field induced optical absorption in ZnO, CLEO/QELS 2011.} in single crystal ZnO driven by mid-infrared pulses in a new regime where the field approaches the bandgap per lattice constant. In this limit, the field cannot be considered as producing a small perturbation to the crystal, instead the material properties are transiently yet nondestructively altered. Here we observe harmonics up to 25th order with a cutoff that scales linearly with applied field. The results are consistent with a simple model for tunnel ionization and radiation from nonlinear and attosecond acceleration of carriers in a band. In addition, we observe dramatic sub-band gap absorption of light nearly 10\% below the gap, due to photon assisted tunneling. We discuss the relationship between these two effects.