High order harmonic generation in a strongly-driven periodic solid*

We present results of high order harmonic generation from the wide band gap (3.2eV) semiconductor ZnO driven by mid-infrared (0.34 eV) laser pulses in the strongly non-perturbative limit where the field approaches a band-gap per lattice constant. We measure harmonics up to 17$^{th}$ order\textit{---extending well above the band edge}. The spectrum comprises odd harmonics when the electric field is polarized along a centrosymmetric orientation and both odd and even harmonic when polarized along a non-centrosymmetric orientation. We present a simple semi-classical model for emission based on the nonlinear driven oscillation of laser-induced carriers in a periodic lattice potential and discuss its limitations. In addition, we study the laser-dressed band structure using dynamical absorption of a time delayed frequency doubled Ti: Sapphire (3.0 eV) probe.