Harmonic generation (HHG) is a general feature of driven nonlinear systems and is well known to occur for strong field laser interactions with atomic systems. This is the basis for producing attosecond pulses in the VUV. The mechanism is well understood in terms of a simple three step model consisting of strong-field ionization, acceleration of the free electron in the laser field, and recombination upon returning to the parent atom. In solids, we expect the process to be fundamentally different due to the high density and periodicity of the system. We have observed for the first time nonperturbative HHG in a bulk crystalline solid. The results appeared in Nature Physics this week ( link to article ) and were the results of a collaboration between Stanford and the Ohio State University.
We measure harmonics up to the 25th order, well above the band-gap of the ZnO crystal we used. We observe several fundamental differences between the solid and atomic case. For example, the scaling of the high-energy cutoff is linearly proportional to the electric field (as opposed to quadratic) and the lack of inversion symmetry gives rise to even and odd harmonics. The results can be understood at least qualitatively in terms of a two step process consisting of strong field ionization across the band gap, followed by radiation due to a nonlinear current driven by the strong field laser. This has important implications for the understanding of attosecond electron dynamics and other non-equilibrium band-structure-related phenomena in strongly driven bulk solids.