Atomic and Molecular Dynamics
Strong Field Processes in Solids
Observation of high-order harmonic generation in a bulk crystal
Shambhu Ghimire, Anthony D. DiChiara, Emily Sistrunk, Pierre Agostini, Louis F. DiMauro and David A. Reis
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. In this work, we have observed for the first time, HHG in a bulk crystalline solid. Harmonics up to the 25th order were measured, well above the band-gap of the ZnO crystal we used. Several fundamental differences were observed, including that the scaling of the high-energy cutoff is linearly proportional to the electric field (as opposed to quadratic) and that 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. The results have implications for the understanding of attosecond electron dynamics and other non-equilibrium band-structure-related phenomena in strongly driven bulk solids.