Nonadiabatic Mechanism of High-Order Harmonic Generation in Solids

Abstract

We theoretically investigate mechanisms of high-order harmonic generation (HHG) in solids. A new theoretical framework presented here holds legitimacy of Bloch theorem even under an influence of the high-intensity electric field. Our theoretical framework shows the nonadiabatic processes, namely, AC Zener tunneling and semimetallization of semiconductors, are key factors for non-perturbative mechanisms of HHG. These mechanisms are classified by the field intensity with respect to the band gap energy and could be understood by an extended simple man model based on the analogy between tunnel ionization in gaseous media and Zener tunneling in semiconductors. These conclusions would stimulate universal understandings of HHG mechanisms in both atomic and solid cases.