  We theoretically investigate mechanisms of higher-order harmonic generation
in solid-state materials under a high-intensity ac electric field. A new
theoretical framework presented in this Letter holds the legitimacy of Bloch
theorem even under the influence of the high-intensity electric field and
provides an exact treatment of the diabatic processes of Bloch electrons.
Utilizing this framework, we first discovered that the diabatic processes,
namely, ac Zener tunneling and semimetallization of semiconductors, are key
factors for nonperturbative mechanisms of HHG. These mechanisms are classified
by the field intensity and could be understood by an extended simple man model
based on an analogy between tunnel ionization in gaseous media and Zener
tunneling in semiconductors. These conclusions would stimulate the universal
understanding of HHG mechanisms in both atomic and solid cases.
