Emission of high-order harmonics from solids provides a new avenue inattosecond science. On one hand, it allows to investigate fundamental processesof the non-linear response of electrons driven by a strong laser pulse in aperiodic crystal lattice. On the other hand, it opens new paths towardefficient attosecond pulse generation, novel imaging of electronic wavefunctions, and enhancement of high-order harmonic generation (HHG) intensity. Akey feature of HHG in a solid (as compared to the well-understood phenomena ofHHG in an atomic gas) is the delocalization of the process, whereby an electronionized from one site in the periodic lattice may recombine with any other.Here, we develop an analytic model, based on the localized Wannier wavefunctions in the valence band and delocalized Bloch functions in the conductionband. This Wannier-Bloch approach assesses the contributions of individuallattice sites to the HHG process, and hence addresses precisely the question oflocalization of harmonic emission in solids. We apply this model to investigateHHG in a ZnO crystal for two different orientations, corresponding to wider andnarrower valence and conduction bands, respectively. Interestingly, fornarrower bands, the HHG process shows significant localization, similar toharmonic generation in atoms. For all cases, the delocalized contributions toHHG emission are highest near the band-gap energy. Our results pave the way tocontrolling localized contributions to HHG in a solid crystal, with hard tooverestimate implications for the emerging area of atto-nanoscience.
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