Multi-channel contributions to High Harmonic Generation (HHG) in solids.

While HHG from gaseous atoms is well understood [1], HHG from solids is discussed theoretically for decades [2] and still debated [3-5], but scrutinized experimentally only recently [3]. We investigated intra- and inter-band contributions to HHG in ZnO and MgO model solids within a single-active-electron approximation and an adiabatic basis-set approach. We compare HHG spectra after integration over initial states from entire BZ with contributions from the band center ($\Gamma $ point) alone over a range of IR-driver-pulse field strengths. In addition, we use our numerical spectra and intensity-dependent cut-off frequencies for ZnO and MgO crystals to benchmark analytical approximations. [1] Le \textit{et al.}, Phys. Rev. A \textbf{80}, 013401 (2009). [2] Plaja and L. Roso-Franco, \textit{Phys. Rev. B} \textbf{45}, 8334 (1992). [3] Ghimire \textit{et al}., Nat\textit{. Phys,} \textbf{7}, 138 (2011). [4] Wu \textit{et al.,} Phys. Rev. A \textbf{91}, 043839 (2015). [5] Vampa \textit{et al}., \textit{Phys. Rev. Lett,} \textbf{113}, 073901 (2014).