Linking High-Harmonic Generation and Strong-Field Ionization in Bulk Crystals

The generation of high-order harmonics in bulk solids subjected to intense ultrashort laser pulses has opened up new avenues for research in extreme nonlinear optics and light-matter interaction on subcycle time scales. Despite significant advancement over the past decade, a complete understanding of the involved phenomena is still lacking. High-harmonic generation in solids is currently understood as arising from nonlinear intraband currents, interband recollision, and ionization-related phenomena. As all of these mechanisms involve or rely upon laser-driven excitation, we combine measurements of the angular dependence of nonlinear absorption and high-order harmonic generation in bulk crystals to demonstrate the relation between high-harmonic emission and nonlinear, laser-induced ionization in solids. An unambiguous correlation between the emission of harmonics and laser-induced ionization is found experimentally, which is supported by numerical solutions of the semiconductor Bloch equations and an analytical model of orientation-dependent ionization rates using maximally localized Wannier functions.