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.

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