Disentangling interface and bulk contributions to high-harmonic emission from solids

This is the authors' abstract. We don't add key points for this paper.

Due to strong absorption of above-bandgap high harmonics generated from solids, the surface should play an important role. However, surface-induced contributions have yet to be identified. In this work, we show that the presence of the interface between a Si crystal and a native SiO2 layer is encoded in the appearance of weak even-order high harmonics. The even harmonics arise from a surface-induced static space charge that perturbs the laser-driven motion of electrons and holes in the bulk of the material. Our demonstration extends surface-induced nonlinear optical effects to the non-perturbative regime, characterized by unbound large-amplitude motion of charge carriers, and therefore paves the way for new methodologies to probe surfaces and their chemistry.