Microscopic electron dynamics in nonlinear optical response of solids

We investigate the microscopic properties of the nonlinear optical response of crystalline solids, and demonstrate that optically-induced microscopic charge distributions display complex spatial structure and nontrivial properties. Their spatial symmetry and temporal behavior are governed by crystal symmetries. We find that even when a macroscopic optical response of a crystal is forbidden, the microscopic optical response can, in fact, be nonzero. In such a case, the optically-induced charge redistribution can be considerable, even though the corresponding Fourier component of the time-dependent dipole moment per unit cell vanishes. We develop a method that makes it possible to completely reconstruct the microscopic optically-induced charge distributions by means of subcycle-resolved x-ray-optical wave mixing. We also show how, within this framework, the direction of the instantaneous microscopic optically-induced electron current flow can be revealed.