  In strong laser fields, the electron density in solids can show up tidal
motions caused by the laser ``pulling'' on the electrons, which forms an
electron density tide (EDT). However, the strong field processes in solids are
always explained by the single-active-electron (SAE) model and the fluctuation
of background electrons is neglected previously. Here, we demonstrate the
strong field induced EDT effect and propose a model for revealing its role in
high harmoinc generation (HHG) from solids. We show that the EDT effect induces
an additional polarization current beyond the SAE approximation. It gives a new
mechanism for HHG and leads to new anisotropic structures, which are
experimentally observed with MgO. Our experiment indicates that the EDT effect
becomes more obvious with increasing the laser intensity. Our work establishes
the bridge between the HHG and the microscopic EDT in solids induced by the
strong laser field, which paves the way to probe the ultrafast electron
dynamics.
