  An interesting property of high harmonic generation in solids is its laser
polarization dependent nature which in turn provides information about the
crystal and band structure of the generation medium. Here we report on the
linear polarization dependence of high-order harmonic generation from a gallium
arsenide crystal. Interestingly, we observe a significant evolution of the
anisotropic response of above bandgap harmonics as a function of the laser
intensity. We attribute this change to fundamental microscopic effects of the
emission process comprising a competition between intraband and interband
dynamics. This intensity dependence of the anisotropic nature of the generation
process offers the possibility to drive and control the electron current along
preferred directions of the crystal, and could serve as a switching technique
in an integrated all-solid-state petahertz optoelectronic device.
