Nonlinear optical anisotropy due to freed electrons

We present a computational study of the pump-probe response of a single atom to assess any microscopic nonlinear optical anisotropy due to freed electrons for non-resonant optical excitation. Using simulations of the Schr\"odinger equation for an atom exposed to a strong, short-duration linearly polarized pump, we calculate the induced dipole moment along probe directions parallel and perpendicular to the pump polarization. Our simulations show birefringence ratios of approximately 0.78-0.9 on the timescale of tens of femtoseconds following the excitation pulse.