Optical Response of Atomic Gases to Ultrafast Pump-Probe Pulses

We present a computational study of the pump-probe response of a single atom to assess any microscopic anisotropy induced by strong, non-resonant optical fields. Using simulations of the Schrödinger equation for an atom exposed to a linearly polarized ultrafast pump pulse, we calculate the induced dipole moment along the probe polarization directions parallel and perpendicular to the pump polarization. Our simulations show birefringence ratios of approximately 0.7-0.9 on the timescale of tens of femtoseconds following the excitation pulse. In the regime studied, we conclude that excited bound states prepared by the pump are primarily responsible for the birefringence observed, but that the precise response reflects the probe pulse properties.