Simulations of femtosecond atmospheric filaments enhanced by dual pulse molecular alignment

A laser pulse propagating through the atmosphere self-focuses due to the nonlinear index of refraction modifications from the instantaneous electronic and delayed rotational responses of the air molecules. If the pulse power is sufficient, the focused pulse intensity can surpass the ionization threshold, resulting in a plasma filament. The balance between defocusing due to plasma refraction and focusing due to the instantaneous and delayed responses results in extended propagation at high intensities. Because the rotational response induced by the first pulse (the pump pulse) is periodic in time, owing to quantum-mechanical discreteness of the rotational eigenfrequencies of the molecules, a subsequent laser pulse (the probe pulse), delayed at the recurrence period, experiences a propagating wake of index modification left behind by the previous pulse. Here, we present propagation simulations based on a recent experiment [Varma et al. (unpublished)] showing that axial extension of the plasma filament and probe pulse shaping imposed by the molecular alignment wake are sensitive to probe delay changes of as little as 10 fs.