Dynamic behavior of postfilamentation Raman pulses.

We report on the postfilamentation behavior of a Stokes pulse created from intense and collimated ultrashort pulses propagating in air. A systematic analysis of the pulse propagation revealed that the redshifted Raman pulse produced during filamentation had a larger divergence than the postfilamentation intense pump pulse. Also, the analysis of the far-field Stokes transverse ring revealed that the intensity in this ionization-free light channel is still sufficiently high to induce stimulated Raman scattering after ionization had ended. This behavior further extends the potential of filamentation to remotely induce third-order nonlinearities.