Experimental and theoretical analyses of a correlation between pump-pulse propagation and harmonic yield in a long-interaction medium

We performed experimental and theoretical analyses of a correlation between pump pulse propagation and harmonic yield in Ar and Xe, in a long static gas cell, at high pumping energies, for loose focusing geometry, using 35-fs pulses from a Ti:sapphire laser. Axial and transverse profiles of the plasma column were measured under several experimental conditions with the aim of characterizing the pulse propagation. From the plasma fluorescence profiles observed, it was found that self-guided propagation of the pump pulse in Xe resulted in the channel formation. A three-dimensional nonadiabatic propagation model was used to calculate the field configuration in the interaction region and to explain the channel formation. A model developed to calculate the transverse plasma profiles can adequately reproduce the experimental data and confirm the channel formation. Correlated measurements of harmonics generation reveal that channel formation greatly enhances the harmonic yield as harmonics propagate under phase-matched conditions.