GENERATION AND PROPAGATION OF HIGH-ORDER HARMONICS IN A RAPIDLY IONIZING MEDIUM

We present the results from simulations of harmonic generation using i ntense ultrashort laser pulses, both for single atoms and in an extend ed gaseous medium. Using a windowing technique on the single-atom spec trum, we show that the highest-order harmonics are generated simultane ously with rapid ionization, and that these harmonics have a phase dep endence which is consistent with a quasiclassical tunneling model. By solving the atomic dynamics simultaneously with the propagation equati ons, we have obtained spectra that directly show the effects of phase mismatching and blueshifting produced by an ionizing medium. At experi mental pressures, the plasma-induced phase mismatch is only important for the highest-order harmonics near the plateau cutoff. All harmonic orders, however, are seen to be blueshifted, by an amount consistent w ith a blueshifting of the fundamental as it propagates through the ion izing medium.