Generation and propagation of high-order harmonics in a rapidly ionizing medium

We present the results from simulations of harmonic generation using intense ultrashort laser pulses, both for single atoms and in an extended gaseous medium. Using a windowing technique on the single-atom spectrum, we show that the highest-order harmonics are generated simultaneously with rapid ionization, and that these harmonics have a phase dependence which is consistent with a quasiclassical tunneling model. By solving the atomic dynamics simultaneously with the propagation equations, we have obtained spectra that directly show the effects of phase mismatching and blueshifting produced by an ionizing medium. At experimental 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 with a blueshifting of the fundamental as it propagates through the ionizing medium.