Tuning high-order harmonic signals via variation of the Porras factor

We study, theoretically and numerically, the macroscopic high harmonic generation process in atoms for focused short Gaussian laser pulses. The process is subject to the coherent build-up of microscopic radiation and therefore depends on the spatial phase distribution of the driving laser pulse which scales with the Porras factor. In this talk we present our findings, namely, that set-ups with positive Porras factors provide higher conversion efficiency compared to those with negative values. In particular, a Porras factor near 1.2 yields optimal features, such as conversion efficiency, narrowness and extension of the cut-off.