Title: Tunable high-harmonic generation by chromatic focusing of few-cycle laser pulses,
Abstract: In this work we study the impact of chromatic focusing of few-cycle laser
pulses on high-order harmonic generation (HHG) through analysis of the emitted
extreme ultraviolet (XUV) radiation. Chromatic focusing is usually avoided in
the few-cycle regime, as the pulse spatio-temporal structure may be highly
distorted by the spatiotemporal aberrations. Here, however, we demonstrate it
as an additional control parameter to modify the generated XUV radiation. We
present experiments where few-cycle pulses are focused by a singlet lens in a
Kr gas jet. The chromatic distribution of focal lengths allows us to tune HHG
spectra by changing the relative singlet-target distance. Interestingly, we
also show that the degree of chromatic aberration needed to this control does
not degrade substantially the harmonic conversion efficiency, still allowing
for the generation of supercontinua with the chirped-pulse scheme, demonstrated
previously for achromatic focussing. We back up our experiments with
theoretical simulations reproducing the experimental HHG results depending on
diverse parameters (input pulse spectral phase, pulse duration, focus position)
and proving that, under the considered parameters, the attosecond pulse train
remains very similar to the achromatic case, even showing cases of isolated
attosecond pulse generation for near single-cycle driving pulses.