Tunable high-harmonic generation by chromatic focusing of few-cycle laser pulses

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 spatiotemporal 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 for 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 focusing. We back up our experiments with theoretical simulations reproducing the experimental HHG results depending on diverse parameters (input pulse spectral phase, pulse duration, and 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.We acknowledge support from Consejería de Educación,Junta de Castilla y Le n (Projects No. SA116U13 and No. SA046U16), Spanish MINECO (Grants No. FIS2009-09522, No. FIS2013-44174 P, No. FIS2015-71933-REDT, and No. FIS2016-75652-P). This work was partly supported by Fundacio para a Ciencia e Tecnología, Portugal, co-funded by COMPETE and FEDER, via Grants No. PTDC/FIS/122511/2010 and No. UID/NAN/50024/2013. F.S. acknowledges support from Grant No. SFRH/BD/69913/2010 and B.A. acknowledges support from Post-Doctoral Fellowship Grant No. SFRH/BPD/88424/2012; H.C. acknowledges support from Sabbatical Leave Grant No. SFRH/BSAB/105974/2015. C.H.-G. acknowledges support from a Marie Curie International Outgoing Fellowship within the EU Seventh Framework Programme for Research and Technological Development, under REA Grant Agreement No. 328334