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

## TL;DR

This study demonstrates that chromatic focusing of few-cycle laser pulses can be used as a novel control parameter to tune high-order harmonic generation spectra without significantly degrading efficiency, enabling new possibilities for attosecond pulse generation.

## Contribution

It introduces the use of chromatic focusing as a control method for HHG, showing experimental and theoretical evidence of its effectiveness and minimal impact on conversion efficiency.

## Key findings

- Chromatic focusing allows tuning of HHG spectra by adjusting focus position.
- Chromatic aberration does not substantially reduce harmonic conversion efficiency.
- Supercontinua and isolated attosecond pulses can be generated using this method.

## 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.