Abstract

High-order harmonic generation (HHG) arising from the non-perturbative interaction of intense light fields with matter constitutes a well-established tabletop source of coherent extreme ultraviolet and soft X-ray radiation, which is typically emitted as attosecond pulse trains. However, when driven by sub-cycle waveforms –among other proper strategies [1,2]– the entire HHG process is confined to less than one optical cycle, resulting in the direct emission of high-frequency isolated attosecond pulses (IAPs), which offer great promise for advancing real-time observation and precision control of electron dynamics at the atomic scale [3,4]. Near-infrared (NIR) sub-cycle drivers for HHG can be readily generated by extreme self-compression of femtosecond pulses in gas-filled hollow capillary fibers (HCFs) [5], specially when these are negatively pumped, i.e., filled with a decreasing pressure gradient [6].

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