  High-harmonic generation (HHG) in liquids is opening new opportunities for
attosecond light sources and attosecond time-resolved studies of dynamics in
the liquid phase. In gas-phase HHG, few-cycle pulses are routinely used to
create isolated attosecond pulses and to extend the cut-off energy. Here, we
study the properties of HHG in liquids, including water and several alcohols,
by continuously tuning the pulse duration of a mid-infrared driver from the
multi- to the sub-two-cycle regime. Similar to the gas phase, we observe the
transition from discrete odd-order harmonics to continuous extreme-ultraviolet
emission. However, the cut-off energy is shown to be entirely independent of
the pulse duration. This observation is confirmed by ab-initio simulations of
HHG in large clusters. Our results support the notion that the cut-off energy
is a fundamental property of the liquid, independent of the driving-pulse
properties. Combined with the recently reported wavelength-independence of the
cutoff, these results confirm the direct sensitivity of HHG to the mean-free
paths of slow electrons in liquids. Our results additionally imply that
few-cycle mid-infrared laser pulses are suitable drivers for generating
isolated attosecond pulses from liquids.
