  Gas-filled hollow-core photonic crystal fibre (PCF) is being used to generate
ever wider supercontinuum spectra, in particular via dispersive wave (DW)
emission in the deep and vacuum ultraviolet, with a multitude of applications.
DWs are the result of the resonant transfer of energy from a self-compressed
soliton, a process which relies crucially on phase matching. It was recently
predicted that, in the strong-field regime, the additional transient anomalous
dispersion introduced by gas ionization would allow phase-matched DW generation
in the mid-infrared (MIR)-something that is forbidden in the absence of free
electrons. Here we report for the first time the experimental observation of
such MIR DWs, embedded in a 4.7-octave-wide supercontinuum that uniquely
reaches simultaneously to the vacuum ultraviolet, with up to 1.7 W of total
average power.
