  In this Letter, we show theoretically that the nonlinear photoionisation
process of a noble gas inside a hollow-core photonic crystal fibre can be
exploited in obtaining broadband supercontinuum generation via pumping close to
the mid-infrared regime. The interplay between the Kerr and photoionisation
nonlinearities is strongly enhanced in this regime. Photoionisation
continuously modifies the medium dispersion, in which the refractive index
starts to significantly decrease and approach the epsilon-near-zero regime.
Subsequently, the self-phase modulation induced by the Kerr effect is boosted
because of the accompanied slow-light effect. As a result of this interplay, an
output spectrum that comprises of a broadband light with multiple
dispersive-wave emission is obtained.
