  Dispersive wave emission (DWE) in gas-filled hollow-core dielectric
waveguides is a promising source of tuneable coherent and broadband radiation,
but so far the generation of few-femtosecond pulses using this technique has
not been demonstrated. Using in-vacuum frequency-resolved optical gating, we
directly characterise tuneable 3fs pulses in the deep ultraviolet generated via
DWE. Through numerical simulations, we identify that the use of a pressure
gradient in the waveguide is critical for the generation of short pulses.
