  Ultrafast broadband ultraviolet radiation is of importance in spectroscopy
and photochemistry, since high photon energies enable single-photon excitations
and ultrashort pulses allow time-resolved studies. Here we report the use of
gas-filled hollow-core photonic crystal fibers (HC-PCFs) for efficient
ultrafast nonlinear optics in the ultraviolet. Soliton self-compression of 400
nm pulses of (unprecedentedly low) ~500 nJ energies down to sub-6-fs durations
is achieved, as well as resonant emission of tunable dispersive waves from
these solitons. In addition, we discuss the generation of a flat supercontinuum
extending from the deep ultraviolet to the visible in a hydrogen-filled HC-PCF.
Comparisons with argon-filled fibers show that the enhanced Raman gain at high
frequencies makes the hydrogen system more efficient. As HC-PCF technology
develops, we expect these fiber-based ultraviolet sources to lead to new
applications.
