  Coherent ultraviolet (UV) light has many uses, for example in the study of
molecular species relevant in biology and chemistry. Very few if any laser
materials offer UV transparency along with damage-free operation at high photon
energies and laser power. Here we report efficient generation of deep and
vacuum UV light using hydrogen-filled hollow-core photonic crystal fiber
(HC-PCF). Pumping above the stimulated Raman threshold at 532 nm, coherent
molecular vibrations are excited in the gas, permitting highly efficient
thresholdless wavelength conversion in the UV. The system is uniquely
pressure-tunable, allows spatial structuring of the out-coupled radiation, and
shows excellent performance in the vacuum UV. It can also in principle operate
at the single-photon level, when all other approaches are extremely
inefficient.
