Thresholdless deep and vacuum ultraviolet Raman frequency conversion in H$_2$-filled photonic crystal fiber

Abstract: 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.