High-energy, few-cycle light pulses tunable across the vacuum ultraviolet

In the last few decades the development of ultrafast lasers has revolutionized our ability to gain insight into light-matter interactions. The appearance of few-cycle light sources available from the visible to the mid-infrared spectral range and the development of attosecond extreme ultraviolet and x-ray technologies provide for the first time the possibility to directly observe and control ultrafast electron dynamics in matter on their natural time scale. However, few-fs sources have hardly been available in the deep ultraviolet (DUV; 4-6 eV, 300-200 nm) and are unavailable in the vacuum ultraviolet (VUV; 6-12 eV, 200-100 nm) spectral range, corresponding to the photon energies required for valence excitation of atoms and molecules. Here, we generate VUV pulses with $\mu$J energy tunable between 160 and 190 nm via resonant dispersive wave emission during soliton self-compression in a capillary. We fully characterize the pulses in situ using frequency-resolved optical gating based on two-photon photoionization in noble gases. The measurements reveal that in most of the cases the pulses are shorter than 3 fs. These findings unlock the potential to investigate ultrafast electron dynamics with a time-resolution that has been hitherto inaccessible when using VUV pulses.