Researchers at the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy fully characterized few-femtosecond (fs)-long light pulses tunable in the vacuum ultraviolet (VUV) region. Their achievement will facilitate the study of valence electron dynamics in materials in the VUV. With the emergence of few-cycle light sources that operate from the visible to the MIR spectral range, it is now possible for scientists to directly observe and control the dynamics of ultrafast electrons on their natural timescale. However, the ability to temporally characterize few-fs light sources in the deep-ultraviolet (DUV) and VUV spectral regions remains elusive. Lack of few-cycle UV pulses has inhibited studies in the UV spectral range, which is the range where most materials exhibit electronic resonances. Measured VUV pulses. Top row: measured spectra with retrieved phase. Bottom row: retrieved pulse shapes. Courtesy of the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy. The team aimed to enable the controlled generation of well-characterized, few-cycle VUV pulses. Members built on a technique developed at Heriot-Watt University, which uses resonant dispersive wave (RDW) emission to generate microjoule-level, tunable, few-fs UV pulses with a wide tunability down to 110 nm. The Heriot-Watt technique can be applied to a wide spectral range down to the DUV. Extending the RDW technique to the VUV spectral region, the researchers fully characterized the temporal shape of microjoule-energy, VUV pulses tuned between 160 and 190 nm. The researchers tuned the few-fs pulses using a technique called electron-FROG, a variation of frequency-resolved optical gating (FROG). They generated the pulses via RDW emission, in a cascaded capillary arrangement seeded by an 800-nm Ti:sapphire laser. Next, the researchers compressed the laser pulses to ~10 fs duration by passing them through a gas-filled, stretched flexible hollow-core fiber (SF-HCF). The compressed pulses were then coupled into a second gas-filled SF-HCF, where soliton self-compression and RDW emission took place. The researchers injected helium at the entrance of the second capillary while the output side was directly connected to a vacuum beamline yielding a gradually decreasing gas pressure along the waveguide. The beam emerging from the second SF-HCF was re-collimated, and the broadband soliton content was filtered out. Using either a VUV spectrometer and a power meter, or sending them into the electron FROG apparatus for full temporal characterization, the researchers measured the filtered RDW pulses. In the electron FROG apparatus, a pair of mirrors split the pulse front in half, and introduced a delay between the two halves. The two half-beams were focused into a velocity-map imaging spectrometer, and used to ionize atoms of a noble gas. While scanning the delay between the two pulse replicas, the researchers recorded the velocity distribution of the photoelectrons resulting from two-photon ionization of the atomic gas. The in-situ measurements revealed that, in most cases, the RDW-generated VUV pulses were shorter than 3 fs, in accordance with predictions made based on simulations. The researchers also used the electron FROG apparatus for pump-probe measurements on a series of small organic molecules, such as ethylene. These measurements, carried out with exceptional temporal resolution, could provide insight into the early-time relaxation dynamics after photo-excitation. Currently, the researchers are analyzing the measured data and comparing it with molecular dynamical simulations. The development of ultrafast lasers has enabled the in-depth study of light-matter interactions. The controlled generation of well-characterized, few-cycle VUV pulses could increase scientific understanding of the relation between light and matter, by allowing valence electron dynamics in materials in the VUV to be studied with unprecedented temporal resolution. The results of the MBI study provide a way to investigate ultrafast electron dynamics and valence excitation of a large class of atoms and molecules with a time-resolution that, until now, has been inaccessible when using VUV pulses. The research was published in Nature Photonics (www.doi.org/10.1038/s41566-025-01770-6).