  Vacuum ultraviolet (VUV) light is critical for the study of molecules and
materials, but the generation of femtosecond pulses in the VUV region at high
repetition rates has proven difficult. Here, we demonstrate the efficient
generation of VUV light at MHz repetition rates using highly cascaded four-wave
mixing processes in a negative-curvature hollow-core fiber. Both even and odd
order harmonics are generated up to the 15th harmonic (69 nm, 18.0 eV), with
high energy resolution of ~40 meV. In contrast to direct high harmonic
generation, this highly cascaded harmonic generation process requires lower
peak intensity and therefore can operate at higher repetition rates, driven by
a robust ~10 W fiber-laser system in a compact setup. Additionally, we present
numerical simulations that explore the fundamental capabilities and
spatiotemporal dynamics of highly cascaded harmonic generation. This VUV source
can enhance the capabilities of spectroscopies of molecular and quantum
materials, such as photoionization mass spectrometry and time , angle , and
spin-resolved photoemission.
