  Developing a widely tunable vacuum ultraviolet (VUV) source with sub-100
femtoseconds (fs) pulse duration is critical for ultrafast pump-probe
techniques such as time- and angle-resolved photoemission spectroscopy
(TrARPES). While a tunable probe source with photon energy of 5.3 - 7.0 eV has
been recently implemented for TrARPES by using a KBe$_2$BO$_3$F$_2$ (KBBF)
device, the time resolution of 280 - 320 fs is still not ideal, which is mainly
limited by the duration of the VUV probe pulse generated by the KBBF device.
Here, by designing a new KBBF device which is specially optimized for fs
applications, an optimum pulse duration of 55 fs is obtained after systematic
diagnostics and optimization. More importantly, a high time resolution of 81 -
95 fs is achieved for TrARPES measurements covering the probe photon energy
range of 5.3 - 7.0 eV, making it particularly useful for investigating the
ultrafast dynamics of quantum materials. Our work extends the application of
KBBF device to ultrafast pump-probe techniques with the advantages of both
widely tunable VUV source and ultimate time resolution.
