  Characterizing and controlling the out-of-equilibrium state of nanostructured
Mott insulators hold great promises for emerging quantum technologies while
providing an exciting playground for investigating fundamental physics of
strongly-correlated systems. Here, we use two-colour near-field ultrafast
electron microscopy to photo-induce the insulator-to-metal transition in a
single VO2 nanowire and probe the ensuing electronic dynamics with combined
nanometer-femtosecond resolution. We take advantage of a femtosecond temporal
gating of the electron pulse mediated by an infrared laser pulse, and exploit
the sensitivity of inelastic electron-light scattering to changes in the
material dielectric function. By spatially mapping the near-field dynamics of
an individual nanowire of VO2, we observe that ultrafast photo-doping drives
the system into a metallic state on a time scale of about 150 fs without yet
perturbing the crystalline lattice. Due to the high versatility and sensitivity
of the electron probe, our method would allow capturing the electronic dynamics
of a wide range of nanoscale materials with ultimate spatio-temporal
resolution.
