Ultrafast nano-imagining of the photoinduced phase transition dynamics in VO$_{\mathrm{2}}$

Many quantum phase transitions in correlated matter exhibit spatial inhomogeneities with expected yet unexplored effects on the associated ultrafast dynamics. Here we demonstrate the combination of ultrafast non-degenerate pump-probe spectroscopy with scattering scanning near-field optical microscopy ($s$-SNOM) for ultrafast nano-imaging. In a femtosecond near-field non-degenerate near-IR (NIR) pump and mid-IR (MIR) probe experiment, we study the photoinduced insulator-to-metal (IMT) transition in nominally homogeneous VO$_{\mathrm{2}}$ micro-crystals using far-from equilibrium excitation. We observe spatial heterogeneity on 50-100 nm length scales in the fluence dependent IMT dynamics, ranging from sub-100 fs to 1 ps. With pump fluences as high as nominally 10 mJ/cm$^{\mathrm{2}}$ we can reach distinct excitation and saturation regimes. These results suggest a large sensitivity of the IMT with respect to local variations in strain, doping, or defects difficult to discern microscopically.