  We establish time-resolved high harmonic generation (tr-HHG) as a powerful
spectroscopy for photoinduced dynamics in strongly correlated materials through
a detailed investigation of the insulator-to-metal transitions in vanadium
dioxide. We benchmark our technique by comparing our measurements to
established momentum-resolved ultrafast electron diffraction, and theoretical
density functional calculations. Tr-HHG allows distinguishing of individual
dynamic channels, including a transition to a thermodynamically hidden phase.
In addition, the HHG yield is shown to be modulated at a frequency
characteristic of a coherent phonon in the equilibrium monoclinic phase over a
wide range of excitation fluences. These results demonstrate that tr-HHG is
capable of tracking complex dynamics in solids through its sensitivity to the
band structure.
