Disorder-dominated Ultrafast Dynamics in Vanadium Dioxide

We have performed nondegenerate ultrafast pump-probe spectroscopy of the insulator-to-metal phase transition in vanadium dioxide ($\mathrm{VO_2}$) on several different samples of varying disorder as functions of temperature and pump fluence. Our results are inconsistent with the commonly used assumption of homogeneous nucleation and growth of metallic islands in the parent semiconducting phase and indicate the strong role of disorder on the ultrafast dynamical response.