Beyond structural bottleneck: the nature of nonthermal optically induced ultrafast phase switching in VO$_2$

Ultrafast manipulation of the electronic states of strongly correlated electronic crystals near room temperature, such as VO$_2$, encompasses enormous opportunities in high-speed electronics and photonics. However, its strong coupling to the first-order structural phase transition presents a bottleneck effect, which leads to cracking and various instabilities. Here, we show that repetitive ps or even sub-ps phase switching can be initiated by using mid-infrared photons where the rapid transformation is driven by instantaneous shift of chemical potential, rather than lattice or electronic heating. Using fs electron crystallography, we establish the cooperative doping-induced multi-step atomic pathway, which leads to a metal-insulator transition at a fractional energy dose.