Fig. 18: Elementary steps of the photoinduced electronic phase transition.
From: Recent progress in the phase-transition mechanism and modulation of vanadium dioxide materials

(1) Photoexcitation promotes electrons to the Conduction Band (CB) and leaves holes in the Valence Band (VB). (2) Intraband transitions increase the screening of the CIA and thereby close the band gap. (3) Thermalization of carriers within the energetic region of the former gap occurs slower than this band gap collapse126. Copyright© 2014 the American Physical Society