Charge transfer model of metal–insulator phase transition and ultrafast optical response in VO2

Metal–insulator phase transition (PT) and ultrafast optical response both in VO2 thin films are studied theoretically and experimentally. Three time scales for optical response, namely, (i) femto–pico-second, (ii) nano-second, and (iii) micro-second scales are manifested here. Such a hierarchy of characteristic times had been related to successive (i) light-induced electrons and holes relaxation to near Fermi-level region, (ii) electron and hole transitions to polaronic states, and (iii) their following transitions to the charge transfer vibronic exciton (CTVE) states. The phenomenological model of this phenomenon is suggested. The charge transfer–lattice instability in VO2 metallic phase which is under the competition with Habbard repulsion effect are considered as a basis for the first order metal–insulator PT. Here the charge transfer related to V–O–V triads had been treated as an order parameter.