Metal-like monoclinic phase and terahertz characteristics in ultrafast phase transition of photoexcited VO2

Photoexcitation is a powerful way to induce phase transition of strongly correlated materials and dynamically control terahertz (THz) devices integrated with photoinduced phase transition (PIPT) materials. To clarify controversies over the physical mechanism between electronic insulator-metal transition (IMT) and structural phase transition (SPT) of photoexcited vanadium dioxide (VO2), the underlying atomic and electronic state changes during photoinduced monoclinic-to-rutile phase transition are illustrated, and the separation with different thresholds between the quasi-instantaneous IMT and the ultrafast SPT is discovered. Below the SPT threshold, there exist metastable states exhibiting the metal-like monoclinic phases, i.e., the strongest metallicity and weak monoclinic phases, when the bond lengths of the V–V pairs are closest. By analyzing the electronic transport properties of these metal-like monoclinic phases, the THz response of the whole phase transition process can be characterized for first time through the quantum-electromagnetic dispersion modeling method. The THz properties of the practical VO2 film are simulated and the great alignments between the measurements and the simulations verify the proposed analysis method, which provides a powerful exploration path and insights for the theoretical analysis and design verification of PIPT materials and their optoelectronic THz devices.