A Hidden Photoinduced Phase-Transition Pathway in Strain-Engineered VO₂

Photoexcitation provides a versatile route to drive quantum materials into nonequilibrium states, opening opportunities for phase engineering beyond conventional tuning parameters such as temperature, magnetic field, pressure, or chemical doping/substitution. VO₂, a prototypical correlated oxide, has long served as a model system for understanding photoinduced insulator-metal transitions, yet the sequence of structural and electronic transitions remains intensely debated. Here, we uncover a hidden photoinduced transition pathway in epitaxially strained VO₂ thin films, in which the structural transition precedes the electronic insulator-metal transition, reversing the canonical temporal order. Femtosecond X-ray diffraction reveals a transient structural state characterized by the disappearance of vanadium dimers generating dynamic tensile strain, while time-resolved terahertz spectroscopy shows that the electronic gap closes only after the strain relaxation. This lattice-driven transition highlights the pivotal role of Mott correlations in dictating electronic properties under nonequilibrium conditions. Our findings establish strain-light coupling as a design principle for ultrafast control of phase transitions, offering new avenues for reconfigurable electronic and photonic devices based on correlated oxides.