Ultrafast Phase Transition in a VO ₂ Membrane: The Role of Mechanics

The mechanics imposed by the substrate, such as epitaxial strain and clamping, provide a freedom for tuning the oxide films’ lattice and properties eventually. Nevertheless, their influence on the ultrafast phenomena in oxide films has rarely been explored. In this work, we investigated the ultrafast phase transition characteristics of a typical correlated oxide, VO2, for both epitaxial films and freestanding membranes released from the substrate. The optical-pump terahertz-probe method revealed that the phase transition threshold of VO2 freestanding membranes can be decreased by around 41%. Furthermore, the extraction of ultrafast response time in picosecond scale and carrier dynamics indicated remarkable reduced restriction in the optical-induced phase transition for freestanding membranes. This is because, after substrate removal, the VO2 membrane is liberated from external stress constraints, allowing the lattice to adjust freely, thereby making it easier to undergo structural reorganization during the phase transition. This approach provides a strategy for achieving lower-power phase transitions in oxides and offers insights into the role of substrate mechanics in ultrafast phenomena.