Anisotropy of Ultrafast Strain in $V_2{O}_3$ Thin Films: Out-of-Equilibrium Phase Transitions under Interfacial Clamping

Ultrafast photoinduced insulator-to-metal transitions in correlated materials are often mediated by lattice distortions, yet the role of interfacial lattice constraints in shaping nonequilibrium pathways remains largely unexplored. We use azimuth-resolved time-resolved X-ray diffraction to track orientation-dependent strain dynamics in granular V${}_2$O${}_3$ thin films on c-cut sapphire, where thermal-expansion mismatch imposes anisotropic interfacial strain. Across the thermal transition, the azimuthal profile of the (110)${}_H$ strain inverts curvature, providing direct evidence of partial clamping of the hexagonal basal-plane lattice (a${}_H$,b${}_H$). After photoexcitation of the antiferromagnetic insulating phase, the structural response remains clamp-limited: weakly constrained grain families reach the full basal-plane contraction characteristic of the metallic-like state, whereas strongly constrained families exhibit a strongly reduced distortion. Fluence-dependent measurements further disentangle transformed fraction from clamping-limited lattice distortion. Our results show that interfacial clamping acts as a static selector for ultrafast phase switching and provide a general route to quantify anisotropic strain dynamics in heterostructures.