Spatiotemporal Imaging of Thermally Driven Metal-Insulator Phase Domain Propagation in VO2

Achieving a detailed picture of the nature and dynamics of phase boundaries remains a major challenge, particularly solid-solid transitions central to phase-change materials. Here, we use stroboscopic optical scattering microscopy and finite-element simulations to study the nonlinear thermally driven motion of the mesoscale insulator-metal phase boundary in VO2 thin films. We visualize the lateral spatiotemporal evolution of a diffraction-limited photoinduced metallic phase domain, resolving its outward propagation, contraction, and ultimate disappearance. The phase boundary dynamics are governed by heat flow, which can be controlled by excitation fluence, film thickness, and substrate thermal conductivity. Parameter-free agreement between the data and nonlinear thermal transport simulations identify the phase boundary transport as locally diffusive under this regime. We quantify how the latent heat acts as a thermal inertia, suppressing transport and prolonging the phase coexistence. Paradoxically, despite being driven by the underlying thermal gradient locally described by Fourier's law, the mesoscale phase front propagates faster than expected from thermal diffusion. This phenomenon arises from the distinction between temperature-field evolution and phase-boundary isotherm motion. These results establish a link between heat dissipation and mesoscale phase-front motion, providing a framework for controlling phase-transition kinetics relevant to photonics applications of VO2. We anticipate that this approach generalizes to a broad class of first- and second-order transitions.