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Vanadium dioxide (VO2) is a promising candidate for intelligent optical limiting owing to its sharp and reversible metal–insulator transition (MIT). Here, we investigate the thermal-optical synergistic phase transition dynamics of epitaxial VO2 thin films under continuous-wave 3.8 μm laser excitation. Thermal prebias is found to significantly reduce the optical activation threshold and compress the macroscopic optical response time scale by nearly 4 orders of magnitude. By performing spatially resolved transient measurements under Gaussian-beam illumination, we further reveal that the observed millisecond-scale response does not reflect the intrinsic speed of the phase transition but instead arises from a coupled process involving rapid local nucleation at the beam center and subsequent lateral propagation of the metallic phase across the illuminated area. While submillisecond phase nucleation is achieved locally under thermal-optical synergy, the effective optical limiting response is governed by beam-scale phase-front propagation. These results clarify the physical origin of the effective response time in VO2-based optical limiters and provide practical guidance for optimizing mid-infrared optical limiting through combined thermal biasing and spatial excitation engineering.
Ji et al. (Fri,) studied this question.