Microstructure Modulation and Mid-Infrared Optical Properties of VOX Films

Objective: Vanadium dioxide (VO₂) thin films have emerged as a promising candidate for Mid-Infrared laser protection due to their reversible metal-insulator transition (MIT), which enables dynamic modulation of optical properties. This study aims to investigate the effect of annealing time on the phase evolution, microstructure, and mid-infrared laser protection performance of vanadium oxide (VOX) thin films, to provide a simple method for fabricating VOX-based optical limiters with tunable properties, it also provides experimental basis for the design of materials to meet different laser protection requirements (low phase transition threshold or fast response).Methods: VOX thin films were deposited on quartz substrates using a DC magnetron sputtering system using a V₂O₃ target, followed by post-deposition annealing in an argon atmosphere at different durations (5, 15, and 120 minutes). The crystalline structure, elemental composition, chemical states, and surface morphology of the films were characterized by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The optical modulation performance was evaluated by Fourier transform infrared (FTIR) spectroscopy and a self-built optical limiting test system.Results and Discussions: The phase of the VOX films undergoes a significant evolution with increasing annealing time: a mixed phase of V₂O₃ and VO₂ with microcracks after 5 min, a well-crystallized and dense pure VO₂ phase after 15 min, and a mixed phase of VO₂ and sheet structures V₂O₅ after 120 min. The phase composition and microstructure drastically influenced the optical limiting performance. The pure VO₂ phase film exhibited the highest optical modulation contrast (OD value of 1.2) and the highest phase transition threshold (100 W/cm²). In contrast, the mixed-phase films (5-min and 120-min annealed) demonstrated a faster phase transition response speed and a lower transition threshold.Conclusions: The composition and microstructure of the same material directly determine the laser protection characteristics of the film: the obtained high-purity and densified VO₂ phase film has excellent optical modulation ability and high protection threshold; while the mixed-phase films containing V₂O₃ or V₂O₅ have slightly reduced limiting performance, but a faster phase transition response, which is suitable for scenarios requiring high dynamic range protection and has certain potential in protection applications that require rapid startup. This research provides a practical basis for the directional design and optimization of VOX film performance through precise control of process parameters.