Microstructure modulation and mid-infrared optical properties of VO_X films (invited)

Objective The proliferation of high-power laser systems, characterized by exceptional directionality and energy density, presents a significant threat to both personnel and sensitive photodetectors through dazzling and blinding effects. Consequently, the development of optical limiters for laser protection is of paramount importance. Vanadium dioxide (VO₂) has emerged as a leading candidate for such applications due to its reversible, temperature-driven insulator-to-metal transition (IMT) near 68 ℃, which is accompanied by a drastic modulation of its optical properties in the mid-infrared (MIR) regime. This allows high transmission of low-intensity signals while suppressing high-intensity threats. However, the performance of VO₂-based limiters is often constrained by factors like limited modulation depth and a fixed IMT threshold. Given the multivalent nature of vanadium, which can form various oxides (e.g., V₂O₃, VO₂, V₂O₅) with distinct electronic and optical characteristics, tailoring the phase composition offers a promising route to engineer film 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 (VO_X) thin films, to provide a simple method for fabricating VO_X-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 VO_X thin films were deposited onto quartz substrates via DC magnetron sputtering using a V₂O₃ target in an argon atmosphere. The as-sputtered films were subsequently subjected to post-annealing treatments in an argon environment at varying durations (5, 15, and 120 min). The crystallographic phase and structure were characterized by X-ray diffraction (XRD) and Raman spectroscopy. Surface morphology and microstructure were examined using scanning electron microscopy (SEM). The chemical composition and valence states of vanadium and oxygen were analyzed by X-ray photoelectron spectroscopy (XPS). The MIR optical switching characteristics were evaluated by measuring temperature-dependent (25 ℃ and 80 ℃) transmittance and reflectance spectra across the 2.5-5 µm range using a fourier transform infrared spectrometer. Finally, the laser-induced optical limiting performance was assessed using a self-built test system with a 3.8 μm continuous-wave laser, measuring the transmittance and phase transition response time under varying incident power densities.

Results and Discussions The structural analysis revealed a clear dependence of phase composition on annealing time (Fig.2). The film annealed for 5 min consisted of a mixed phase of V₂O₃ and VO₂. Extending the annealing to 15 min yielded a single-phase, well-crystallized VO₂ film. Further prolonging the treatment to 120 min induced the formation of a secondary V₂O₅ phase, resulting in a VO₂/V₂O₅ mixed structure. Correspondingly, SEM images (Fig.1) showed an evolution from a cracked, fine-grained morphology (5 min) to a denser, smoother surface with larger grains (15 min), and finally to a coarse, plate-like structure (120 min). XPS analysis (Fig.3, Tab.1 and Tab.2) confirmed the evolution of surface chemical states, showing an optimal V⁴⁺ concentration for the 15-minute annealed film and a significant increase in V⁵⁺ associated with V₂O₅ formation after 120 min. The phase composition and microstructure drastically influenced the optical limiting performance. All films exhibited optical switching behavior between their insulating (25 ℃) and metallic (80 ℃) states at 3.8 µm (Fig.4). The single-phase VO₂ film (15 min) demonstrated the most pronounced modulation, with transmittance plunging from 62.1% to 4.5% and reflectance surging from 24.2% to 79.8%, corresponding to the highest optical density (OD = 1.2) (Fig.4(c)). The mixed-phase films showed compromised performance: the V₂O₃/VO₂ film (5 min) had a moderate OD of 0.83, while the VO₂/V₂O₅ film (120 min) exhibited the weakest modulation (OD=0.56). The laser-induced optical limiting performance (Fig.5, Tab.3) provided critical performance metrics. The single-phase VO₂ film possessed the highest phase transition threshold (~100 W/cm²), desirable for allowing uninterrupted passage of low-intensity signals. Once activated, it achieved deep limiting (T from 64% to 4%). In contrast, the V₂O₃/VO₂ film activated at a much lower threshold (41.6 W/cm²) and displayed the fastest response time (Fig.5(c)), attributed to its defective microstructure facilitating localized heating. The VO₂/V₂O₅ film showed an intermediate threshold (58 W/cm²) and response speed, alongside inferior limiting depth.

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 VO_X film performance through precise control of process parameters.