Preparation and Nonlinear Optical Properties of Multivalent Vanadium Oxide Composite Films

Flexible optical limiting materials that combine high transparency, environmental stability, and strong nonlinear optical responses are highly desirable for laser protection applications. In this work, three composite films based on vanadium oxides with different valence states, namely VO 2 /PMMA, V 2 O 3 /PMMA, and V 2 O 5 /PMMA, were fabricated by a solution-casting method using PMMA as the polymer matrix. The morphology, phase composition, valence states, and interfacial interactions of the powders and composite films were systematically characterized by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. The results show that the three vanadium oxides retain their characteristic structural features in the PMMA matrix. Nanosecond open-aperture Z-scan measurements at 532 and 1064 nm reveal pronounced wavelength- and composition-dependent nonlinear optical responses. At 532 nm, all three composite films exhibit valley-shaped transmission attenuation and optical limiting behavior, among which V 2 O 3 /PMMA shows the strongest response and the lowest optical limiting threshold of 128.28 mJ/cm 2 , outperforming VO 2 /PMMA and V 2 O 5 /PMMA. At 1064 nm, VO 2 /PMMA and V 2 O 3 /PMMA still display strong attenuation-type responses, whereas V 2 O 5 /PMMA exhibits a peak-shaped response characteristic of saturable absorption. These results demonstrate that the nonlinear optical behavior of VO x /PMMA films can be effectively tuned by the vanadium valence state, providing useful guidance for the design of flexible materials for optical limiting and saturable absorption applications.