Femtosecond Laser-Induced Vanadium Oxide Metamaterial Nanostructures and the Study of Optical Response by Experiments and Numerical Simulations

Surface patterning is a popular approach to produce photonic metasurfaces that are tunable when electro-optic, thermo-optic, or magneto-optic materials are used. Vanadium oxides (V_yO_x) are well-known phase change materials with many applications, especially when used as tunable metamaterial photonic structures. Particularly, VO₂ is a well-known thermochromic material for its near-room-temperature phase transition from the insulating to the metallic state. One-dimensional (1D) VO₂ nanograting structures are studied by numerical simulation, and the simulation results reveal that the VO₂ nanograting structures could enhance the luminous transmittance (T_lum) compared with a pristine flat VO₂ surface. It is worth mentioning that T_lum is also polarization-dependent, and both larger grating height and smaller grating periodicity give enhanced T_lum, particularly at TE polarization in both insulating (20 °C) and metallic (90 °C) states of VO₂. Femtosecond laser-patterned VO₂ films exhibiting nanograting structures with an average periodicity of ≈500–700 nm have been fabricated for the first time to enhance thermochromic properties. Using X-ray photoelectron spectroscopy, it is shown that at the optimum laser processing conditions, VO₂ dominates the film composition, while under extra processing, the existence of other vanadium oxide phases such as V₂O₃ and V₂O₅ increases. Such structures show enhanced transmittance in the near-infrared (NIR) region, with an improvement in NIR and solar modulation abilities (ΔT_NIR = 10.8%, ΔT_sol = 10.9%) compared with a flat VO₂ thin film (ΔT_NIR = 8%, ΔT_sol = 10.2%). The slight reduction in transmittance in the visible region is potentially due to the scattering caused by the imperfect nanograting structures. This new patterning approach helps understand the polarization-dependent optical response of VO₂ thin films and opens a new gateway for smart devices.