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 y O 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 (Tlum) compared with a pristine flat VO2 surface. It is worth mentioning that Tlum is also polarization-dependent, and both larger grating height and smaller grating periodicity give enhanced Tlum, 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 (ΔTNIR = 10.8%, ΔTsol = 10.9%) compared with a flat VO2 thin film (ΔTNIR = 8%, ΔTsol = 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.