Surface patterning is a popular approach to produce photonic metasurfaces that are tunable when electro-optic, thermooptic, 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, VO 2 is a well-known thermochromic material for its near-room-temperature phase transition from the insulating to the metallic state. One-dimensional (1D) VO 2 nanograting structures are studied by numerical simulation, and the simulation results reveal that the VO 2 nanograting structures could enhance the luminous transmittance (T lum ) compared with a pristine flat VO 2 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 VO 2 . Femtosecond laser-patterned VO 2 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, VO 2 dominates the film composition, while under extra processing, the existence of other vanadium oxide phases such as V 2 O 3 and V 2 O 5 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 VO 2 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 VO 2 thin films and opens a new gateway for smart devices.

No abstract

Smart windows based on VO2 can control the infrared radiation entering the building based on the temperature, however, the visible part of the spectrum is not controlled. Liquid crystal (LC) based privacy windows, on the other hand, control the visibility either with temperature or applied voltage, however, the total transparency remains fixed as the scattering is mainly in the forward direction. To be able to control both the visibility and the temperature in the house, here we combine both layers in which the LC layer is made of a composite of nanoporous organic microparticles called Cochleates at small concentrations embedded in the LC matrix, thus acting as a tunable scattering metamaterial. The VO2-LC interface has less Fresnel reflectivity and therefore higher solar modulation is expected in an optimized window. In addition, being hidden under the LC layer, VO2 will be protected from oxidation. Electro-optic and thermo-optic properties of the device are investigated including the response time measurements. A non-reciprocity effect is observed showing better performance when the VO2 layer is facing the outside world, in which the window becomes more transparent from inside than from outside and also showing higher solar modulation. Response time is 1 ms for the rise and 10 ms fall time at 70 V. This approach opens up a new possibility of thermochromic VO2 and LC-based systems to satisfy the real-life requirements on smart window applications.