## 2 Vanadium Dioxide Filter Elaboration Procedure
The narrow (\(\sim\)5K) thermal hysteresis loops depicted in Figure 1 for both measurements can be attributed to the first-order structural (monoclinic to tetragonal (rutile)) transition accompanying the SMT transition. Figure 1: (**a**) Temperature dependence of the electrical resistance of a VO\({}_{2}\) thin film on glass at T = 300–380 K, as has been experimentally measured; (**b**) temperature dependence of the optical transmittance of a VO\({}_{2}\) thin film on glass at \(\lambda\) = 1550 nm for T = 300–350 K, as has been experimentally measured. The results of a typical measurement of the optical transmittance within the spectral range 400 nm \(\leq\lambda\leq\) 1000 nm at T \(\approx\) 300 K and at T = 370 K are shown in Figure 2. As presented in Figure 2, the rutile (metallic) VO\({}_{2}\) film (T = 370 K) exhibits remarkably low transmittance between 350 nm \(\leq\lambda\leq\) 500 nm, combined with a progressive decrease of the transmittance in the near infrared region upon an increasing wavelength. In the monoclinic insulating phase, i.e., at T = 300 K \(<\)\(T_{SMT}\), the transmittance increases steadily for any wavelength, while it is almost identical to that of the rutile phase in the UV region, see Figure 2. Accordingly, the overall spectral variation of the transmittance attains a \"hanging bell\" shape in the visible light region, with a broad peak centered at 650 nm--where an overall transmittance of 25% is obtained--suggesting a unique radiation-shielding ability of VO\({}_{2}\) thin films extending on both sides of the visible light region. Therefore, rutile VO\({}_{2}\) films appear highly promising as absorption/reflectance materials for solar radiation shielding performance. The experimental results shown in Figures 1 and 2 are in good agreement with recent experimental [34; 35; 36; 37; 38], as well as theoretical, literature reports [39]. The sharp decrease of both the electrical resistance and T-dependent IR-transmittance observed around the SMT transition indicates the high-phase purity and adequate crystallinity of the VO\({}_{2}\) films, in accordance with the obtained XRD patterns. The spectral variation of the transmittance of rutile VO\({}_{2}\) shown in Figure 2 at \(\lambda\geq\) 750 nm reflects a synergetic combination of simultaneously increasing reflectivity and absorption [40] and is thus governed by the intrinsic properties of the material. The strong absorption in the IR region is likely due [41] to the plasma oscillation of the conduction electrons in the metallic phase, indicating a direct correlation with the electrical conductivity of the material in the metallic phase. This, in turn, can be reasonably closely related to the tunability of the filter, as an increase of the electrical conductivity of metallic VO\({}_{2}\) can be expected to result in a higher rejection (i.e., lower transmittance) of near IR radiation. On the other hand, it should be remarked that the steep increase of the optical transmittance at 450 nm \(\leq\)\(\lambda\leq\) 600 nm shown in Figure 2 is insensitive to the electrical conduction, as well as changes of the crystal symmetry and the structural transition (monoclinic to rutile) of the material. This effect should thus be related to fundamental band-structural features of VO\({}_{2}\), like the band gap between valence bands formed by O 2p states and the \(\pi^{\star}\) band, in accordance with previous work [28]. Within this scenario, narrowing the band pass width of the proposed filter might be attempted by band gap engineering of the material. A decrease of the band gap width would be desirable for this specific application, which could involve the doping of VO\({}_{2}\) with properly selected cations. However, it should be emphasized that no such effort has been reported yet, to the best of our knowledge. Figure 2: Optical transmittance of a VO\({}_{2}\) thin film on glass below (300 K) and above (370 K) \(T_{SMT}\) upon wavelength variation in the range 400–1000 nm.