## 2 Vanadium Dioxide Filter Elaboration Procedure  
Bulk vanadium dioxide undergoes a fully reversible semiconductor to metal transition (SMT) when heated above \(T_{SMT}\) = 340 K, which is accompanied by a structural transition. The dramatic changes of the electrical, optical, and thermal properties associated with the SMT transition have also been experimentally verified in thin VO\({}_{2}\) films with thicknesses down to tens of nm [23; 24]. Above \(T_{SMT}\), VO\({}_{2}\) thin films are quite transparent at around 650 nm, while they exhibit enhanced reflectance/absorption both in the UV and near-IR region. For wavelengths around 650 nm, the absolute transparency of VO\({}_{2}\) films can be enhanced by tailoring the chemical composition and microstructure of the material, as well as its geometrical features (thickness). This spectral variation of optical transparency appears very beneficial for an optical communication system employing a VO\({}_{2}\) thin film as an optical filter in front of the receiver in order to reduce the background noise. The implementation of a VO\({}_{2}\) optical filter might require the integration of a resistor in the form of a metal film as a heater in order to retain the film in its metallic phase and thus take advantage of its full filtering capability. Alternatively, the transition temperature \(T_{SMT}\) might be tuned to the operation temperature of the device by the appropriate doping of VO\({}_{2}\), as extensively researched in the literature [25; 26; 27; 28; 29; 30; 31; 32; 33]. Within the present investigation, high-quality VO\({}_{2}\) films have been obtained by physical vapor deposition (PVD), which was employed to deposit V\({}_{2}\)O\({}_{5}\) films on transparent, i.e., glass, substrates and conduct subsequent thermal reduction. Specifically, the V\({}_{2}\)O\({}_{5}\) films, with an estimated thickness of 240 nm, were reduced to monoclinic VO\({}_{2}\) (M1) by annealing under steady nitrogen (N\({}_{2}\)) gas flow with a pressure of 0.4 mbar, at an annealing temperature of about 500 \({}^{\circ}\)C for 3 h. The transformation of V\({}_{2}\)O\({}_{5}\) to pure monoclinic VO\({}_{2}\) (M1) phase was monitored by x-ray diffraction (XRD) analysis on a Siemens D 5000 diffractometer with Cu K\(\alpha\) radiation at a 2\(\theta\)-step of 0.03\({}^{\circ}\) and a counting time of 2 s/step. To investigate the changes of the electrical and optical properties of VO\({}_{2}\) films around the SMT, electrical resistance was measured in the temperature range of 300-380 K where the SMT takes place, using a 225 Keithley current source and a DMM 2000 Keithley voltmeter. For the same purpose, optical transmittance was measured on the same specimen, at a wavelength of \(\lambda\) = 1550 nm within the SMT temperature region, i.e., 300-350 K. We used an LED diode emitting at 1550 nm as a source, while the detection was performed by an InGaAs photodiode. In addition, optical transmittance was measured within the spectral region 400-1000 nm at room temperature and \(T>T_{SMT}\). A Xe lamp monochromated through an Oriel monochromator was used as a light source for the transmittance and a calibrated silicon photodiode was used for the detection of the transmitted light. In our calculations, we took into account the contribution of the bare substrate. For the transmittance experiments, we designed a special heating stage to achieve isothermal conditions over the film area and their temperature was monitored by a NiCr-Constantam thermocouple in contact with the films. According to the results shown in Figure 1a, the low-current DC-electrical resistance of a VO\({}_{2}\) thin film on a glass substrate exhibits a sharp SMT at \(T_{SMT}\) = 340K, resulting in a drop of approximately three orders of magnitude. The same film exhibited an abrupt decrease of 40% of the optical transmittance in the near infrared region, i.e., \(\lambda\) = 1550 nm, at \(T_{SMT}\), as shown in Figure 1b.