## 3 Results and Discussion  
The optical transmittance of the sample in the mid-wave IR region (3-5 \(\upmu\)m) was measured using a Fourier transform infrared spectrometer (FTIR-650S, Gangdong Sci & Tech, Tianjin, China) under ambient conditions. The IR transmit  
Figure 2: (**a**) Fabricating process of the IR transparent EMI shielding window. (**b**) Micrograph of metallic meshes under magnification of 50\(\times\). (**c**) Measurement of linewidth under magnification of 400\(\times\). window, the sapphire window with mesh coatings, and the sapphire window with mesh coatings and the covering layer are plotted in Figure 3. It can be seen that the bare sapphire window itself is highly transparent to IR light, exhibiting a high transmittance of 88.89% at 3.210 \(\upmu\)m. Since the mesh has large effective apertures for light transmission, the infrared transmittance of the window decreased by only 4.14% after the mesh coating was fabricated. The most interesting aspect of the result is that the transmittance of the electromagnetic shielding window with a covering layer is slightly increased by 1.42% and reaches 86.17%, which can be attributed to the destructive interference [37]. As a result, the electromagnetic shielding layer reduces the window's infrared transmittance by only 2.72%. With such a high transmission rate, the employment of the electromagnetic shielding window does not affect the detecting and imaging functions of optoelectronic system. It is worth noting that the IR transmittance starts to decrease at wavelengths greater than 4 \(\upmu\)m and drops to -48% at 5 \(\upmu\)m. This is due to the increased IR absorption of Al\({}_{2}\)O\({}_{3}\) in this region. In addition, it is worth mentioning that the transmission short-wave cutoff limit is about 0.19 \(\upmu\)m [31]; the EMI shielding window possesses high visible light transmittance as well. Attenuation of electromagnetic waves by shielding materials is quantified by shielding effectiveness, which is the logarithmic ratio of the incident power (\(P_{i}\)) to the transmitted power (\(P_{i}\)) [13]. Measurements of EMI shielding performance in this study were conducted over a wide frequency range of 1.7-18 GHz using the waveguide method [38], which is schematically illustrated in Figure 4a. The EMI shielding measurement setup consists mainly of a vector network analyzer (VNA, AV3672B, Ceyear, Qingdao, China) and waveguides, as shown in Figure 4b. The system was calibrated using the Transmission-Reflection-Load (TRL) technique before testing. As-prepared samples were firmly clamped between the commissures of waveguides, and different sets of waveguides operating in L-, S-, C-, X-, and Ku-band were utilized to achieve shielding performance across a broad frequency spectrum. EMI SE values can be obtained through \(S\)-parameters collected by the VNA. Since the ratio of \(P_{i}\) to \(P_{t}\) is the measure of electromagnetic transmittance, the total SE can be expressed as [13; 29]:  
\[SE=-10\lg(P_{t}/P_{i})=-10\lg(|S_{21}|^{2}) \tag{2}\]  
Figure 3: Optical properties of the transparent EMI shielding window. According to transmission line theory, EMI Shielding effectiveness of a conductive film is mainly dependent on the sheet resistance [25], and the metallic mesh performs as a continuous conductive film at low frequencies. The sheet resistance of a mesh film can be controlled by the structural parameters of mesh thickness, period, and linewidth [39]. In this study, linewidth of the metal mesh is of 2.5 \(\upmu\)m and the mesh period is 110 \(\upmu\)m. The thickness of Cr and Au mesh layers are 35 nm and 285 nm, respectively. Figure 4c presents the measured EMI shielding performance of the mesh-coated sapphire window. The average EMI SE in the wide frequency range of 1.7-18 GHz is measured to be 23.5 dB, which can block more than 99% electromagnetic power and is sufficient for practical applications [40]. It has been reported that mesh film exhibits obvious EMI shielding degradation at higher frequencies [24; 25; 32; 41; 42].