## 4 Manufacture and Measurement  
To further verify its feasibility, a medium wave filter with high transmittance and ultra-wideband electromagnetic shielding was manufactured, with the following parameters: \(P=4\)\(\upmu\)m; the radius \(r=1.5\)\(\upmu\)m; the substrate material \(h_{2}=0.5\) mm; and the surface layer is coated with gold with a thickness \(h_{1}=100\) nm.  
This study utilized mature processes of ultraviolet (UV) photolithography [39; 40] to prepare the samples for the fabrication of metallic micro/nano structures. Other methods such as nano-imprinting [41] and laser direct-writing technology [42] can also be used for this purpose. The process flow, as shown in Figure 7a, begins with the deposition of a layer of metal film consisting of 30 nm Cr (as an adhesive layer) and 100 nm Au onto a clean substrate surface. Next, a layer of photoresist is spin-coated onto the metal surface and stoved. The stepper process is then used for exposure and development, followed by IBE etching of the sample. Finally, the remaining photoresist is removed. SEM images of the micro/nano structure are presented in Figure 7b,c.  
The transmission spectrum under different conditions are presented in Figure 7d. The brown curve represents the actual measurement result. After comparing the cascaded calculation and fitting results, it was discovered that the transmission curve distribution remains relatively consistent within the 3-5 \(\upmu\)m band. Further, through analysis, it was found that, due to the errors in exposure time and etching time during the preparation process, the metal linewidth of some of the obtained structures was larger than that of the simulation, and the structural period also experienced errors due to changes in linewidth.  
Figure 6: (**a**) The transmission spectrum of medium and long infrared filter in visible light band; (**b,c**) CIE1931 color space inversion map distribution of medium and long infrared filters, wherein the illustration is a partial enlarged view of the color presented. (**d**) EMI SE spectrum of medium and long infrared filter in 1–18 GHz band.  
Therefore, the transmission peak undergoes a peak width decrease phenomenon at 3-5 \(\upmu\)m, which is consistent with the results in Figure 3b,c. Furthermore, during the etching process, due to the small thickness of the metal film, it is difficult to control the etching time, resulting in uneven pits on the substrate surface after etching, thereby reducing the overall transmittance of the sample.  
The rectangular waveguide is connected to the coaxial line through the waveguide coaxial converter and connected to the vector network analyzer to collect \(S\) parameters as shown in the Figure 8a. According to the measured result in Figure 8b, the average EMI SE of 1.7-18 GHz is 45 dB. It follows that, combined with Figures 6d and 8b, the dependence of EMI SE on the cycle plays a dominant role, indicating that the subwavelength structure is conducive to reducing the dependence of EMI SE on the frequency, especially the electromagnetic shielding stability in the high-frequency range. It cannot be ignored that, during the measurement process, there are errors such as the stability of the connection between the waveguide and the sample, as well as manual operations, which result in certain fluctuations in the final measurement results, but they are basically consistent.  
Figure 7: (**a**) Fabricating process flowchart of the structure; (**b**,**c**) SEM image with periodically arranged micro/nano structure and the partial enlarged view; (**d**) the transmission spectrum.