###### Abstract  
Metal mesh films have been shown to be a promising strategy to effectively mitigate the growing issue of electromagnetic interference (EMI) in optoelectronic systems. To achieve superior shielding effectiveness, it is common to increase the thickness of the mesh film. However, mesh-based shielding materials have frequency-dependent shielding effectiveness that decreases as the frequency increases. Simply increasing the thickness of the mesh cannot effectively enhance the EMI shielding effectiveness at high frequencies. This will further lead to challenges such as increased processing difficulties and higher costs. In this paper, we present an infrared transparent electromagnetic shielding window based on metal mesh with irregular patterns and proper thickness. The mesh coating is fabricated on a sapphire substrate using ultraviolet photolithography technology and exhibits an efficient electromagnetic shielding effectiveness of more than 20 dB over the wide frequency range of 1.7-18 GHz while maintaining high infrared optical transparency. More importantly, there is no distinct variation in shielding effectiveness between low and high frequency ranges, demonstrating a balanced shielding characteristic across a broad frequency band. This work could be crucial in designing cost-effective and efficient EMI shielding windows for optoelectronic systems. metal mesh; infrared transparent; electromagnetic shielding: balanced shielding properties; wide frequency range +
Footnote †: journal: Applied Sciences  
need for the development of infrared transparent electromagnetic shielding windows [10]. Common strategies for achieving infrared transparent electromagnetic shielding include the application of ultra-thin metal films or metal mesh films to the outer surfaces of the windows or the utilization of infrared transparent conductive oxides [11; 12; 13; 14; 15; 16]. Additionally, some novel materials, such as carbon nanomaterials, have demonstrated favorable conductive and infrared optical properties as well [10; 17]. Continuous or quasi-continuous transparent conducting films such as transparent conductive oxides, ultra-thin metals, and films of carbon nanotubes can provide efficient EMI shielding effectiveness (SE) [18; 19; 20]. However, a well-known issue with these films is that their transparency is compromised as the loading amount or film thickness increases [21; 22]. As a result, it is difficult to decouple the optical transmittance from the EMI SE. One solution to this problem is the use of perforated metallic micro-structures or networks, such as metallic meshes, which can partially alleviate the conflict between optical transmittance and electrical conductivity by increasing the mesh thickness [23]. Metallic meshes with micron/submicron line widths and submillimeter grid spacings have been demonstrated to effectively transmit high levels of Vis-IR light while simultaneously reflecting microwaves [24; 25]. In these materials, the void aperture allows for optical transparency while the interconnected fine metallic wires provide electrical conductivity. While the main focus is on EMI shielding effectiveness, it is important to consider the shielding frequency spectrum as well, especially in the case of broadband EMI shielding. Mesh-based shielding materials often exhibit frequency-dependent shielding effectiveness, with the effectiveness decreasing as the frequency increases [23; 24; 26]. As a result, the shielding effectiveness of metal meshes over a wide frequency range is commonly evaluated by their performance at high frequencies. To achieve superior shielding effectiveness, it is customary to increase the thickness of the mesh film. Nevertheless, this practice is not without challenges. For instance, although increasing the thickness of the mesh can lead to a certain degree of improvement in shielding effectiveness at low frequencies, the enhancement at high frequencies is limited, resulting in a significant difference between the shielding effectiveness of high and low frequency bands.