Researchers develop a new shield to block EMI and allow wireless optical signals

The grid-patterned mesh blocks electromagnetic interference while allowing optical communication channels. (Image Credit: Optical Materials Express (2022). DOI: 10.1364/OME.478830)

Electronic devices are everywhere, including factories, medical facilities, and households. These devices have electromagnetic interference shielding to block electromagnetic radiation from disrupting other devices. However, this shielding also keeps out optical communication channels essential for device detection, sensing, or operation. A shield capable of blocking interference without obstructing optical communication channels can optimize device performance.  Zhejiang University researchers recently developed a transparent flexible silver mesh allowing high-quality infrared wireless optical communication while blocking electromagnetic interference in the microwave radio region's X band portion. 

"We take the advantage of the ultrabroad transparency and low haze of a metallic micromesh to demonstrate efficient electromagnetic shielding, visible transparency and high-quality free-space optical communication," said research team leader Liu Yang from Zhejiang University in China. "Sandwiching the mesh between transparent materials improves the chemical stability and mechanical flexibility of the silver mesh while also imparting a self-cleaning quality. These properties will enable our silver mesh to be applied widely both indoors and outdoors, even on corrosive and free-form surfaces."    

The silver mesh is made of a duplicated grid pattern applied atop a transparent, flexible polyethylene substrate. Flexibility is achieved through stress releases as it's bent. The measurement of the mesh's holes, called the opening ratio, helps determine the silver mesh's transparency, making it "independent of the incident light wavelength." 

"A large opening ratio, for example, is beneficial for a high broadband transparency and low haze but is detrimental to high conductivity and thus electromagnetic shielding performance," said Yang. "Because the physical parameters for our mesh can be easily optimized by changing the grid period, line width and thickness, it is easier to achieve well-balanced optical, electrical and electromagnetic properties compared with what is possible with other kinds of transparent conductive films such as silver nanowire networks, ultrathin metallic films and carbon-based materials."

The researchers fabricated a silver mesh on a polyethylene substrate. The mesh had a 6 μm grid line width, 150 μm grid period and measured 59 to 220 nm thick. Afterward, they coated it with a 60 μm thick polydimethylsiloxane layer. This exhibited high transmission "for a broad wavelength range from 400 nm to 2000 nm" and a 7.12 Ω/sq sheet resistance to enable a 26.2 dB (maximum) electromagnetic shield in the X band. In addition, the resulting film shields low-frequency mobile phone signals.

However, this is just a prototype, so they still need to implement improvements. Adding more conductive materials can boost the electromagnetic shielding effectiveness. Meanwhile, more transparent and less hazy materials can improve the overall transparency and the free space optical communication quality. The researchers are looking into mid-infrared transparent conductive materials to expand the FSO communication to longer wavelengths where the atmospheric interference decreases and to achieve higher communication quality. The mesh also needs to be more practical to install and affordable for commercialization.

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