by Researchers have experimentally demonstrated for the first time a mechanically flexible silver mesh that is visibly transparent, enables high-quality wireless optical infrared communications, and efficiently shields electromagnetic interference in the X-band portion of the microwave radio range. Optical communication channels are important for the operation of many devices and are widely used for remote sensing and detection.
Electronic devices are now found everywhere in our homes, on factory floors, and in medical facilities. EMI shielding is often used to prevent electromagnetic radiation from these devices from interfering with each other and affecting device performance.
Electromagnetic shielding, also used in the military to hide equipment and vehicles from the enemy, can also block the optical communication channels needed for remote sensing, detection, or equipment operation. A shield that can block interference but allow optical communication channels could help optimize device performance in a variety of civilian and military environments.
“Many traditional transparent electromagnetic interference shields only let visible light signals through,” said research team leader Liu Yang of Zhejiang University in China. “However, visible wavelengths are not well suited for optical communications, particularly free-space or wireless optical communications, due to the enormous background noise.”
In the diary Express for optical materials, the researchers describe their new network. They show that in combination with transparent silicone and polyethylene, it can achieve a high average electromagnetic shielding effect of 26.2 dB in X-band with good optical transmittance in a wide range of wavelengths, including infrared.
“We take advantage of the ultra-wide transparency and low turbidity of a metallic micromesh to demonstrate efficient electromagnetic shielding, visible transparency, and high-quality free-space optical communications,” Yang said. “Embedding the fabric between transparent materials improves the chemical stability and mechanical flexibility of the silver fabric while also giving it a self-cleaning property. These properties allow a wide application of our silver fabric both indoors and outdoors, even on corrosive and free form surfaces.”
A flexible and transparent network
Researchers designed the new silver mesh using a very simple structure – a repeating square grid pattern applied to a transparent and flexible polyethylene substrate. The continuous lattice structure makes the silver mesh very flexible by releasing stresses when bending. Since the transparency of the silver fabric is primarily determined by the aperture ratio, a measure of the size of the holes in the fabric, it is independent of the wavelength of the incident light.
“A large aperture ratio, for example, is beneficial for high, broadband transparency and low haze, but is disadvantageous for high conductivity and therefore electromagnetic shielding performance,” Yang said. “Because the physical parameters for our tissue can be easily optimized by changing the grating period, linewidth and thickness, it is easier to achieve balanced optical, electrical and electromagnetic properties compared to what is possible with other types of transparent conductive materials.” films, such as silver nanowire networks, ultra-thin metal films, and carbon-based materials.”
To demonstrate their new technology, the researchers fabricated a silver mesh on a polyethylene substrate. The mesh had a grating period of approximately 150 µm, a grating line width of approximately 6 µm and a thickness ranging from 59 to 220 nm. This was then covered with a layer of 60 µm thick polydimethylsiloxane. The resulting film showed high transmission for a wide wavelength range from 400 nm to 2000 nm and sheet resistance as low as 7.12 Ω/sq, enabling high electromagnetic shielding effectiveness of up to 26.2 dB in X-band. The researchers also showed that the film can shield low-frequency cellphone signals.
The researchers note that this work is only a prototype demonstration, so there is still a lot of room for improvement. For example, the use of more conductive materials would improve the effectiveness of electromagnetic shielding, and materials that are more transparent and have less haze could improve not only visible transparency but also the quality of free-space optical communications.
They are also exploring transparent conductive materials in the mid-infrared, which would extend FSO communications to longer wavelengths where atmospheric interference can be reduced and higher communications quality can be achieved. In order to be commercialized, the network would also need to be more practical to install and less expensive.
