Interview

Abstract

Professor Yongmeng Liu from the Harbin Institute of Technology in China talks about the research behind his Letter ‘Optical-transparent Wi-Fi bandpass mesh-coated frequency selective surface’ on page 381. Professor Yongmeng Liu Our group's work is focused on designing novel frequency selective surfaces (FSSs) as security wallpaper for optical windows of electromagnetic filtering and interference shielding. A FSS is an array of periodic apertures perforated on a conducting sheet or metallic patches which exhibits bandpass or bandstop responses. A traditional FSS generally has very low optical transmittance and cannot be easily applied as a dual-mode filter to achieve simultaneous high optical transparency and Wi-Fi bandpass filtering. The solution is to create novel periodic structures that have unique and fascinating features attracting us to this field of research. We have reported a FSS periodic structure fabricated using a UV lithography technique to achieve simultaneous high optical transparency and Wi-Fi bandpass filtering. The FSS is a resonance cavity structure with a periodic array of apertures removed from an optically transparent conductive mesh coating. Simulation and experimental results show that the resonance centre frequency of the mesh-coated FSS is about 5.3 GHz with transmittance loss of −2dB, while its optical transmittance is greater than 87.5% in a waveband of 1.35 to 3.3 µm. Such a FSS has the merits of ameliorable electromagnetic shielding effectiveness and improvable optical transmittance by optimising the thickness of the mesh layer. Our FSS can be used as security wallpaper for optical windows to solve the problem of transmission security of Wi-Fi communications. Generally, transparent conductive mesh coating is widely used to provide both strong RF/microwave shielding and optical transmittance, however this seriously attenuates Wi-Fi transmittance. A bandpass FSS is used to provide spatial filtering, but suffers from low optical transmittance due to the large fraction of the area obscured by metallic films. Neither of these existing technologies can meet the dual requirement of optics/Wi-Fi filtering for security wallpaper. It is a significant advancement to develop a periodic structure to achieve simultaneous optical transparency and stable Wi-Fi bandpass. One potential application is as security wallpaper for securing confidential business data. Another application is to enhance the transmission of the specific signal of interest through optical windows. State-of-the-art FSS technology includes indium tin oxide (ITO). This kind of ITO-FSS is used for energy saving windows since the ITO layer reflects a significant portion of long and short infrared wave energy. However, the restrictive relationship between conductivity and optical transparency of ITO is the fatal flaw in this implementation. We are working on a project to design conductive metallic meshes to shield radio-frequency/microwave interference for optical windows to optimise mesh patterns and parameters to improve optical transmission and to minimise the high-order optical diffraction effect, as well as enhance electromagnetic shielding effectiveness. Challenges will be to further optimise the structure of the mesh-coated FSS to improve the performance, specifically the sheet resistance, bandwidth and the transmission stability of incidence angles and polarisations. The FSS has high sheet resistance which will impact Wi-Fi transmission, however the FSS has adjustable thickness to enhance its conductivity and improve Wi-Fi transmission performance while having no impact on optical transparency. Additional challenges include developing ways to integrate the mesh-coated FSS with optical windows of existing buildings. Fortunately we can fabricate the FSS on flexible PET films and add the composite films to each side of existing optical windows. Over the next few years novel periodic electromagnetic structures will integrate metamaterials, active circuit elements and other new techniques to develop security wallpapers to protect communications security. The trend of periodic electromagnetic structures for electromagnetic filtering and shielding will evolve from expensive, heavy and passive to low-cost, lightweight, active and self-tuning. We would like to see increasing numbers of new periodic electromagnetic structures being developed throughout numerous frequency ranges, perhaps even the entire electromagnetic spectrum. We believe all these beautiful periodic electromagnetic structures will be brought into real-world use and will make our lives better!