A Screen-printed, Silver-nanowire-based Optically Transparent Wideband Reconfigurable Intelligent Surface for 5G mmWave Applications

The reconfigurable intelligent surface (RIS) can help establish secondary line-of-sight links for 5G mmWave and beyond applications to cater for the large path loss and blockage in millimeter-wave direct links. It is desirable that the RISs are optically transparent so that they can be placed on building windows, etc. However, traditional transparent designs with indium tin oxide films have challenges of high loss and low transparency, while metallic meshes suffer from a large feature size for millimeter wave designs. In this letter, we propose an optically transparent RIS using screen-printed silver nanowires (AgNW) and PIN diodes. The AgNW ink has more than 80% optical transparency and a decent conductivity of $5\times10^{6}$ S/m. The RIS array size is 9x20, and each column serially connects 9 unit cells. The RIS can work from 23.5 GHz to 29.5 GHz with 30° angular stability. The measurement results of the RIS show up to 15 dB gain enhancement within 30° angle range, while maintaining high optical transparency and large bandwidth.