Specular Reflection Reduction of passive RIS for Single Beam Anomalous Reflection

Reflective reconfigurable intelligent surfaces (RISs) are renowned for their capability to reflect incoming waves towards a chosen anomalous direction. These surfaces consist of arrays of unit cells with linearly varying phase profiles, which can be implemented using two or more unit cells per period. However, using fewer unit cells per period can result in specular reflection besides anomalous reflection. This study demonstrates that increasing the number of states in RIS design can significantly reduce specular reflection. The proposed designs include 18 × 18-unit cell passive RISs of four-state and five-state, which effectively redirect incoming waves towards an anomalous direction, with suppressed specular reflection. Comparative reflection patterns for two- to five-state passive RISs are included to show that the five-state design can provide the highest suppression of about 12.4 dB.