Physical Optics and Reflection Locality in Designing Reconfigurable Intelligent Surfaces

This paper discusses the applicability conditions of the Physical Optics (PO) approximation and investigates its predictive power for far-field diffraction patterns of Reconfigurable Intelligent Surfaces (RISs) based on binary metasurfaces (BMSs). We compare the PO model with the approximation of socalled Reflection Locality (RL). Although these two approximations are conceptually different, in the case of BMSs, RL turns out to be a prerequisite of the applicability of the PO. If RL holds for such RISs, PO adequately predicts the diffraction pattern only in the case of large periods, which restricts the applicability of the PO to rather small deflection angles. Recently, it was shown that the RL approximation and the popular approximation of so-called angular stability for periodically non-uniform reflecting metasurfaces are equivalent. Therefore, we conclude that BMSs with angular stability can be successfully designed using the PO approximation if the required deflection angles are restricted. In the absence of angular stability, the accuracy of PO is very poor. This finding highlights the importance of considering both the angular stability and the electromagnetic dimensions in RISs designs.