Angle-invariant scattering in metasurfaces

Metasurfaces are efficient and versatile electromagnetic structures that have already enabled the implementation of a wide range of microwave and photonic wave shaping applications. Despite the extensive research into metasurfaces, a rigorous and comprehensive understanding of their angular dispersion remains vastly under-explored. Here, we use the generalized sheet transition conditions (GSTCs) to model and analyze the angular dispersive properties of metasurfaces. Based on this theoretical framework, we demonstrate that a metasurface may exhibit either partial or complete co- and cross-polarized transmission and reflection coefficients that are angle-invariant, meaning that their amplitude, phase, or both remain unchanged with varying incidence angles. We show that these angle-independent responses exist only when specific conditions, given in terms of the metasurface effective susceptibilities, are met. Using the GSTCs formalism, we derive several of these conditions and illustrate their scattering properties. Among other findings, this analysis reveals that, contrary to common assumptions, nonlocality (spatial dispersion) does not only increase the angular dispersion of a metasurface, but may also be used to achieve complete angle-invariant scattering. Additionally, this work demonstrates that fully efficient extrinsic chirality is possible with a pseudochiral metasurface in a partially angle-invariant fashion. We expect our work to provide a general strategy for eliminating, or at least reducing, angular-dependent scattering responses of metasurfaces, which may prove instrumental for applications that are highly sensitive to the detrimental effects of angular dispersion.