Independent bandwidth control of a spin-decoupled metasurface enabled by a non-Hermitian topological phase

Spin-decoupled metasurfaces have attracted extensive attention in recent years due to their broad applicability in diverse fields, such as reflector antennas, vortex beam detection, and advanced imaging systems. Typically, these metasurfaces rely on the interplay between geometric and propagation phases. However, these two phases exhibit fundamentally different behaviors regarding wavelength dependence: geometric phase remains relatively stable across varying wavelengths, whereas propagation phase, governed by structural resonances and material dispersion, is inherently wavelength-sensitive. As a result, achieving spin-decoupled metasurfaces with independent bandwidth control remains a significant challenge. In this work, we utilize the topological phase associated with non-Hermitian exceptional points (EPs) to propose an innovative strategy for designing spin-decoupled metasurfaces. By systematically tuning the structural parameters of the unit cells, we achieve a complete and continuous 2 π modulation of the topological phase across various pre-designed spectral regions. When combined synergistically with the conventional geometric phase, we propose a spin-decoupled metasurface with independently controllable bandwidth properties. Specifically, the metasurface exhibits broadband behavior under right-handed circular polarization (RCP) illumination and controllable narrowband operation under left-handed circular polarization (LCP) illumination. This novel approach, to our knowledge, offers unprecedented flexibility in tailoring the spectral response of spin-decoupled metasurfaces. This advancement opens new possibilities for dynamically tunable metasurface devices, facilitating diverse practical applications, such as polarization modulation, adaptive filtering, optical communications, and sensing technologies.