Extreme Diffraction Control in Metagratings Leveraging Bound States in the Continuum and Exceptional Points

Abstract Coupled resonances in non‐Hermitian systems can lead to exotic optical features, such as bound states in the continuum (BICs) and exceptional points (EPs), which have been recently emerged as powerful tools to control the propagation and scattering of light. Yet, similar tools to control diffraction and engineer spatial wavefronts have remained elusive. Here, it is shown that, by operating a metagrating around BICs and EPs, it is possible to achieve an extreme degree of control over coupling to different diffraction orders. Subwavelength metallic slit arrays stacked on a metal‐insulator‐metal waveguide, enabling a careful control of the coupling between localized and guided modes are explored. By tuning the coupling strength from weak to strong, the overall spectral response can be tailored and the emergence of singular features, like BICs and EPs can be enabled. Perfect unitary diffraction efficiency with large spectrum selectivity is achieved around these singular features, with promising applications for selective wavefront shaping, filtering, and sensing.