Mirror-symmetry-protected metasurfaces enable ultra-broadband tailored transmission with invariant reflection

In this work, we demonstrate a unique class of ultra-broadband mirror-symmetry-protected metasurfaces capable of achieving independent control over transmission and unaffected specular reflection. By leveraging mirror symmetry between two metaatoms, they are capable of generating out-of-phase transmission and in-phase reflection with different polarizations in an ultra-broad spectrum. This enables the flexible control of the transmission via designing the arrangement of the meta-atoms while, at the same time, retaining the same specular reflection regardless of the arrangement. Through microwave experiments, we have verified the functionalities of the mirror-symmetry-protected metasurfaces in a broad spectrum, such as beam splitting and diffusing in transmission, with undistorted specular reflection. This function, strictly protected by the mirror symmetry between the meta-atoms, is independent of detailed structure, material loss, and frequency. Extension to higher frequencies, like the infrared regime, has also been verified by simulations. Our work paves a road towards ultra-broadband metasurfaces, offering unprecedented broadband asymmetric control of electromagnetic waves.