Tunable strong circular dichroism at large-angle oblique incidence via plasmonic bound states in the continuum

Plasmonic metasurfaces with chiral responses have attracted significant research interest because of their capacity to manipulate light spin. However, existing research indicates that strong circular dichroism (CD) responses are typically limited to normal incidence or small-angle oblique incidence (≤60^∘), hindering applications under highly oblique illumination. To overcome this, we propose a plasmonic metasurface featuring cross-shaped metal-insulator-metal unit cells for creating strong extrinsic chiral quasi-bound states in the continuum. Simulations demonstrate near-perfect absorption around 75^∘ oblique incidence, achieving a high quality factor (∼76) and strong reflected CD (∼0.84). Crucially, the chiral response is reversible simply by adjusting the azimuth angle of incidence, facilitating dynamic CD modulation in a wide range from -0.82 to 0.84. This work presents a scheme for constructing and manipulating intense CD responses under large-angle oblique incidence, advancing the design of high-performance systems for on-chip spin selection and chiral biosensing.