Highly efficient and completely controllable optical activity for cross-polarization conversion of arbitrary linear...

Although large optical activity (OA) with negligible circular dichroism (CD) has been widely explored in metasurface platforms, concurrent realization of the theoretical limit of OA and near-unity efficiency remains a long-standing challenge. Here, we propose a bilayer dielectric metasurface that overcomes the trade-off between OA and CD and achieves ±90^∘ OA with efficiency above 0.93 and negligible CD. This design offers continuous control of OA from -90^∘ to 90^∘ by changing the twist angle of the nano-structures in different layers. We further demonstrate polarization-independent cross-polarization conversion (CPC) with high efficiency for arbitrary polarization incidence. Our design offers a versatile route for polarization manipulation with promising applications in vectorial optical field generation and beyond.