Manufacturable blazed metasurface gratings designed by 3D topology optimization model

We present the generalization of our FEM-based topology optimization framework to 3D blazed metasurfaces operating in reflection over the visible and near-infrared range [400-1,500]nm. The design region is described through a density-based SIMP interpolation and optimized using the adjoint method, enabling the treatment of several tens of thousands degrees of freedom. A first approach directly applies topology optimization to the 3D Finite Element mesh (mesh-based), yielding a freeform structure that achieves an average diffraction efficiency of 62% in order -1 over two octaves under the targeted incidence. However, such patterns remain difficult to manufacture. We therefore introduce a pillar-based parameterization, embedding fabrication constraints within the optimization loop. The resulting binary metasurface, compatible with e-beam lithography and Reactive Ion Etching techniques, achieves an average efficiency of 57% over the same spectral band in s-polarization, with low polarization dependence. This work demonstrates that large-scale 3D topology optimization can bridge the gap between broadband optical performance and realistic nanofabrication constraints for blazed metasurfaces.