Manufacturable blazed metasurface gratings designed by a 3D topology optimization model

We present an open-source finite element method-based topology optimization framework for 3D blazed metasurfaces operating in reflection over the [400-1500] nm visible and near-infrared range. The design region is described through a density-based interpolation and optimized using the adjoint method, enabling the treatment of several tens of thousands of 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 the -1st order over two octaves under the targeted incidence. However, the freeform patterns obtained 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 p-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.