Robust design of topology-optimized metasurfaces

Topology-optimized metasurfaces are thin film optical devices that have received much interest because they support ultra-high diffraction efficiencies. An important design consideration is ensuring that devices are insensitive to imperfections arising from realistic fabrication processing. We show that topology-optimized metasurfaces can be made robust by incorporating the performance of geometrically eroded and dilated devices directly into the iterative optimization algorithm. We additionally apply topology optimization to refine conventional designs and make them more robust. Unexpectedly, we find that devices robust to systematic erosion and dilation variations are also robust to random periodic perturbations. An analysis of the optical modes of robust devices indicates that robustness is enforced via highly complex and non-intuitive interactions between these modes and cannot be enforced using simple design rules. These concepts can more generally address other fabrication imperfections, such as thickness and refractive index variation, and can extend to other diffractive and nanophotonic platforms.