Optimal High Efficiency 3D Plasmonic Metasurface Elements Revealed by Lazy Ants

Recent transmissive optical metamaterials that leverage a generalized form of Snell's law to induce an anomalous refraction of light have garnered considerable interest in both optical and materials communities. However, most of these designs have primarily centered around parametric studies of planar canonical structures for their low profile and relative ease of manufacturing. In many of these cases, all-dielectric designs are preferred over metallodielectrics due to their low loss characteristics. Moreover, considering modern advances in nanofabrication techniques, these canonical structures represent only a small portion of the design space that is explorable. In this work, we exploit a generalized Multi-Objective Lazy Ant Colony Optimization (MOLACO) algorithm and a modified Pareto locus search mechanism to optimize arbitrary three-dimensional metamaterial unit cells in the optical regime based on the Membrane Projection Lithography technique. Our exploration has revealed unintuitive metallodielectric structures for phase-gradient metasurface applications in the midwave infrared (MWIR) regime that achieve transmission magnitudes comparable to the highest-performance all-dielectric designs found in the literature. As a proof-of-concept, a beam-steering metasurface is synthesized using these unintuitive unit cell geometries and is shown to achieve over 84% diffraction efficiency, which is among the highest performing metallodielectric metasurfaces in the MWIR reported to date.