Polymeric Optical Metasurfaces by Two-Photon Lithography: Practical Designs for Beam Steering

Metasurfaces have shown immense potential for wave control in diverse frequency regimes ranging from microwaves to the visible. Optical metasurfaces are particularly challenging since materials with low loss and the ability to tightly confine light (high-index dielectrics, plasmonic metals) become limited. In addition, from a practical point of view they require nanofabrication techniques with high resolution and precision. Here, we consider polymer-based optical metasurfaces to be fabricated with two-photon lithography, which is a fast, scalable, and cost-effective nanofabrication technique. We focus on a beam steering scenario in transmission. The principle of operation is based on phase accumulation within short vertical waveguide segments. We start from idealized, perfectly-cylindrical meta-atom shapes and proceed to conical shapes, which exhibit increased mechanical stability, and smooth half-ellipsoid-like structures that are compatible with the voxels of the laser writing process. We show that the adopted realistic meta-atom shapes lead to only a small deterioration of the steering performance and by employing numerical optimization we are able to recover the performance obtained with the idealized ones. Our work aspires to enable high-performance, practical optical metasurfaces taking fabrication limitations and particularities thoroughly into account.