Large-angle deflection lens using supercell metasurfaces

Abstract

A metasurface is an ultra-thin flat optical component, containing nanostructures that scatter light. Due to the planar structure it can be monolithically integrated with a vertical cavity surface-emitting laser (VCSEL), which removes alignment issues. With metasurfaces, full control of the wavefront is possible and hence a light beam can be shaped in almost anyway. For instance focusing metalenses, lenses that deflect light or vortex generators can be made. In many applications, for example in bio-photonics, it is desirable to have a lens with high numerical aperture, which entails deflection of light with large angels. The conventional way of designing a metasurface is to use phase mapping, however that approach has a fundamental limitation when it comes to creating steep angles. Therefor another type of metasurface based on diffracting elements have been implemented in this project. More specifically, these metasurfaces are made in GaAs to enable integration with GaAs VCSELs and the diffracting elements (supercells) are structured by nano-holes to get the desired light shape and circumvent the issue of aspect ratio dependent etching (ARDE). The metasurfaces were first simulated in COMSOL, then fabricated using electronbeam lithography and dry etching, and finally characterized. Metalenses utilizing supercells that deflect light up to 85◦ have been achieved. The lenses deflect with the desired angle and show a focusing effect. However the focus of the lenses is not as tight as desired and calls for further investigation.