Design of continuously tunable varifocal metalenses

Active metasurfaces with tunable optical properties enable reconfigurable or programmable photonic systems that can swiftly adapt to different functional needs. Realizing continuous tuning of phase-gradient metasurfaces, however, remains elusive. This is because active modulation of individual or few meta-atoms is traditionally deemed essential to continuous tuning, which nonetheless introduces significant technical difficulties associated with addressing and control of large meta-atom arrays. In this paper, we propose a novel active metasurface architecture to circumvent this challenge. The key notion is to modulate the gradients of metasurface phase profiles over large regions instead of setting the phase at individual locations, thereby enabling continuous tuning with only a minimal number of independently controlled elements. The design principle can be generically applied to different active tuning mechanisms. As an example, we modeled a varifocal metalens with focal length continuously tunable between 4 and 10 mm based on electrically switched optical phase change materials.