Full-Color Non-Dispersive Visible Metalens by Sequentially Stacked Bilayers of HAR SiN Mold and TiO₂ Pillars

We present a new visible metalens, which works as an ideal non-dispersive diffractive lens by providing equal transmission phase profiles over the full-color broadband of light (wavelengths from 400 to 700nm). To implement such engineered dispersion, we sequentially stacked bilayers of meta-structures, i.e., high-index TiO₂ pillars in SiO₂ mold and low-index air holes in SiN mold of high-aspect-ratio (HAR, ~30). When the metalens phase profile is slowly-varying across the device (i.e., low numerical aperture), the dispersion at each location of the metalens through which light propagates can be designed by optimizing the bilayers’ structural and material dispersions. As a result, the metalens has polarization independence and high focusing efficiencies (~83%, experiment, improving), which overcomes the limitations of the previous metalens approaches. The high-aspect ratio structuring of ~2um thick SiN layer is a key new material opportunity for visible meta-optics. This is an important advancement of the diffractive lens for many emerging applications; Fresnel lenses that have been used for DSLR cameras with refractive lenses are bulky, expensive and suffer various diffractive artifacts, whereas this metalens is thin, mass-producible and suppresses the artifacts significantly. The non-dispersive nature of the metalens allows it to be a basic building block for more complicated dispersion engineered optical systems. Furthermore, the sequentially stacked bilayers will enable new functional metasurfaces at visible frequencies by harnessing its structural and material compositions.