High-efficiency rectangular transverse air-gap metalens for visible light based on a dual-coupled PB phase

Pancharatnam-Berry (PB) phase modulation leverages the precise linear correspondence between output phase and nanopillar rotation angle, enabling wide application in modulating focal spherical waves. However, the intrinsic material absorption of waveguide structure inevitably brings energy loss within the visible light spectral range, severely limiting focusing efficiency over broad bandwidths under general models of nanopillars. Here, we introduce a dual-coupled metalens composed of an air slot-etched rectangular TiO₂ nanopillar, and the extremely narrow air gap along its long axis of nanopillar can be regarded as a high-aspect-ratio rectangular waveguide for strictly confining the incident light. In the visible spectrum of 440∼670 nm, the optical performance of the dual-coupled metalens is tested at the numerical apertures (NAs) of 0.35, 0.44, 0.6, and 0.82. Simulation results indicate that the structure not only meets the basic requirements of PB phase control, but also exhibits ultra-low loss and ultrahigh focusing efficiency. The novel, to the best of our knowledge, structure proposed in this work provides new design inspiration for general phase modulations and efficient micro-optical devices including wide-spectral metalens.