Antireflection by design in bilayer metasurfaces
Antireflection coatings are ubiquitous in optical systems, where they maximize transmission and suppress undesirable reflections by impedance-matching uniform interfaces. Extending this principle to metasurfaces, however, is fundamentally more challenging because wavefront control requires a library of geometrically distinct meta-atoms, each locally imposing a prescribed phase that is tethered to its transmittance. Here, we show that vertical integration resolves this constraint by allowing bilayer meta-atoms to operate simultaneously as a phase shifter and an impedance-matching stack. Using an effective thin-film model, we derive a design rule that links the effective indices of two independently patterned layers and identifies antireflective bilayer libraries with full $0$-$2π$ transmission-phase coverage. We realize this concept in a free-standing TiO$_2$/TiO$_2$ metalens operating at 1310 nm, which suppresses reflectance below that of bare glass while preserving diffraction-limited focusing. These results establish bilayer metasurfaces as a framework for co-engineering optical impedance and wavefront response at the meta-atom level.