Advances in wavefront shaping with bilayer metasurfaces

Cascaded meta-optics have enabled complex wavefront shaping beyond their single layer counterpart owing to the additional design variables afforded by each plane. For instance, complex amplitude modulation, generalized polarization transformations, and wide field of view are key attributes that fundamentally require multi-plane wavefront matching. Nevertheless, existing embodiments of bilayer metasurfaces have relied on configurations which suffer from Fresnel reflections, low mode confinement, or undesired resonances which inevitably compromise the intended response. We review recent progress in this area and introduce the first bilayer metasurface made of free-standing titanium dioxide meta-atoms in the visible regime. We demonstrate its use in wavefront shaping of linearly polarized light using pure geometric phase with diffraction efficiency reaching 80% — expanding previous literature on Berry phase metasurfaces which often assumed circularly polarized illumination. We discuss the potential of this configuration in structured light generation, integration on chip, imaging and polarization control.