Wave propagation in twisted metamaterials

Twisted metamaterials, or arrays of identical planar metasurfaces stacked with a sequential rotation, have been recently introduced to realize broadband circular dichroism. Here we develop a generalized Floquet analysis to obtain the exact modal solutions for eigenwaves supported by these structures. The dispersion relation and wave propagation in twisted metamaterials are discussed in detail. Our analysis shows how the modal dispersion in these metamaterials becomes inherently different from the one of conventional periodic structures and how the eigenmodes support specific circular polarization properties based on a lattice effect, even when achiral inclusions are considered. These wave properties are ideal to realize optical devices that manipulate the polarization state of light over broad bandwidths. By analyzing the physical nature of these modes, including complex modes, we also extend the application of twisted metamaterials to realize passband and stop-band nanophotonic structures with strong polarization manipulation properties.