Using optical absorption to reduce cross-talk in spatially multiplexed waveguiding metasurfaces

Dual-wavelength metasurfaces often employ spatial multiplexing design concepts where two interleaved lattices of meta-atoms, each designed for a specific wavelength, occupy the same layer. However, this arrangement incurs efficiency losses as each wavelength inevitably "sees" unintended meta-atoms designed for the other wavelength, causing spurious interference (crosstalk) effects, which are especially affecting the shorter wavelength. In this paper, we numerically demonstrate that the crosstalk can be reduced by introducing some optical absorption at the shorter wavelength, unveiling the near-field mechanisms at play. Then, through designing and simulating a dual-wavelength beam-steering metasurface, we demonstrate the benefits of certain level of absorption in terms of wavefront purity (characterized by the diffraction efficiency into a desired blazed order) and beam-steering efficiency. This method also presents some advantages in its simplicity as it allows to use the traditional phase-mapping approach based on the simulations of two independent meta-atom libraries, compared to more complex methods that require accounting for the crosstalk between meta-atoms.