Metasurfaces Atop Metamaterials: Surface Morphology Induces Linear Dichroism in Gyroid Optical Metamaterials.

Abstract

Optical metamaterials offer the tantalizing possibility of creating extraordinary optical properties through the careful design and arrangement of sub-wavelength structural units. Gyroid-structured optical metamaterials possess a chiral, cubic, and triply-periodic bulk morphology, and exhibit a red-shifted effective plasma frequency. They also exhibit a strong linear dichroism (i.e. a variation in optical properties—reflectance and transmittance—as a function of azimuthal orientation of linearly polarized incident light), the origin of which is not yet understood. Here, we study the reflection and transmission of light from and through gold gyroid optical metamaterials and find a strong correlation between the surface morphology of the gyroid metamaterial and its linear dichroism. The termination of the gyroid surface breaks the cubic symmetry of the bulk lattice and gives rise to the observed wavelengthand polarization-dependent reflection. Our results show that incident light couples into both localized and propagating plasmon modes associated with anisotropic surface protrusions and the gaps between such protrusions. The localized surface modes give rise to the anisotropic optical response, i.e. the linear dichroism. Simulated reflection spectra are highly sensitive to minute details of these surface terminations, down to the nanometer level, and may be understood with analogy to the optical properties of a two-dimensional anisotropic metasurface atop a three-dimensional isotropic metamaterial. This pronounced sensitivity of the metamaterial response to the sub-wavelength surface morphology has significant consequences for both the design and application of optical metamaterials.