Linear and Circular Dichroism in Gyroid Optical Metamaterials

Optical metamaterials offer the possibility of accessing extraordinary optical properties through the specific arrangement of their sub-wavelength structural units. A particularly interesting morphology is the gyroid due to its chiral, cubic, and triply-periodic geometry. Gyroid optical metamaterials with unit cell sizes of tens of nanometers can be conveniently fabricated via electrodeposition of gold into voided, self-assembled polymer templates. This bottom-up self-assembly approach allows the fabrication of gold gyroids with different orientations and unit cell sizes. We show here that gyroid metamaterials exhibit a depressed effective plasma frequency, as well as a circular and linear dichroism, a surprising result given the underlying cubic symmetry. By combining electron microscopy and optical goniometry, we show that the reflection of light from gold gyroids is strongly polarization-dependent, and is sensitive to the orientation of the gyroid and the terminations of its top surface. Our results highlight the influence of orientation and surface terminations on the optical properties of metamaterials, and have significant consequences for both their design and applications.