Over the past two decades, metamaterials have led to an increasing number of biosensing and nanophotonic applications due to the possibility of a careful control of light propagating through subwavelength features. Chiral nanostructures (characterized by the absence of any mirror symmetry), in particular, give rise to unique chiro-optical properties such as circular dichroism and optical activity. Here, we present a gyroid optical metamaterial with a periodicity of 65 nm exhibiting a strong circular dichroism at visible wavelengths. Our bottom-up approach, based on metallic replication of the gyroid morphology in triblock terpolymer films, generates a large area of periodic optical metamaterials. We observe a strong circular dichroism in gold and silver gyroid metamaterials at visible wavelengths. We show that the circular dichroism is inherently linked to the handedness of the gyroid nanostructure, and demonstrate its tuneability. The optical effects are discussed and compared to other existing systems, showing the potential of bottom-up approaches for large-scale circular filters and chiral sensing.

The bespoke optical properties of a metamaterial are controlled by a careful design of the material's subwavelength nanostructure, which typically consists of noble metals. [1] Despite the fixed plasma frequency of the bulk metal, [2] the plasmonic response of metamaterials can be tuned through their nanostructure, [3] which results in a drastic change in color. A well-studied nanostructured metal is nanoporous gold, a 3D network of gold in air with pore sizes on the order of tens of nanometers. These materials are prepared by selective etching of gold-containing alloys, e.g., by dealloying bimetallic Au-Ag films, [4,5] or by electrochemical deposition into a porous template. [6] The surface plasmon polariton properties of nanoporous gold are well understood, and the material has shown excellent use in surface-enhanced Raman scattering substrates. [7-10] The optical properties of nanoporous gold are predominantly determined by the metal fill fraction, with little control over the optical response apart from a red-or blue-shift of the polarization-independent spectra, which can be qualitatively understood on the basis of simple effective medium approximations. [6,11] Controlling the arrangement and orientation of plasmonic nanostructures enables a versatile tuning of the plasmonic response. This can lead to emerging nontrivial optical properties in the visible wavelength range, such as linear and circular dichroism, and negative refraction. In this context, the use of block copolymer (BCP) templates is particularly suitable as they self-assemble into periodic nanostructures, including spheres, cylinders, lamellae, and the 3D gyroid morphology. [12] Such nanostructured BCP films can be replicated into metals by selectively removing one of the BCP phases followed by metal deposition into the voided BCP film. [13-16] While unprocessed gyroid BCP films only lead to optical metamaterials with short-range order and isotropic optical properties, [6] long-range-ordered gyroid metamaterials have been shown to give access to new optical properties such as linear [14,15] and circular dichroism. [16,17] Moving from the gyroid morphology to intrinsically anisotropic structures, such as lamellar [18] and cylindrical morphologies, is expected to create stronger dichroism if they are aligned in a specific direction.