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 subwavelength structural units. Gyroid-structured optical metamaterials possess a chiral, cubic, and triply periodic bulk morphology that exhibits a redshifted effective plasma frequency. They also exhibit a strong linear dichroism, the origin of which is not yet understood. Here, the interaction of light with gold gyroid optical metamaterials is studied and a strong correlation between the surface morphology and its linear dichroism is found. The termination of the gyroid surface breaks the cubic symmetry of the bulk lattice and gives rise to the observed wavelength-and polarization-dependent reflection. The results show that 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, creating the linear dichroism. Simulated reflection spectra are highly sensitive to minute details of these surface terminations, down to the nanometer level, and can be understood with analogy to the optical properties of a 2D anisotropic metasurface atop a 3D isotropic metamaterial. This pronounced sensitivity to the subwavelength surface morphology has significant consequences for both the design and application of optical metamaterials. Optical MetamaterialsThe ORCID identification number(s) for the author(s) of this article can be found under https://doi.

Strong Circular Dichroism in Single Gyroid Optical Metamaterials

“…We first focus on silver gyroid metamaterials that have a stronger optical activity compared to gold. Top-view SEM images show that the fabricated free-standing silver single gyroid has a periodic unit cell of circa 65 nm and a 110 out-of-plane orientation (Figure 1b), in agreement with previous work using the same triblock terpolymer [40,41]. The periodicity the gyroid morphology was confirmed throughout the entire film thickness by focused ion beam SEM (Figure S3).…”

“…Here, we present an optical metamaterial fabricated by replication of a self-assembled gyroid in block copolymer (BCP) films [39,40] exhibiting strong CD at visible frequencies. Triblock terpolymer films with an alternating gyroid morphology exhibit an intricate nanostructure with a cubic unit cell of approximately 65 nm [41]. Large single gyroid gold domains, inclinated along their cubic 110 direction, display a linear polarizationdependent optical response [39,41,42], which has only very recently been explained [41].…”

“…Triblock terpolymer films with an alternating gyroid morphology exhibit an intricate nanostructure with a cubic unit cell of approximately 65 nm [41]. Large single gyroid gold domains, inclinated along their cubic 110 direction, display a linear polarizationdependent optical response [39,41,42], which has only very recently been explained [41]. While the same sample displayed no discernible CD when illuminated at normal incidence, a weak gyrotropic effect was observed when the sample was tilted to align its 111 axis with the illumination direction [39].…”

Hyperbolic Optical Metamaterials from Shear‐Aligned Block Copolymer Cylinder Arrays

“…[ 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.…”

Hyperbolic Metamaterials via Hierarchical Block Copolymer Nanostructures

“…Among the various nanopatterning methods, self-assembly (SA) of block copolymers (BCPs) has emerged as a cost-effective and scalable bottom-up approach, with a high throughput and able to provide highly dense and periodic patterns at the nanometric scale [16][17][18] . The extensive research over the last years has proved self-assembly of BCPs as a valid alternative to conventional nanopatterning techniques for many different applications including optics 19,20 , biosensing 21 , lithography 22,23 , and electronics 24,25 .…”