Hyperbolic Optical Metamaterials from Shear‐Aligned Block Copolymer Cylinder Arrays

Abstract

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.

Metamaterial Eigenmodes beyond Homogenization

“…We now consider a wire metamaterial [38], consisting of aligned metal wires arranged on an isotropic 2D lattice. An example of such a metamaterial was fabricated in [39]. It consists of gold or silver cylinders with radius R = 10 nm and lattice constant a = 30 nm, arranged on a hexagonal lattice with (10) inclination, as illustrated in Fig.…”

“…In this section, we concentrate on the hyperbolic wire metamaterial fabricated in [39], to investigate the accuracy of the approximate MGA predictions Eq. ( 8) and Eq.…”

“…1. We here focus on the physically richer silver metamaterial with lattice constant a = 30 nm and cylinder radius R = 10 nm of [39]. More cases with gold wires and different radii are considered in Supplementary Section S4.…”

Hyperbolic Metamaterials via Hierarchical Block Copolymer Nanostructures

“…Although a number of hierarchical BCP patterns obtained by sequentially self-assembled BCP, [37] shear-induced alignment, [38] soft graphoepitaxy, [39] and thickness-modulated BCP films [40] are reported in the literature, the realization of plasmonic substrates and metasurfaces after metal loading or deposition [41][42][43] still remains challenging due to the relatively low aspect ratio of the metallic nanofeatures. Recently, Kilchoer et al [44] developed a methodology to create horizontally aligned metal nanowires, formed by electrodeposition inside cylinder phase PS-b-PMMA block copolymers. With this method, the authors obtained a broadband Type I hyperbolic metamaterial with inplane optical axis showing macroscopic effects such as strong linear dichroism observed by polarization-resolved optical spectroscopy.…”

“…Recently, Kilchoer et al. [ 44 ] developed a methodology to create horizontally aligned metal nanowires, formed by electrodeposition inside cylinder phase PS‐ b ‐PMMA block copolymers. With this method, the authors obtained a broadband Type I hyperbolic metamaterial with in‐plane optical axis showing macroscopic effects such as strong linear dichroism observed by polarization‐resolved optical spectroscopy.…”