Surface Structure Dependent Circular Dichroism in Single and Double Gyroid Metamaterials

Abstract

Optical properties of interest in gyroids include circular dichroism (CD), Kuhn's dissymmetry factor, [10] and polarizationselective reflection and transmission.3D, chiral metamaterials are often fabricated in complex and intensive processes such as two-photon direct laser writing or multilayer electron-beam lithography. [11,12] In contrast to these top down fabrication techniques, block copolymer self-assembly and templating holds promise for large scale gyroid fabrication, producing macroscale structures with subwavelength and tunable periodicities, typically on the order of 10 s of nanometers. [13,14] At specific composition windows, triblock copolymers self-assemble into an alternating gyroid structure, consisting of two single gyroid networks surrounded by a matrix. Plasmonic, single gyroid metamaterials can then be created through the selective removal of one of the network structures, followed by infilling with a plasmonic metal as depicted in Figure 1a. [15][16][17] The resonant behavior of these plasmonic structures can therefore be tuned by typical parameters for plasmonic structures, including the unit cell dimensions, infilling fraction, and refractive index contrast with the surrounding medium.Previous theoretical investigation into the resonant properties of single gyroid metamaterials found that they support chiral modes along the 〈100〉 and 〈111〉 families of directions. [18] Unfortunately, these modes were predicted to only support low levels of CD due to the connectivity of the screw axes within the single gyroid structure. More recently, however, there is experimental evidence that silver infilled single gyroid metamaterials exhibit strong circular dichroism in the visible and near-infrared (NIR). [19] This, combined with evidence that the linear dichroism observed in single gyroid metamaterials can be attributed to resonances on the gyroid's top surface, [20] suggests that surface resonances may also significantly contribute to a gyroid metamaterial's overall chiral optical response, in addition to the resonant interactions with the bulk that have been previously investigated. Many emerging applications of gyroid metamaterials will utilize thin films, which are naturally terminated with specific surface structures. This presents an opportunity for tuning the properties of gyroid metamaterials: as we show in this manuscript using finite-difference timedomain (FDTD) simulations, the surface orientation, thickness of the metamaterial, excitations on the front and rear surfaces, and surface defects all emerge as strategies to tune the optical Gyroid optical metamaterials consist of triply periodic chiral networks that are attractive photonic structures due to the combination of intriguing optical properties and spontaneous self-assembly-based fabrication routes using materials such as block copolymers. A previous experimental investigation found that gyroid metamaterials support strong circular dichroism, beyond what simulations only considering bulk interactions predict. In this work, simulations a...

Core–Shell Gyroid in ABC Bottlebrush Block Terpolymers

“…Network materials with interpenetrating nanodomains have attracted significant interest because of their unique properties, , affording a versatile tool for numerous applications, from ion transport media, nanoporous membranes, therapeutic delivery vehicles, templated mesoporous inorganic materials, − to large-scale photonics and metamaterials. , The desired pore diameter may range from several nanometers to above 100 nm. Compared with time-consuming top-down strategies such as lithography, which is limited in material size, the self-assembly of block copolymers may be considered a facile bottom-up path for network preparation.…”

Boundary Frustration in Double-Gyroid Thin Films

“…17−26 For instance, DG has been proposed as a template for optical metamaterials whose circular dichroism is predicted to vary with the termination plane at the film's exposed surface. 18 DG has also been considered for filtration membranes 8 and as a template for electronic materials 16 owing to its uniform domain size and three-dimensionally cocontinuous morphology. However, the effects of thin-film confinement on the DG are poorly understood.…”

“…Many promising applications of self-assembled block polymers, including their use as nanolithographic templates, − nanoporous membranes, − and advanced optical and electronic material templates, − require thin-film confinement. The double-gyroid (DG) phase in thin films has received particular interest recently because of its potential utility in these applications. − For instance, DG has been proposed as a template for optical metamaterials whose circular dichroism is predicted to vary with the termination plane at the film’s exposed surface . DG has also been considered for filtration membranes and as a template for electronic materials owing to its uniform domain size and three-dimensionally cocontinuous morphology.…”