Surface Morphology Induces Linear Dichroism in Gyroid Optical Metamaterials
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COMMUNICATION Optical Metamaterials www.advmat.de Metasurfaces Atop Metamaterials: Surface Morphology Induces Linear Dichroism in Gyroid Optical Metamaterials James A. Dolan, Raphael Dehmel, Angela Demetriadou, Yibei Gu, Ulrich Wiesner, Timothy D. Wilkinson, Ilja Gunkel, Ortwin Hess, Jeremy J. Baumberg, Ullrich Steiner, Matthias Saba, and Bodo D. Wilts* Metamaterials are artificially engineered Optical metamaterials offer the tantalizing possibility of creating materials whose optical properties are extraordinary optical properties through the careful design and arrangement dependent on both the geometry of their of subwavelength structural units. Gyroid-structured optical metamaterials structural units and their chemical com- [1] possess a chiral, cubic, and triply periodic bulk morphology that exhibits position. The ability to design an effec- tive permittivity ε (ω) and permeability a redshifted effective plasma frequency. They also exhibit a strong linear eff µeff(ω) by careful choice of these subwave- dichroism, the origin of which is not yet understood. Here, the interaction length structural units offers the potential of light with gold gyroid optical metamaterials is studied and a strong for intriguing applications, such as super- correlation between the surface morphology and its linear dichroism is found. lenses and cloaking devices.[2,3] Associ- The termination of the gyroid surface breaks the cubic symmetry of the bulk ated material properties include those lattice and gives rise to the observed wavelength- and polarization-dependent otherwise unavailable in nature, such as a negative refractive index and extreme reflection. The results show that light couples into both localized and “hyperbolic” optical anisotropy.[4] The propagating plasmon modes associated with anisotropic surface protrusions observation of these unique properties and the gaps between such protrusions. The localized surface modes give at optical frequencies, however, requires rise to the anisotropic optical response, creating the linear dichroism. structural control on the length scale Simulated reflection spectra are highly sensitive to minute details of these of just a few nano meters. “Top down” techniques are either unable to produce surface terminations, down to the nanometer level, and can be understood bulk 3D structures (e.g., electron beam with analogy to the optical properties of a 2D anisotropic metasurface atop a lithography[5,6]), or cannot produce such 3D isotropic metamaterial. This pronounced sensitivity to the subwavelength structures on the nanoscale (e.g., direct surface morphology has significant consequences for both the design and laser writing[7–9]), with the uniformity and application of optical metamaterials. efficiency necessary for a truly macro- [10,11] scopic εeff(ω) and µeff(ω). Dr. J. A. Dolan[+], Dr. R. Dehmel[++], Prof. J. J. Baumberg Dr. J. A. Dolan[+], Dr. I. Gunkel, Prof. U. Steiner, Dr. B. D. Wilts Cavendish Laboratory, Department of Physics Adolphe Merkle Institute University of Cambridge University of Fribourg J.J. Thomson Avenue, Cambridge CB3 0HE, UK Chemin des Verdiers 4, 1700 Fribourg, Switzerland Dr. J. A. Dolan[+], Prof. T. D. Wilkinson E-mail: [email protected] Department of Engineering Dr. A. Demetriadou University of Cambridge School of Physics and Astronomy J.J. Thomson Avenue, Cambridge CB3 0FA, UK University of Birmingham Edgbaston, Birmingham B15 2TT, UK The ORCID identification number(s) for the author(s) of this article [+++] can be found under https://doi.org/10.1002/adma.201803478. Dr. Y. Gu , Prof. U. Wiesner Department of Materials Science and Engineering [+]Present address: Institute for Molecular Engineering, Argonne Cornell University National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA 214 Bard Hall, Ithaca, NY 14853-1501, USA [++]Present address: Papierfabrik Louisenthal GmbH, 83701 Gmund a.T., Prof. O. Hess, Dr. M. Saba Germany Department of Physics [+++]Present address: The Dow Chemical Company, 2301 N. Brazosport Imperial College London Blvd., Freeport, TX 77541, USA Prince Consort Road, London SW7 2AZ, UK © 2018 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and re- production in any medium, provided the original work is properly cited. DOI: 10.1002/adma.201803478 Adv. Mater. 2019, 31, 1803478 1803478 (1 of 8) © 2018 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.advmat.de Alternative “bottom up” approaches overcome these limita- oriented with the 〈110〉 direction normal to the substrate.[20,28] tions and allow efficient fabrication of otherwise inaccessible This is also consistent with previous results using the same 3D nanoscale structures. One successful method employs self- and similar triblock terpolymers under identical and near- assembling triblock terpolymers, and results in optical meta- identical fabrication conditions, which were additionally char- materials with a single gyroid morphology.[12–15] The chemical acterized by (grazing-incidence) small-angle X-ray scattering dissimilarity between the different “blocks” of the terpolymer (SAXS).[24,28] Linearly polarized optical microscopy (Figure 1b) (i.e., the constituent polymer chains) leads to microphase- reveals domains randomly oriented in-plane with lateral sizes separation into various morphologies on the nanometer length of ≈50–200 µm, where the color and intensity contrast arises scale. These morphologies are used as sacrificial templates to from the linear dichroism of the gyroid metamaterial.[12] fabricate bulk functional nanomaterials. Here, the resulting The reflectance spectra of single domains of gyroid optical structures are 3D metamaterials with macroscopic optical metamaterials were acquired under linearly polarized light properties which emerge directly from the templated network (Figure 1c,d; for a plot on a linear scale see Figure S1 in the morphology.[12,16] Supporting Information). The spectra clearly depend on the azi- The single gyroid structure is a chiral, cubic, and triply muthal angle of the incident light ϕ, showing a strong linear periodic geometry found in a variety of natural and synthetic dichroism. As ϕ increases from 0° to 180°, the reflectance of the self-assembled systems.[17–20] Its unique morphology[17,21,22] metamaterial oscillates around a minimum at 0° (λ ≳ 600 nm); imparts a range of optical properties to gyroid-structured at shorter wavelengths the minimum reflectance is found at optical metamaterials that are dependent on the unit cell size, slightly larger ϕ (Figure 1c), hence the mismatch between the volume fill fraction, surrounding refractive index, and degree 45° and 135° spectra in Figure 1d. The azimuthal angles are of order.[12–15,20,23] Intriguingly, gyroid optical metamaterials defined relative to the in-plane [110] direction of the gyroid, give rise to a linear and circular dichroism (i.e., a variation in the orientation of maximum reflectance at long wavelengths. the optical properties as a function of azimuthal orientation of Figure S2 shows the equivalent transmittance data, which is linearly polarized light and handedness of circularly polarized complementary to the reflectance data of Figure 1c,d and simi- light, respectively), both of which are sensitive to the structure larly exhibits a strong linear dichroism. The same data is shown of the metamaterial.[12,13] However, this optical anisotropy is on a linear scale in Figure S3 in the Supporting Information. only readily observable (e.g., by standard optical microscopy Unlike the idealized representation of the gyroid optical techniques) when the metamaterial possesses long-range order, metamaterial in Figure 1e, the fabricated metamaterial is not whereby domains of the self-assembled template span at least uniformly terminated and exhibits a significant variety of ape- tens of micrometers. If the metamaterial possesses only short- riodic surface morphologies (Figure 1a). This arises from the range order, its anisotropic optical properties are masked and it nonuniformity of the electrodeposition growth front during responds similarly to disordered nanoporous gold.[15] fabrication, as the electrodeposited gold grows spherically out- Here, we fabricated and characterized exceptionally large ward from randomly distributed nucleation sites at the sub- domains of a gold gyroid optical metamaterial and studied their strate.[29] We approximate the aperiodic surface morphologies reflectance under various polarizations of incident light. These found in the fabricated metamaterial with those periodic sur- domains were manufactured from a self-assembled triblock face morphologies exhibited by uniformly terminated gyroid terpolymer template. Since the terpolymer partially crystal- structures. Possible uniform terminations for a 〈110〉-oriented lizes during solvent vapor annealing, giving rise to a contrast gyroid are shown in Figure 1f,g (more terminations are shown in polarized light microscopy, it is possible to optimize the in Figure S4 in the Supporting Information). The termina- annealing process to produce polymeric templates with gyroid tions are characterized by the parameter τ, the distance of the domains which span hundreds of micrometers.[24] terminating plane from the crystallographic origin in units of Previous work has shown that gyroid optical metamaterials structural periodicity in the [110] direction, i.e.,