Abstract
While metal is the most common conducting constituent element in the fabrication of metamaterials, graphene provides another useful building block, that is, a truly two-dimensional conducting sheet whose conductivity can be controlled by doping. Here we report the experimental realization of a multilayer structure of alternating graphene and Al2O3 layers, a structure similar to the metal-dielectric multilayers commonly used in creating visible wavelength hyperbolic metamaterials. Chemical vapour deposited graphene rather than exfoliated or epitaxial graphene is used, because layer transfer methods are easily applied in fabrication. We employ a method of doping to increase the layer conductivity, and our analysis shows that the doped chemical vapour deposited graphene has good optical properties in the mid-infrared range. We therefore design the metamaterial for mid-infrared operation; our characterization with an infrared ellipsometer demonstrates that the metamaterial experiences an optical topological transition from elliptic to hyperbolic dispersion at a wavelength of 4.5 μm.
Highlights
While metal is the most common conducting constituent element in the fabrication of metamaterials, graphene provides another useful building block, that is, a truly two-dimensional conducting sheet whose conductivity can be controlled by doping
As the thinnest material imaginable, graphene makes an ideal building block for multilayer structures, as it enables the minimum possible period and the highest possible cutoff for the high k-modes[14,25], which has been limited in metal and semiconductor-based Hyperbolic metamaterials (HMMs) by the non-negligible thickness of those materials
Similar graphene-dielectric multilayer structures have been proposed and analysed theoretically by different groups and shown to function as a HMM operating at terahertz (THz) and mid-infrared frequencies[13,14,15,16,17,18,19,20,21]
Summary
While metal is the most common conducting constituent element in the fabrication of metamaterials, graphene provides another useful building block, that is, a truly two-dimensional conducting sheet whose conductivity can be controlled by doping. Hyperbolic metamaterials (HMMs) are artificially structured materials designed to attain an extremely anisotropic optical response, in which the permittivities associated with different polarization directions exhibit opposite signs[1,2,3] Such anisotropic behaviour results in an isofrequency surface in the shape of a hyperboloid, which supports propagating high k-modes and exhibits an enhanced photonic density of states. The propagating high k-modes supported by HMM are exploited to achieve sub-diffraction-limited images using a hyperlens[7] Some natural materials such as bismuth, graphite and hexagonal boron nitride exhibit hyperbolic dispersion in specific spectral ranges[8,9,10], while artificial HMMs are most commonly realized with two categories of structures such as metal-dielectric multilayers[4,7] and metallic nanorod arrays[11]. We characterize the effective permittivities of the fabricated metamaterial with ellipsometry to demonstrate the hyperbolic dispersion in the mid-infrared range
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