Nanowire metamaterials with extreme optical anisotropy

Abstract

We study perspectives of nanowire metamaterials for negative-refraction waveguides, highperformance polarizers, and polarization-sensitive biosensors. We demonstrate that the behavior of these composites is strongly influenced by the concentration, distribution, and geometry of the nanowires, derive an analytical description of electromagnetism in anisotropic nanowire-based metamaterials, and explore the limitations of our approach via three-dimensional numerical simulations. Finally, we illustrate the developed approach on the examples of nanowire-based high energy-density waveguides and non-magnetic negative index imaging systems with far-field resolution of one-sixth of vacuum wavelength.The anisotropy of effective dielectric permittivity is widely used in optical, infrared (IR), THz and GHz sensing, spectroscopy, and microscopy 1,2,3,4 . Strongly anisotropic optical materials can be utilized in nonmagnetic, non-resonant optical media with negative index of refraction, and have the potential to perform subdiffraction imaging and to compress the radiation to subwavelength areas 2,5,6,7 . The performance of these polarization-sensitive applications can be related to the relative difference of the dielectric constant along the different directions. In the majority of natural anisotropic crystals this parameter is below 30% 8 . While it may be sufficient for some applications, a number of exciting phenomena ranging from high-performance polarization control 4 to subwavelength light guiding 2,5,6 to planar imaging 7 require different components of a permittivity tensor to be of different signs.In this Letter we study the perspectives of using nanowire composites as meta-materials with extreme optical anisotropy. We demonstrate that even 10% stretching/compression of the nanowire structures may dramatically affect the electromagnetic properties of these systems and change the sign of components of the permittivity tensor. We present an analytical description of wave propagation in anisotropic nanowire composites -Generalized Maxwell-Garnett approach (GMG), and verify our technique via three-dimensional (3D) numerical simulations. Finally, we illustrate our approach on the examples of several nanowire-based systems for light compression below the diffraction limit and negative refractionimaging with far-field resolution of λ 0 /6 (with λ 0 being free-space wavelength).The use of metallic wire mesh as anisotropic lowfrequency plasma has been proposed in 9 and experimentally realized for normal light incidence in 4,10 . However, the applicability of these nanowire-based materials for any non-trivial geometry involving oblique light incidence or wave-guiding is still considered to be questionable due to strong nonlocal interactions 11 , that may potentially result in positive components of the permittivity tensor. Furthermore, the majority of existing effective-medium theories (EMTs) 11,12,13,14 are limited to the optical response of nanowires that are isotropically distributed in the host material. The predicted respo...

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