Silver nanowire hyperlenses

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Due to the hyperbolic dispersion relation for electromagnetic waves, hyperbolic metamaterials like silver nanowire arrays allow the transformation of evanescent waves into propagating waves. This is of special importance for subwavelength imaging and focussing of light. Although several hyperlens geometries have been theoretically studied, only one early design based on bent metal/dielectric multilayers has experimentally been realised up to now. Particularly, the use of metal nanowire arrays for hyperlenses was not experimentally demonstrated yet although theory suggests a multitude of advantages for these nanowire lenses, e.g. large operating bandwidth, low loss, easy fabrication, and plane surfaces. We therefore intend to theoretically investigate and experimentally demonstrate the subwavelength focusing and imaging ability of such nanowire hyperlenses using two different designs of hyperlenses based on silver nanowire arrays. By electrochemical deposition of silver into nanoporous alumina templates and structuring of the top surface, the silver nanowire hyperlenses will exhibit a plane bottom surface for imaging and focus creation. The lenses allow the focusing of incident plane waves and, by reversing the light path, the formation of far-field images from near-field subwavelength-sized objects. Absorption is expected to be lower than for multilayer hyperlenses, which mainly operate close to the surface plasmon resonance. Due to the broadband hyperbolic dispersion relation of the nanowire arrays, lenses for different wavelengths of choice can be realised, which are not bound to a specific limiting frequency, given by a particular material combination. Moreover, the fabrication of the nanowire hyperlenses promises to be robust and less demanding than the deposition of ultra-low-roughness metal/dielectric multilayers. Applying white light SNOM and multi-photon lithography, the subwavelength focus formation will be studied. The influence of spatial dispersion on focus size and imaging will be investigated by adjusting period and diameter of the silver nanowires. Furthermore, the impact of absorption, especially for large wave vectors, as another limiting parameter will be evaluated. Besides the direct experimental results on nanofocusing and subwavelength imaging the work will be important for the newly sparked interest in enhanced thermal radiation due to the offered large photonic density of states in the hyperbolic metamaterial. This might further open ways to apply silver nanowire metamaterials with specifically shaped top surfaces for enhanced radiative cooling.