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Investigating spatial-frequency power flow and evanescent-wave amplification in metamaterial superlenses Aghanejad, Iman


It has been shown that a homogeneous and isotropic material with simultaneously negative values of permittivity and permeability exhibits counter-intuitive responses such as refraction to the same side of normal (negative refraction) and evanescent wave amplification. Consequently, a flat slab of this material can focus an object to the image plane and overcome the diffraction limit associated with conventional dielectric lenses. Such double-negative materials do not exist in nature, however, artificial structures comprised of arrays of resonant inclusions have been designed to mimic their responses and realize a flat superlens with sub-diffractive resolution. This thesis rigorously investigates the behaviour of these artificial heterogeneous structures known as negative-index metamaterials. In the first part of this work, we present spatial frequency maps of power flow in order to provide deeper insights into the flow of propagating waves in negative-index metamaterials. We show through k-space diagrams that the distribution of power can vary considerably across these structures. These observations allow us to categorize different metamaterials and identify the mechanism behind negative refraction at a given interface. We also discuss how k-space maps of power flow can be used to explain the high or low transmittance of the power into different superlenses made from negative-index metamaterials. In the second part of this work, we study evanescent wave amplification in negative-index metamaterial lenses and propose design strategies to improve their imaging resolution and robustness. We show how resonant modes arising in metamaterial superlenses cause imaging artifacts and reduce imaging fidelity. We choose a well-studied periodic structure consisting of an array of magnetodielectric cylinders for the lens medium under test. We demonstrate that the presence of these artifacts can lead to erroneous interpretation of the standard two-source resolution test for lenses. We show that artifacts can be mitigated by introducing point defects into the array, which move the resonant modes to higher spatial frequencies and in the case of finite lenses, suppress their amplitudes through radiation losses. This strategy enables more robust and reliable subwavelength imaging performance, improves the spatial resolution of the metamaterial lens, and reduces the deleterious effects of material losses.

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