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Thursday, October 1st 2009

1:25 pm:

Metamaterials are artificially engineered structures that have properties, such as negative refractive index, n, nonexistent in natural materials. The recent development of metamaterials with negative n confirms that structures can be fabricated and interpreted as having a negative permittivity, ε, and a negative permeability, μ, simultaneously. Since the original microwave experiments for the demonstration of negative index behavior in split ring resonators (SRRs) and wire structures, new designs have been introduced, such as fishnet, that have pushed the existence of the negative refraction at optical wavelengths [1, 2]. Most of the experiments with the fishnet structure measure transmission, T, and reflection, R, and use the retrieval procedure [3,4] to obtain the effective parameters, ε, μ, and n. Although, stacking of three [5], four [6], five [7] functional layers and recently fabricated [8] ten-functional layer fishnets (21 layers of silver and MgF2) have been realized, they do not constitute a bulk metamaterial. Even the thickest fabricated [8] fishnet structure only has a total thickness, 830 nm, half of the wavelength (λ=1700 nm). However, it is very important to study how the optical properties change as one increases the number of layers. How many layers are needed to achieve convergence of the optical properties? How do optical properties behave as one change the distance between two neighboring fishnets? If the distance is small, we have a strong coupling case. The convergence of optical properties is slow, and more importantly, it does not convergence to the isolated fishnet case. What is the mechanism for negative n in the strong coupling limit? Here, we report a detailed study of the weakly and strongly coupled fishnets to understand the origin of negative n, as well the mechanism of low losses (that is, high figure of merit (FOM)) for the strongly coupled fishnets. We also study the convergence of the retrieval parameter (ε, μ, and n) as the number of unit cells (layers) increases. For the weakly coupled structures, the convergence results for n and FOM are close to the single unit cell. As expected, for the strongly

coupled structures, hybridization is observed and the retrieval results for n and FOM are completely different from the single unit cell. We demonstrate that the high value of FOM for the strongly coupled structure is due to periodicity

effects.

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