Optical Metamagnetism and Negative-Index Metamaterials
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optical frequencies; hence, to obtain an optical NIM, an artificial magnetic response must be achieved as a prerequisite. Following from the previous discussion, two types of NIMs can be introduced. A double-negative NIM (DN-NIM) is a metamaterial with simultaneously negative real parts of the effective permeability and permittivity constants (µ′ < 0 and ε′ < 0). A single-negative NIM (SN-NIM) has a negative refractive index with either µ′ < 0 or ε′ < 0 (but not both). At optical wavelengths, obtaining ε′ < 0 is easy compared to obtaining µ′ < 0, as noble metals naturally have a negative ε′ value above the plasma wavelength. The ratio −n′/n′′ is often taken as a figure of merit (FOM) of NIMs because lowloss NIMs are desired. The FOM can be rewritten as
Metamagnetism and Negative-Index Metamaterials
Uday K. Chettiar, Shumin Xiao, Alexander V. Kildishev, Wenshan Cai, Hsiao-Kuan Yuan, Vladimir P. Drachev, and Vladimir M. Shalaev
FOM = −(⎪µ⎪ε′ + ⎪ε⎪µ′)/ (⎪µ⎪ε′′ + ⎪ε⎪µ′′),
Abstract A new class of artificially structured materials called metamaterials makes it possible to achieve electromagnetic properties that do not exist in nature. In this article, we review the recent progress made in the area of optical metamaterials, specifically artificial magnetism and negative-index metamaterials. It was predicted that nanostructured metamaterials could provide magnetic responses and negative refractive indexes at optical frequencies. To date, optical metamagnetics have been fabricated to demonstrate magnetic responses in the infrared range and across the entire visible spectrum. Metamaterials showing negative refractive indexes, also called negative-index materials (NIMs), have also been demonstrated in the infrared range and at the border with the visible spectral range. Additionally, we report the results of a sample that displays NIM behavior for red light at a wavelength of 710 nm and another sample that displays double-negative NIM behavior at 725 nm. Both observations represent the shortest wavelengths so far at which NIM behavior has been observed for light. We also discuss the fabrication challenges and the impact of fabrication limitations, specifically the effect of surface roughness of the fabricated structures, on the optical properties of the metamaterials.
Introduction The refractive index, n, is one of the most important optical characteristics of any material. It can be written as n = n′ + in′′,
(1)
where n′ is the real part and n′′ is the imaginary part of the refractive index. The same convention is used throughout this article, where a single prime (′) denotes the real part and a double prime (′′) denotes the imaginary part. Conventionally, the real part of the refractive index (n′) is assumed to be positive, but a negative refractive index (n′ < 0) does not violate any fundamental laws of physics. Negative-index materials (NIMs), with n′ < 0, have some remarkable properties that make them can-
didates for a number of potential applications such as super-resolution.1 In a landmark work in 1968, Vesela
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