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Ultralow-Loss Optical Diamagnetism in Silver Nanoforests Journal of Optics A: Pure and Applied Optics Related content - Metallo-dielectric core–shell nanospheres Ultralow-loss optical diamagnetism in silver as building blocks for optical three- dimensional isotropic negative-index metamaterials nanoforests R Paniagua-Domínguez, F López-Tejeira, R Marqués et al. To cite this article: J J H Cook et al 2009 J. Opt. A: Pure Appl. Opt. 11 114026 - Negative index meta-materials based on two-dimensional metallic structures Gennady Shvets and Yaroslav A Urzhumov - Negative refractive index metamaterials View the article online for updates and enhancements. from inherently non-magnetic materials Vassilios Yannopapas and Alexander Moroz Recent citations - Effective medium theory for two- dimensional non-magnetic metamaterial lattices up to quadrupole expansions Ioannis Chremmos et al - The reflectivity of carbon fiber reinforced polymer short circuit illuminated by guided microwaves A. Bojovschi et al - The strong diamagnetic behaviour of unidirectional carbon fiber reinforced polymer laminates A. Galehdar et al This content was downloaded from IP address 128.179.254.55 on 21/05/2018 at 12:44 IOP PUBLISHING JOURNAL OF OPTICS A: PURE AND APPLIED OPTICS J. Opt. A: Pure Appl. Opt. 11 (2009) 114026 (9pp) doi:10.1088/1464-4258/11/11/114026 Ultralow-loss optical diamagnetism in silver nanoforests J J H Cook, K L Tsakmakidis and O Hess Advanced Technology Institute and Department of Physics, Faculty of Engineering and Physical Sciences, University of Surrey, Guildford GU2 7XH, UK E-mail: [email protected] Received 7 April 2009, accepted for publication 4 June 2009 Published 17 September 2009 Online at stacks.iop.org/JOptA/11/114026 Abstract A comprehensive investigation of the optical properties of a composite metamaterial in which silver nanowires are aligned inside a finite-thickness dielectric host medium is presented. Using a rigorous finite-element based modelling approach, together with a self-consistent process for the extraction of effective-medium parameters, we find that this structure can enable an effective optical diamagnetic response that is orders of magnitude stronger compared to that of naturally occurring diamagnetic materials. Interestingly, our analysis reveals that there is a frequency region where the nanoforest exhibits a strong diamagnetic response while simultaneously allowing for high transmission of incident electromagnetic waves. We examine the physical origin behind the magnetic properties of this structure, as well as its resilience to fabrication imperfections. Our analysis shows that the structure is robust to the presence of disorder, in the occurrence of which it can still facilitate high figure-of-merit diamagnetic responses. Keywords: diamagnetism, metamaterials, plasmonics, disorder, finite element modelling (Some figures in this article are in colour only in the electronic version) 1. Introduction Obtaining a metamaterial response (e.g., diamagnetism or NRI) at higher frequencies is challenging, not least because Metamaterials are engineered media that owe their electromag- of the modification of the electromagnetic properties of netic properties to their chemical composition and the structure metals at optical frequencies, the absence of high-permittivity of their (subwavelength) ‘metamolecules’. As the periodic ta- dielectric hosts [7] and the fabrication difficulties associated ble only has a finite number of chemical elements, engineering with the correspondingly small periodicities and features of the electromagnetic response of a medium by suitable structur- the metamolecules [8]. Moreover, considerations such as ing of its ‘metamolecules’ allows for overcoming a variety of bandwidth, dissipative losses and disorder also need to be limitations pertaining to naturally occurring materials, and en- included in the analysis before a useful metamaterial design tering regimes where, e.g., the (effective) refractive index of a can be reached. medium can now become negative—a property never found in The aforementioned realizations have led to the inves- nature. tigation and proposal of a variety of metamaterial designs. The first metamaterial was a combination of magnetic Structures investigated thus far towards obtaining optical mag- metamolecules, made of so-called split-ring resonators [1], and netism and/or negative refractive index include the so-called electric metamolecules made of thin metal wires [2]. That fishnet design (metal/dielectric layers) [9], short (cut) metal- structure was shown to exhibit an effective negative refractive lic wires [10], plasmonic nanorings [11] and chiral metallic index (NRI) in the microwave regime [2]. The experimental crosses [12]. demonstration of a NRI medium in the microwave regime led Significantly less research has focused on the potential to a profound resurgence of interest in the properties of these of metamaterials for enabling low-loss diamagnetism, with composite media, as well as to continuous efforts towards previous studies [13, 14] focusing on well ordered and scaling them to higher frequencies in order to realize and infinitely periodic arrays of plasmonic nanoparticles, in which harness their unique optical properties [3–6] across a broad the obtained figures of merit (Re{μ}/ Im{μ}, μ being the spectrum. effective magnetic permeability of the metamaterial) were 1464-4258/09/114026+09$30.00 1 © 2009 IOP Publishing Ltd Printed in the UK J. Opt. A: Pure Appl. Opt. 11 (2009) 114026 J J H Cook et al typically of the order of unity. Considering that most naturally occurring diamagnetic media are characterized by magnetic susceptibilities χ of the order of only ∼−10−5 (with the exception of superconductors, which are perfect diamagnets with χ =−1), low-loss diamagnetic metamaterials may find important potential applications, e.g., in enabling room- temperature (dia)magnetic levitation. In the following we are studying a metamaterial nanocomposite that can facilitate ultralow-loss diamagnetic responses, even in the presence of disorder, by virtue of its suitably engineered structure. The metamaterial is an anisotropic formation of silver nanowires (‘silver nanoforest’) embedded in a dielectric host medium, and can be straightforwardly realized using existing, mature metamaterial technology [15, 16]. In this structure the Figure 1. Schematic illustration of the three-dimensional geometrical area of the metal–dielectric interfaces is large (compared structure of the diamagnetic metamaterial. Also shown is the with the volume of a metal wire); hence, it is expected thickness of a unit cell (tcell) along the direction of light propagation. that the effective electromagnetic response of this medium should be greatly affected by the presence of surface plasmon polaritons (SPPs) [17, 18]—quasi-particles formed by light but not negative effective refractive index or negative effective being coupled to oscillating electrons on the surface of a permittivity or permeability. negative-permittivity material, such as a plasmonic wire. The real (ε) and imaginary (ε) parts of the permittivity The coupling of light to supported SPP modes confines of the silver nanowires were, respectively, modelled using the it to the interface of the metallic–dielectric inclusions and following Drude–Sommerfeld relations: allows unusual subwavelength features to emerge, including extraordinary transmission of light through subwavelength ω2τ 2 ε = 1 − p , (1) holes in metal surfaces [19]. As we shall discover in the 1 + ω2τ 2 following, placing aligned silver nanowires inside a finite- thickness dielectric host allows for the existence and mutual ω2τ 2 p coupling of SPP modes, the latter having a profound effect ε = , (2) ω(1 + ω2τ 2) on the overall (averaged/effective) magnetic response of the 15 −1 structure, leading ultimately to frequency regions with high with ωp = 9.8 × 10 s being the plasma frequency of the transmission and negative magnetic susceptibilities (χ<0and metal, τ = 3.1 × 10−14 s the relaxation time of the electrons 0 < Re{μ} < 1). in the metal and ω the angular frequency of the incoming EM wave. The values for ωp and τ were derived from the 2. Computational methodology experimental data of Johnson and Christy [22]. The dielectric background was assumed to be lossless and have a constant, To rigorously analyse the optical properties of the nanostruc- frequency independent, permittivity of εb = 6.52 (glass). tured plasmonic arrangements, we deploy a vectorial, adaptive finite-element simulation methodology [20], combined with proper perfectly matched layer (PML) absorbing, radiation and 3. Retrieval of the metamaterial effective constitutive periodic boundary conditions, which allow us expediently and parameters with high accuracy to obtain the scattering spectra of our struc- ture. The nanowires shown in figure 1 are assumed to extend The adaptive-mesh finite-element simulation methodology [20] infinitely in the horizontal (x) and vertical (z) directions, with allows one to expediently obtain the (steady state) electric and a horizontal centre-to-centre spacing fixed to x = 36 nm (in magnetic field spatial distributions, as well as the reflection and all cases), radius r = 5 or 10 nm, and longitudinal spacing transmission of a perpendicularly (to our structure) incident of y = 12, 22 or 24 nm. Crucial for the manifestation of electromagnetic wave for a variety of optogeometric set-ups low-loss diamagnetism in this structure is, as we shall discover and wavelengths. From these results it is possible to com- in the following, the fact that its
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