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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 14 — Jul. 2, 2012
  • pp: 15781–15796
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Plasmon nanoparticle superlattices as optical-frequency magnetic metamaterials

Hadiseh Alaeian and Jennifer A. Dionne  »View Author Affiliations


Optics Express, Vol. 20, Issue 14, pp. 15781-15796 (2012)
http://dx.doi.org/10.1364/OE.20.015781


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Abstract

Nanocrystal superlattices have emerged as a new platform for bottom-up metamaterial design, but their optical properties are largely unknown. Here, we investigate their emergent optical properties using a generalized semi-analytic, full-field solver based on rigorous coupled wave analysis. Attention is given to superlattices composed of noble metal and dielectric nanoparticles in unary and binary arrays. By varying the nanoparticle size, shape, separation, and lattice geometry, we demonstrate the broad tunability of superlattice optical properties. Superlattices composed of spherical or octahedral nanoparticles in cubic and AB2 arrays exhibit magnetic permeabilities tunable between 0.2 and 1.7, despite having non-magnetic constituents. The retrieved optical parameters are nearly polarization and angle-independent over a broad range of incident angles. Accordingly, nanocrystal superlattices behave as isotropic bulk metamaterials. Their tunable permittivities, permeabilities, and emergent magnetism may enable new, bottom-up metamaterials and negative index materials at visible frequencies.

© 2012 OSA

1. Introduction

Advances in self–assembly have enabled development of nanocrystal superlattices - metallic, semiconducting, or dielectric nanoparticles arranged in periodic arrays [1

1. R. M. Erb, H. S. Son, B. Samanta, V. M. Rotello, and B. B. Yellen, “Magnetic assembly of colloidal superstructures with multipole symmetry,” Nature (London) 457, 999–1002 (2009). [CrossRef]

11

11. W. H. Evers, H. Friedrich, L. Filion, M. Dijkstra, and D. Vanmaekelbergh, “Observation of a ternary nanocrystal superlattice and its structural characterization by electron tomography,” Angew. Chem. Int. Ed. 48, 9655–9657 (2009). [CrossRef]

]. Such lattices are typically composed of one, two, or three types of nanocrystals, forming unary, binary, and ternary superlattices, respectively. By tuning the interplay of electrostatic interactions with entropic, van der Waals, steric, and dipolar forces, nanocrystal superlattices can exhibit a structural diversity transcending that of natural lattices. For example, more than fifteen distinct binary nanocrystal superlattice arrangements have been realized, many with no isostructural intermetallic counterparts [8

8. E. V. Shevchenko, D. V. Talapin, N. A. Kotov, S. O’Brien, and C. B. Murray, “Structural diversity in binary nanoparticle superlattices,” Nature (London) 439, 55–59 (2006). [CrossRef]

]. More recently, self-assembly of ternary nanocrystal superlattice bilayers has been shown to yield arrangements unique from any three-dimensional lattice [11

11. W. H. Evers, H. Friedrich, L. Filion, M. Dijkstra, and D. Vanmaekelbergh, “Observation of a ternary nanocrystal superlattice and its structural characterization by electron tomography,” Angew. Chem. Int. Ed. 48, 9655–9657 (2009). [CrossRef]

]. To date, nanocrystal superlattice assemblies have been achieved over millimetre-scales, both substrate-supported and free-standing [8

8. E. V. Shevchenko, D. V. Talapin, N. A. Kotov, S. O’Brien, and C. B. Murray, “Structural diversity in binary nanoparticle superlattices,” Nature (London) 439, 55–59 (2006). [CrossRef]

, 10

10. A. Dong, J. Chen, P. M. Vora, J. M. Kikkawa, and C. B. Murray, “Binary nanocrystal superlattice membranes self-assembled at the liquid-air interface,” Nature (London) 446, 474–477 (2010). [CrossRef]

].

The structural diversity of nanocrystal superlattices gives rise to intriguing electronic and magnetic properties distinct from their constituent nanoparticles. For example, binary superlattices of PbTe and Ag2Te nanoparticles in AB2 lattices have been shown to exhibit strongly enhanced conductance, with over a 100-fold increase in conductivity compared to unary PbTe or Ag2Te superlattices [9

9. J. J. Urban, D. V. Talapin, E. V. Shevchenko, C. R. Kagan, and C. B. Murray, “Synergism in binary nanocrystal superlattices leads to enhanced p-type conductivity in self-assembled PbTe/Ag2Te thin films,” Nat. Mater. 6, 115–121 (2007). [CrossRef] [PubMed]

]. Fast and efficient optical modulators have also been demonstrated based on enhanced electro-optical interactions and nonlinearities in self-assembled nanocrystal superlattices [12

12. Y. G. Zhao, A. Wu, H. L. Lu, S. Chang, W. K. Lu, S. T. Ho, M. E. van der Boom, and T. J. Marks, “Traveling wave electro-optic phase modulators based on intrinsically polar self-assembled chromophoric superlattices,” Appl. Phys. Lett. 79, 587–589 (2001). [CrossRef]

, 13

13. D. G. Suna, Z. Liub, J. Mab, and S. T. Hob, “Design and fabrication of electro-optic waveguides with self-assembled superlattice films,” Opt. Laser Technol. 39, 285–289 (2007). [CrossRef]

]. Since the emergent properties of nanocrystal superlattices are highly dependent on the nanoparticle size, shape, composition, and the lattice stoichiometry, they may provide the framework for new, bottom-up metamaterials at visible frequencies [7

7. A. L. Rogach, “Binary superlattices of nanoparticles: self-assembly leads to metamaterials,” Adv. Funct. Mater. 43, 148–149 (2004).

]. In particular, electromagnetic coupling between superlattice constituents could enable tunable electric permittivities, magnetic permeabilities, and refractive indices spanning positive, negative, and near-zero values. Combined with the unique optical properties of quantum-confined nanostructures, such tunability could lead to profound applications for superlensing, cloaking, or optical communications [14

14. A. Alu and N. Engheta, “The quest for magnetic plasmons at optical frequencies,” Opt. Express 17, 5723–5730 (2009). [CrossRef] [PubMed]

, 15

15. A. Alu and N. Engheta, “Plasmonic and metamaterial cloaking: physical mechanisms and potentials,” J. Opt. A: Pure Appl. Opt. 10, 093002 (2008). [CrossRef]

].

In recent years, periodic nanoparticle-based structures have received increasing attention [17

17. B. Auguie and W. L. Barnes, “Collective resonances in Gold nanoparticle arrays,” Phys. Rev. Lett. 101, 143902 (2008). [CrossRef] [PubMed]

27

27. I. Romero and F. J. Garcia de Abajo, “Anisotropy and particle-size effects in nanostructured plasmonic metamaterials,” Opt. Express 17, 22012–22022 (2009). [CrossRef] [PubMed]

]. For example, at microwave frequencies, cubic lattices of large-permittivity polaritonic spheres have been shown to exhibit a negative permittivity and permeability [28

28. C. L. Holloway, E. F. Kuster, J. B. Jarvis, and P. Kabos, “A double negative (DNG) composite medium composed of magnetodielectric spherical particles embedded in a matrix,” IEEE Trans. Antennas Propag. 51, 2596–2603 (2003). [CrossRef]

, 29

29. J. Liu and N. Bowler, “Analysis of double-negative (DNG) bandwidth for a metamaterial composed of magnetodielectric spherical particleseEmbedded in a matrix,” IEEE Antennas Wirel. Propag. Lett. 10, 399–402 (2011). [CrossRef]

]. At optical frequencies, metallic nanoparticle lattices have been shown to support propagating modes characterized by tunable positive refractive indices and permittivities [18

18. A. L. Fructos, S. Campione, F. Capolino, and F. Mesa, “Characterization of complex plasmonic modes in two-dimensional periodic arrays of metal nanospheres,” J. Opt. Soc. Am. B 28, 1446–1458 (2011). [CrossRef]

,21

21. A. Alu, A. Salandrino, and N. Engheta, “Negative effective permeability and left-handed materials at optical frequencies,” Opt. Express 14, 1557–1567 (2006). [CrossRef] [PubMed]

,23

23. A. Valecchi and F. Capolino, “Metamaterials based on pairs of tightly-coupled scatterers,” in Metamaterials Handbook (CRC Press, 2009).

,24

24. S. Steshenko, F. Capolino, S. Tretyakov, C. Simovski, and F. Capolino, “Super resolution with layers of resonant arrays of nanoparticles,” in Metamaterials Handbook (CRC Press, 2009).

,27

27. I. Romero and F. J. Garcia de Abajo, “Anisotropy and particle-size effects in nanostructured plasmonic metamaterials,” Opt. Express 17, 22012–22022 (2009). [CrossRef] [PubMed]

]. Close-packed trimer, quadramer, and heptamer-based plasmonic assemblies have also been shown to support optical-frequency magnetic resonances and Fano resonances of electric and magentic modes [30

30. N. Liu, S. Mukherjee, K. Bao, L. Brown, J. Dorfmuller, P. Nordlander, and N. J. Halas, “Magnetic plasmon formation and propagation in artificial aromatic molecules,” Nano Lett. 12, 364–369 (2012). [CrossRef]

33

33. J. B. Lassiter, H. Sobhani, J. A. Fan, J. Kundu, F. Capasso, P. Nordlander, and N. J. Halas, “Fano resonances in plasmonic nanoclusters: geometrical and chemical tunability,” Nano Lett. 10, 3184–3189 (2010). [CrossRef] [PubMed]

]. In this paper, we theoretically determine the effective permittivity and permeability of close-packed nanoparticle superlattices. Attention is given to unary Au and binary Au/silica nanoparticle superlattices composed of varying nanoparticle size, shape, and lattice geometries. We base our superlattice structures on lattices that have already been experimentally demonstrated, or that promise to exhibit a bulk permeability at optical frequencies based on the unit-cell geometry [30

30. N. Liu, S. Mukherjee, K. Bao, L. Brown, J. Dorfmuller, P. Nordlander, and N. J. Halas, “Magnetic plasmon formation and propagation in artificial aromatic molecules,” Nano Lett. 12, 364–369 (2012). [CrossRef]

]. To determine their optical properties, we use a three-dimensional, analytic algorithm based on rigorous coupled wave analysis (RCWA) [34

34. M. G. Moharam and T. K. Gaylord, “Three-dimensional vector coupled-wave analysis of planar-grating diffraction,” J. Opt. Soc. Am. A 73, 1105–1112 (1983). [CrossRef]

]. We modify and generalize the method so that it is unconditionally stable for two- and three-dimensional periodic arrays with constituents of arbitrary shape and material.

The method generates the full electromagnetic fields of the lattice as well as its scattering parameters, which in turn can be used to extract the superlattice effective refractive index, normalized impedance, permittivity, and permeability [35

35. D. R. Smith, S. Schultz, P. Markos, and C. M. Soukoulis, “Determination of effective permittivity and permeability of metamaterials from reflection and transmission coefficients,” Phys. Rev. B 65, 195104 (2002). [CrossRef]

37

37. C. Menzel, C. Rockstuhl, T. Paul, and F. Lederer, “Retrieving effective parameters for metamaterials at oblique incidence,” Phys. Rev. B 77, 195328 (2008). [CrossRef]

]. Our results indicate that the optical properties of superlattices can be broadly tuned at visible frequencies via variation of nanoparticle size, separation, shape, and lattice structure. Notably, superlattices of non-magnetic particles exhibit emergent paramagnetism, which to the best of our knowledge is the first report of optical magnetism in plasmonic nanoparticle superlattices. Furthermore, the optical properties of nanoparticle superlattices are largely angle- and polarization-independent, suggesting their potential as isotropic, three-dimensional magnetic or negative index metamaterials.

2. Theoretical framework

Rigorous coupled wave analysis is a well-known method for solving Maxwell’s equations in periodic media. To date, it has been successfully employed in the analysis of surface relief gratings, corrugated surfaces, and photonic crystals [38

38. Q. Cao, P. Lalanne, and J. P. Hugonin, “Stable and efficient Bloch-mode computational method for one-dimensional grating waveguides,” J. Opt. Soc. Am. A 19, 335–338 (2002). [CrossRef]

41

41. M. Shahabadi, S. Atakaramians, and N. Hojjat, “Transmission line formulation for the full-wave analysis of two-dimensional dielectric photonic crystals,” IEE Proc.-Sci. Meas. Technol. 151, 327–334 (2004). [CrossRef]

]. Compared with other numerical techniques such as finite difference time domain, finite difference frequency domain and finite element methods, RCWA provides an accurate analytical, full-vectorial solution to Maxwell’s equations in periodic structures. It is therefore well-suited for analysis of nanocrystal superlattices. Here, we briefly present our extended and generalized RCWA formulation for two and three dimensional periodic nanoparticle arrays.

2.1. Two-dimensional superlattices

Consider an infinite two dimensional array of arbitrarily shaped objects periodically arranged in the xy-plane. The materials are assumed to have a linear electromagnetic response. According to Bloch’s theorem [42

42. R. E. Collin, Foundations for Microwave Engineering (IEEE, 2001). [CrossRef]

], the eigensolutions of the array are pseudo-periodic given by the product of a periodic function and a plane wave with wavevector k. Assuming a complex exponential time dependence of the form exp(−iωt), the pseudo-periodic electric and magnetic fields can be written as:
E(r+L)=E(r)exp(ik.L)H(r+L)=H(r)exp(ik.L)
(1)
where E(r) and H(r) are vector fields with the same spatial periodicity as the lattice. Using a Fourier expansion, E(r) and H(r) can be represented as:
E(r)=m,n=Emn(z)exp(i[(2mπLx+Kx0)x+(2nπLy+Ky0)y])H(r)=m,n=Hmn(z)exp(i[(2mπLx+Kx0)x+(2nπLy+Ky0)y])
(2)
where (Lx, Ly) and (Kx0, Ky0) are the lattice parameters and the Bloch wavevectors in the x and y directions, respectively. Due to the periodicity of the lattice, the dielectric function is also periodic in the xy-plane, and can be expanded as a double Fourier series:
ε(r)=m,n=εmn(z)exp(i(2mπLxx+2nπLyy))
(3)

Here, εmn(z) is the mnth Fourier coefficient of the dielectric constant at any position z orthogonal to the xy-plane. For numerical implementation, the series must be truncated. Including up to the Nth harmonic in each direction, Maxwell’s equations can be written in terms of the unknown expansion coefficients of the electric and magnetic fields:
ddz(Ex)=iωμ0[([Kx][N]2[Kx]I)(Hy)+[Kx][N]2[Ky](Hx)]ddz(Ey)=iωμ0[[Ky][N]2[Kx](Hy)+([Ky][N]2[Ky]I)(Hx)]ddz(Hy)=iωε0[([Ky]2[N]2)(Ex)[Ky][Kx](Ey)]ddz(Hx)=iωε0[([Kx][Ky])(Ex)+([Kx]2[N]2)(Ey)]
(4)
where [Kx] and [Ky] are wavevector-type matrices of the form:
[Kx]=[(2mπLx+Kx0)000Kx0000(2mπLx+Kx0)][Ky]=[(2nπLy+Ky0)000Ky0000(2nπLy+Ky0)]
(5)
and [N] is a permittivity-type matrix:
[N]2=[e0e2Ne2Ne0].
(6)
In this representation, ek denotes an NxN Toeplitz matrix of the permittivity Fourier expansion coefficients, starting from εk0 up to εkN.

To simplify Eq. (4), the components of the four coupled first order differential equation can be grouped into (V) and (I) vectors:
(I)=[(Hy)(Hx)](V)=[(Ex)(Ey)]
(7)
reducing Eq. (4) to two coupled equations:
ddz(V)=iωμ0[L](I)ddz(I)=iωε0[C](V)
(8)

Matrices μ0[L] and ε0[C] have dimensions of inductance and capacitance per unit length, respectively, and are given by:
[L]=[[Kx][N]2[Kx][1][Ky][N]2[Kx][Kx][N]2[Ky][Ky][N]2[Ky][1]][C]=[[Ky]2[N]2[Kx][Ky][Ky][Kx][Kx]2[N]2]
(9)

For any illumination, (Ex,y) and (Hx,y) in Eq. (10) can be determined by expanding the corresponding components of the incident fields in a Fourier series. Consequently, the unknown coefficients for any two-dimensional periodic array can be determined via field matching at the boundaries.

2.2. Extension to three-dimensional lattices of arbitrarily-shaped constituents

In general, a three-dimensional periodic structure can be modeled as stacks of two-dimensional arrays along the z direction. For example, single layers of spherical and polyhedral nanoparticles reported in this work are modelled as stacks of 25 and 50 layers of 2D arrays of circular disks and square patches, respectively. Cascaded S-matrices of these layers can then be used to determine the optical properties of the full three-dimensional lattice.

Fig. 1 Schematic of a three-dimensional unary superlattice composed of nanoparticles. Lx and Ly are the periods of the lattice in the x and y directions, respectively. The illumination is assumed to be a planewave with incident angle θ and wavevector k. The scattering coefficients of a single unit layer are defined by f1+, f1, f2+, and f2.

Note that fn denotes the transmission coefficient T of the lattice while f1+ denotes the reflection coefficient R. These quantities can be directly used to extract the superlattice optical parameters, as described below.

2.3. Permittivity, permeability, and refractive index retrieval

The imaginary component of the refractive index, n″, can be uniquely determined from the first equation. The real component of the refractive index, n′, has infinite solutions separated by λ/d in the complex plane. The appropriate choice of n′, corresponding to a physical, causal material, can be determined by imposing Kramers-Kronig criteria on n″. Among two possible answers for the normalized impedance, the solution Z′ > 0 must be chosen to ensure a passive medium. Subsequently, the effective permittivity and permeability can be determined directly:
μ=nZε=n/Z
(18)
For off-normal illumination at an angle θ1, n must be replaced with ncosθ2 while Z must be substituted with either Zcosθ2 or Z/cosθ2 for in-plane and out-of-plane polarizations, respectively. Here, θ2 is the refracted angle in the second medium and satisfies Snell’s law: n1sinθ1 = n2sinθ2.

3. Results

3.1. Unary nanoparticle superlattices

First, we consider arrays of gold spherical nanoparticles, as illustrated in Fig. 1. Three superlattices are explored, composed of nanoparticles with radii R of 5 nm, 10 nm, and 30 nm. Particles are assumed to be separated by 2 nm, approximating the interparticle separation of self-assembled superlattices due to ligands. The refractive index between particles is set to 1.5, typical of organic solvents and ligands; the regions above and below the lattice are taken to have an index n=1 (air). The empirical Johnson and Christy dataset is used to describe permittivity of the gold nanoparticles [43

43. P. B. Johnson and R. W. Christy, “Optical constants of the noble metals,” Phys. Rev. B 6, 4370–4379 (1972). [CrossRef]

]. Using the method presented in Section 2, good convergence is obtained with 21 harmonics.

Figure 2 presents the extinction cross section (1−PT) of Au nanoparticles superlattices composed of 1, 2, 3, and 4 layers. The transmitted power PT is determined by integrating the Poynting vector of the transmitted fields across the unit cell area. The incident light is assumed to be polarized in the x-direction.

Fig. 2 Normalized extinction cross sections for N= 1, 2, 3 and 4 layers of gold nanoparticles of varying radii R in a superlattice. Particle radii are (a) 5 nm, (b) 10 nm and (c) 30 nm. Illumination at normal incidence is assumed.

As seen in Fig. 2, for all particle radii, extinction cross sections increase with the number of layers. For superlattices composed of R=5 nm particles, the extinction spectra exhibits a dominant resonance peak around 528 nm. The response is not unlike that of an individual R=5 nm Au particle, which predominantly radiates as an electric dipole. However, the superlattice resonance is ∼36 meV lower in energy than an isolated Au particle (which peaks at 520 nm), due to energetically-favorable dipole-dipole interactions in-plane. As the number of lattice layers increases, the resonance energy of the superlattice slightly increases. This blue-shift can be attributed to the interaction of retarded multipolar modes in different z-layers of the lattice, which become out-of-phase as the wave propagates through the lattice. As particle size increases to R=10 nm, the dominant peak broadens, increases, and red-shifts compared to the R=5 nm lattice. The red-shift, from λ= 528 nm for R = 5 nm particles to λ= 542 nm for this R=10 nm particle lattice, arises from the lower-energy resonance of individual particles. Further, the broadening arises from increased ohmic and radiation losses. As the particle size increases, the superlattice spectra begins to deviate significantly from a single resonant peak. For example, for superlattices composed of R=30 nm particles, the extinction cross-section increases and resonant peaks are substantially broadened. As has been shown in a series of articles by McPhedran [44

44. R. C. McPhedran and D. R. McKenzie, “Exact modelling of cubic lattice permittivity and conductivity,” Nature (London) 265, 128–129 (1977). [CrossRef]

46

46. R. C. McPhedran, D. R. McKenzie, and G. H. Derrick, “The conductivity of lattices of spheres II. the body centred and face centred cubic lattice,” Proc. R. Soc. London Ser. A 362, 211–232 (1978). [CrossRef]

] the inclusions in a tightly coupled array cannot be described merely by dipoles, even for deeply subwavelength particles.

Figure 3 shows the calculated optical parameters of each unary superlattice, including the refractive index, normalized impedance, effective relative permittivity, and effective relative permeability. Optical properties are calculated for 1 through 4 unit layers, in addition to the semi-infinite, half-space case (i.e, a lattice with an infinite number of layers in the lower half-space). For simplicity, only results for one layer and the semi-infinite case are presented. Notably, the results show striking agreement between the parameters - consistent with recent results of spherical and elliptical plasmonic nanoparticle lattices [27

27. I. Romero and F. J. Garcia de Abajo, “Anisotropy and particle-size effects in nanostructured plasmonic metamaterials,” Opt. Express 17, 22012–22022 (2009). [CrossRef] [PubMed]

].

Fig. 3 Effective refractive index, normalized impedance, relative permittivity and permeability of gold nanoparticle superlattices with radii of (a)5 nm, (b)10 nm and (c)30 nm. The red dots indicate the effective parameters for a semi-infinite array while the lines show the parameters for one layer of the lattice. Note that the scales are different for each case and are also expanded around the permeability resonance in panel (b).

As seen in the upper panels, the real component of n can be readily tuned via different particle sizes. Similarly, the imaginary part of n monotonically increases with increasing particle size; this superlattice behavior is not unlike that of individual Au particles, which exhibit increased absorption as their size increases. Note the peaks in the extinction cross section of Fig. 2 are strongly correlated with the peaks of n″.

Through calculations of the lattice index and impedance, shown in the upper panels, the lattice permittivity and permeability can be determined via Eq. (18). Interestingly, despite having constituents with large negative permittivity, Au superlattices composed of R=5 nm spheres have a positive relative permittivity throughout visible frequencies. As particle size increases to R=30 nm, the permittivity can be tuned from −1.5 to +10 at visible frequencies [18

18. A. L. Fructos, S. Campione, F. Capolino, and F. Mesa, “Characterization of complex plasmonic modes in two-dimensional periodic arrays of metal nanospheres,” J. Opt. Soc. Am. B 28, 1446–1458 (2011). [CrossRef]

, 27

27. I. Romero and F. J. Garcia de Abajo, “Anisotropy and particle-size effects in nanostructured plasmonic metamaterials,” Opt. Express 17, 22012–22022 (2009). [CrossRef] [PubMed]

]. More interestingly, as particle size increases, the array begins to exhibit emergent magnetic behavior. For lattices of R=5 nm particles, the effective permeability is unity. However, as particle size increases to R = 30 nm, the array exhibits paramagnetic behavior where μ can be tuned from 0.7 to 1.1 at visible frequencies. Increasing the particle size to R= 50 nm particles increases the span of μ from 1.3 to 0.4 for wavelengths between 570 nm and 730 nm (data not shown).

Surprisingly, this magnetic response arises even if the lattice consists of only one layer of Au nanoparticles, where any magnetic dipolar moment would not be expected to align with the driving magnetic field. Such a response is actually a magnetic antiresonance, characterized by / opposite to /. Similar to the response of an infinite array of metallic wires in the microwave range [47

47. T. Koschny, P. Marko, D. R. Smith, and C. M. Soukoulis, “Resonant and antiresonant frequency dependence of the effective parameters of metamaterials,” Phys. Rev. E 68, 065602 (2003). [CrossRef]

], these visible-frequency magnetic antiresonances arise from large dispersion in the electric permittivity near the resonance frequency [48

48. R. Merlin, “Metamaterials and the Landau Lifshitz permeability argument: Large permittivity begets high-frequency magnetism,” Proc. Nat. Acad. Sci. USA 106, 1693–1698 (2009). [CrossRef] [PubMed]

].

To further elucidate the emergent electric and magnetic properties of unary Au nanoparticle superlattices, Fig. 4 illustrates a time snapshot of the electric and magnetic fields of a R=30 nm nanoparticle superlattice composed of two layers. Wavelengths of λ=350 nm (a,b) and λ=650 nm (d,e) are considered. For both wavelengths, the electric field distribution (panels a and d), clearly exhibits strong localization in the inter-particle gaps. However, the electric field is enhanced at λ=650 nm compared to λ=350 nm, due to a strong electric resonant mode in array. This electric resonance results in a strong dispersion of the lattice permittivity, which in turn produces a magnetic antiresonance. As seen in panels b and e, the magnetic field within the lattice is enhanced at λ=650 nm, where the magnetic permeability of the lattice is 0.7, compared to λ=350 nm, where μ=1. Magnetic behavior is also evidenced in the displacement current between layers, defined as −iωD, shown as arrows in panels (c) and (f). As can be observed, enhanced circulation of the displacement current density is present for λ=650 nm compared to λ=350nm, pointing to the enhanced magnetic response of Au superlattices at this wavelength.

Fig. 4 Electric (a, d) and magnetic (b, e) field profiles and displacement current quiver plots (c, f) of Au nanoparticle superlattices composed of R=30 nm particles. Wavelengths of λ=350 nm (a–c) and λ= 650 nm (d–f) are included.

Since emergent magnetism relies on the coupling of electrically-resonant elements, variations of the lattice geometry should change the emergent permeability. To investigate this dependence, we consider rectangular lattices of R=30 nm particles. By varying the lattice dimensions along x or y, the interparticle coupling can be tuned. Figure 5 illustrates n, ε, and μ for three different lattice geometries: a square lattice (Lx=62 nm, Ly=62 nm) and two rectangular lattices, (Lx=162 nm, Ly=62 nm) and (Lx=62 nm, Ly=162 nm). In all cases, the incident illumination is assumed to be normal to the lattice and the electric field is polarized along the x-direction.

Fig. 5 Effective refractive index and relative permittivity and permeability of 30-nm-radii gold nanoparticles arranged in rectangular lattices with periodicities L in the x and y directions. The incident electric field is polarized along the x-direction. The effective parameters of the square array are also included for comparison.

Compared to the square array, denoted with black curves in the figure, rectangular lattices exhibit a smaller real and imaginary refractive index, due to the increased void space between the particles. As Lx increases, the coupling between Au particles decreases. This reduced coupling is evidenced by the reduced magnitude of the resonance in permittivity and the near-unity permeability calculated for the lattice of (Lx=162 nm, Ly=62 nm), denoted in red in Fig. 5. The particles, however, are less sensitive to lattice deviations perpendicular to the polarization direction. For example, the (Lx=62 nm, Ly=162 nm) lattice maintains the strong coupling of the particles in the x-direction while decreasing the coupling in the y-direction. As the particles are mainly coupled in the x-direction, this lattice still exhibits a strong resonant permittivity hence a non-unity magnetic permeability.

To serve such as bulk metamaterials, superlattices should be fairly insensitive to the angle and polarization of the incident light. Figure 6 illustrates the spectral behavior of the effective optical parameters for various illumination angles θ. The upper panels present the retrieved parameters for an incident electric field polarized in-plane while the lower panels show results for an out-of-plane polarization. As Fig.6 reveals, n, ε, and μ are nearly independent of incident angle for each polarization implying that the array has an angle independent behavior. Correspondingly, the results for each polarization exhibit similar trends, indicating that the lattice is also almost polarization independent. Such results are consistent with the symmetric and isotropic geometry of the lattice unit cell. Accordingly, unary Au superlattices are indeed nearly-isotropic bulk magnetic metamaterials.

Fig. 6 Effective refractive index and relative permittivity and permeability of R=30 nm gold nanoparticles arranged in a square lattice with Lx=Ly=62 nm as a function of incident angle, θ, and wavelength. The upper panels show the parameters for out-of-plane polarization and the lower panels show the results for in-plane polarization.

3.2. Polyhedral and binary nanoparticle superlattices

In recent years, considerable attention has been given to the synthesis of more complex superlattice structures - including superlattices of non-spherical particles and binary nanocrystal superlattices [2

2. J. Henzie, M. Grunwald, A. W. Cooper, P. L. Geissler, and P. Yang, “Self-assembly of uniform polyhedral silver nanocrystals into densest packings and exotic superlattices,” Nat. Mater. 11, 131–137 (2011). [CrossRef] [PubMed]

,10

10. A. Dong, J. Chen, P. M. Vora, J. M. Kikkawa, and C. B. Murray, “Binary nanocrystal superlattice membranes self-assembled at the liquid-air interface,” Nature (London) 446, 474–477 (2010). [CrossRef]

]. In this section, we illustrate how these more complex superlattice structures can enable enhanced optical parameter tunability.

Fig. 7 Effective parameters of 50 nm octahedral gold nanoparticles arranged in a cubic lattice with Lx=Ly=52 nm, illuminated at normal incidence.
Fig. 8 (a) The optical properties of an AB2-type binary superlattice composed of R=30nm gold and silica nanoparticles. (b,c) Ex field profiles of the lattice in xy, xz and yz cross sections at λ= 300 nm (b) and 613 nm (c).

In addition to non-spherical particle lattices, considerable attention has been given to the synthesis of compound nanocrystal superlattices composed of more than one type of material. Among them, the simplest binary lattice is characterized by AB2-type packing. This lattice, also known as a Kagome lattice, has been the subject of many studies spanning solid state physics and photonics [49

49. J. L. Atweeod, “Kagome’ lattice: a molecular toolkit for magnetism,” Nat. Matter. 1, 91–92 (2002). [CrossRef]

51

51. Q. Chen, S. C. Bae, and S. Granick, “Directed self-assembly of a colloidal Kagome lattice,” Nature (London) 469, 381–384 (2010). [CrossRef]

]. For example, the flat band structure of Kagome lattices renders them a promising candidate for lasers, as they provide a platform for coherent lasing in all directions [52

52. A. V. Giannopoulos, C. M. Long, and K. D. Choquette, “Photonic crystal heterostructure cavity lasers using kagome lattices,” Electron. Lett. 44, 38–39 (2008). [CrossRef]

]. Further, these lattices are known to develop weak long-range ferromagnetic order from paramagnetic building blocks [53

53. X. Li, J. Zhou, Q. Wang, X. Chen, Y. Kawazoe, and P. Jena, “Magnetism of two-dimensional triangular nanoflake-based kagome lattices,” New J. Phys. 14, 033043 (2012). [CrossRef]

]. Here, we analyze the emergent optical magnetism of binary AB2-type lattices composed of gold and silica nanoparticles. Each unit cell has one R=30 nm silica particle surrounded by six R=30 nm gold nanoparticles. As before, particles are assumed to be embedded in a medium with index n=1.5 and are separated by 2 nm.

Figure 8(a)–8(c) shows the effective parameters of one unit layer of a gold/silica nanoparticle superlattice, illuminated at normal incidence by an x-polarized planewave. Although not shown, y-polarized illumination excites the same spectral response. For visible wavelengths, the permittivity ε varies between −7 and 20 while the permeability μ is tunable between 0.5 and 1.2. An AB2 array of R=50 nm gold and silica nanoparticles exhibits even stronger tunability, with ε ranging from −6.5 to +30 and μ between 0.2 and 1.7 at visible frequencies (data not shown). Upon illumination, two distinct types of plamonic modes are excited: ‘surface’ modes propagating in the plane of the lattice, and ‘gap’ modes propagating normal to the lattice plane between the particles. While surface modes carry power in the transverse lattice planes, gap modes carry power through the lattice in z via the interparticle spacings. Figures 7(d) and 7(e) compare the electric field profiles of the lattice at wavelengths of λ=300 nm (away from the resonance) and λ= 613 nm (on resonance). As seen, the electric fields are mostly confined between the metallic particles. However, the xy-field cross section on resonance is significantly enhanced, due to strong coupling to the electric resonant mode. Further, the electric field distribution in xz and yz-cross sections at each wavelength reveals substantial field transmission at λ=300 nm (corresponding to excitation of ‘gap’ modes) and nearly-suppressed transmission at λ= 613 nm (corresponding to excitation of ‘surface’ modes). Engineering the excitability of surface versus gap modes should provide further optical tunability, selectively exploiting the waveguided or resonant character of nanoparticle superlattices.

4. Outlook and conclusions

We have developed a generalized and stable RCWA method for the analysis of two and three dimensional periodic structures, and applied the method to the analysis of plasmonic nanocrystal lattices. To our knowledge, this is the first report determining the calculated refractive index, electric permittivity and magnetic permeability of superlattices using analytic, full-vectorial solutions to Maxwell’s equations. Our results indicate that the effective permittivity of superlattices can be tuned throughout a broad range of large positive and negative values. Furthermore, this tunability can yield a paramagnetic response of the lattice, despite all constituents being non-magnetic. The emergent electric and magnetic response is highly dependent on the constituent particle size, shape, separation and lattice arrangement, but is largely independent of illumination angle and polarization. It is predicted that combination of magnetically resonant elements with metallic nanoparticles in unary or binary arrays will generate simultaneous resonances in the electric permittivity and magnetic permeability. Such materials could form the basis for new, negative index metamaterials at visible frequencies, enabled by advances in colloidal chemistry and self-assembly.

Acknowledgments

The authors are grateful to Dr. Sassan Sheikholeslami and Dr. Aitzol Garcia for fruitful scientific discussions. This work is supported in part by a SLAC National Accelerator Laboratory LDRD award in concert with the Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under contract DE-AC02-76SF00515. J.A.D. also acknowledges support from an Air Force Office of Scientific Research Young Investigator Grant.

References and links

1.

R. M. Erb, H. S. Son, B. Samanta, V. M. Rotello, and B. B. Yellen, “Magnetic assembly of colloidal superstructures with multipole symmetry,” Nature (London) 457, 999–1002 (2009). [CrossRef]

2.

J. Henzie, M. Grunwald, A. W. Cooper, P. L. Geissler, and P. Yang, “Self-assembly of uniform polyhedral silver nanocrystals into densest packings and exotic superlattices,” Nat. Mater. 11, 131–137 (2011). [CrossRef] [PubMed]

3.

A. Tao, P. Sinsermsuksakul, and P. Yang, “Tunable plasmonic lattices of silver nanocrystals,” Nat. Nanotechnol. 2, 435–440 (2007). [CrossRef]

4.

Z. L. Wang, “Structural analysis of self-assembling nanocrystal superlattices,” Adv. Mater. 10, 13–30 (1998). [CrossRef]

5.

N. A. El-Masry, M. K. Behbehani, S. F. LeBoeuf, M. E. Aumer, J. C. Roberts, and S. M. Bedair, “Self-assembled AlInGaN quaternary superlattice structures,” Appl. Phys. Lett. 79, 1616–1618 (2001). [CrossRef]

6.

R. P. Andres, J. D. Bielefeld, J. I. Henderson, D. B. Janes, V. R. Kolagunta, C. P. Kubiak, W. J. Mahoney, and R. G. Osifchin, “Self-assembly of a two-Dimensional superlattice of molecularly linked metal clusters,” Science 273, 1690–1693 (1996). [CrossRef]

7.

A. L. Rogach, “Binary superlattices of nanoparticles: self-assembly leads to metamaterials,” Adv. Funct. Mater. 43, 148–149 (2004).

8.

E. V. Shevchenko, D. V. Talapin, N. A. Kotov, S. O’Brien, and C. B. Murray, “Structural diversity in binary nanoparticle superlattices,” Nature (London) 439, 55–59 (2006). [CrossRef]

9.

J. J. Urban, D. V. Talapin, E. V. Shevchenko, C. R. Kagan, and C. B. Murray, “Synergism in binary nanocrystal superlattices leads to enhanced p-type conductivity in self-assembled PbTe/Ag2Te thin films,” Nat. Mater. 6, 115–121 (2007). [CrossRef] [PubMed]

10.

A. Dong, J. Chen, P. M. Vora, J. M. Kikkawa, and C. B. Murray, “Binary nanocrystal superlattice membranes self-assembled at the liquid-air interface,” Nature (London) 446, 474–477 (2010). [CrossRef]

11.

W. H. Evers, H. Friedrich, L. Filion, M. Dijkstra, and D. Vanmaekelbergh, “Observation of a ternary nanocrystal superlattice and its structural characterization by electron tomography,” Angew. Chem. Int. Ed. 48, 9655–9657 (2009). [CrossRef]

12.

Y. G. Zhao, A. Wu, H. L. Lu, S. Chang, W. K. Lu, S. T. Ho, M. E. van der Boom, and T. J. Marks, “Traveling wave electro-optic phase modulators based on intrinsically polar self-assembled chromophoric superlattices,” Appl. Phys. Lett. 79, 587–589 (2001). [CrossRef]

13.

D. G. Suna, Z. Liub, J. Mab, and S. T. Hob, “Design and fabrication of electro-optic waveguides with self-assembled superlattice films,” Opt. Laser Technol. 39, 285–289 (2007). [CrossRef]

14.

A. Alu and N. Engheta, “The quest for magnetic plasmons at optical frequencies,” Opt. Express 17, 5723–5730 (2009). [CrossRef] [PubMed]

15.

A. Alu and N. Engheta, “Plasmonic and metamaterial cloaking: physical mechanisms and potentials,” J. Opt. A: Pure Appl. Opt. 10, 093002 (2008). [CrossRef]

16.

A. Alu and N. Engheta, “Theory of linear chains of metamaterial/plasmonic particles as subdiffraction optical nanotransmission line,” Phys. Rev. B 74, 205436 (2006). [CrossRef]

17.

B. Auguie and W. L. Barnes, “Collective resonances in Gold nanoparticle arrays,” Phys. Rev. Lett. 101, 143902 (2008). [CrossRef] [PubMed]

18.

A. L. Fructos, S. Campione, F. Capolino, and F. Mesa, “Characterization of complex plasmonic modes in two-dimensional periodic arrays of metal nanospheres,” J. Opt. Soc. Am. B 28, 1446–1458 (2011). [CrossRef]

19.

A. Taleb, V. Russier, A. Courty, and M. P. Pileni, “Collective optical properties of silver nanoparticles organized in two-dimensional superlattices,” Phys. Rev. B 59, 13350–13358 (1999). [CrossRef]

20.

D. V. Tapalin, E. V. Shevchenko, C. B. Murray, A. V. Titov, and Petr Kral, “Dipole-dipole interactions in nanoparticle superlattices,” Nano Lett. 7, 1213–1219 (2007). [CrossRef]

21.

A. Alu, A. Salandrino, and N. Engheta, “Negative effective permeability and left-handed materials at optical frequencies,” Opt. Express 14, 1557–1567 (2006). [CrossRef] [PubMed]

22.

C. Rockstuhl, F. Lederer, C. Etrich, T. Pertsch, and T. Scharf, “Design of an artificial three-dimensional composite metamaterial with magnetic resonances in the visible range of the electromagnetic spectrum,” Phys. Rev. Lett. 99, 017401 (2007). [CrossRef] [PubMed]

23.

A. Valecchi and F. Capolino, “Metamaterials based on pairs of tightly-coupled scatterers,” in Metamaterials Handbook (CRC Press, 2009).

24.

S. Steshenko, F. Capolino, S. Tretyakov, C. Simovski, and F. Capolino, “Super resolution with layers of resonant arrays of nanoparticles,” in Metamaterials Handbook (CRC Press, 2009).

25.

S. Campione, S. Steshenko, and F. Capolino, “Complex bound and leaky modes in chains of plasmonic nanospheres,” Opt. Express 19, 18345–18363 (2011). [CrossRef] [PubMed]

26.

A. Alu and N. Engheta, “Optical nanotransmission lines: synthesis of planar left-handed metamaterials in the infrared and visible regimes,” J. Opt. Soc. Am. A 23, 571–583 (2006). [CrossRef]

27.

I. Romero and F. J. Garcia de Abajo, “Anisotropy and particle-size effects in nanostructured plasmonic metamaterials,” Opt. Express 17, 22012–22022 (2009). [CrossRef] [PubMed]

28.

C. L. Holloway, E. F. Kuster, J. B. Jarvis, and P. Kabos, “A double negative (DNG) composite medium composed of magnetodielectric spherical particles embedded in a matrix,” IEEE Trans. Antennas Propag. 51, 2596–2603 (2003). [CrossRef]

29.

J. Liu and N. Bowler, “Analysis of double-negative (DNG) bandwidth for a metamaterial composed of magnetodielectric spherical particleseEmbedded in a matrix,” IEEE Antennas Wirel. Propag. Lett. 10, 399–402 (2011). [CrossRef]

30.

N. Liu, S. Mukherjee, K. Bao, L. Brown, J. Dorfmuller, P. Nordlander, and N. J. Halas, “Magnetic plasmon formation and propagation in artificial aromatic molecules,” Nano Lett. 12, 364–369 (2012). [CrossRef]

31.

S. N. Sheikholeslami, A. Garcia-Etxarri, and J. A. Dionne, “Controlling the interplay of electric and magnetic modes via Fano-like plasmon resonances,” Nano Lett. 11, 3927–3934 (2011). [CrossRef] [PubMed]

32.

J. A. Fan, C. Wu, K. Bao, J. Bao, R. Bardhan, N. J. Halas, V. N. Monaharan, P. Nordlander, G. Shvets, and F. Capasso, “Self-assembled plasmonic nanoparticles clusters,” Science 328, 1135–1137 (2010). [CrossRef] [PubMed]

33.

J. B. Lassiter, H. Sobhani, J. A. Fan, J. Kundu, F. Capasso, P. Nordlander, and N. J. Halas, “Fano resonances in plasmonic nanoclusters: geometrical and chemical tunability,” Nano Lett. 10, 3184–3189 (2010). [CrossRef] [PubMed]

34.

M. G. Moharam and T. K. Gaylord, “Three-dimensional vector coupled-wave analysis of planar-grating diffraction,” J. Opt. Soc. Am. A 73, 1105–1112 (1983). [CrossRef]

35.

D. R. Smith, S. Schultz, P. Markos, and C. M. Soukoulis, “Determination of effective permittivity and permeability of metamaterials from reflection and transmission coefficients,” Phys. Rev. B 65, 195104 (2002). [CrossRef]

36.

A. M. Nicolson and G. F. Ross, “Measurement of the intrinsic properties of materials by time-domain techniques,” IEEE Trans. Instrum. Meas. 19, 377–379 (1970). [CrossRef]

37.

C. Menzel, C. Rockstuhl, T. Paul, and F. Lederer, “Retrieving effective parameters for metamaterials at oblique incidence,” Phys. Rev. B 77, 195328 (2008). [CrossRef]

38.

Q. Cao, P. Lalanne, and J. P. Hugonin, “Stable and efficient Bloch-mode computational method for one-dimensional grating waveguides,” J. Opt. Soc. Am. A 19, 335–338 (2002). [CrossRef]

39.

P. Lalanne and E. Silberstein, “Fourier-modal methods applied to waveguide computational problems,” Opt. Lett. 25, 1092–1094 (2000). [CrossRef]

40.

M. G. Moharam, E. B. Grann, D. A. Pommet, and T. K. Gaylord, “Formulation for stable and efficient implementation of the rigorous coupled-wave analysis of binary grating,” J. Opt. Soc. Am. A 12, 1068–1076 (1995). [CrossRef]

41.

M. Shahabadi, S. Atakaramians, and N. Hojjat, “Transmission line formulation for the full-wave analysis of two-dimensional dielectric photonic crystals,” IEE Proc.-Sci. Meas. Technol. 151, 327–334 (2004). [CrossRef]

42.

R. E. Collin, Foundations for Microwave Engineering (IEEE, 2001). [CrossRef]

43.

P. B. Johnson and R. W. Christy, “Optical constants of the noble metals,” Phys. Rev. B 6, 4370–4379 (1972). [CrossRef]

44.

R. C. McPhedran and D. R. McKenzie, “Exact modelling of cubic lattice permittivity and conductivity,” Nature (London) 265, 128–129 (1977). [CrossRef]

45.

R. C. McPhedran and D. R. McKenzie, “The conductivity of lattices of spheres I. the simple cubic lattice,” Proc. R. Soc. London Ser. A 359, 45–63 (1978). [CrossRef]

46.

R. C. McPhedran, D. R. McKenzie, and G. H. Derrick, “The conductivity of lattices of spheres II. the body centred and face centred cubic lattice,” Proc. R. Soc. London Ser. A 362, 211–232 (1978). [CrossRef]

47.

T. Koschny, P. Marko, D. R. Smith, and C. M. Soukoulis, “Resonant and antiresonant frequency dependence of the effective parameters of metamaterials,” Phys. Rev. E 68, 065602 (2003). [CrossRef]

48.

R. Merlin, “Metamaterials and the Landau Lifshitz permeability argument: Large permittivity begets high-frequency magnetism,” Proc. Nat. Acad. Sci. USA 106, 1693–1698 (2009). [CrossRef] [PubMed]

49.

J. L. Atweeod, “Kagome’ lattice: a molecular toolkit for magnetism,” Nat. Matter. 1, 91–92 (2002). [CrossRef]

50.

U. Schlickum, R. Decker, F. Klappenberger, G. Zoppellaro, S. Klyatskaya, W. Auwarter, S. Neppl, K. Kern, H. Brune, M. Ruben, and J. V. Barth, “Chiral Kagome’ lattice from simple ditopic molecular bricks,” J. Am. Chem. Soc. 130, 11778–11782 (2008). [CrossRef] [PubMed]

51.

Q. Chen, S. C. Bae, and S. Granick, “Directed self-assembly of a colloidal Kagome lattice,” Nature (London) 469, 381–384 (2010). [CrossRef]

52.

A. V. Giannopoulos, C. M. Long, and K. D. Choquette, “Photonic crystal heterostructure cavity lasers using kagome lattices,” Electron. Lett. 44, 38–39 (2008). [CrossRef]

53.

X. Li, J. Zhou, Q. Wang, X. Chen, Y. Kawazoe, and P. Jena, “Magnetism of two-dimensional triangular nanoflake-based kagome lattices,” New J. Phys. 14, 033043 (2012). [CrossRef]

OCIS Codes
(160.4760) Materials : Optical properties
(240.6680) Optics at surfaces : Surface plasmons
(160.3918) Materials : Metamaterials
(160.4236) Materials : Nanomaterials
(250.5403) Optoelectronics : Plasmonics
(050.6624) Diffraction and gratings : Subwavelength structures

ToC Category:
Metamaterials

History
Original Manuscript: April 16, 2012
Revised Manuscript: June 16, 2012
Manuscript Accepted: June 19, 2012
Published: June 27, 2012

Citation
Hadiseh Alaeian and Jennifer A. Dionne, "Plasmon nanoparticle superlattices as optical-frequency magnetic metamaterials," Opt. Express 20, 15781-15796 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-14-15781


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References

  1. R. M. Erb, H. S. Son, B. Samanta, V. M. Rotello, and B. B. Yellen, “Magnetic assembly of colloidal superstructures with multipole symmetry,” Nature (London)457, 999–1002 (2009). [CrossRef]
  2. J. Henzie, M. Grunwald, A. W. Cooper, P. L. Geissler, and P. Yang, “Self-assembly of uniform polyhedral silver nanocrystals into densest packings and exotic superlattices,” Nat. Mater.11, 131–137 (2011). [CrossRef] [PubMed]
  3. A. Tao, P. Sinsermsuksakul, and P. Yang, “Tunable plasmonic lattices of silver nanocrystals,” Nat. Nanotechnol.2, 435–440 (2007). [CrossRef]
  4. Z. L. Wang, “Structural analysis of self-assembling nanocrystal superlattices,” Adv. Mater.10, 13–30 (1998). [CrossRef]
  5. N. A. El-Masry, M. K. Behbehani, S. F. LeBoeuf, M. E. Aumer, J. C. Roberts, and S. M. Bedair, “Self-assembled AlInGaN quaternary superlattice structures,” Appl. Phys. Lett.79, 1616–1618 (2001). [CrossRef]
  6. R. P. Andres, J. D. Bielefeld, J. I. Henderson, D. B. Janes, V. R. Kolagunta, C. P. Kubiak, W. J. Mahoney, and R. G. Osifchin, “Self-assembly of a two-Dimensional superlattice of molecularly linked metal clusters,” Science273, 1690–1693 (1996). [CrossRef]
  7. A. L. Rogach, “Binary superlattices of nanoparticles: self-assembly leads to metamaterials,” Adv. Funct. Mater.43, 148–149 (2004).
  8. E. V. Shevchenko, D. V. Talapin, N. A. Kotov, S. O’Brien, and C. B. Murray, “Structural diversity in binary nanoparticle superlattices,” Nature (London)439, 55–59 (2006). [CrossRef]
  9. J. J. Urban, D. V. Talapin, E. V. Shevchenko, C. R. Kagan, and C. B. Murray, “Synergism in binary nanocrystal superlattices leads to enhanced p-type conductivity in self-assembled PbTe/Ag2Te thin films,” Nat. Mater.6, 115–121 (2007). [CrossRef] [PubMed]
  10. A. Dong, J. Chen, P. M. Vora, J. M. Kikkawa, and C. B. Murray, “Binary nanocrystal superlattice membranes self-assembled at the liquid-air interface,” Nature (London)446, 474–477 (2010). [CrossRef]
  11. W. H. Evers, H. Friedrich, L. Filion, M. Dijkstra, and D. Vanmaekelbergh, “Observation of a ternary nanocrystal superlattice and its structural characterization by electron tomography,” Angew. Chem. Int. Ed.48, 9655–9657 (2009). [CrossRef]
  12. Y. G. Zhao, A. Wu, H. L. Lu, S. Chang, W. K. Lu, S. T. Ho, M. E. van der Boom, and T. J. Marks, “Traveling wave electro-optic phase modulators based on intrinsically polar self-assembled chromophoric superlattices,” Appl. Phys. Lett.79, 587–589 (2001). [CrossRef]
  13. D. G. Suna, Z. Liub, J. Mab, and S. T. Hob, “Design and fabrication of electro-optic waveguides with self-assembled superlattice films,” Opt. Laser Technol.39, 285–289 (2007). [CrossRef]
  14. A. Alu and N. Engheta, “The quest for magnetic plasmons at optical frequencies,” Opt. Express17, 5723–5730 (2009). [CrossRef] [PubMed]
  15. A. Alu and N. Engheta, “Plasmonic and metamaterial cloaking: physical mechanisms and potentials,” J. Opt. A: Pure Appl. Opt.10, 093002 (2008). [CrossRef]
  16. A. Alu and N. Engheta, “Theory of linear chains of metamaterial/plasmonic particles as subdiffraction optical nanotransmission line,” Phys. Rev. B74, 205436 (2006). [CrossRef]
  17. B. Auguie and W. L. Barnes, “Collective resonances in Gold nanoparticle arrays,” Phys. Rev. Lett.101, 143902 (2008). [CrossRef] [PubMed]
  18. A. L. Fructos, S. Campione, F. Capolino, and F. Mesa, “Characterization of complex plasmonic modes in two-dimensional periodic arrays of metal nanospheres,” J. Opt. Soc. Am. B28, 1446–1458 (2011). [CrossRef]
  19. A. Taleb, V. Russier, A. Courty, and M. P. Pileni, “Collective optical properties of silver nanoparticles organized in two-dimensional superlattices,” Phys. Rev. B59, 13350–13358 (1999). [CrossRef]
  20. D. V. Tapalin, E. V. Shevchenko, C. B. Murray, A. V. Titov, and Petr Kral, “Dipole-dipole interactions in nanoparticle superlattices,” Nano Lett.7, 1213–1219 (2007). [CrossRef]
  21. A. Alu, A. Salandrino, and N. Engheta, “Negative effective permeability and left-handed materials at optical frequencies,” Opt. Express14, 1557–1567 (2006). [CrossRef] [PubMed]
  22. C. Rockstuhl, F. Lederer, C. Etrich, T. Pertsch, and T. Scharf, “Design of an artificial three-dimensional composite metamaterial with magnetic resonances in the visible range of the electromagnetic spectrum,” Phys. Rev. Lett.99, 017401 (2007). [CrossRef] [PubMed]
  23. A. Valecchi and F. Capolino, “Metamaterials based on pairs of tightly-coupled scatterers,” in Metamaterials Handbook (CRC Press, 2009).
  24. S. Steshenko, F. Capolino, S. Tretyakov, C. Simovski, and F. Capolino, “Super resolution with layers of resonant arrays of nanoparticles,” in Metamaterials Handbook (CRC Press, 2009).
  25. S. Campione, S. Steshenko, and F. Capolino, “Complex bound and leaky modes in chains of plasmonic nanospheres,” Opt. Express19, 18345–18363 (2011). [CrossRef] [PubMed]
  26. A. Alu and N. Engheta, “Optical nanotransmission lines: synthesis of planar left-handed metamaterials in the infrared and visible regimes,” J. Opt. Soc. Am. A23, 571–583 (2006). [CrossRef]
  27. I. Romero and F. J. Garcia de Abajo, “Anisotropy and particle-size effects in nanostructured plasmonic metamaterials,” Opt. Express17, 22012–22022 (2009). [CrossRef] [PubMed]
  28. C. L. Holloway, E. F. Kuster, J. B. Jarvis, and P. Kabos, “A double negative (DNG) composite medium composed of magnetodielectric spherical particles embedded in a matrix,” IEEE Trans. Antennas Propag.51, 2596–2603 (2003). [CrossRef]
  29. J. Liu and N. Bowler, “Analysis of double-negative (DNG) bandwidth for a metamaterial composed of magnetodielectric spherical particleseEmbedded in a matrix,” IEEE Antennas Wirel. Propag. Lett.10, 399–402 (2011). [CrossRef]
  30. N. Liu, S. Mukherjee, K. Bao, L. Brown, J. Dorfmuller, P. Nordlander, and N. J. Halas, “Magnetic plasmon formation and propagation in artificial aromatic molecules,” Nano Lett.12, 364–369 (2012). [CrossRef]
  31. S. N. Sheikholeslami, A. Garcia-Etxarri, and J. A. Dionne, “Controlling the interplay of electric and magnetic modes via Fano-like plasmon resonances,” Nano Lett.11, 3927–3934 (2011). [CrossRef] [PubMed]
  32. J. A. Fan, C. Wu, K. Bao, J. Bao, R. Bardhan, N. J. Halas, V. N. Monaharan, P. Nordlander, G. Shvets, and F. Capasso, “Self-assembled plasmonic nanoparticles clusters,” Science328, 1135–1137 (2010). [CrossRef] [PubMed]
  33. J. B. Lassiter, H. Sobhani, J. A. Fan, J. Kundu, F. Capasso, P. Nordlander, and N. J. Halas, “Fano resonances in plasmonic nanoclusters: geometrical and chemical tunability,” Nano Lett.10, 3184–3189 (2010). [CrossRef] [PubMed]
  34. M. G. Moharam and T. K. Gaylord, “Three-dimensional vector coupled-wave analysis of planar-grating diffraction,” J. Opt. Soc. Am. A73, 1105–1112 (1983). [CrossRef]
  35. D. R. Smith, S. Schultz, P. Markos, and C. M. Soukoulis, “Determination of effective permittivity and permeability of metamaterials from reflection and transmission coefficients,” Phys. Rev. B65, 195104 (2002). [CrossRef]
  36. A. M. Nicolson and G. F. Ross, “Measurement of the intrinsic properties of materials by time-domain techniques,” IEEE Trans. Instrum. Meas.19, 377–379 (1970). [CrossRef]
  37. C. Menzel, C. Rockstuhl, T. Paul, and F. Lederer, “Retrieving effective parameters for metamaterials at oblique incidence,” Phys. Rev. B77, 195328 (2008). [CrossRef]
  38. Q. Cao, P. Lalanne, and J. P. Hugonin, “Stable and efficient Bloch-mode computational method for one-dimensional grating waveguides,” J. Opt. Soc. Am. A19, 335–338 (2002). [CrossRef]
  39. P. Lalanne and E. Silberstein, “Fourier-modal methods applied to waveguide computational problems,” Opt. Lett.25, 1092–1094 (2000). [CrossRef]
  40. M. G. Moharam, E. B. Grann, D. A. Pommet, and T. K. Gaylord, “Formulation for stable and efficient implementation of the rigorous coupled-wave analysis of binary grating,” J. Opt. Soc. Am. A12, 1068–1076 (1995). [CrossRef]
  41. M. Shahabadi, S. Atakaramians, and N. Hojjat, “Transmission line formulation for the full-wave analysis of two-dimensional dielectric photonic crystals,” IEE Proc.-Sci. Meas. Technol.151, 327–334 (2004). [CrossRef]
  42. R. E. Collin, Foundations for Microwave Engineering (IEEE, 2001). [CrossRef]
  43. P. B. Johnson and R. W. Christy, “Optical constants of the noble metals,” Phys. Rev. B6, 4370–4379 (1972). [CrossRef]
  44. R. C. McPhedran and D. R. McKenzie, “Exact modelling of cubic lattice permittivity and conductivity,” Nature (London)265, 128–129 (1977). [CrossRef]
  45. R. C. McPhedran and D. R. McKenzie, “The conductivity of lattices of spheres I. the simple cubic lattice,” Proc. R. Soc. London Ser. A359, 45–63 (1978). [CrossRef]
  46. R. C. McPhedran, D. R. McKenzie, and G. H. Derrick, “The conductivity of lattices of spheres II. the body centred and face centred cubic lattice,” Proc. R. Soc. London Ser. A362, 211–232 (1978). [CrossRef]
  47. T. Koschny, P. Marko, D. R. Smith, and C. M. Soukoulis, “Resonant and antiresonant frequency dependence of the effective parameters of metamaterials,” Phys. Rev. E68, 065602 (2003). [CrossRef]
  48. R. Merlin, “Metamaterials and the Landau Lifshitz permeability argument: Large permittivity begets high-frequency magnetism,” Proc. Nat. Acad. Sci. USA106, 1693–1698 (2009). [CrossRef] [PubMed]
  49. J. L. Atweeod, “Kagome’ lattice: a molecular toolkit for magnetism,” Nat. Matter.1, 91–92 (2002). [CrossRef]
  50. U. Schlickum, R. Decker, F. Klappenberger, G. Zoppellaro, S. Klyatskaya, W. Auwarter, S. Neppl, K. Kern, H. Brune, M. Ruben, and J. V. Barth, “Chiral Kagome’ lattice from simple ditopic molecular bricks,” J. Am. Chem. Soc.130, 11778–11782 (2008). [CrossRef] [PubMed]
  51. Q. Chen, S. C. Bae, and S. Granick, “Directed self-assembly of a colloidal Kagome lattice,” Nature (London)469, 381–384 (2010). [CrossRef]
  52. A. V. Giannopoulos, C. M. Long, and K. D. Choquette, “Photonic crystal heterostructure cavity lasers using kagome lattices,” Electron. Lett.44, 38–39 (2008). [CrossRef]
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