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Plasmonic Fano resonances in metallic nanorod complexes Zhong-Jian Yang,†ab Zhong-Hua Hao,a Hai-Qing Lin*b and Qu-Quan Wang*a Plasmonic Fano resonances (FRs) in nanostructures have been extensively studied in recent years. Nanorod-based complexes for FRs have also attracted much attention. The basic optical properties and fabrication technology of different kinds of plasmonic nanorods have been greatly developed over the

Received 8th December 2013 Accepted 6th February 2014

last several years. The mutipole plasmon resonances and their flexible adjustment ranges on nanorods make them promising for FR modifications and structure diversity. In this paper, we review some recently studied plasmonic nanorod based nanostructures for FRs, including single nanorods, dimers,

DOI: 10.1039/c3nr06502b

mutipole rods and nanorod–nanoparticle hybrids. The corresponding applications of the FRs are also

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briefly discussed.

1. Introduction Ever since the explanation of an abnormal asymmetric lineshape by Fano,1 the Fano resonances (FRs) have been investigated in many systems2 such as quantum dots,3,4 atoms,5 photonic structures6–8 and Bose–Einstein condensates.9 In recent years, Fano resonance in plasmonic nanostructures has also attracted much attention10–12 and this is mainly due to the a

Key Laboratory of Articial Micro- and Nano-structures of Ministry of Education, School of Physics and Technology, Wuhan University, Wuhan 430072, China. E-mail: [email protected]

b

Beijing Computational Science Research Center, Beijing 100084, China. E-mail: [email protected] † Present address: Department of Applied Physics, Chalmers University of Technology, 412 96 G¨ oteborg, Sweden.

Prof. H.Q. Lin started his career in computational physics when he was a graduate student at University of California at San Diego. His main research areas are condensed matter and computational physics with a focus on many-body systems. He was elected to the Fellow of American Physical Society, Computational Physics Division in 2003. In recent years, Prof. Lin leads a research group working in the area of simulating optical properties of nanostructures. Currently he is the director of Beijing Computational Science Research Center and he serves on several editorial boards of international journals in computational physics.

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following reasons: (1) the fundamental physics in these FR structures. For example, people have been trying to construct bright and dark modes in order to obtain FRs in designed structures.13,14 On the other hand, for some FR-like systems one needs to nd out the involved modes and their coupling behaviors due to the interactions.15 The general explanations of FRs in plasmonic structures are now attracting attentions.2,15–22 (2) The rapid progress in the experimental techniques and corresponding simulation methods enables the realization of these studies at the nanoscale. (3) The potential applications are so important that they strongly drive people to devote their efforts in studying them. These potential applications include chemical and biological sensors,23–27 surface enhanced Raman scattering (SERS)28–30 and nonlinear optics.31,32 Many different kinds of structures have been explored and one big family is the nanoparticle based system which includes single particles,10 dimers33,34 and nanoparticle clusters,15,35–43 where a particle is generally of a dipole surface plasmon (SP) resonance in the FRs. Some discussions about this family can be found in recent reviews.10,11 Here in this review we will discuss the FRs in nanorod based plasmonic systems, which is another big family of Fano resonant nanostructures. The plasmon properties of metal nanorods have been extensively studied since the early years of plasmonics.44–48 Both physical and chemical fabrication techniques have been greatly developed,47–50 as have the theoretical descriptions of the nanorod systems.51–53 Generally, for a nanorod with a small ratio of length to diameter (or length to width for a rectangle shape), there is a strong dipole SP resonance mode. When the ratio is increased, a nanorod could also have higher-order SP resonance modes.53–55 There are two properties of nanorod structures that make them advantageous for FRs. One is the highly developed fabrication techniques for nanorods. The other one is the highly tunable plasmon modes of individual rods for the couplings.

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Nanorods as well as nanoparticles are probably the most popular simple basic elements for studying the optical properties of plasmonic structures. More complicated structures, such as nanorings, also have high tunable plasmon modes, but these structures cannot be realised easily in experiments. As for particles such as nanodisks or spheres, the plasmon modes of individual particles are not as tunable as the modes of nanorods, which makes it difficult to obtain higher order SP modes. These SP modes on nanorods are also highly tunable with rod geometries. The combination of two or more nanorods will induce couplings between their SPs and are usually coherent. Even in a single nanorod, different SP modes could also couple with each other. Among them, one important result caused by the plasmon couplings is the FR, which will be the focus of this paper. Aer a brief introduction of the theory background of plasmonic FR, we will discuss the FRs in nanorod-based systems that include single rods, dimers, mutipole rods and nanorod–nanoparticle hybrids. The applications of FRs in these systems will also be discussed at the end.

2. Theory background of plasmonic FR FR was rst studied by quantum mechanical theory, where the interference of a discrete state with a continuum state induces a distinctly asymmetric optical response lineshape. A formula for the FR shape was also obtained:1,2 FðEÞ ¼ A

ðb þ qÞ2 ; b2 þ 1

x€b + gbx_ b + ub2xb + n12xd ¼ a1eiut,

(2)

x€d + gdx_ d + ud2xd + n12xb ¼ 0,

(3)

and the amplitude of the highly damped oscillator is then given by: cb ¼

ud 2 þ igd u  u2   a1 : ub 2 þ igb u  u2 ud 2 þ igd u  u2  n12 2

(4)

Usually in FR plasmonic systems, we have gd  gb  ub, ud, and one can obtain that |cb|2 has an FR formula similar to eqn (1). Other optical properties, for example absorption, can also be studied under this model. In earlier studies, this model was used to investigate some specic systems.19,57–59 Recently, an extended coupled oscillator model was developed.16 It has general validity and one could recover the earlier models with specic parameters. Several other models have also been reported recently.17–20,22 More physical information can be obtained from these models. For example, the dependence of the FR lineshape on the plasmon resonance width, positions, and the coupling strength, respectively. They are accurate but require more complex mathematical derivations.

3.

FRs in single nanorods

(1)

where the Fano factor q is the shape parameter which describes the degree of asymmetry, A is the amplitude constant and b ¼ (E  E0)/G. E0 is the resonant energy and G is the width of resonance. This theory has also been extended to explain lots of classical systems, where the discrete and continuum states are replaced by modes with a narrow and wide spectrum width, respectively. A direct analogue is the small Au–Ag sphere nanoparticle dimer.56 But in most plasmonic structures, the narrow and wide modes are obtained by constructing bright (|Bi) and dark (|Di) SP resonances.10 The bright mode then can be excited by two pathways: |Ii / |Bi and |Ii / |Bi / |Di / |Bi, where |Ii is the excitation source. The interference between these two pathways can induce an FR. This description gives a clear physical picture that aids in understanding FR. The classical coupled oscillator model is also usually used to analyse plasmonic FRs.16,19,57–59 Although this model is simple, it provides signicant insight into the understanding of FRs. In this model there are two harmonic oscillators, each has its own resonance frequency and damping rate. There is a coupling between the two oscillators. Then the motion equations of them can be given. The oscillation amplitudes can be obtained analytically and used to study the system's optical responses. Suppose the rst oscillator corresponds to a bright mode, which has a resonance frequency ub, high energy dissipation rate gb and displacement xb ¼ cbeiut. The second oscillator corresponds to a dark mode with frequency ud, low energy dissipation rate gd

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and displacement xd ¼ cdeiut. The coupling between them is denoted by n12. The rst one is driven by an external force a1eiut. The equations of motion can be written as:

Two or more modes are usually involved to undergo Fano interferences, so for a single nanorod one should excite different modes on it. One way is to excite different longitudinal modes along the rod.60–62 Generally, the dipole SP mode of a plasmonic nanorod can be excited easily by a normal incidence plane wave. When the rod length is very large compared with the diameter (or wide for rectangular shape), the higher mode with a three half wavelength resonance (denoted by n ¼ 3, the dipole mode corresponds to n ¼ 1) can be excited as shown by Fig. 1(a). Fano resonance can be found due to the coherent coupling between these two modes. Fano proles were also carried out to t the electromagnetic calculation results and they show good agreement.61 It has been shown recently that even higher modes (n ¼ 4 and 5) can also been observed.62 The experimental results of the rod scattering spectra show clear asymmetric peaks for the high resonance modes (n ¼ 3, 4 and 5), and these lineshapes can be tted by the Fano prole function well. Here it should be noted that the mode with an even number n cannot be excited with a normal incidence plane wave due to the symmetry. But these modes can be excited with an oblique incidence plane wave or a dipole point source. Another way is to bring symmetry breaking to the system by putting a nanorod on a high index dielectric,63 as shown in Fig. 1(b), and this will induce multipolar plasmon modes. The excitation white light with oblique incidence has both p- and spolarizations (Fig. 1(b)). So the excitation electric eld can be decomposed into three components, which are so called

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Fig. 1 The FRs in single nanorods. (a) The scattering spectrum and near-field distributions for a single silver rod with a surrounding medium dielectric constant of 2.25. The dashed line is the Fano profile function fitting result. (b) A schematic showing the structure with different excitation manners. Left: the white light source at an oblique incidence angle. Right: top: HL; middle: HT; bottom: V. (c) Left: the experimental (black circles) and calculated (colored curves) scattering spectra of the Au nanorod on the glass substrate. Right: the results for the Au nanorod on the silicon substrate. The green, pink and blue curves represent the HL, HT, and V excitation, respectively. The red curve represents the sum of the three curves. (Reproduced from ref. 61 (a) and ref. 63 (b and c).)

horizontal and longitudinal (HL), horizontal and transverse (HT) and vertical (V) excitations, respectively. The high index dielectric can induce image charges of the plasmonic modes. This will make the plasmon mode with the V excitation stronger, and a clear new higher-order mode appears with HL excitation. Finite-difference time-domain (FDTD) calculations show that these two modes correspond to quadrupolar and

octupolar resonance modes. The coherent couplings between these two modes result in the Fano resonance as shown in Fig. 1(c). The low index dielectric does not make this happen because it cannot induce the above modes. Small nanorods do not show Fano resonance either, even with a high index dielectric, because the new mode with HL excitation cannot be excited.

Fig. 2 (a) The absorption spectra of each rod in the Au–Ag nanorod dimer. The fittings with Fano functions are shown by the dashed lines. (b) The far field optical responses of a dipole–quadrupole plasmon coupling nanorod dimer as a schematic shown in the top of (c). The near field distributions near FR are also shown in the bottom of (c). (d) The scattering spectrum and the plasmon hybridization diagram for a nanorod heterodimer. The nanorod aspect ratios are 3.5 and 5. (e) The SP resonance energies of individual rods. (f) The charge distributions for the longer nanorod and the dimer. (Reproduced from ref. 64 (a), ref. 66 (b and c) and ref. 70 (d–f).)

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It is seen that although there are FRs in single nanorods in Fig. 1(a)–(c), their origins are quite different. In the rst structure, the modes that are involved for the Fano interference are both simple longitude plasmon resonances, while in the second one, FR is caused by the longitude and transverse plasmon modes. Here, the high index dielectric plays an important role for the excitation of the transverse mode (with V excitation), because without the dielectric this transverse mode is usually too weak to be coupled for FRs.

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4. FRs in nanorod dimers

Fig. 3 (a) The nonradiative enhancements of a dipole by a dimer and an individual short nanorod. The inset shows a schematic of the dipole–dimer coupling configuration. (b) The absorption spectra of the T-shaped nanodimer with different gaps. The insets show field distributions. (c) Left: a schematic of a dimer structure. Right: the field distribution of the individual long rod SP mode and the locations defined along the rod. (d) The absorption spectra of the short rod in the dimer with different locations along the long rod. (e) The absorption spectrum splitting of the short rod as a function of its location. (Reproduced from ref. 68 (a), ref. 71 (b) and ref. 72 (c–e).)

As it has been shown that there could be several longitudinal SP modes on a single plasmonic nanorod. So when two nanorods form a dimer structure, there could be many different couplings between the SPs on the two rods.64,65 A simple dimer is a structure consisting of two dipole resonant SP modes64 as shown in Fig. 2(a). In this structure, one dipole mode is on an Au nanorod and the other mode is on an Ag nanorod. Hybridization of the two dipole SP modes occurs, and this results in a bonding mode and an antibonding one in the dimer. Near the antibonding mode, one can nd twinned Fano-like lineshapes on the optical spectra of the two rods. These spectra were also tted well with Fano line-shape function. It is interesting that the Fano factor (q) for the two rods always have opposite signs, which means their lineshapes are of reversed asymmetry. The congurations with two rods side by side were also investigated, and they show similar Fano interferences. To construct a bright and dark SP mode with their spectra overlapping each other is an effective way to get Fano resonance

Fig. 4 FR in asymmetric Pi-shape nanorod structures. (a) The experimental setup for the near-field imaging in the reflection mode. (b) The numerically calculated reflection spectra for the horizontal (blue) and vertical (red) polarization as indicated by the schematics. The letters mark the positions on the spectra where near-field imaging was performed. (c) The experimental (upper row) and calculated (lower row) amplitude |Ez| and phase images for horizontal polarization, recorded at 10.2 mm wavelength. (d) The experimental (upper row) and calculated (lower row) amplitude |Ez| and phase images for vertical polarization, recorded at the positions A–D in (b). (Reproduced with permission from ref. 73.)

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Fig. 5 (a) Top: a schematic of the plasmonic system consisting of a radiative element and a dark element with a separation d. Bottom: The field plot of an uncoupled radiative atom (left) and the coupled system (right) at the FR dip of the optical spectrum. (b) The numerical reflection spectra for a cascaded structure where the individual p-shaped structures are asymmetrically placed. The distance between the constituent elements are 100 nm (red curve) and 700 nm (black curve). The result for an individual p-shaped structure is shown in (c). (d and e) The experimentally and numerically obtained extinction spectra of an individual dolmen structure, respectively, with polarization as defined by the arrows in panel d. (f and g) The calculated surface charge distributions of the dipolar mode and the Fano extinction dip, respectively. (h) Scanning electron micrographs of a fabricated EIT metamaterial structure. (i) The experimental transmittance and reflectance spectra for the structure in (h). (j) The electric field distribution near the FR dip of the reflectance spectrum in (i). (Reproduced from ref. 13 (a), ref. 77 (b and c), ref. 14 (d–g) and ref. 76 (h–j).)

in a plasmonic structure. In nanorod systems, a dipole SP mode can couple with a plane wave easily and it could serve as a bright mode. However, it is also known that a quadrupole SP resonance could hardly couple with a plane wave, especially for normal incidence, so it could serve as a dark mode. Now, the two can form a dimer66–69. Fig. 2(b) and (c) show such a kind of structure,66 and Fano resonance is indeed observed. The bright dipole mode |Bi has a low quality factor Q, while that of the dark quadrupole mode is high. They are coupled with each other through near eld interaction. When the energy is transferred from low Q |Bi to high Q |Di, it will go back easily. Then the pathway |Ii / |Bi / |Di / |Bi forms and FR occurs. We can also understand the Fano interference in this system with respect to the structure as a whole. As it is known that in some structures, the bright and dark modes obviously come from different segments of them,13,14 while in some other structures, the bright and dark modes are both supported by the structure as a whole.15 For the structure shown in Fig. 2(b) and (c), the bonding and antibonding hybridizations of the dipole and quadrupole plasmons occur near the right and le peaks, respectively. These make the structure as a whole bright to excitation, while at the dip, the total structure is dark. So the Fano dip can be taken as a result of the coupling between the bright and dark modes of the dimer. Similarly, the FR lineshape in Fig. 2(a) could also be understood from this point of view. The bonding and antibonding coupling of two dipole SPs will induce bright and dark modes of the dimer as a whole, and so FR occurs. FRs involving more complex SP hyridization modes in some other dimers have also been studied.70 Fig. 2(d)–(f) show such a dimer structure. The two nanorods have aspect ratios, L/(2R)

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(where L is the rod length and R is the radius) of 3.5 and 5, and both have a diameter of 20 nm. When put together, the SP modes on the two rods will be hybridized. The two peaks of the dimer are corresponding to the hybridized dipole–dipole bonding and dipole–quadrupole bonding modes. Their interferences result in an FR dip, where the dipole–dipole bonding mode is bright and the dipole–quadrupole mode is dark. It should be noted that although there has been a great development in the fabrication techniques for plasmonic structures, not all of the theoretical structures could be fabricated ideally in the experiments. For example, in Fig. 2(b) and (c), the gap between two rods is 10 nm, which is still a challenge to fabricate. Usually a small gap between elements is necessary to achieve relatively stronger FRs. Thus, there is still room for improvement in the realization of strong coupling FRs in experiments. The dimers shown in Fig. 2 are two rods being positioned in a line. Some other congurations, such as structures with two rods vertical to each other, have also been studied.68,71,72 Fig. 3(a) and (b) show similar dimer structures where there is a dipole SP on the short rod and a quadrupole SP mode on the long rod. When the short rod is placed near the apex of the long rod, a weak FR is seen in Fig. 3(a).68 The FR in this system is similar to that in Fig. 2(b). When the short rod is placed at the center of the long rod, the FR is very strong as shown by Fig. 3(b).71 Another similar conguration consisting of a short rod with dipole SP mode and a long rod with higher-order mode (3 halfwave SP resonance) was also studied72 as shown in Fig. 3(c). The dipole SP is a bright mode, while the higher mode is dark in the FRs. The FR strength can be greatly modied by the near eld distribution on the long rod. As is seen in Fig. 3(d) and (e) that

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Fig. 6 (a) The absorption spectrum of the short rod at z2 ¼ 83 nm in the system with a long rod and two identical short rods. The configuration is similar to that in Fig. 3(c). The black curve is the result of one single short rod located at z2 ¼ 83 nm coupled to a long rod. The two short rod locations along the long rod are (z1, z2) ¼ (225 nm, 83 nm). (b) and (c) The z1 is changed and the configurations are (z1, z2) ¼ (83 nm, 83 nm) and (z1, z2) ¼ (116 nm, 83 nm), respectively. (d) The field distribution at the right peak in (a). (e) The field distribution at the peak in (b). (f) A schematic of a plasmonic structure with four rods. (g) A numerical simulation (red line) and extended coupled oscillator model fit (black line) of the scattering spectrum for a structure in (f). (h) A schematic of another plasmonic structure with four rods. (i) The calculated transmission spectra from uncoupled (blue solid), symmetrically coupled (red dashed), and asymmetrically coupled (black solid) optimized structures in (h). The corresponding experimental results are shown in (j). (k) The top-view charge distributions representing the excitation of the dark modes at respective FR in asymmetrically coupled configurations. (Reproduced from ref. 72 (a–e); with permission from ref. 16 (f and g) and ref. 81 (h–k).)

FR is very strong for the short rod locating near the standing wave antinodes, and the corresponding spectra splittings are large. When the short rod is near the standing wave nodes, no FR occurs, and there is no spectrum splitting. So the spectrum splitting, which is closely related to the FR strength, shows an oscillation with short rod position along the long rod. Fig. 4 shows another kind of asymmetric p-shaped nanorod “dimer” structure which shows FRs.73 It seems a little reluctant to call this structure a dimer as it is more like three nanorods (two long and one short) with two being welded together. However, one can also take this structure as a straight nanorod and a bent one; more importantly, the plasmon resonance properties of this structure make it behave like a heterodimer. An important useful experiment setup called scattering-type near-eld optical microscopy (see Fig. 4(a)) was used to directly map the spatial eld distribution of the Fano modes in the infrared wavelength region for this system. This technique, together with others74,75 that can measure the near elds directly are very exciting and useful to study coherent couplings in plasmonic structures experimentally. Fig. 4(b) shows the reection spectra for this structure with different excitation polarizations (horizontal (blue) and vertical

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(red)). For the horizontal excitation, a peak is seen which is the quadrupole plasmon resonance on the whole structure. This mode can be veried by near eld distribution with amplitude and phase information from both experimental and theoretical calculation methods (Fig. 4(c)). For vertical excitation, the two dipole resonances in the quadrupole mode show a 180 phase difference, so it cannot be excited directly and it is dark. But the dipole mode on a long rod can be excited directly and it is bright (see image A in Fig. 4(d)). The dark mode can be excited by the bright one through near eld coupling (see image C in Fig. 4(d)). The Fano interference between these two modes makes a dip on the reection spectrum. This interference can also be demonstrated by the near eld intensity toggling at different wavelengths around the dip (Fig. 4(d)).

5. FRs in multipole nanorods structures FRs in plasmonic nanostructures with more than two nanorods as building blocks have been studied extensively. Many efforts

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Fig. 7 (a) The calculated scattering spectra of the heterodimers. The inset shows the definition of the displacement, h, of the nanosphere. (b) The

charge distributions on the central cross sections of the heterodimers for the higher-energy (peak 1) and lower-energy (peak 2) scattering peaks and the dip between them. (c) The plasmon hybridization diagram for the nanorod–nanosphere heterodimer. (d) A schematic of the standing Au nanorod array on the percolating Au nanoparticle film (side view). (e) The absorption spectra of an Au nanorod array with a different incidence angle. (f) The transmittance spectra of the percolating Au nanoparticle film with (blue line) and without (magenta line) an Au nanorod array. (g) The normalized difference of the transmittance (DT/T) of the nanorod-film hybrid involving the percolating nanoparticle film. (Reproduced with permission from ref. 87 (a–c) and ref. 88 (d–g).)

Fig. 8 The experimental extinction spectra of nanorod heptamers with (a) azimuthal arrangement, and (c) radial arrangement for the parallel and perpendicular polarizations. (b and d) The charge distributions of the eigenmodes for an azimuthal nanorod heptamer and a radial one, respectively. (Reproduced from ref. 82.)

have been devoted into the investigations of the FR based plasmonic metamaterials.13,76–79 Fig. 5(a) shows a pioneer theoretically proposed plasmonic electromagnetically induced transparency (EIT) metamaterial based on the FR in this system.13 The structure consists of three dipole SP resonant nanorods. This kind of arrangement is also called a dolmentype structure. The right one can be excited by the light directly and works as a bright mode. The le two rods cannot be excited by the light directly, but the SP modes on them can be driven by

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the bright mode. Thus, the two SP modes will always have opposite dipole moments. They together form a quadrupole SP resonance and work as a dark mode. The FR in this system is caused by the couplings between the bright and dark modes. The FR in this dolmen-type structure was also experimentally realized and studied later14 as shown by Fig. 5(d)–(g). The structure is shown by the inset in Fig. 5(d). The experimental extinction spectra with two different excitation eld polarizations (as denoted by the red and blue arrows) are shown in Fig. 5(d). FDTD calculations were also carried out (Fig. 5(e)) and there is excellent agreement between the experimental and theoretical results. With horizontal excitation polarization (red arrow), a broad dipolar mode of the whole structure is excited and a wide peak is seen on the spectrum (red line in Fig. 5(e)). The surface charge distribution at the peak (l ¼ 780 nm) veries the dipolar mode here, where the three dipole modes are in phase and make the whole a broad dipolar mode (Fig. 5(f)). While for perpendicular polarization (blue arrow), near l ¼ 780 nm the dipole modes on the two parallel rods are out of phase. For individual dimers such as this, the mode would be dark due to the very weak coupling to the excitation. However, it can be coupled to the bright dipole mode on the right rod through near eld interaction (Fig. 5(g)). The dispersive coupling between the bright and dark modes results in a dip on the spectrum. It is seen that the plasmon couplings in FR here are the same as that in Fig. 5(a). But the EIT phenomenon is a special kind of FR, because its dip is very sharp, which requires more efforts to optimize the structure and accurate nanofabrication in experiments. Similar to this kind of structure, a three-dimensional (3D) plasmonic EIT metamaterial was then proposed and realized in experiment76 as illustrated by Fig. 5(h). The two-layer structure consists of a single rod with dipole SP resonance and two rods working together to form a quadrupole SP mode (Fig. 5(j)). This system experimentally shows a good coupling efficiency

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Fig. 9 (a) The analytic and measured reflectivity of a Pi-shape before (solid lines) and after (dashed lines) functionalization with a 3 nm thick protein A/G monolayer. The corresponding analytical and experimental reflectivity difference is shown in (b). (c) Schematic representations of protein mono- and bilayers binding to the structure (not to scale) and the equivalent dielectric model. The theoretical (d) and experimental (e) normalized reflectivity DR/RQ spectra before (dashed lines) and after (solid lines) the binding of IgG antibodies to three different structures immobilized by the protein A/G. (Reproduced with permission from ref. 23.)

(Fig. 5(i)). The plasmonic EIT metamaterials with more than three nanorods have also been theoretically investigated77 as shown in Fig. 5(b). They consist of two cascaded p-shaped structures. For a single p-structure, an FR dip appears on the reection spectrum (Fig. 5(c)). When another structure is put nearby with an asymmetric design, the FR dip is enhanced due to the cooperative effect between the two structures. As the distance between them is increased to 700 nm, the near eld interaction decreases and the spectrum becomes to be that of a single one. It should be pointed out that in lots of the FR structures mentioned above (for example those in Fig. 2, 3 and 5), there are also plasmon hybridizations around the FRs. Although there are both spectrum splittings and common plasmon hybridizations80 in these structures, near the spectra dips (between splittings) their plasmon response behaviors are different. For the FRs, there are two excitation pathways |Ii / |Bi and |Ii / |Bi / |Di / |Bi near the dips. For common plasmon hybridizations a dip between splits means that no mode is there (there is no or very weak optical response). As one can see there are clear plasmon modes on the dark cavities (see for example Fig. 2(c), 3(b) 5(a) and (j)) at the FR dips, this is due to the |Ii / |Bi / |Di / |Bi excitation pathway. But if there were only common hybridizations in these structures no plasmon modes should be observed at the dips. In addition to the plasmonic metamaterials, there are also some other complex plasmonic structures that consist of more than two nanorods and show FRs.18,65,72,81,82 Generally, SP modes on some rods will work collectively and the phase differences between them are very important. Fig. 6(a)–(e) show FRs of a system consisting of three nanorods.72 The SP properties of the individual elements are the same as those in Fig. 3(c)–(e). Two of

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them are dipole SP resonant optical antennas, and the long nanorod is a F-P cavity with a 3 half-wave SP resonance. The coupling between one antenna and the F-P cavity was also discussed above. Here, when there are two antennas in this structure, the FRs can be greatly modied due to the cooperative effects of them. For two antennas both being located near the standing wave antinodes with distance along the cavity being equal to a SP wavelength (2p phase difference), a constructive effect occurs and the FR is enhanced (Fig. 6(a)). While the distance is changed to be half of a SP wavelength (p phase difference), a destructive effect occurs and the FR is destroyed (Fig. 6(b)). It was also found that the eld on the F-P cavity with a constructive effect is much stronger than that with a destructive effect (Fig. 6(d) and (e)). When one antenna is placed at the standing wave node, there is no plasmon coupling between it and the cavity. So it has no inuence on the other antenna (Fig. 6(c)). Fig. 6(f) shows a plasmonic nanostructure with four nanorods.16 Each of them is dipole SP resonant. So they are all bright modes, and their hybridizations produce FRs in this system. It is noted that Fig. 2(a) also shows FRs in the hybridized systems with only bright modes, but the structure here is better for the generation and modications of the FRs. The center two rods work together and form an independent resonator, and two nanorods on two sides form the other resonator. They can couple together and the whole structure is superradiant when all of the dipole modes are in phase and subradiant when the center ones are out of phase with the side ones. As a result, the FR can be induced by the interference between the superradiant and subradiant modes. A simple extended coupled oscillator model was introduced in this work and tted the optical spectra very well (Fig. 6(g)). Fig. 6(h) also shows another structure with four nanorods,81 and each of them is dipole SP resonant too. But

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Fig. 10 (a) A schematic of a triangular nanoprism dipole–quadrupole

plasmon coupling dimer. (b) The D + Q (red) and D–Q (black) modes of the dimer as a function of the environment refractive index; the blue line shows the case of the symmetric nanoprism dimer composed of two identical prisms with length 200 nm. (c) The experimental spectra of differential reflectance DR/R acquired by spatial modulation spectroscopy for a dipole–quadrupole plasmon coupling dimer. (d) The corresponding nonlinear signal for pumping energy of 60 pJ probed with a second pulse at 5 ps delay after the pump pulse arrives. (e and f) The corresponding spectra obtained with FDTD simulations. (Reproduced with permission from ref. 92 (a and b) and ref. 32 (c–f).)

there are obvious differences in the structure congurations between the above system and this one. In the latter system, the two side rods are on one side, and they can also be moved apart from the center of the two collinear rods. These variations cause signicant differences in the optical responses including the FR properties as shown by Fig. 6(i) and (j). Double FRs can be obtained in this system, and the corresponding charge distributions at the two FR dips are also shown in Fig. 6(k). It is seen that any of the dark modes here are different from the above one, where the charge distributions of the side rods or center rods are always the same. Another property in this double FR system is that the radiations near the FRs show directional patterns.

6. FRs in nanorods–nanoparticle hybrids In addition to the above FRs in the single or coupled rod systems, the FRs induced by the coupling between the rods and other plasmonic geometry structures have also reported.83–88 For

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example, the systems with couplings between a nanorod and a nanoparticle have been extensively studied.87 Fig. 7(a) shows this type of structure, where there is a small Au nanoparticle and an Au nanorod. The structure was experimentally realized, and its optical properties were also studied. In comparison to the single rod situation which shows only one resonance peak, the optical spectrum shows a great change in the coupling structure, and two peaks with a clear dip appear. FDTD simulations were also carried out to analyze this system. The nanoparticle shows a dipole plasmon resonance, and both the dipole and quadrupole plasmon modes of the nanorod are involved in the couplings (Fig. 7(b) and (c)). It was found that there is a bonding dipole–dipole coupling mode near the main peak and a bonding dipole–quadrupole plasmon coupling mode near the dip (Fig. 7(b)). These pronounced dips are induced by the Fano interference between these two modes. It was also shown that the FR lineshapes can be changed a lot with the particle location. Fig. 7(d) shows another nanorod-nanoparticle coupled structure, which consists of an Au nanorod array and a lm formed by Au nanoparticles.88 The rod array shows a clear long-axis SP resonance on the optical spectrum (Fig. 7(e)). The percolating Au nanoparticle lm shows a constant optical transmittance for the large wavelength region (Fig. 7(f)). When these two are coupled with each other through near eld interactions, an FR occurs and a strong transmission enhancement is observed (Fig. 7(f) and (g)). FDTD calculations were also used to study this system, and the results also show a similar phenomenon. In this system, the plasmon modes on the Au particle lm can be excited by the normal incidence (qin ¼ 0 ) p-polarized plane wave directly, and there is a large wavelength range response which can be seen from the low transmission region. For the nanorods, as it is seen from the absorbance spectrum, there is no effective excitation of the plasmon modes with normal incidence. But in fact there is a long-axis surface plasmon mode for them and it can be effectively excited by a p-polarized wave with a high incidence angle (qin ¼ 80 , see Fig. 7(e)). So now when these two parts are put together and excited by a normal incidence plane wave, the plasmon mode on the particle lm is bright (|Bi) and that on the nanorods is dark (|Di). They couple with each other through near eld interaction, and the Fano interference between these modes appears, which causes a large enhancement of the transmission. The calculations also show that the near eld on the rods is enhanced due to the couplings in FR. This causes an enhancement of the avalanche multiphoton luminescence magnitude by 2 orders. Recently, the FR properties have been studied in the heptamer type structures82 which consist of a central circular nanoparticle and six surrounding nanorods as shown by the insets in Fig. 8. Two kinds of nanorod orientations were investigated and they are azimuthal (Fig. 8(a)) and radial (Fig. 8(c)) arrangements. Fig. 8(a) shows the experimental extinction spectra of an azimuthal nanorod heptamer with the parallel (x-axis) and perpendicular (y-axis) polarizations. The FR dips were obtained for both polarizations. The dip positions are different because of fabrication error. For a heptamer with perfect symmetry, the two spectra should be the same according

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to the simulation results. The eigenmodes were obtained by the boundary integral eigenvalue calculations to understand these results. Two eigenmodes are shown in Fig. 8(b). They both have a net dipole moment with x-dipoles which can interact with x-polarized light (there are also similar eigenmodes with y-dipoles). The lower energy mode is a dark mode, where the center dipole moment is opposite to the combined surrounding ones. The higher energy mode is bright because the dipoles are in phase. Although this method reveals the nature of each mode, it is valid only in the electrostatic limit and the retardation effect must be taken into account for large structures here. The retardation effect causes a broadening and redshi of the bright mode, which causes it to overlap and Fano interfere with the relatively stationary dark mode. As for radial nanorod heptamers, there is no FR dip on the spectra as shown in Fig. 8(c). There are also two eigenmodes with x-dipoles, but the difference here is that the low energy is bright and the high energy mode is dark (Fig. 8(d); there are also similar eigenmodes with y-dipoles). The redshi bright mode with a retardation effect cannot overlap the dark mode. So no FR is found here. Here it should be pointed out that in large scale FR plasmonic oligomers including the structures above, the effect of the Young's interference is very important to obtain a complete understanding of the plasmonic responses.89

7. Applications The FRs in plasmonic nanostructures can nd a lot of applications, where nanorod based systems also show good performances. One obvious application is in chemical and biological sensors. Fig. 9 shows a kind of FR nanorod structure for molecule sensing.23 The structure conguration is the same as that discussed in Fig. 4, so the FR is also the same. The sensing properties were studied by measuring the relative change of the FR reection spectrum amplitude. As we know, the FR lineshape appears for a vertical excitation polarization (Fig. 4 and solid lines in Fig. 9(a)). Aer functionalization with a 3 nm thick protein A/G monolayer, the reectivity changes as shown in Fig. 9(a). Theoretical analyses were also carried out and showed a good agreement with the experimental results. Interestingly, it is seen that near the subradiant (Q) mode the difference of reectivity (DR) is relatively larger due to the Fano interference and is easily detectable (Fig. 9(b)). The biosensing is then performed by measuring the normalized differential reectivity DR/RQ (where RQ is the reectivity at the subradiant mode). For different structures, the spectra differ due to a different subradiant mode position (uQ). In Fig. 9(e), the dashed lines show the experimental results with a monolayer of a single recognition protein A/G of thickness h1. The dashed lines in Fig. 9(d) show the corresponding theoretical results. From these results the value of h1 can be obtained and it agrees well with the known value. The systems with adding another protein nanolayer were also studied as shown schematically in Fig. 9(c). The bilayer of thickness h1 + h2 consists of goat antibody IgG of thickness h2 on top of the protein A/G (h1). The DR/RQ are further enhanced as shown by

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the solid lines in Fig. 9(d) and (e) and h2 can also be estimated (h2 z 5.1 nm) from this method. In some other sensor devices, the optical FR spectra can be changed a lot with the nanorod locations90,91 because the coherent constructive or destructive interferences are sensitive to the geometry congurations. There are also some structures where the narrow FR spectrum feature makes them sensitive to the surrounding environment.92–94 Fig. 10(a) shows a nanorod dimer with a high sensitivity for a refractive index sensor.92 The dimer is dipole–quadrupole plasmons coupled and is similar to that in ref. 66. But here, the cross section is of triangular shape instead of circular. The bonding and antibonding couplings between the dipole (D) and quadrupole (Q) modes are denoted by D–Q and D + Q, respectively, and this coupling will also induce FR. It is seen that the D + Q mode shows a high sensor performance, and the gure of merit (FoM) is as large as 16.1 (Fig. 10(b)). The nonlinear optical properties in some nanorod-based FR systems were also investigated as shown by Figs. 10(c)–(f), where a short and long rod formed an asymmetric dimer structure.32 FR appears when the bright mode on the short rod and the dark mode on the long rod are coupled, as shown by the experimental spectra of differential reectance (Fig. 10(c)) and simulated differential extinction spectra (Fig. 10(e)). The nonlinear responses of the dimer with ITO nearby are shown in Fig. 10(d) for a delay of 5 ps between the arrival of the pump and the probe pulses. An overall red shi of the spectrum is clearly observed and this is caused by the nonlinear refractive index change of the ITO. The simulations also show similar results (Fig. 10(f)).

8. Conclusions and discussion In summary, we have reviewed the recent progress in the eld of FRs in nanorod-based plasmonic structures. Aer a brief introduction on the theoretical background of FR, the structures with single nanorod, dimers, and mutipole rods were discussed. The FRs in single rods can be observed by excitations of higher order SP modes besides dipole SP or breaking symmetry with dielectric. In nanorod dimers, the dipole SP of one rod will generally hybridize with the dipole or higher order modes of the other rods. FRs can be obtained with the interference of the hybridized modes. The plasmon couplings in the mutipole nanorods are usually complicated. Two or more rods can work collectively and form a bright or dark mode, and the interference of these modes induce FRs. The adjustment range of the FR lineshapes in these multipole rod structures is relatively larger. FRs in some nanorod-nanoparticle hybrids have also been discussed. Finally, we discussed the applications of plasmonic nanorod-based FRs, such as chemical and biological sensors and nonlinear optics. A lot of research topics based on FRs in nanorod-based structures, such as sensors, metamaterial, nonlinear optics, optical forces95 and magnetic properties, are expected to be developed further in the future. Some of them have been discussed above. As to the magnetic properties, in this paper we have discussed the FRs in nanorod structures all based on electric eld interactions. It would also be very interesting to

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study the magnetic properties of FR nanorod structures. In fact, there have been some reports about the magnetic properties in Fano resonant nanoparticle clusters.96,97 For example, in a kind of Metal-Dielectric-Metal (MDM) oligomers, magnetic plasmon resonances can also be excited for each MDM element structure. This could lead to an additional FR besides the electric FR in planar oligomers. These study results are instructive for investigating nanorod structures.

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Acknowledgements This work was supported by the MOST of China (Grant no. 2011CB922200) and the Natural Science Foundation of China (11174229, 11204221 and 11374236).

Notes and references 1 U. Fano, Phys. Rev., 1961, 124, 1866–1878. 2 A. E. Miroshnichenko, S. Flach and Y. S. Kivshar, Rev. Mod. Phys., 2010, 82, 2257–2298. 3 A. C. Johnson, C. M. Marcus, M. P. Hanson and A. C. Gossard, Phys. Rev. Lett., 2004, 93, 106803. 4 M. Kroner, A. O. Govorov, S. Remi, B. Biedermann, S. Seidl, A. Badolato, P. M. Petroff, W. Zhang, R. Barbour, B. D. Gerardot, R. J. Warburton and K. Karrai, Nature, 2008, 451, 311–314. 5 I. Bloch, J. Dalibard and W. Zwerger, Rev. Mod. Phys., 2008, 80, 885–964. 6 S. Fan, P. R. Villeneuve, J. D. Joannopoulos and H. A. Haus, Phys. Rev. Lett., 1998, 80, 960–963. 7 S. Fan and J. D. Joannopoulos, Phys. Rev. B: Condens. Matter, 2002, 65, 235112. 8 A. E. Miroshnichenko and Y. S. Kivshar, Nano Lett., 2012, 12, 6459–6463. 9 R. A. Vicencio, J. Brand and S. Flach, Phys. Rev. Lett., 2007, 98, 184102. 10 B. Luk'yanchuk, N. I. Zheludev, S. A. Maier, N. J. Halas, P. Nordlander, H. Giessen and C. T. Chong, Nat. Mater., 2010, 9, 707–715. 11 M. Rahmani, B. Luk'yanchuk and M. Hong, Laser Photonics Rev., 2012, 1–21. 12 N. J. Halas, S. Lal, W. S. Chang, S. Link and P. Nordlander, Chem. Rev., 2011, 111, 3913–3961. 13 S. Zhang, D. A. Genov, Y. Wang, M. Liu and X. Zhang, Phys. Rev. Lett., 2008, 101, 047401. 14 N. Verellen, Y. Sonnefraud, H. Sobhani, F. Hao, V. V. Moshehalkov, P. Van Dorpe, P. Nordlander and S. A. Maier, Nano Lett., 2009, 9, 1663–1667. 15 J. A. Fan, C. Wu, K. Bao, J. Bao, R. Bardhan, N. J. Halas, V. N. Manoharan, P. Nordlander, G. Shvets and F. Capasso, Science, 2010, 328, 1135–1138. 16 A. Lovera, B. Gallinet, P. Nordlander and O. J. F. Martin, ACS Nano, 2013, 7, 4527–4536. 17 V. Giannini, Y. Francescato, H. Amrania, C. C. Phillips and S. A. Maier, Nano Lett., 2011, 11, 2835–2840. 18 Y. Francescato, V. Giannini and S. A. Maier, ACS Nano, 2012, 6, 1830–1838. This journal is © The Royal Society of Chemistry 2014

Nanoscale

19 B. Gallinet and O. J. F. Martin, Phys. Rev. B: Condens. Matter Mater. Phys., 2011, 83, 235427. 20 B. Gallinet and O. J. F. Martin, ACS Nano, 2011, 5, 8999–9008. 21 T. Pakizeh, C. Langhammer, I. Zoric, P. Apell and M. K¨ all, Nano Lett., 2009, 9, 882–886. 22 C. Forestiere, L. Dal Negro and G. Miano, Phys. Rev. B: Condens. Matter Mater. Phys., 2013, 88, 155411. 23 C. Wu, A. B. Khanikaev, R. Adato, N. Arju, A. A. Yanik, H. Altug and G. Shvets, Nat. Mater., 2012, 11, 69–75. 24 F. Hao, P. Nordlander, Y. Sonnefraud, P. V. Dorpe and S. A. Maier, ACS Nano, 2009, 3, 643–652. 25 N. Liu, T. Weiss, M. Mesch, L. Langguth, U. Eigenthaler, M. Hirscher, C. Sonnichsen and H. Giessen, Nano Lett., 2010, 10, 1103–1107. 26 F. Neubrech, A. Pucci, T. Walter Cornelius, S. Karim, A. Garc´ıa-Etxarri and J. Aizpurua, Phys. Rev. Lett., 2008, 101, 157403. 27 H. Aouani, H. ˇ S´ıpov´ a, M. Rahmani, M. Navarro-Cia, K. Hegnerov´ a, J. Homola, M. Hong and S. A. Maier, ACS Nano, 2012, 7, 669–675. 28 M. K¨ all, H. Xu and P. Johansson, J. Raman Spectrosc., 2005, 36, 510–514. 29 H. Xu and M. K¨ all, Top. Appl. Phys., 2006, 103, 87–104. 30 B. Gallinet, T. Siegfried, H. Sigg, P. Nordlander and O. J. F. Martin, Nano Lett., 2013, 13, 497–503. 31 Y. Zhang, F. Wen, Y. R. Zhen, P. Nordlander and N. J. Halas, Proc. Natl. Acad. Sci. U. S. A., 2013, 110, 9215–9219. 32 M. Abb, Y. Wang, P. Albella, C. H. de Groot, J. Aizpurua and O. L. Muskens, ACS Nano, 2012, 6, 6462–6470. 33 L. V. Brown, H. Sobhani, J. B. Lassiter, P. Nordlander and N. J. Halas, ACS Nano, 2010, 4, 819–832. 34 O. Pe˜ na-Rodr´ıguez, U. Pal, M. Campoy-Quiles, L. Rodr´ıguezFern´ andez, M. Garriga and M. I. Alonso, J. Phys. Chem. C, 2011, 115, 6410–6414. 35 J. A. Fan, K. Bao, C. Wu, J. Bao, R. Bardhan, N. J. Halas, V. N. Manoharan, G. Shvets, P. Nordlander and F. Capasso, Nano Lett., 2010, 10, 4680–4685. 36 M. Rahmani, D. Y. Lei, V. Giannini, B. Lukiyanchuk, M. Ranjbar, T. Y. F. Liew, M. H. Hong and S. A. Maier, Nano Lett., 2012, 12, 2101–2106. 37 S. J. Barrow, X. Wei, J. S. Baldauf, A. M. Funston and P. Mulvaney, Nat. Commun., 2012, 3, 1275. 38 F. Shaei, F. Monticone, K. Q. Le, X. X. Liu, T. Hartseld, A. Al` u and X. Li, Nat. Nanotechnol., 2013, 8, 95–99. 39 M. Hentschel, M. Saliba, R. Vogelgesang, H. Giessen, A. P. Alivisatos and N. Liu, Nano Lett., 2010, 10, 2721–2726. 40 D. Dregely, M. Hentschel and H. Giessen, ACS Nano, 2011, 5, 8202–8211. 41 J. B. Lassiter, H. Sobhani, M. W. Knight, W. S. Mielczarek, P. Nordlander and N. J. Halas, Nano Lett., 2012, 12, 1058– 1062. 42 Y. Cui, J. Zhou, V. A. Tamma and W. Park, ACS Nano, 2012, 6, 2385–2393. 43 M. Hentschel, D. Dregely, R. Vogelgesang, H. Giessen and N. Liu, ACS Nano, 2011, 5, 2042–2050. 44 S. Link, M. B. Mohamed and M. A. El-Sayed, J. Phys. Chem. B, 1999, 103, 3073–3077. Nanoscale, 2014, 6, 4985–4997 | 4995

View Article Online

Published on 15 April 2014. Downloaded by Virginia Tech on 19/10/2014 06:38:09.

Nanoscale

45 S. Link and M. A. El-Sayed, J. Phys. Chem. B, 1999, 103, 8410– 8426. 46 Y. Yu, S. Chang, C. Lee and C. R. C. Wang, J. Phys. Chem. B, 1997, 101, 6661–6664. 47 S. E. Lohse and C. J. Murphy, Chem. Mater., 2013, 25, 1250– 1261. 48 H. Chen, L. Shao, Q. Li and J. Wang, Chem. Soc. Rev., 2013, 42, 2679–2724. 49 P. Muhlschlegel, H. J. Eisler, O. J. F. Martin, B. Hecht and D. W. Pohl, Science, 2005, 308, 1607–1069. 50 M. W. Knight, H. Sobhani, P. Nordlander and N. J. Halas, Science, 2011, 332, 702–704. 51 L. Novotny, Phys. Rev. Lett., 2007, 98, 266802. 52 G. W. Bryant, F. J. Garc´ıa de Abajo and J. Aizpurua, Nano Lett., 2008, 8, 631–636. 53 E. Cubukcu and F. Capasso, Appl. Phys. Lett., 2009, 95, 201101. 54 H. Ditlbacher, A. Hohenau, D. Wagner, U. Kreibig, M. Rogers, F. Hofer, F. R. Aussenegg and J. R. Krenn, Phys. Rev. Lett., 2005, 95, 257403. 55 J. Dorfm¨ uller, R. Vogelgesang, R. T. Weitz, C. Rockstuhl, C. Etrich, T. Pertsch, F. Lederer and K. Kern, Nano Lett., 2009, 9, 2372–2377. 56 G. Bachelier, I. Russier-Antoine, E. Benichou, C. Jonin, N. Del Fatti, F. Vall´ ee and P.-F. Brevet, Phys. Rev. Lett., 2008, 101, 197401. 57 C. L. G. Alzar, M. A. G. Martinez and P. Nussenzweig, Am. J. Phys., 2002, 70, 37. 58 M. Rahmani, B. Luk'yanchuk, B. Ng, A. Tavakkoli, K. G. Y. F. Liew and M. H. Hong, Opt. Express, 2011, 19, 4949–4956. 59 H. Chen, L. Shao, Y. C. Man, C. Zhao, J. Wang and B. Yang, Small, 2012, 8, 1503–1509. 60 J. M. Reed, H. Wang, W. Hu and S. Zou, Opt. Lett., 2011, 36, 4386–4388. 61 F. L´ opez-Tejeira, R. Paniagua-Dom´ınguez, R. Rodr´ıguezOliveros and J. A. S´ anchez-Gil, New J. Phys., 2012, 14, 023035. 62 S. Zhang, L. Chen, Y. Huang and H. Xu, Nanoscale, 2013, 5, 6985–6991. 63 H. Chen, L. Shao, T. Ming, K. C. Woo, Y. C. Man, J. Wang and H. Q. Lin, ACS Nano, 2011, 5, 6754–6763. 64 Z. J. Yang, Z. S. Zhang, W. Zhang, Z. H. Hao and Q. Q. Wang, Appl. Phys. Lett., 2010, 96, 131113. 65 S. I. Bozhevolnyi, A. B. Evlyukhin, A. Pors, M. G. Nielsen, M. Willatzen and O. Albrektsen, New J. Phys., 2011, 13, 023034. 66 Z. J. Yang, Z. S. Zhang, L. H. Zhang, Q. Q. Li, Z. H. Hao and Q. Q. Wang, Opt. Lett., 2011, 36, 1542–1544. 67 T. R. Liu, Z. K. Zhou, C. Jin and X. Wang, Plasmonics, 2013, 8, 885–890. 68 Z. J. Yang, Z. S. Zhang, Z. H. Hao and Q. Q. Wang, Appl. Phys. Lett., 2011, 99, 081107. 69 N. W. Bigelow, A. Vaschillo, J. P. Camden and D. J. Masiello, ACS Nano, 2013, 7, 4511–4519. 70 K. C. Woo, L. Shao, H. Chen, Y. Liang, J. Wang and H. Q. Lin, ACS Nano, 2011, 5, 5976–5986.

4996 | Nanoscale, 2014, 6, 4985–4997

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71 Z. Xi, Y. Lu, W. Yu, P. Wang and H. Ming, J. Opt., 2013, 15, 025004. 72 Z. J. Yang, Q. Q. Wang and H. Q. Lin, Nanoscale, 2012, 4, 5308–5311. 73 P. Alonso-Gonzalez, M. Schnell, P. Sarriugarte, H. Sobhani, C. Wu, N. Arju, A. Khanikaev, F. Golmar, P. Albella, L. Arzubiaga, F. Casanova, L. E. Hueso, P. Nordlander, G. Shvets and R. Hillenbrand, Nano Lett., 2011, 11, 3922– 3926. 74 M. Rahmani, E. Yoxall, B. Hopkins, Y. Sonnefraud, Y. Kivshar, M. Hong, C. Phillips, S. A. Maier and A. E. Miroshnichenko, ACS Nano, 2013, 7, 11138–11146. 75 S. J¨ ager, A. M. Kern, M. Hentschel, R. J¨ ager, K. Braun, D. Zhang, H. Giessen and A. J. Meixner, Nano Lett., 2013, 13, 3566–3570. 76 N. Liu, L. Langguth, T. Weiss, J. K¨ aste, M. Fleischhauer, T. Pfau and H. Giessen, Nat. Mater., 2009, 8, 758–762. 77 A. E. Çetin, A. Artar, M. Turkmen, A. A. Yanik and H. Altug, Opt. Express, 2011, 19, 22607–22618. 78 A. Artar, A. A. Yanik and H. Altug, Nano Lett., 2011, 11, 1685– 1689. 79 X. R. Su, Z. S. Zhang, L. H. Zhang, Q. Q. Li, C. C. Chen, Z. J. Yang and Q. Q. Wang, Appl. Phys. Lett., 2010, 96, 043113. 80 E. Prodan, C. Radloff, N. J. Halas and P. Nordlander, Science, 2003, 302, 419–422. 81 A. Artar, A. A. Yanik and H. Altug, Nano Lett., 2011, 11, 3694– 3700. 82 V. A. Tamma, Y. Cui, J. Zhou and W. Park, Nanoscale, 2013, 5, 1592–1602. 83 N. Verellen, P. Van Dorpe, C. Huang, K. Lodewijks, G. A. E. Vandenbosch, L. Lagae and V. V. Moshchalkov, Nano Lett., 2011, 11, 391–397. 84 J. Wang, C. Fan, J. He, P. Ding, E. Liang and Q. Xue, Opt. Express, 2013, 21, 2236–2244. 85 H. Liu, B. Li, L. Zheng, C. Xu, G. Zhang, X. Wu and N. Xiang, Opt. Lett., 2013, 38, 977–979. 86 J. Chen, P. Wang, C. Chen, Y. Lu, H. Ming and Q. Zhan, Opt. Express, 2011, 19, 5970–5978. 87 L. Shao, C. Fang, H. Chen, Y. C. Man, J. Wang and H. Q. Lin, Nano Lett., 2012, 12, 1424–1430. 88 Z. K. Zhou, X. N. Peng, Z. J. Yang, Z. S. Zhang, M. Li, X. R. Su, Q. Zhang, X. Shan, Q. Q. Wang and Z. Zhang, Nano Lett., 2011, 11, 49–55. 89 M. Rahmani, A. E. Miroshnichenko, D. Y. Lei, B. Luk'yanchuk, M. I. Tribelsky, A. I. Kuznetsov, Y. S. Kivshar, Y. Francescato, V. Giannini, M. Hong and S. A. Maier, Small, 2014, 10, 576. 90 F. Shaei, C. Wu, Y. Wu, A. B. Khanikaev, P. Putzke, A. Singh, X. Li and G. Shvets, Nat. Photonics, 2013, 7, 367–372. 91 N. Liu, M. Hentschel, T. Weiss, A. P. Alivisatos and H. Giessen, Science, 2011, 332, 1407–1410. 92 T. R. Liu, Z. K. Zhou, C. Jin and X. Wang, Plasmonics, 2013, 8, 885–890. 93 B. Gallinet and O. J. F. Martin, ACS Nano, 2013, 7, 6978– 6987.

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View Article Online

Feature Article

96 N. Liu, S. Mukherjee, K. Bao, L. V. Brown, J. Dorfm¨ uller, P. Nordlander and N. J. Halas, Nano Lett., 2012, 12, 364– 369. 97 J. Yang, M. Rahmani, J. H. Teng and M. H. Hong, Opt. Mater. Express, 2012, 2, 1407–1415.

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94 A. B. Evlyukhin, S. I. Bozhevolnyi, A. Pors, M. G. Nielsen, I. P. Radko, M. Willatzen and O. Albrektsen, Nano Lett., 2010, 10, 4571–4577. 95 Q. Zhang, J. J. Xiao, X. M. Zhang, Y. Yao and H. Liu, Opt. Express, 2013, 21, 6601–6608.

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Nanoscale, 2014, 6, 4985–4997 | 4997

Plasmonic Fano resonances in metallic nanorod complexes.

Plasmonic Fano resonances (FRs) in nanostructures have been extensively studied in recent years. Nanorod-based complexes for FRs have also attracted m...
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