Localization, Hybridization, and Coupling of PlasmonResonances in an Aluminum Nanomatryushka

Arash Ahmadivand & Nezih Pala

Received: 19 October 2014 /Accepted: 11 December 2014 /Published online: 24 December 2014# Springer Science+Business Media New York 2014

Abstract In this study, we investigate the plasmon response oftwo concentric aluminum (Al) nanoshells as a nanomatryushkaunit to introduce a novel compositional structure that has astrong potential to employ in designing practical nanoscaleplasmonic devices. Herein, we employed Al nanoshells witha coverage of oxide (Al2O3) layer with certain and homoge-nous size of thickness in inner and outer sides. Using plasmonhybridization theory and finite-difference time-domain(FDTD) method as numerical model, we calculated andsketched the optical response and energy level diagram forthe studied structure. Strong plasmon resonances are reportedin the UV and visible wavelengths that can be supportedefficiently by using the proposed nanomatryushka unit com-posed of Al/Al2O3 on a SiO2 surface. Utilizing presentednanomatryushka in designing an artificial dimer configuration,the possibility of appearing of dark modes and formation ofFano resonances in such a symmetric structure in the UV andvisible spectra are verified numerically. Immersing the present-ed dimer in various liquids with different refractive indices, thebehavior of Fano dip is investigated and corresponding figureof merit (FoM) is quantified based on the plasmon resonanceenergy shifts over the refractive index variations. Thisunderstating opens novel avenues to obtain sharp and deepFano resonances in simple and low-cost structures that havestrong potentials in fabrication of biochemical sensors,superlensing, and biological agents.

Keywords Aluminum . Plasmon hybridization theory .

Fano resonance . Numerical modeling .

Figure ofmerit (FoM)

Introduction

Aluminum has been considered as an alternative plasmonicsubstance for conventional noble metallic materials such asAg, Cu, and Au at the certain optical bandwidths [1, 2].Hence, designing precise sensors and light harvesting andphotovoltaic devices, nanoantennas, routers, and modulatorsare some of practical fields in using Al subwavelength struc-tures [3–6]. Natural abundance, low cost, and CMOS com-patible during fabrication processes are some of key featuresthat persuaded numerous researchers to use Al in designingplasmonic devices [1, 2]. Besides, comparing the plasmonresponse of Al and conventional noble metallic substances,an Al nanosize particle with a certain percentage of oxidecoverage (Al2O3) yields pronounced plasmon resonancepeaks as dipolar and multipolar modes, which is free ofintense internal transitions, and rapid oxidation at the UVspectrum [1, 6, 7]. Further, besides the chemical characteris-tics, the structural properties of utilized nanoparticles havesignificant influences on the quality of plasmon resonanceexcitation in molecular levels. In terms of physical properties,several shapes of nanoparticles with spherical and non-spherical geometries have been employed in designing plas-monic structures [8, 9]. Nanorod, sphere, ring, disk, shell,pyramid, cube, rice, star-like, and core-shell particles withAu and Ag substances are some of conventional types ofconfigurations that have wide range of utilization in variousfields of optical sciences [1, 2, 10–12]. As a specific case, it isshown that concentric nanoshells or “nanomatryushka” con-figuration provides an extra degree of flexibility in the geo-metrical parameters that can be exploited in tuning the posi-tion and intensity of plasmon resonance extremes at the de-sired bandwidths [13]. Moreover, this structure has been ex-tensively used to characterize the theory of plasmon resonancehybridization as an initial configuration due to its capability tosupport strong bonding and antibonding resonant modes [14].The behavior of this spherical and symmetric configuration

A. Ahmadivand (*) :N. PalaDepartment of Electrical and Computer Engineering, FloridaInternational University, 10555 W Flagler St.,Miami, FL 33174, USAe-mail: aahma011@fiu.edu

Plasmonics (2015) 10:809–817DOI 10.1007/s11468-014-9868-z

has been investigated for gold (Au) substances comprehen-sively, and it is verified that nanomatryushka reflects uniquefeatures during illuminating by a light source in the visible andnear infrared frequencies [13, 14].

In addition, it is well understood that plasmonic molecularstructures are the potential arrangements in designing differenttypes of ultrasensitive sensors, routers, antennas, etc. [15, 16].In molecular level, a couple of identical nanoparticles or “di-mer” is one the simplest and regular configuration of molecularclusters, and various shapes of nanoparticles have beenemployed in designing dimers that are able to support differentresonance energies depend on the chemical and physical char-acteristics [17, 18]. Recently, researchers proved that symmetrybreaking in the structural properties of a simple dimer or usingantisymmetric nanoparticles give rise to appearing of antibond-ing dark modes besides the systematic bonding bright modes inthe calculated plasmon hybridization profile as an energy leveldiagram [19]. In this regime, appearing of dark modes and aconstructive interference between dark and bright modescauses formation of resonant dips or Fano resonances alongthe extinction diagram [6, 14, 15]. The behavior and quality ofFano resonances can be employed in designing plasmonicnanosensors with significant sensitivity for a wide range ofpurposes [20, 21]. On the other hand, hybridization of plasmonresonances at the UV spectrum yields a method to exploitnanostructures in designing various biological devices, gassensors, DNA-related applications, and biological agents.Low cost, reliable responsivity, and reasonable efficiency arethe major and fundamental advantageous of discussed Al-based subwavelength devices.

In this article, the plasmon response of the Al/Al2O3 con-centric nanoshells at the UV and visible spectra is studied byemploying plasmon hybridization theory, where the structure isdeposited on a SiO2 substrate and the medium between twonanoshells is the vacuum. To this end, we used experimentallydetermined Palik constants for Al, Al2O3, and SiO2 materials[22]. Using both numerical and theoretical methods, we inves-tigated the optical features for the proposed concentric nano-scale configuration. We also determined the plasmon responseof the dimer structure composed of Al/Al2O3 nanomatryushkaunits; hence, we utilized two nanomatryushka units in a closeproximity to each other as a dimer. The quality of appeareddark modes and the resulted Fano dip are investigated byplotting corresponding spectral responses. Finally, examinedthe accuracy of the dimer structure is evaluated during immers-ing by various liquids, where plotting and quantifying the FoMdefined the sensitivity of the structure.

Theory

Figure 1 shows a three-dimensional diagram for the proposedmultilayer structure or nanomatryushka with the description

of geometrical parameters as following: a/b/c/d, which are theinner and outer radii of the interior and exterior nanoshells,respectively. Obviously, the oxide layer entirely covered theconfiguration in both inner and outer parts of nanoparticles.Herein, the accurate and appropriate geometrical sizes aredetermined based on the plasmon response of proposed nano-structure during illumination with a linear plane wave as alight source with transverse polarization (φ=0°) direction. It iswell understood that a certain coverage of oxide layer anddielectric substrate are required to induce dipolar and quadru-polar peaks along the scattering cross-sectional diagram [6,24]. For an Al nanomatryushka, we assumed that the structureis deposited on a silica substrate and the thickness of Al2O3

layer is determined based on the optical response of configu-ration during exposing by a light source. To this end, wecomputed the dielectric function in the UV bandwidth forthe particles unit using Drude model [25]:

εAl ¼ ε∞−ω2p

ω ωþ iγð Þ ð1Þ

where ωp is the bulk plasmon frequency, ε∞ is the highfrequency response, and γ is the collision frequency. In thecurrent computations, all of the above parameters are set to thefollowing: ωp=1.2×10

16 rad/s, ε∞=3.7, and γ=19.79×1012

(s−1). On the other hand, Choy [26] verified that by using theBruggeman model, determination of dielectric function for agiven compositional structure at a certain bandwidth is possi-ble. What is more, the dielectric functions for compositional

Fig. 1 Three and two-dimensional snapshots for an Al/Al2O3

nanomatryushka structure with description of geometrical parameters

810 Plasmonics (2015) 10:809–817

metallic nanostructures can be defined by employing themethods that are suggested by [22, 26] include presentedexperimental data by Argall et al. [27].

To calculate the scattering cross-sectional profile for thepresented nanomatryushka, Mie scattering theory is employedto compute the scattering efficiency for the electromagneticfield interaction with the structure [28]. First, we assume thatthe background refractive index is n=1, while thenanomatryushka is deposited on a SiO2 surface with thepermittivity of ε=2.05. Using Mie theory for spherical ob-jects, the incident and scattered electromagnetic fields can beexpanded in vector spherical wave functions around the par-ticles. Therefore, the entire excited field around the nanopar-ticles can be obtained using below equations, which is the sumof the incident, refracted and scattered fields:

Etot ¼ Einc þ Ere f þ Esca ð2Þ

where:

Einc ¼ bzE0eiωt ð3Þ

and the cross section of scattered field is [28] as follows:

Csca ¼ 4π

k2XT

t¼1

XMt

n¼1

Xn

m¼−nℜe pt*mna

tmn þ qt*mna

tmn

� � ð4Þ

where k is the wavevector modulus, t is the number of shells ina nanomatryushka system, pmn

− t ,qmn− t are the expansion coeffi-

cients of the incident plane wave (light source) and amn is thepartial scattered field expansion coefficient as below:

atmn ¼ a−tn p−tmn−a

−tn

X1;Tð Þ

j≠t

XNt

v¼1

Xv

s¼−nAt jmnsva

jmn þ Bt j

mnsvbjsv

� � ð5Þ

btmn ¼ b−tn q−tmn−b

−tn

X1;Tð Þ

j≠t

XNt

v¼1

Xv

s¼−nBt jmnsva

jmn þ At j

mnsvbjsv

� � ð6Þ

where Bmnsvij ,Amnsv

ij are the vector coefficients based on Hankelfunctions for spherical objects. To this end, we employedfinite-difference time-domain (FDTD) method (LumericalFDTD Solutions 8.9) as a numerical tool that is employed toextract the optical response of the studied subwavelengthstructures. The spatial cell sizes are set to dx=dy=dz=1.2 nm, with 25,000 number of cells, and perfectly matchedlayers (PMLs) are the boundary condition with 64 layers.Considering numerical stability for the employed compo-nents, simulation time step is set to the 0.03 fs according tothe Courant stability condition [14, 23].

An Isolated Aluminum Nanomatryushka

Figure 2a exhibits the scattering cross-sectional profilefor a nanomatryushka with the following geometries: 45/75/110/155 nm in two different regimes. The first simu-lation is performed for a pure Al on an infinite SiO2

surface (Al@SiO2), where two distinct shoulders areappeared at λ=335 and 520 nm correspond to the quad-rupole and dipole extremes, respectively. The secondstate is correlated to the Al nanomatryushka that iscovered with an oxide layer on a SiO2 substrate(Al@Al2O3@SiO2), where three pronounced peaks areappeared along the diagram at λ=210, 405, and635 nm correspond to the octupole, quadrupole, anddipole resonant extremes, respectively. In the latest re-gime, particles have been covered with an oxide layerwith the thickness of 15 nm homogenously (see the insetgraph in Fig. 2a). Moreover, a red shift to the visiblespectrum is realized in this state, which includes a no-ticeable decrement in the amplitude of the scatteringefficiency (dipole and quadrupole peaks). Figure 2b, cillustrates two-dimensional snapshots for the examinednanomatryushka for the absence and presence of oxidelayer under transverse polarization excitation. Obviously,the number of appeared modes in the second state ismore than the previous regime due to the presence ofoxide layers around the nanomatryushka unit and itseffect in excitation of plasmon resonances.

In terms of chemical properties, Al and Al2O3 havealmost opposite absorbing behaviors in the range of UVto the visible region [29–31]. To determine the normal-ized absorption over the photon energy (eV) at a certainfrequency (ω), the number of photons that are absorbedby an individual nanomatryushka unit must be deter-mined. Theoretical and numerical methods can beemployed in this calculation. Considering the theoreticalviewpoint, the optical power that is absorbed per unitvolume from a monochromatic source (here is a linearplane wave) at angular frequency “ω” can be calculatedfrom the divergence of the complex Poynting vector:

P r;ωð Þ ¼ −1

2Re ∇:P r;ωð Þf g ¼ −

1

2ως ε r;ωð Þf g E r;ωð Þj j2 ð7Þ

Herein, employing multi-coefficient material model inFDTD solutions, we are able to determine the index of utilizedsubstances against the illumination bandwidth. In this regime,the number of absorbed photon energy (eV) at a certainfrequency can be determined numerically.

A ¼ −ως ε r;ωð Þf g E r;ωð Þj j2

2ℏω¼ −

πhς ε r;ωð Þf g E r;ωð Þj j2 ð8Þ

Plasmonics (2015) 10:809–817 811

Besides, in terms of numerical calculations, we employedFDTD numerical model, while Poynting vector is utilized tocalculate the absorbed optical power as below [32]:

Pabs baz;ω� �

¼ −1�2ℜ ∇:P baz;ω

� �� �V=m2� � ð9Þ

to calculate the absorption power by the equations above, theeffect and intensity of light source must be included, andhence, for a monochromatic linear plane wave source, thenormalized absorption cross-section can be defined usingbelow relation:

Cabs ¼ −1�2

ℜ ∇:P baz;ω� �� �

ℏωPsource ωð Þ

0@

1A ð10Þ

Technically, considering the nonretarded limit in plasmonresonance computations for Al/Al2O3 nanomatryushka struc-ture, the normalized absorption profile can be figured out in

different regimes. Figure 3 illustrates the normalized absorp-tion cross-section for the aluminum nanomatryushka in com-positional regime over the photon energy (eV) variations. Inthis profile, we specified the position of energy levels in anisolated nanomatryushka unit, where four different extremesappeared according to the plasmon resonance energy localiza-tion around 1.19, 1.59, 2.21, and 2.38 eV. It should be notedthat the resonance energy is localized strongly in the vacuumspace between nanoshells as well as the inner offset distance ofthe interior Al/Al2O3 nanoshell. In this case, three lower ener-gies are produced by the vacuum space between nanoshells,while the other higher resonance energy rises from the inter-ference of strong dipole modes of the nanoshells at the outsideof the exterior nanoshell. The inset diagram is the absorptionefficiency profile for the examined nanomatryushka over thewavelength variations, which is in complete agreement withthe absorbed photons at different wavelengths. The ratio ofabsorption has been decreased significantly for λ>700 nm,which originates from the behavior of oxide layer in visible

Fig. 2 a Calculated scatteringcross-sectional profile for an Alnanomatryushka unit with thepresence and absence of oxidecover layer under transverseelectric polarization excitationand b, c two-dimensionalsnapshots for the plasmonresonance excitation andhybridization in ananomatryushka unit without(Al@SiO2) and with(Al@Al2O3@SiO2) the oxidelayer, respectively, which aredeposited on a SiO2 substrate

812 Plasmonics (2015) 10:809–817

and near infrared region. In addition, this figure reflects theimportance of oxide layer in the performance of the structureduring practical applications.

Next, we examine the plasmon response of the structure bysketching the energy level diagram for the structure based onplasmon hybridization theory (see Fig. 4a). This figure helpsus to realize the origin of performed plasmon resonant modes.This diagram is calculated for different three states of ananomatryushka unit which includes separated nanoshellsand a nanomatryushka unit. To this end, we illuminated theinterior and exterior nanoshells individually to plot the energydiagram, and corresponding resonant modes are denoted by|ω±>ins and |ω±>ens (both bonding and antibonding modes)for interior and exterior nanoshells, respectively. Consideringtwo nanoparticles in a sole system such as a nanomatryushkaunit, an interference between mentioned modes gives rise tothe strong hybridization of localized resonant modes (see themid column in Fig. 4a). For the hybridized regime, onenonbonding energy level appeared (|ω−

+>nonbonding), whichresulted from the interaction of antibonding modes of bothof the nanoshells in a nanomatryushka system (|ω−>

ins↔ |ω+>ens). In addition, four energy levels are observedat different energy levels, and the mode that appeared at1.19 eV corresponds to the dipole resonant mode (|ω−

−>unit),which is formed based on the plasmon bonding modes ofinner nanoshell and exterior nanoshells (|ω−>ins↔ |ω−>ens).The second energy level is 1.59 eV corresponds to the dipoleresonant mode (|ω−

+>unit). This mode is performed upon theinterference of bonding mode of interior nanoshell and amixture of antibonding and bonding modes of exterior shell

(|ω−>ins↔{|ω+>ens+|ω−>ens}). The other resonant mode

(with the energy level at 2.23 eV) is the third plasmon resonantmode (|ω+

−>unit) that is formed based on the interaction of amixture of bonding and antibonding modes of interior shelland bonding plasmon modes of the exterior shell ({|ω−>ins+|ω+>ins}↔ |ω+>ens). The last mode is the resonant mode thatis detected at the energy band of 2.38 eV (|ω+

+>unit), whichresulted upon the interference of antibonding modes of interi-or and exterior nanoshells (|ω+>ins↔ |ω+>ens). It should benoted that the production of nonbonding resonant modes isnot observable in the scattering efficiency profile. Figure 4b isa schematic for the calculated charge distribution diagram fornanomatryushka unit composed of Al/Al2O3/SiO2 materialsbased on plasmon hybridization diagram.

Aluminum Nanomatryushka Dimer

In this section, we investigate the plasmon response for anartificial subwavelength dimer structure composed of twoidentical Al/Al2O3 nanomatryushka units that are suited onan infinite SiO2 substrate. The edge to edge offset distancebetween adjacent nanounits is indicated by D2d, where theother geometrical parameters are based on the calculatedquantities in the prior section. Figure 5a, b illustrates twoand three-dimensional schematic diagrams for the dimer struc-ture with the geometrical description inside. The value of D2d

defines the intensity of resonance coupling and plasmon res-onance hybridization in the dimer structure between proximalnanomatryushka units. Figure 5c shows numerically calculat-ed scattering cross-sectional profile for the dimer structureunder transverse (φ=0°) and longitudinal (φ=90°) electricpolarization directions, where D2d=12 nm. Obviously, forthe transverse mode, strong bright (λ~300 nm) and dark(λ~900 nm) modes appeared along the scattering diagram thatare indicated inside the profile, while for the longitudinalmode, the dark mode is absent, and only the bright mode israised. Appearing of two opposite modes and a constructiveinterference between them caused to formation of a distinctminimum (dip) along the scattering profile for transversepolarization excitation mode. This unique feature can beemployed in designing polarization-dependent Fanoswitches.As we discussed already, formation of Fano dips in plasmonicmolecular structures needs for a major symmetry breaking inthe structure with high complexity in the structural properties.In the presented dimer, a strong Fano dip is performed aroundthe UV spectrum (λ~435 nm) without the expense of symme-try breaking and high complexity in geometrical components.Figure 5d presents hybridization and localization of plasmonresonance modes in a dimer system in decimal scale as a two-dimensional snapshot. Noticing in this profile, hybridizationof plasmons is clear between nanounits, while a similar en-hancement in the plasmon energies is obvious in a vacuum

Fig. 3 Normalized absorption cross-sectional profile for an Al/Al2O3

nanomatryushka structure on the SiO2 surface over the photon energyvariations with the description of energy levels. Inset is the absorptionefficiency diagram for the structure over the wavelength variations fromUV to the visible spectra

Plasmonics (2015) 10:809–817 813

space between interior and exterior nanoshells of an individualnanomatryushka. One of the fundamental parameters thatplays important role in intensifying the energy of Fano reso-nance dip is the offset gap distance parameter, where Fig. 5eexhibits the scattering profile for several gap distances of D2d

<12. Obviously, decreasing the size of the edge to edgedistance gives rise to significant enhancement in the qualityof Fano dip by formation of a deeper and narrower Fanoresonant mode. The optimal condition for the Fano dip withthe highest possible quality is obtained forD2d=2.5 nm, whilethe Fano dip is red shifted to the longer wavelengths. The

noteworthy point here is that more decrements in the size ofgap distance cause to degradations in the quality of Fanominimum due to the lower energy of darkmode in this regime.In this case, the dark mode cannot be coupled to the brightmode efficiently; therefore, the required interference to for-mation of a Fano mode cannot perform. Using numericaltechniques, Fig. 6a exhibits the absorption cross-sectionalprofile for a dimer structure composed of two Al/Al2O3

nanomatryushka units over the photon energy (eV) variations.In this profile, we detected a strong energy peak in the offsetdistance region (3.19 eV), which corresponds to the strong

Fig. 4 a Plasmon hybridizationdiagram for multilayernanostructure and b chargedistribution diagram for thestudied nanomatryushka atdifferent energy levels

814 Plasmonics (2015) 10:809–817

hybridization result of the interference between bright anddark modes. This energy appeared at the outside of the dimerstructure between proximal nanoparticles. The other two en-ergy extremes (2.28 and 2.69 eV) are related to the excitedenergy modes in individual nanomatryushka units that areappeared in a unique extreme. Figure 6b shows a snapshotof the power absorption by nanoscale dimer.

Ultimately, we examine the plasmon response of the Al-based multilayer dimer structure by using perturbations in therefractive index of the surrounding media. To this end, weutilized the equation below:

FoM ¼Δλ=Δn

FWHMð11Þ

where Δλ=Δn is the sensitivity of the configuration to thesurrounding perturbations, and FWHM is the full width at half

maximum for symmetric Fano resonances. Conventionally,Fano dips are highly narrow and sharp and this sharpnessallows for an accurate measurement of minor shifts in theposition of extremes that can originate from the medium alter-ations. In this case, we detected antisymmetric Fano resonantdips along the scattering spectra; therefore, the midpoint of theminimum and maximum resonance energies must be definednumerically, and additionally, the FWHM can be determinedbymeasuring the energy difference between spectral features ofthe last and initial Fano dips. Figure 7 presents the scatteringefficiency profile for the final dimer structure with the follow-ing geometries: (155/110/75/45)nm and D2d=2.5 nm, duringimmersing by different liquids with dissimilar refractive indi-ces. In this regime, three different surrounding media condi-tions have been considered as the following: air with n=1.000293, ethyl alcohol (ethanol) with n=1.361, and benzenewith n=1.501 (all of the refractive indices are measured

Fig. 5 a, b Two- and three-dimensional figures for a dimer structurecomposed of Al/Al2O3 nanomatryushka units on a SiO2 surface, cnumerically calculated scattering cross-sectional profile for the dimerstructure under transverse and longitudinal polarization excitations, d

two-dimensional snapshot for the dimer structure under transversepolarization excitation in a decimal scale, and e the effect of offset gapdistance variations on the position and quality of Fano dip

Plasmonics (2015) 10:809–817 815

experimentally at λ~590 nm [33, 34]). Obviously, increasingthe refractive index of the environmental substance gives rise to

large red shifts of Fano dip to the longer spectra. The insetdiagram is the calculated linear figure of merit (FoM), which isthe energy deviations (ΔE eV) over the refractive index vari-ations (n). Quantifying the FoM for the Al-based multilayerstructure yields a remarkable FoM~6.8, which opens newvenues for designing precise plasmon resonance and bio/chemical sensors based on simple nanoscale arrangements.Evaluating the performance of investigated structure with re-cently proposed dimer and even more complex nanoclusters,the proposed nanostructure provides strong localization ofplasmon resonances in a small volume, where the sharpnessof Fano dip is interesting. For instance, a simple Al/Al2O3/SiO2

nanoshell dimer with the FoM of 5.9 [2], and a complexoctamer composed of Al/Al2O3/SiO2 nanodisks with the FoMof 7.2 [6] are the comparable examples for Al-based molecularorientations. Hence, proposed configurations yield acceptableFoM despite of its simple orientation. Also, periodic arrays ofthe studied Al/Al2O3 dimer can be employed in designingmetamaterial structures with negative refractive indices thatcan provide ultra-sensitivity in detecting subtle perturbationsin the medium, that will be investigated in future publications.

Conclusions

In this work, we investigated the plasmon response of an Al/Al2O3 nanomatryushka structure on a SiO2 substrate in bothisolated and dimer regimes. The effect of oxide layer on thescattering efficiency of an isolated nanomatryushka is studiednumerically. Using plasmon hybridization theory, the energylevel diagram for the particle unit is plotted based on thenormalized absorption cross-sectional profile. Then, we ad-justed two nanomatryushka units in a close proximity to eachother to provide strong hybridization of plasmon resonances.

Fig. 6 a Normalized absorption diagram over the photon energy for thealuminum dimer structure, b two-dimensional snapshots for the dimerstructure under transverse mode excitation, and absorption efficiencydiagram over the wavelength variations

Fig. 7 Scattering efficiencyprofile for the dimer configurationduring immersing by variousliquids with different refractiveindices. Inset id the numericallycalculated FoM profile for thedimer structure

816 Plasmonics (2015) 10:809–817

Considering calculated scattering cross-sectional diagram fortransverse polarization mode excitation, we observed twostrong bright and dark modes along the diagram. We provedthat a constructive interference between these opposite modesgives rise to formation of a Fano resonance dip around the UVspectrum, and also the behavior of Fano dip is investigatedduring structural modifications. Ultimately, the performanceof Fano dip during perturbations in the refractive index of thesurrounding medium is studied, and corresponding FoM isplotted and quantified numerically. In this understanding, wepaved a method to appear Fano resonances around UV spec-trum in a symmetric structure without the expense of symme-try breaking.

Acknowledgments This work is supported by NSF CAREER programwith the Award number: 0955013.

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