Rhodium Plasmonics for Deep-Ultraviolet Bio-Chemical Sensing

Arash Ahmadivand1& Raju Sinha1 & Serkan Kaya1 & Nezih Pala1

Received: 15 September 2015 /Accepted: 7 October 2015# Springer Science+Business Media New York 2015

Abstract Rhodium (Rh) has been recently introduced as aperfect metal for ultraviolet (UV) applications with theadvantages of its oxide-free nature and support of strongplasmon resonant modes at very short wavelengths. Wereport on a simple platform of nanoplasmonic structuresto support strong plasmonic Fano resonances across thedeep-UV spectrum for biochemical sensing applications.We investigate the plasmonic response of several typesof Rh nanoparticles and designed dimer-type antennasusing nanorings with geometrical tunability in both sym-metric and antisymmetric assemblies. Using numerical andtheoretical methods, it is shown that Rh-based dimer an-tennas with broken symmetry can be tailored to supportstrong plasmon resonant modes at the deep-UV region(E > 6eV ). We also propose a complex infinity-shapedstructure composed of a pair of split rings with a nanodiskin between with extra degree of tunability to push theplasmon resonant modes further in deep-UV spectrum.Plasmon hybridization theory is used to describe formationof plasmonic Fano-resonant dips in simple nanoscale as-semblies. We calculate the corresponding figure of meritfor the Rh-based nanostructure around 11.5 which showsan excellent sensitivity to the refractive index perturbationsof the surrounding medium at very short wavelengths forsensing applications.

Keywords Nanoplasmonics . Deep UV . Rh nanoring . Fanoresonant mode . Infinity-shaped nanoantenna

Introduction

There have been rapid and remarkable progresses in designand fabrication of nanoscale photonic devices for biologicaland chemical sensing applications with excellent accuracy andhigh performance [1–4]. Several techniques have beenemployed to enhance the accuracy and speed of detection insubwavelength regime, such as surface-enhanced Raman scat-tering (SERS) [5], room temperature ring resonators [6], fiberoptic technology [7], and hybridization of localized surfaceplasmon resonances (LSPRs) [8, 9]. However, all of thesemethods have various weaknesses for practical sensing oper-ations. Considering the performance of the nanoscale devicesreported in the past few years, hybridization of LSPRs inmetallic particles is one of the most reliable and promisingtechniques for sensing applications including biological sens-ing and DNA detection [10–13]. Plasmon resonance hybridi-zation can be achieved efficiently in closely packed metallicmolecular and atomic size clusters in both symmetric andantisymmetric geometries [14, 15]. Noble metals (Au, Ag,and Cu) have extensively been used as conventional materialsfor plasmon resonance excitation across the visible to the far-infrared region for various applications including sensing [14,15]. However, biochemical sensing applications are not limit-ed to visible and infrared regions, while inducing plasmonresonant modes at the ultraviolet (UV) (E=3.1–12 eV, λ=100–400 nm) spectra provides effective techniques for variouspurposes from UV astronomy to gas sensing [16–21].Introducing the same procedure for LSPR enhancement atthe UV band is challenging due to the dramatic dissipativefactors correlating with regular noble metals that have been

* Arash Ahmadivandaahmadiv@fiu.edu

1 Department of Electrical and Computer Engineering, FloridaInternational University, 10555 W Flagler St., Miami, FL 33174,USA

PlasmonicsDOI 10.1007/s11468-015-0117-x

discussed broadly [16, 17]. To resolve the problems with no-ble metals in this spectral range, recently, aluminum (Al) andrhodium (Rh) have been proposed [16, 18]. In terms of mate-rial properties, a thin layer (a few nanometers) of oxide layer(Al2O3) grows naturally around Al nanoparticles, while Rh isfully oxide-free metal. It is well accepted that the natural oxidelayer around Al nanoparticles affects the spectral response ofAl-based structures [16]. Despite its fast oxidation, its com-patibility with complementary metal-oxide (CMOS) process-es, natural abundance, and lack of internal transitions make Althe most promising plasmonic metal for deep-UVapplication,with a wide range of utilizations in fabricating deep-UV light-emitting diodes (LEDs) and UV photodetectors [19, 20] andnanoantennas for cathodoluminescence [21]. On the otherhand, oxide-free Rh nanostructures provide unique featuresat the deep-UV bandwidth that have not been studied for sens-ing purposes. Considering chemical properties, unlike Al withthree electrons in the valence band (electron configuration: 1s2

2s2 2p6), Rh just provides one electron per atom in the fieldlocalization and interactions, analogous with conventional no-ble metals (electron configuration: [Kr] 5s1 4d8). Rh nanopar-ticle with excellent size tunability can be deposited on a di-electric surface uniformly by simple sonochemical method[22], and also, high-resolution electron microscopy (HREM)can be employed to synthesize and characterize Rh nanopar-ticles [23].

Plasmon hybridization in nanoscale structures has beenemployed for several applications from sensing towaveguiding [24–26]. Hybridization of plasmons includesformation of new collective electric and magnetic plas-monic modes arising as a result of strong near-field inter-ferences of resonant modes, supporting by metallic assem-blies. Interaction of the incident light with a metallicnanoassembly gives rise to excitation of superradiantbright and subradiant dark modes in the extinction profile,where any weak and destructive interference betweensuch modes can lead to formation of “Fano resonant(FR) modes” [27, 28]. Technically, the result of this inter-ference is the suppression of the dipolar superradiantmode by the subradiant mode to the shorter spectra andformation of a distinct minimum in the extinction profilecorrelating with hotspots, where a high amount of opticalpower can be absorbed at this resonance frequency (FRdip position). Several parameters such as geometrical andchemical ones may influence the energy and quality ofplasmonic FR modes. Spherical nanoparticles are the mostcommon types of structures, because of their easy fabri-cation facility and simple analytical computations. Simplenanodisks, nanorings, and complex nanomatryoshka par-ticles are the popular types of spherical structures thathave a wide range of usage in various optical devices.On the other hand, the material of nanoparticles also hasa significant influence on the plasmon response of the

proposed device. Recently, specific methods have beenproposed to analyze the plasmon response of Al-basednanoclusters, resulted by inducing FR dip in the near-UV spectrum [28]. However, inducing Fano minima isnot limited to clusters; core-shell particles with an oxidecore and Al coating have also been proposed for the samepurpose [29].

Here, we report on a systematic investigation of the plas-monic properties of a complex Rh nanoantenna consisting of acouple of peripheral split nanorings and a nanodisk in betweenas an infinity-shaped nanostructure for biochemical sensingapplications at the deep-UV region. Using hybridization ofplasmons in strongly coupled nanoparticles, we tailored aunique configuration of Rh structures to support pronouncedand deep Fano minima across the deep-UV bandwidth. Inaddition, adopting a step-by-step method, we compared theplasmon resonance localization in symmetric and antisymmet-ric dimer-type antennas with the proposed infinity-shapednanoantenna. Depositing the final nanoantenna on ametasurface in periodic arrays as a plasmonic metamaterial,we tailored a high-sensitivity platform for molecular detectionacross the deep-UV spectrum.

Numerical Setup

The plasmon responses, scattering cross sections, chargedistributions, and E-field maps are simulated by usingf in i t e -d i f f e rence t ime-domain (FDTD) method(Lumerical package). In the simulation, to determine theplasmon response for the proposed UV photodetector, thefollowing simulation parameters were employed: The spa-tial cell (grid) sizes were set to dx=dy=dz=0.1 nm (1 A°),and 32 perfectly matched layers (PMLs) were the bound-aries to absorb the scattered fields in the workplace.Additionally, simulation time step was set to the 0.01 fsaccording to the Courant stability. The light source was alinear plane wave source with a pulse length of 2.6533 fsand offset time of 7.5231 fs. Applying transverse polari-zation (E┴) direction (φ=0°) normal to the x-axis, we setthe intensity of the incident electric field to |E0|=1 V/m.In terms of chemical properties, the empirical dielectricfunction of bulk Rh is calculated based on Palik constantsfor solids [30].

Results and Discussion

Similar to other plasmonic subwavelength structures, plas-monic response of the Rh nanostructures strongly dependson the geometrical dimensions and type of the dielectric sub-strate that they are deposited on. Here, we first consider thespectral response of the isolated Rh nanosphere, disk, and ring

Plasmonics

with varying dimensions deposited on a glass (SiO2) substratewith the permittivity of ε=2.1. Using electrostatic approxima-tion for a sphere in subwavelength regime, proposed byBohren [31], the scattering efficiency can be rewritten as

Qscat ¼ 12π5R4s

λεp�εmεpþ2εm

���

���

� �2, where Rs is the radius of Rh sphere and

εp and εm are the permittivities of the sphere and medium,respectively. Figure 1a shows the scattering efficiency profilefor a Rh nanosphere with varying radius, in the range of20 nm<Rs<100 nm with 20-nm alteration steps. For Rs=20 nm, we detected a single dipolar peak (indicated by PD)at the deep-UV band at 6.42 eV. By increasing the radius ofthe sphere, PD becomes stronger (the amplitude increasing)and the quadrupolar shoulder (indicated by PQ) becomes vis-ible for Rs≥40 nm with a very small amplitude around6.74 eV. In the numerically calculated spectral response, byincreasing the radius of the sphere, a noticeable redshift in theposition of PD is identified, while the position of PQ shoulder

has a very minor redshift (around ∼0.12 eV). For instance, forRs=40 nm, we observed the PD at 6.19 eV, while for Rs>60 nm, the peak ofPD is induced between 5 and 6 eV. It shouldbe noted that increasing the size of the sphere leads PD and PQextremes to have growing amplitude including a dramatic red-shift in the position of the peaks to the lower energies (longerwavelengths). Let us proceed to the Rh nanodisk with bettergeometrical tunability in comparison to a simple nanosphere.Utilizing the earlier theoretical model, the scattering efficiency

can be rewritten as Qscat ¼ 12π6R4dh

λεp�εmεpþ2εm

���

���

� �2, where Rd and h are

the radius and height of the sample Rh nanodisk, respectively.Figure 1b depicts the scattering efficiency spectra for the pro-posed Rh nanodisk with varying geometrical dimensions as afunction of photon energy. Here, besides PD (at 6.24 eV), apeak correlating with PQ appears (at 6.89 eV) for the smallernanodisk with the size of D1=[Rd, h]=[20, 20]nm. Increasingthe radius and height of nanodisk gives rise to redshifts in the

Fig. 1 a–c The scattering efficiency spectra for Rh nanosphere, nanodisk, and nanorings, respectively, deposited on a glass substrate. d, e Numericallyplotted E-field maps for the plasmon resonance excitation in Rh nanosphere and nanoring at the dipole peak positions, respectively

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position of PD, while the PQ reflects trivial displacements forthese variations in the same way as nanosphere case. Theresults presented so far prove the claim that it is possible tohave resonant peaks at deep UV with acceptable amplitudes.However, the performance of the resonant modes can be en-hanced further by using more complex nanoparticles with su-perior geometrical tunability. Employing the proposedmethodby Averitt et al. [32], the scattering efficiency for a ring is

given by Qscat ¼ 128π5R6soε

3m

3λ4εpεi�εmε j

εpεiþ2εmε j

���

���

� �2, where εi ¼ εm

1þ 2 RsiRso

� ��3Þ + εp 2þ Rsi

Rso

� ��3Þ and ε j ¼ εm 1� Rsi

Rso

� ��3

Þ + εp 2þ RsiRso

� ��3Þ. Utilizing this method, we plotted the

plasmon response for a Rh nanoring with varying geometricalparameters as shown in Fig. 1c. In this regime, we have acouple of structural parameters to revise and adjust the posi-tion of resonant peaks at the desired frequency. In the depictedprofile, we considered only the inner and other radii to inducedipolar and multipolar peaks at deep UV. For the smallernanoring with [Rsi, Rso]=[15, 35]nm, we observed two dis-tinct extremes for PD (at 6.27 eV) and PQ (at 6.92 eV). Byincreasing the size of the ring, both PD and PQ redshift to thelower energies. Further modifications in the geometrical com-ponents directly led to formation of the third peak correlatingwith the octupolar mode (indicated by PO) at the deep-UVband, while the position of PQ is redshifted to longer wave-lengths. For instance, for the nanoring with the size of [Rsi,

Rso]=[45, 65]nm, three extremes are appeared at 5.44, 6.35,and 6.92 eV, correlating with PD, PQ, and PO, respectively.Considering the energy of dipolar and multipolar extremes,engineering the interaction of these modes in the near-fieldcoupling regime would be useful to induce new hybridizedenergy states in the deep-UV region. Also, it is well acceptedthat the plasmon response of nanoring is highly sensitive tothe environmental alterations [33, 34], which is useful forsensing purposes. Figure 1d, e displays the E-field maps forthe plasmon resonance excitation in Rh nanospheres andnanorings with different sizes at different energy levels ofdipolar resonant modes at the deep-UV spectrum. In both ofthe snapshots, increasing the overall size of nanoparticlescauses dramatic shifts to the lower energies (near-UV region),including a significant enhancement in the electric field inten-sity |E|. Evaluating the resonance condition in all three typesof simulated particles, nanoring provides the highest order ofplasmon modes with significant energies across the deep UV,opening paths to induce new electric dark plasmon modes andFRs at this region in more complex nanostructures. One of theconventional approaches to provide strong near-field couplingbetween nanoparticles is the use of particles in molecular as-semblies or clusters across the visible to the near-infrared re-gion (NIR). Considering several types of nanoparticle assem-blies from a simple symmetric two-member dimer [35, 36] toan antisymmetric ten-member decamer [37], inducing newmodes in plasmonic nanostructures requires specific

Fig. 2 a Schematic picture of a symmetric dimer antenna consisting ofRh nanorings. b Normalized scattering spectra of the Rh dimer duringchanging of the gap distance between proximal nanorings. c Energy-levelprofile for plasmon resonant mode hybridization of Rh dimer for the

strong coupling regime. d Electric field excitation and hybridization ina symmetric dimer system as E-field plots for different offset gapdistances

Plasmonics

conditions such as breaking the symmetry or adding dielectricnanoparticles in the offset gaps between particles in a plas-monic system [37–39]. To understand the plasmon responseof Rh nanoparticles at the deep-UV spot, we consider andcompare the spectral response of various types of dimers com-posed of a pair of molecular Rh particles in a closely spacedregime with symmetric and antisymmetric features. Here, dueto presence of high number of multipolar resonant modes andalso because of having excellent geometrical tunability, weutilized Rh nanorings with the determined dimensions in theprior analysis.

Figure 2a illustrates a classical dimer nanoantenna com-posed of two identical rings in the closely spaced regime ona glass host. Figure 2b exhibits the plasmon response for asimple symmetric dimer antenna composed of a couple ofrings with the size of [Rsi, Rso]=[35, 65]nm, while the edge-to-edge distance is varied from weak to strong coupling re-gime. In the proposed mechanism for hybridization of plas-mons in a dimer composed of nanospheres [40, 41], the

retardation effects can be ignored because of the subwave-length dimension of the structure. In the depicted scatteringcross section for the studied dimer (calculated in the retarda-tion limit) for Dg=5 nm, we observed three distinct maximawith intensified energies at 5.69, 6.36, and 6.88 eV, correlatingwith PD, PQ, and PO, respectively, that are denoted in theprofile. Decreasing the size of Dg to 1.5 nm, a noticeableredshift is detected to the longer wavelengths (lower energies),while the energy of the resonant modes intensified. In thisregime, PD, PQ, and PO appeared at 5.41, 6.25, and 6.78 eV,respectively. It should be noted that despite of the dramaticredshift in the position of plasmon modes, these plasmon ex-tremes remained at the deep-UV region. For the strong cou-pling regime with the small gap distance between nanoparti-cles, the sub- and superradiant plasmon modes become muchstronger and distinct in comparison to weak coupling regime.Appearing of dipolar and multipolar modes, the interferencebetween different multipolar modes results in hybridization ofplasmons and formation of new dark modes during the

Fig. 3 a Schematic picture for anantisymmetric dimer antennaconsisting of Rh nanorings. bNormalized scattering spectra ofthe antisymmetric Rh dimer witha dip at 6.5 eV correlating withFano resonance. c Electric fieldexcitation and hybridization in theantisymmetric dimer system as anE-field plot. d Charge distributiondiagrams of the dipolarmomentum inside theantisymmetric dimer system forboth bright and dark plasmonicmode peaks

Plasmonics

breaking of the symmetry, as shown in Fig. 2c as an energylevel profile. Figure 2d exhibits the E-field maps (numericalsnapshots) for plasmon resonance hybridization and couplingin three different offset gap distances at the deep-UV wave-lengths. Here, because of having a symmetric structure, wehave observed only the superradiant bright plasmon modes inthe spectral response. To induce new modes such as collectivesubradiant darkmodes, we needmore complex structures withbroken symmetry.

Due to simple fabrication techniques of dimer-type struc-tures, we just modify the geometrical features of the dimer. Tothis end, we monitor the plasmon response of an antisymmet-ric dimer that is shown in Fig. 3a as a schematic picture, wherethe sizes of nanorings are S1=[Rsi, Rso]=[35, 65]nm, andS2=[Rsi, Rso]=[25, 45]nm. The gap distance between twoproximal particles is set toDg=1.5 nm. Illuminating the struc-ture with a linear plane wave source and using the hybridiza-tion concept to characterize the plasmon response of the anti-symmetric dimer, the resonant modes can be classified accord-ing to their behavior in the studied system. In the near-fieldcoupling regime, the interaction between small and largenanorings causes excitation of bonding and antibonding ener-gy levels and symmetry cancellation leads to formation of

dipole-dipole and dipole-multipole coupling states.Technically, the bright plasmon resonant modes can effective-ly couple to the incident light with the dipolar in-phase mo-mentum, which is observed at 5.8 eV (see Fig. 3b, d). On theother hand, due to the symmetry cancellation, a subradiantdark mode is formed at 6.9 eV with out-of-phase dipolar mo-mentum via dipole-multipole coupling (see Fig. 3b, d). Itshould be noted that in the retarded limit, presence ofhigher-order plasmon modes (PQ and PO) plays a key role information of subradiant plasmon resonances. Also, excitationof opposite plasmon modes in the near-field coupling regimeleads to formation of destructive and weak interference of darkand bright plasmonic modes, which in turn result in the sup-pression of bright mode creation of a Fano dip (a hotspot) as adistinct narrow minimum in the deep-UV wavelengths (hereat 6.5 eV). Because of the antisymmetric geometry of thestructures, the induced plasmonic FR mode is antisymmetricand anisotropic, which strongly depends on the polarization ofthe incident light and the geometry of the dimer. Figure 3b isthe normalized scattering profile for the antisymmetric Rhdimer nanoantenna with the demonstration of the inducedFano minimum at deep-UV band. Excitation and hybridiza-tion of plasmon resonant modes are depicted in Fig. 3c as a

Fig. 4 a Schematic of theinfinity-shaped Rh nanoantennaon a glass substrate. bNumerically calculated scatteringcross section for three differentRh nanoparticles as a comparativefigure. c E-field distribution of theplasmon resonance confinementinside three differentnanoparticles under transversepolarization excitation

Plasmonics

numerically calculated E-field map, where formation ofhotspot at the offset gap is clearly visible. Figure 3d includesa couple of charge distribution diagram for two oppositemodes at the deep-UV region, which are in accord with thehybridization theory concept. The noteworthy point here is theposition of Fano dip observed at very short wavelengths(higher energies). These obtained results can be even furtherenhanced by using more complex nanostructures with inher-ent antisymmetric geometries. However, to avoid using highlycomplex structures, in continue, we try to induce high-qualityFR modes at deep-UV wavelengths in simple dimer nano-structures by modifying the geometry of the presented anten-na. One of the most popular approaches in this case is splittingcertain parts of nanorings to provide required antisymmetriccondition [11, 42]. It is well accepted that, in the retarded limit,any deconstruction in the geometry of symmetric nanoparticleaggregates can result in excitation of new collective subradi-ant modes, which causes the formation of strong hotspots in

the capacitive gap spots in between the proximal nanoparticles[43, 44]. Employing this method, we introduced a new orien-tation of nanoparticles, resembling an infinity-shaped nanoan-tenna with an excellent degree of freedom in geometrical pa-rameters with significant tunability as shown in Fig. 4a.Noticing in the depicted schematic picture, using the proposedmethod by Liu et al. [11], two split nanorings with certainangle (θ) and analogous sizes are located close to each other,where a Rh nanodisk is located in between with a varyingradius. To show the effect of each one of nanoparticles onthe plasmon response of the final infinity-shaped structure,we plotted the spectral response of the structure step by stepas shown in Fig. 4b. A split nanoring with the angle of θ=60°is able to support three peaks correlating with PD, PQ, and POat the deep-UV region, appeared at 5.23, 6.34, and 6.92 eV,respectively.

Then, by adding a Rh nanodisk in a close proximity to thesplit parts of the ring with the size of Rd=60 nm (see the insets

Fig. 5 Deep-UV tuning of Rhplasmonic infinity-shapednanoantenna. a Scattering cross-sectional profile for FDTDsimulations of the proposednanoantenna for different sizes ofthe middle nanodisk, resemblingweak to strong coupling regimes.b Simulation snapshots of theelectric field excitation inside thenanoantenna. c Chargedistribution in the antenna at theFano dip in the deep-UV region(6.60 eV)

Plasmonics

inside Fig. 4b), we induced new hotspots in the capacitiveregion between nanoring wedges and nanodisk (see Fig. 4c).As shown in Fig. 4b, adding a nanodisk to the structure result-ed in formation of a distinct and narrowerminimum at 6.54 eVdue to the suppression in the position of dipolar peak.Comparing the quality of the last Fano dip with the priorone (which was induced in antisymmetric dimer), we observethat the last one is a deeper and sharper plasmonic FR modewith a considerable blueshift to the deeper-UV band. In thefinal step, we added a split nanoring with the same propertieslike the prior ring to the opposite side of the nanodisk toprovide more complex geometry that can be tailored to sup-port highly strong plasmon resonant modes at deeper UVregion. Again noticing in Fig. 4b, by adding the new nanoringto the structure and forming an infinity-shaped antenna, twopronounced and sharp Fano dips are induced at 6.59 and5.86 eV. Here also, we detected a subtle blueshift in the posi-tion of one of Fano dips which is observed in the deep-UVregion. Figure 4c exhibits the E-field maps for the FDTDsimulation results for each modification of the structure,where formation of hotspots in the capacitive regions andconfinement of electric fields in the gap spots between coupleof ring wedges and disk are recognizable. The capacitive

offset gaps between central nanodisk and peripheral split ringsplay a key role in enhancing the energy of plasmonic FRminima. One simple method to change the size of this spotis modifying the radius of the middle nanodisk. Figure 5aillustrates the scattering spectra for the infinity-shaped nano-structure, while the radius of the central disk is varied in therange of 50 nm≤Rd≤80 nm. For a disk with small radii (Rd≤50 nm), the gap distance between ring wedges and middledisk increases, leading to have a weak coupling regime andvanishing of Fano minima. Of this, multipolar modes corre-lating with split ring do not contain enough energy to coupleefficiently to dipolar mode of central disk; hence, the possibil-ity to detect strong plasmon resonances as distinct shouldersand FRs as pronounced dips is so low (see question marks inthe calculated profile which show the low-energy shallowdips). In contrast, for Rd>50 nm, the offset gap reduces andcauses high confinement of plasmon resonances in the capac-itive region between nanoparticles in an infinity-shaped sys-tem. Obviously, two distinct dips intensified accordingly andshifted to the deep-UV region around 6.60 and 5.95 eV.However, by increasing the radius of nanodisk for Rd≥80,the dipolar peak correlating with the nanodisk redshifts tothe lower energies (longer wavelengths); therefore, FR dips

Fig. 6 a Schematic illustration ofa plasmonic metamaterialcomposed of Rh infinity-shapednanoantennas deposited on ametasurface. b Transmissionprofile for the proposedmetamaterial during changing ofthe RI of the ambience. cNumerically computed complexeffective permittivity. d Plasmonresonance energy shifts as afunction of medium RI

Plasmonics

redshifted to the longer wavelengths. This is because of for-mation of strong dipolar resonant mode correlating with nano-disk at longer spectra (λ∼390 nm), which leads to dramaticredshifts in the position of induced Fano modes. Evaluatingthe scattering profile for these variations, the best possibleFano dip is achieved for Rd=65 nm (see Fig. 5a), while thegap spot between adjacent nanoparticles is 2.5 nm. Figure 5brepresents the electric field snapshots for the confinement andhybridization of plasmons in the proposed Rh infinity-shapedantenna in the deep-UV region for the examined nanodisksizes. Figure 5c is the charge distribution diagram inside theinfinity-shaped nanostructure at the position of Fano dips,which are distributed in the opposite direction in both periph-eral split rings. It should be noted that transverse polarizationexcitation is utilized to provide perpendicular modes to thering wedges, giving rising to the formation of subradiantmodes in nanorings. This condition provides strong polariza-tion dependency for the infinity-shaped structure and can beutilized as a platform for designing Fano routers across thedeep-UV region [45].

On the other hand, there has been growing interest in FRdips with high quality in several types of plasmonic clustersand blocks for biological and environmental sensing [46–48].The mechanism behind this application is the formation ofplasmonic Fano dips that are strongly sensitive to the pertur-bations in the refractive index (RI) of the surrounding ambi-ent. The quality, number, narrowness, and deepness of FRminima are the major factors in designing precise and highlyaccurate plasmonic nanosensors. In practical sensing experi-ments, a microfluidic chip [49] can be employedwith differentglycerol solutions with variant RI to immerse nanoplasmonicsensor in it. Using these environmental variations, the plas-mon resonance shifts (Δλres (nm) or ΔE (eV)) in the calcu-lated extinction spectra determines the accuracy of devices.Herein, to provide a platform for practical deep-UV applica-tions, we utilized experimentally determined RI of variousfluids around λ=193 nm (6.42 eV) [50]. The following liquidswith defined RI are used: AlNH4 (SO4)2·12H2O@20 % (n=1.4374), Li2SO4@16 % (n=1.4559), CsCl@60 % (n=1.561),and AlCl3·6H2O@50 % (n=1.5842).

Figure 6a displays a schematic picture for the Rh infinity-shaped nanoantennas in periodic arrays as meta-atoms depos-ited on a Si/SiO2 substrate, resembling a plasmonicmetamaterial. The conventional method to determine the sen-sitivity of a plasmonic sensor is plotting corresponding figureof merit which is the peak of resonance shifts as a function ofRI alterations, as FoM ¼ δλres

δnð ÞFWHM. To determine the FoM for theproposed structure, we utilize the full width at half minimum(FWHM) of the antisymmetric FR mode line shapes.Figure 6b illustrates the transmission amplitude profile forthe plasmonic UV metamaterial, while the RI of the environ-ment is varied from free space n=1 to n=1.5842. For the freespace condition, a couple of pronounced minima are appeared

at the deep UV (6.55 and 5.75 eV) as transmission windows.Just analogous with regular plasmonic subwavelength sensingdevices that are operating based on plasmonic FR qualities, byincreasing the RI of the surrounding medium, we detected asensible redshift in the position of FR dip to the longer spectraas shown in Fig. 6b. For instance, for n=1.5842, the Fano dipsare induced at 6.25 and 5.45 eV (showing the redshift in theposition of resonant modes). Calculating the resonant modeshifts (ΔE=1.6882) and using the method above, we comput-ed the FoM for the plasmonic metamaterial based on infinity-shaped Rh nanoantenna as approximately ∼11.5 (see Fig. 6c,slope of the linear diagram shows the FoM). Figure 6d alsoshows the complex effective permittivity of the proposing UVplasmonic metamaterial that is calculated numerically for bothreal and imaginary parts at the short wavelengths. It should beunderlined that due to the strong polarization dependency ofthe infinity-shaped nanoantenna to the incident beam, we uti-lized transverse polarization excitation (E┴) to induce hotspotsin the capacitive space between split rings and nanodisk (theE-field is perpendicular to the rings’ wedges). In contrast, forthe longitudinal polarization excitation (E┴), required subra-diant modes cannot be excited, because the E-field is parallelwith the rings’ wedges. This condition can be utilized for fastswitching applications such as designing Fano routers.Ultimately, comparing the precision of the investigated nano-structure during sensing with recently analyzed analogoussensing devices that are operating across the UV to the visiblespectra, we proposed a new structure with excellent FoMacross a unique bandwidth. Considering theoretical and exper-imental predictions for defined FoM for various types of indi-vidual and compact plasmonic structures, the FoM is rangingfrom 5.7 for complex antisymmetric octamer assembly at theNIR [43], 5.9 for Al/Al2O3 nanoring dimers at the UV band[29], and 14.3 for Al/Al2O3 nanomatryoshka trimers acrossthe UV to visible spectra [51].

Conclusions

In conclusion, we presented a comprehensive study of plas-mon responses for several types of simple and complex dimer-type assemblies of Rh-based nanoparticles using plasmon hy-bridization mechanism. Analyzing the spectral response ofvarious structures, we have proposed an infinity-shaped nano-antenna with Rh substance to hybridize the plasmon resonantmodes in the deep-UV region efficiently. It is shown thatstrongly coupled Rh nanodisk and split rings in a unite systemcan be tailored to support strong plasmonic FR modes acrossthe deep UV of the spectrum via coupling of the dark andbright modes in the capacitive region between nanorings’wedges and central nanodisk. Depositing the Rh infinity-shaped nanoantennas as meta-atoms on a metasurface, weinvestigated the sensitivity of the proposed plasmonic device

Plasmonics

for biochemical sensing applications via simply altering thesurrounding dielectric medium characteristics and calculatedthe FoM=11.5 in the very deep-UV region.

Acknowledgments This work is supported by NSF CAREER programwith the award number: 0955013 and by Army Research Laboratory(ARL) Multiscale Multidisciplinary Modeling of Electronic Materials(MSME) Collaborative Research Alliance (CRA) (Grant No. W911NF-12-2-0023, Program Manager: Dr. Meredith L. Reed). Raju Sinha grate-fully acknowledges the financial support provided through presidentialfellowship by the University Graduate School (UGS) at Florida Interna-tional University.

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