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Rsoft diffractmod-Polarization-induced tunability of

Polarization-induced tunability of

localized surface plasmon resonances in

arrays of sub-wavelength cruciform

apertures

Paul G.Thompson,1,2Claudiu G.Biris,2Edward J.Osley,1,2Ophir

Gaathon,3Richard M.Osgood,Jr.,3Nicolae C.Panoiu,2and

Paul A.Warburton1,2,?

1London Centre for Nanotechnology,University College London,17-19Gordon Street,

London,WC1H0AH,UK

2Department of Electronic and Electrical Engineering,University College London,Torrington

Place,London,WC1E7JE,UK

3Department of Applied Physics and Applied Mathematics,Columbia University,New York,

New York10027,USA

?p.warburton@https://www.wendangku.net/doc/0a18963173.html,

Abstract:We demonstrate experimentally that by engineering the struc-

tural asymmetry of the primary unit cell of a symmetrically nanopatterned

metallic?lm the optical transmission becomes strongly dependent on the

polarization of the incident wave.By considering a speci?c plasmonic

structure consisting of square arrays of nanoscale asymmetric cruciform

apertures we show that the enhanced optical anisotropy is induced by the

excitation inside the apertures of localized surface plasmon resonances.

The measured transmission spectra of these plasmonic arrays show a

transmission maximum whose spectral location can be tuned by almost

50%by simply varying the in-plane polarization of the incident photons.

Comprehensive numerical simulations further prove that the maximum

of the transmission spectra corresponds to polarization-dependent surface

plasmon resonances tightly con?ned in the two arms of the cruciform

aperture.Despite this,there are isosbestic points where the transmission,

re?ection,and absorption spectra are polarization-independent,regardless

of the degree of asymmetry of the apertures.

?2011Optical Society of America

OCIS codes:(250.5403)Plasmonics;(230.0250)Optoelectronics;(050.6624)Subwavelength

structures.

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1.Introduction

In recent years much attention has been focused on optical properties of nanopatterned metallic ?lms.One particularly intriguing characteristic of these plasmonic nanostructures is their abil-ity to resonantly trap and tightly con?ne light in spatial domains comparable or smaller than the optical wavelength,leading thus to the formation of so-called localized surface plasmon (LSP)resonances[1–3].These modes are spacially local collective oscillations of conduction electrons at the surface of metals.They can form at the surface of metallic nanoparticles,such as spheres,rods,disks,and rings,or in metallic nanocavities,e.g.apertures in metallic?lms or surface indentations.Because the frequencies of LSPs are strongly dependent on the shape and size of the plasmonic nanoparticles,as well as the properties of the dielectric environment,they can be tuned over the entire visible and infrared domains[4,5].Equally importantly,because of the large electric permittivity of metals,the resonant excitation of LSPs induces a signi?cant ?eld enhancement at metal-dielectric interfaces.These unique phenomena associated with the formation of LSPs have led to many exciting applications,including in chemical and biomedi-cal sensing[6–9],surface-enhanced Raman excitation spectroscopy[10–12],metallic nanotips for near-?eld optical microscopy[13–15],and optical nanoantennae[16–18].

One fascinating property of LSP resonances is that in many aspects they are similar to the electromagnetic response of an atom or a molecule,as a result of which they are sometimes called meta-atoms or meta-molecules.Moreover,similar to the case of interacting atoms or molecules,electromagnetic coupling induces the hybridization of LSP resonances of closely-spaced interacting nanoparticles[19],thus leading to complex plasmonic resonance spectra. This analogy can be extended even further:plasmonic nanoparticles can be assembled into one-dimensional(1D)plasmonic chains[20,21],2D metasurfaces[22,23],and3D bulk meta-materials[24–31]with remarkable properties.Demonstrated phenomena include the induction of a macroscopic magnetic moment in metallic resonators[32]at frequencies as large as optical frequencies;strong optical chirality[22,28,29,32,33];properly designed metallic resonators have remarkably large optical absorption[30,31,34,35];for increasing optical power,LSPs exhibit second-and third-order nonlinear polarizability[23,36,37].

Some of us have recently theoretically predicted[38]that one can tailor the properties of LSPs formed in a2D array of asymmetric apertures in metallic?lms so that the mid-infrared optical transmittance of the corresponding plasmonic metasurface becomes strongly dependent on the polarization of the incoming?eld.Here we experimentally con?rm this prediction and, furthermore,extend our theoretical and experimental analysis to the re?ectance and absorp-tance.In particular,we use a simple design for an asymmetric aperture whose properties can be conveniently tuned,namely,a Swiss cross with asymmetric arms.The plasmonic response of arrays of symmetric cruciform apertures has been extensively studied both theoretically and experimentally[34,39–42].The LSP resonances of such symmetric apertures,however,con-sists of two degenerate modes orthogonally polarized with respect to each other and therefore the optical transmission of the corresponding plasmonic metasurfaces is polarization insensi-tive.Here we demonstrate that by introducing structural asymmetry in the design of the cru-ciform aperture,the optical transmission and re?ectance of a uniform,periodic array of such apertures show enhanced optical anisotropy.In particular,the maximum of the transmission spectra,which corresponds to the resonant excitation of a LSP in the array of asymmetric cru-ciform apertures,can be tuned by almost50%by simply rotating the plane of polarization of the incident wave.Moreover,using Babinet’s principle,the ideas presented in this work can be readily extended to the complementary geometry of metallic crosses placed on a dielectric substrate[43,44].Similar phenomena could be found in arrays of rectangular or elliptical aper-tures.However,in both of these cases the maximum dimensions of such apertures are similar to the size of the wavelength at which resonances occur and so do not truly operate within the metamaterial regime.In the case of our cruciform apertures the lengths of the arms are around 6times smaller than the resonant wavelengths,and therefore function within the metamaterial regime.

2.Fabrication

Our cruciform aperture arrays were fabricated on0.5mm thick single-crystal calcium?uoride substrates,due to the low absorption of this crystal at mid-infrared wavelengths.A30nm thick layer of Au was then thermally-evaporated onto the substrate,preceded in situ by a5nm thick

Fig.1.(a)Schematic of the unit cell also showing the de?nition of the in-plane electric-?eld

polarization angle,θ.(b)Scanning electron micrograph of an array with the inset showing

magni?ed detail.(c)Schematic cross-section through the XY-segment,as shown in(b).

Cr adhesion layer.Arrays of asymmetric cruciform apertures were then milled into the surface of the gold using a30keV gallium focused-ion-beam.The beam current was50pA and the dose50pC/μm2.The device structure and a fabricated array are shown in Fig.1.The devices were designed so as to operate in the mid-infrared band,but we also fabricated several samples for C-band operation(λ～1.55μm).Each array has15×15unit cells and a periodicity in both the x and y directions ofΛ=2μm;thus the array has dimensions30×30μm2.The size of the arrays is large enough so that size-dependent array effects are negligible[2].A similar array of symmetric cruciform apertures was fabricated as a control sample.This array had the same periodicity and same number of unit cells as the arrays of asymmetric apertures.

3.Measurements

3.1.Experimental setup

The transmission spectra of the arrays were measured using Fourier-transform infrared(FTIR) microscopy.The corresponding experimental setup is shown in Fig.2.The optical source is a mid-infrared globar and the signal was detected using a mercury cadmium telluride detector. The spot size of the incident beam on the sample was0.33mm and is thus substantially larger than the array.For re?ection measurements the light is incident upon the top(gold)side of the sample.As expected,in the spectral range considered in our experiments(λ>Λ),measured transmission spectra are independent of which side of the sample was irradiated.The metal ?lms are optically opaque for the thickness used in our experiments and therefore the measured transmission corresponds to the light transmitted through the aperture array.The transmission spectra were normalized to the bare CaF2substrate while the re?ection spectra were normalized to the unpatterned gold surface.Data were obtained for incident in-plane polarization angles

Fig.2.Optical arrangement of FTIR microscope showing optical paths for obtaining both

re?ection and transmission spectra.

(as de?ned in Fig.1a)between θ=0and θ=90?in 15?increments.The minimum extinction ratio of the polarizer across the wavelength measurement range is 100.

Due to increase in noise of the FTIR spectrum below 2μm,a second separate approach was used to measure the transmission spectrum around the telecommunication C-band.An Optical Parametric Ampli?ed (OPA)Ti:Sapphire source was used to generate 100fs laser pulses at a series of wavelengths from 1440nm to 1670nm.At any given wavelength a Berek’s variable waveplate was used to vary the laser polarization.The polarized light was focused onto the plane of the patterned array at an angle normal to the surface.Focusing was accomplished using a 10×objective lens so as to produce a 12μm spot size (1/e 2).The transmitted signal was then reimaged onto a germanium photodetector.Care was taken to ensure that any spurious intensity modulation due to beam ”walk-off”as the waveplate was rotated was eliminated via position averaging.The array in this case,was separately designed for use in the 1.55μm region using the same materials system as for the mid-infrared array described in the text.

3.2.Transmission,absorption,and re?ection spectra

The resulting experimentally measured transmission spectra for the asymmetric cruciform ar-ray,presented in Fig.3a,show two distinct peaks,A and B ,the positions of which,to within the accuracy of the measurement,are invariant with respect to the polarization angle.As the polarization angle is changed from θ=0to θ=90?,the amplitude of peak A decreases from its maximum value reached at θ=0and eventually decays to below the noise level,while the peak B begins to emerge and increases in amplitude to reach its maximum at θ=90?.The spectra in Fig.3a show another intriguing spectral point,I (at λ=4.46μm),at which transmis-sion is independent of polarization.Drawing from an analogy from molecular spectroscopy,this point is termed an isosbestic point [38].By comparison,the spectra for the control array of

0.10.2

0.3

0.4Transmission

0.050.10.15wavelength [μm] 0.20.40.60.8Reflectance 0.50.60.70.8

0.9

wavelength [μm] 0.10.20.30.4

Absorption 0.10.2

0.3wavelength [μm] 0.10.20.30.4Transmission 0.050.10.15wavelength [μm]

Fig.3.(a),(c),and (e)Measured FTIR transmission,re?ection and absorption spec-

tra (respectively)for an array of asymmetric cruciform apertures with L x =1675nm,

L y =1003nm,g x =418nm and g y =165nm.These spectra show polarization angles vary-

ing from θ=0(blue)to θ=90?(brown)in increments of 15?.(b),(d),and (f)Simulation

of FTIR transmission,re?ection and absorption spectra for asymmetric cruciform apertures

with the above dimensions.(g)and (h)Measured transmission and simulation spectra for

the control array of symmetric cruciform apertures with dimensions L x =L y =1264nm

and g x =g y =368nm.

#154021 - $15.00 USD

symmetric cruciform apertures show a single peak (see Fig.3g)and,within the inherent varia-tions introduced by the fabrication process,the transmission is insensitive to the polarization of the electric ?eld.In addition,a transmission minimum,W ,is seen at λ=3.3μm,for arrays of both asymmetric and symmetric apertures.This minimum corresponds to the Wood’s anomaly of the periodic array and is predicted to occur at λW =n d Λ/ i 2+j 2[1–3],where n d is the index of refraction of the dielectric medium and i and j are mode indices.In the case of square arrays the largest wavelength at which the Wood’s anomaly occurs corresponds to i =1and j =0.As such,for an array with Λ=2μm the wavelength of the Wood’s anomaly correspond-ing to Au-air (n d =1)and Au-CaF 2(n d =1.4)is λW =2μm and λW =2.8μm,respectively.The measured re?ection spectra (Fig.3c)are qualitatively anti-correlated with the transmission spectra,with clear re?ection minima at the wavelengths of the transmission peaks A and B .Absorption spectra were obtained using the formula A =1?(T +R ).(This expression is valid at wavelengths exceeding the aperture period (here 2μm)in the case where diffuse scattering is negligible.)Remarkably,for both the re?ection and absorption spectra (Fig.3e)there is an isosbestic point,its wavelength being blue-shifted with respect to that of the isosbestic point in the transmission spectra (the isosbestic point of the re?ection and absorption spectra is at the wavelength λr =4.36μm and λa =4.32μm,respectively).The wavelength of the isosbestic point is dependent on the amplitude and width of the two resonances and,since these may differ in re?ection,transmission and absorption,it is not surprising that the wavelength of the isosbestic point shifts.

3.3.Measured aperture dimensions

In order to investigate theoretically the optical properties of the fabricated arrays,it was neces-sary to know the fabricated dimensions of the arrays,as de?ned in Fig.1a.Owing to fabrication process variability,the dimensions of each aperture vary.Therefore measurements were taken of 10fabricated apertures in each array using scanning electron microscopy and the dimen-sions averaged.For the array of asymmetric cruciform apertures the lengths of the arms of the cruciform were found to be L x =1675nm and L y =1003nm,whereas their width was g x =418nm and g y =165nm.Note also that for the array of symmetric cruciform apertures fabrication tolerances led to a small degree of asymmetry,the corresponding mean values being L x =1270nm,L y =1258nm,g x =362nm,and g y =373nm.Therefore,in this case the values of L x and L y were averaged to arrive at a single mean value:L x =L y =1264nm.Similarly,the values of the width of the arms,g x and g y ,were averaged to the mean value g x =g y =368nm.

4.Simulations

Device simulations were carried out using commercially available software,RSoft’s Diffract-MOD,[45]which implements the rigorous coupled-wave analysis method.Simulated spectra for the arrays of asymmetric and symmetric cruciform apertures are shown in Fig.3.In our simulations numerical convergence was reached when we included N =17diffraction orders for each transverse dimension,which amounts to a total of N 2=289diffraction orders.Fur-thermore,we assume that the frequency-dependent dielectric constant of Au is described by the Drude model,εAu (ω)=ε0 1?ω2p ω(ω+i γ)

,where ωp and γare the plasma and the damping fre-quencies,respectively.In the case of Au,ωp =13.72×1015rad /s and γ=4.05×1013s ?1[46].Note that since our devices operate in the mid-infrared frequency domain the contribution to the dielectric constant of inter-band effects can be neglected.We have observed,however,that in order to achieve a good agreement between the experimental data and the numerical results we had to use an increased damping frequency of γ→1.5γ=6.08×1013s ?1.This fact is not surprising since it is well known that due to electron scattering into surface states the dielectric #154021 - $15.00 USD

y [μm ]?10 1

x [μm]y [μm ]?10 1 ?10 1 x [μm]?10 1 ?10 1 x [μm]?1

0 1 ?10

1 x [μm]

?10 1 y [μm ]y [μm ]2, θ = 90°|E y |2, θ = 90°010

2030

50 01020

2, θ = 0°2, θ = 90°|E y |2, θ = 0°|E y |2, θ = 90°(c)(e)(g)Fig.4.Simulated spatial pro?les of the electric ?eld for θ=0and θ=90?.Panels a ,b ,

c ,an

d d show th

e ?eld pro?les at a wavelength o

f 3.9μm (correspondin

g to peak A in

Fig.3b),while panels e ,f ,g ,and h show the ?eld pro?les at a wavelength of 5.75μm

(corresponding to peak B in Fig.3b).The electric ?eld is normalized to the amplitude of

the incident plane wave.

constant of metallic nanostructures depends on their size when the corresponding characteristic size is comparable to the skin depth.For metallic ?lms,the bulk damping frequency is replaced by γ?lm =γbulk +αv F /d ,where αis a theory-dependent quantity on the order of 1,v F is the Fermi velocity,and d is the thickness of the ?lm [5].Interestingly enough,at optical frequencies the corresponding scaling factor was found to be equal to 3[26].

The two transmission maxima and their polarization-dependence,as well as the spectral lo-cation of the isosbestic point and the Wood’s anomaly in the experimentally-measured spectra are well reproduced in the simulated spectra for the asymmetric apertures.An additional peak,labeled C (λ=2.6μm),is however observed in the simulation,which in the experimental data appears to be only slightly above the noise level.Likewise for the array of symmetric apertures,the simulation reproduces the single peak of the experimental data at λ=4.6μm,but also predicts the existence of an additional peak C at shorter wavelength.This additional peak is at the same wavelength as for the asymmetric apertures.The insensitivity of the po-sition of peak C to the detailed geometry of the unit cell suggests that it is due to extended surface plasmon polariton (SPP)resonances.This is con?rmed by simulations of the elec-tric ?eld at peak C (not shown)which show signi?cant amplitudes in the region away from #154021 - $15.00 USD

y [μm ] ?1

1 510

150

502

4y [μm ] 1

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10x [μm]

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0 (e)(f)Fig.5.Simulated spatial pro?les of the electric ?eld at the isosbestic point (λ=4.75μm,

corresponding to point I in Fig.3b)for θ=0,θ=45?,and θ=90?.The electric ?eld is

normalized to the amplitude of the incident plane wave.

the apertures covered by the gold.Indeed,the wavelength of SPPs is given by the relation λSPP =(Λ/ 2+j 2)Re εd εAu /(εd +εAu )[1–3],which implies that for Au-air and Au-CaF 2interfaces the SPP wavelength is λSPP =2.005μm and λSPP =2.807μm,respectively (here the symbol Re means the real part of a complex number).Since |εAu | εd ,λSPP is only slightly larger than λW .Note that in this analysis the coupling between the plasmons excited at the air-metal and metal-substrate interfaces is neglected.This is a valid approximation because,as our experiments demonstrate,the transmission through a bare metallic ?lm is essentially equal to zero and therefore there is no coupling between the two plasmons.The extended SPP res-onances are signi?cantly weaker in our experimental measurements,as compared to those in simulations,presumably due to losses resulting from the surface roughness of the evaporated metal ?lm [47]and low signal-to-noise ratio of the detector in the lower-wavelength spectral domain.

In contrast,peaks A and B result from local surface plasmon resonances in the shorter and longer arms of the asymmetric aperture,respectively.More speci?cally,they correspond to the cut-off wavelength of the waveguide modes supported by the cruciform apertures.This inter-pretation is further supported by the fact that the simulated spectra for the symmetric apertures are polarization-independent as in this case the two modes are degenerate.Importantly,since the properties of these modes are de?ned entirely by the shape of the apertures,the correspond-ing transmission depends only on the optical coupling between these modes and the incoming plane wave and as such it is not affected by the roughness of the top surface of the metallic ?lm.The linewidths of the two peaks in both our simulations and our experiments are in excellent agreement.Since the simulated spectra are derived from an in?nite array of identical apertures,this con?rms that inhomogeneous broadening plays no signi?cant role in our measurements.The amplitudes of the measured LSP transmission peaks A and B are however suppressed by comparison with the simulated peaks due to similar loss processes which suppress the extended SPP modes as described above.

In order to con?rm our interpretation that peaks A and B correspond to LSP resonances,we #154021 - $15.00 USD

wavelength [μm]t r a n s m i s s i o n Fig.6.Experimentally measured transmission spectra for all fabricated arrays of asym-

metric cruciform apertures at polarization angles of θ=0(black),θ=45?(blue),and

θ=90?(red).The mean values of the other dimensions are L x =1645nm,g x =418nm

and g y =165nm.

show in Fig.4and Fig.5the simulated ?eld distributions within the apertures.The ?eld pro-?les correspond to a depth of half of the thickness of the Au ?lm.Figure 4shows the ?eld distributions at polarization angles of θ=0and θ=90?at the two transmission peaks in Fig.3b.The ?eld pro?les in panels (a)–(d)in Fig.4illustrate the in-plane electric-?eld components at λ=3.9μm (corresponding to peak A ),while panels (e)–(h)in this same ?gure show the ?eld pro?les at λ=5.75μm (corresponding to peak B ).From these simulations it is clear that peak A occurs due to the resonant excitation of a waveguide mode that is primarily polar-ized transverse to the shorter,y -oriented arm of the aperture (as shown in Fig.4a).Similarly,peak B corresponds to the cut-off wavelength of a waveguide mode with polarization primar-ily transverse to the longer,x -oriented arm (as shown in Fig.4h).Switching between these two modes is accomplished by changing the polarization of the incident plane wave.It should be noted that these LSP resonances do not correspond to the cut-off modes of the separate arms of the cruciform apertures,as in this case the cut-off wavelength would obey the relation λc <2max (L x ,L y ).Importantly,this result suggests that the wavelength of the transmission peaks can be readily tuned over a wide spectral range by simply changing the shape of the apertures.

Figure 5shows the in-plane electric ?eld distributions at the transmissive isosbestic point in #154021 - $15.00 USD

Ly [nm]

Fig.7.L y-dependence of the wavelength of the LSP transmission resonances A(blue)and

B(red),and the isosbestic point I(green).Filled points are experimental data;un?lled

points are data from simulations.Error bars in L y correspond to the standard deviation of

the fabricated device dimensions.

our simulations(Fig.3b),λ=4.75μm,at polarization angles ofθ=0,θ=45?,andθ=90?. Unlike the?elds corresponding to transmission maxima,the?elds calculated at the isosbestic

point do not have a predominant polarization state.This phenomenon also explains why such an

isosbestic point exists.Thus,let us denote by T x(λ)and T y(λ)the transmission spectra corre-sponding to an incident plane wave polarized along the x-and y-axis,respectively,and assume

that there is a wavelength,λ0,for which T x(λ0)=T y(λ0).Then,atλ=λ0,the total transmis-sion corresponding to the polarization angleθis T(λ0)=T x(λ0)cos2θ+T y(λ0)sin2θ,i.e., it is independent of the polarization angleθ.In other words,despite the fact that the plas-monic metasurface is anisotropic,atλ=λ0it is optically isotropic(as far as transmission is concerned).A similar argument holds for the isosbestic points in the re?ection and absorption spectra,although the wavelength at which the re?ectivity coef?cients,R x(λ)and R y(λ),and the absorption components,A x(λ)and A y(λ),are mutually equal would differ in the three cases. This is an expected result as the total transmission and re?ection coef?cients and,implicitly, the total absorption,depend in an intricate way on the re?ection and transmission coef?cients at the top and bottom facets of the metal?lm as well as the coupling coef?cients between the LSPs excited in the cruciform apertures and the incoming/outgoing plane waves[2].Going back to the analogy with physical chemistry,we can view the plasmonic metasurface as a2D distribution of meta-molecules whose polarizability,at the isosbestic point,is independent on polarization.

5.Further investigations

In order to further validate our interpretation of the physical origin of the resonant peaks,ex-

perimentally measured transmission spectra are now presented for an ensemble of asymmetric

cruciform aperture arrays,in which one geometrical parameter(in this case the length of the

shorter arm,L y,is systematically varied.Each array has15×15unit cells and a periodicity

ofΛ=2μm.The mean and standard deviation of L y for each array were determined using SEM,with the standard deviation of L y ranging from22nm to43nm.Typical values for the standard deviation in L x were around35nm for each array.The transmission spectra forθ=0,θ=45?,andθ=90?are displayed in Fig.6.The extracted spectral location of the resonant peaks A and B,as well as that of the isosbestic point I,are plotted in Fig.7.As expected,peak

Fig.8.Transmission through a cross array,designed for the 1.55μm region,at several po-

larizations and for different wavelengths was measured using a 100fs tunable optical para-

metric source.Each wavelength point was measured separately and averaged over many

pulses.The dimensions of the array were L x =600nm,L y =500nm,g x =200nm and

g y =100nm with a lattice constant of Λ=700nm.

A (the shorter wavelength peak)shifts to longer wavelengths as the length,L y ,of the shorter arm increases,whereas peak

B (the longer wavelength peak)is invariant with L y .Also,as the length of L y increases,the cruciform apertures tend toward symmetry in L x and L y .Thus peaks A and B ,and the isosbestic point I ,tend to converge toward a single peak with the amplitude of peak A increasing (due to an increasing area of the optical mode)and the amplitude of peak B decreasing.Also plotted in Fig.7are the results of our simulations for varying L y .The values of L x ,g x ,and g y used in the simulations are given by the mean of all these values across all the arrays,as measured by SEM.The results of our numerical simulations agree well with the experimental data,using a single value of the damping frequency γ=6.08×1013s ?1.This con?rms our physical interpretation of the features observed in the experimental spectra.

One important property of the plasmonic arrays investigated here is that the operating wave-length can be varied by simply scaling the size of the asymmetric cruciform apertures.Of course,since the dielectric constant of metals depends on frequency the operating frequency does not scale linearly with the size of the structure.These ideas are illustrated by the experi-mental data presented in Fig.8,which displays measurements of asymmetric cruciform arrays with dimensions L x =600nm,L y =500nm,g x =200nm and g y =100nm and a lattice constant of Λ=700nm.Thus,it can be seen that the main properties of the symmetric plas-monic arrays,namely,the existence of polarization dependent transmission maxima and of an isosbestic spectral point,are preserved when the size of the structure is scaled down so as the operating wavelength is shifted to the telecom C-band.

6.Conclusions

Our ?ndings suggest that the functionality of our proposed plasmonic nanostructures can be greatly enhanced by interspersing arrays whose unit cell consist of cruciform apertures with different sizes or,more generally,apertures with different other shapes.Since the transmission maxima of the arrays and their optical re?ectivity are determined solely by the frequency of the corresponding LSP resonances,the spectral optical response of these plasmonic nanostructures can be tailored for speci?c applications,allowing one to explore new designs of frequency-agile metasurfaces with enhanced functionality.One such potential application is to broadband #154021 - $15.00 USD

negative index metamaterials.Speci?cally,it has been demonstrated that by layering2D plas-monic arrays of symmetric crosses and dielectric thin-?lm spacers one obtains metamaterials with a negative index of refraction[40].In this connection,our study suggests that employing plasmonic arrays made of asymmetric crosses opens up the possibility of achieving negative index of refraction over a broad frequency domain.Moreover,the frequency of LSP resonances changes signi?cantly with the index of refraction of a chemical substance?lling the apertures, an effect that can be used to develop new plasmonic-based nanodevices for parallel,on-chip sensing for chemical and biomedical applications.In particular,it has been recently demon-strated[48]that molecules deposited on an optically thick metallic?lm perforated by a periodic array of holes can dramatically affect the transmission spectra,at wavelengths at which they are strongly absorbent,an effect called absorption induced transparency.In this connection,it can be readily understood that our plasmonic structures can be used as tunable surface?lters for chemical or biological analysis.Equally important,the conclusions of our work can be readily extended to the technically relevant1.55μm wavelength region by simply scaling the size of the apertures.

In summary,we have presented a comprehensive experimental and theoretical study of op-tical properties of plasmonic metasurfaces characterized by strong form-anisotropy of the unit cell.In particular,we have demonstrated that the excitation of LSP resonances strongly affects the transmission spectra of the plasmonic nanostructure by providing polarization-dependent transmission channels.This feature allows the transmission properties of the plasmonic ar-rays to be readily tuned by properly engineering the shape and size of the unit cell of the array.In short,our?ndings can have important relevance to new applications in nanophotonics and plasmonics,including frequency-agile surfaces,polarization-selective absorbers,strongly anisotropic metamaterials,plasmonic-based sensors for chemical and biomedical applications, and broadband negative index metamaterials.

Acknowledgement

This work was supported by an NSF/EPSRC Materials World Network Program Grant DMR#-MWN-0806682.