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APPLIED PHYSICS LETTERS 107, 121110 (2015)Tuning the effective plasma frequency of nanorod metamaterialsfrom visible to telecom wavelengthsM. E. Nasir, a) S. Peruch, N. Vasilantonakis, W. P. Wardley, W. Dickson, G. A. Wurtz,and A. V. ZayatsDepartment of Physics, King’s College London, Strand, London WC2R 2LS, United Kingdom(Received 11 June 2015; accepted 13 September 2015; published online 25 September 2015)Hyperbolic plasmonic metamaterials are important for designing sensing, nonlinear, and emissionfunctionalities, which are, to a large extent, determined by the epsilon-near-zero behaviourobserved close to an effective plasma frequency of the metamaterial. Here, we describe a methodfor tuning the effective plasma frequency of a gold nanorod-based metamaterial throughout thevisible and near-infrared spectral ranges. These metamaterials, fabricated by two-step anodizationin selenic acid and chemical post-processing, consist of nanorods with diameters of around 10 nmand interrod distances of around 100 nm and have a low effective plasma frequency down to awavelength range below 1200 nm. Such metamaterials open up new possibilities for a variety ofapplications in the fields of bio- and chemical sensing, nonlinearity enhancement, and fluorescencecontrol in the infrared. VC 2015 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4931687]The ability to design the optical properties of metamaterialshas already achieved a significant impact in a varietyof photonic, data processing, and sensing applications. 1–6Various types of metamaterials have been explored in theradio-frequency (RF), terahertz, infrared, and optical regionsbased on split-ring resonators, nanorod pairs, coaxial structures,fishnets, and other approaches, allowing both the electricand magnetic resonances to be engineered, providingcontrol of the permittivity and permeability of the nanostructuredmedia. 7–9 The adaptation of the above mentionedapproaches at longer wavelengths is straightforward, whiletheir scaling down to the visible spectral range is difficultdue to challenging top-down fabrication required at the subwavelengthscale.A different class of metamaterials based on arrays ofaligned plasmonic nanorods has also been studied. 10 Thistype of anisotropic metamaterial has unique electromagneticproperties determined by the electromagnetic interactionbetween the rods forming the array and can be describedwithin the effective medium theory (EMT) via the effectivepermittivity tensor components e x ¼ e y 6¼ e z , correspondingto the direction perpendicular to (x,y) and along (z) nanorodaxes. The metamaterial behaves as an indefinite metamaterial(Re(e z ) ¼ 1) at RFs and can exhibit hyperbolic dispersionin the spectral range where Re[e x,y (k)] > 0 andRe[e z (k)] < 0. For a plasmonic nanorod-based metamaterial,the hyperbolic regime has a short-wavelength cut-off typicallyin the visible spectral range, but no long-wavelengthlimit. The optical properties of such anisotropic metamaterialsare governed by their ability to support bulk plasmonpolaritonswhich define the behaviour of the extraordinarymodes supported by the metamaterial. 11 Both ordinary andextraordinary modes are related to the inter-rod couplingvia cylindrical surface plasmons (CSPs) supported by thenanorods in the array. 12,13 These peculiar dispersion propertiesexhibited by the metamaterial have been used fora) mazhar.nasir@kcl.ac.uknegative refraction index engineering and super-resolutionapplications, deep-subwavelength wave guiding, spontaneousemission control, nonlinearity enhancement, ultrasensitiverefractive index sensing, etc. 4–6,10,11,14–16Many of the above described properties are defined by theÞŠ 0) of the metamaterial,which is the onset of hyperbolic dispersion and aroundwhich the so-called epsilon-near-zero (ENZ) regime takesplace. In contrast to other types of metamaterials, which canbe easily fabricated at long wavelengths, but which are problematicin the visible, control over the bulk plasma frequencyof nanorod-based metamaterials can easily be achieved in thevisible spectral range by changing the interaction between thenanorods in the array. This can be accomplished by changingeither the period of the array or the nanorod diameter, using abottom-up, template-based fabrication approach. Typically,metamaterials comprised of arrays of aligned metallic nanorodsare synthesised in a porous alumina template fabricatedusing conventional electrolytes, such as sulphuric or oxalicacid, and have an effective plasma frequency limited to thevisible spectral range of 530–650 nm (Refs. 17 and 18) for Aubasedmetamaterials and to shorter wavelengths for those fabricatedusing Ag. The use of other plasmonic metals results inincreased losses due to the intrinsic material properties. Thus,in contrast to other types of metamaterials, it is not straightforwardto fabricate such nanorod-based metamaterials with aneffective plasma frequency in the infrared and telecom spectralranges.In this paper, we describe the design, fabrication andcharacterisation of metamaterials based on periodic arrays ofvertically aligned Au nanorods which allows the effectiveplasma frequency to be tuned throughout the visible and intothe infra-red spectral range. Highly ordered nanoporous aluminatemplates with 15 nm pore diameter and 120 nminterpore spacing over large cm-sized areas are fabricated bytwo-step anodization in selenic acid. The effective plasmafrequency of Au-nanorod based metamaterials obtained withsuch templates can then be tuned by controlling the nanorodeffective plasma frequency (Re½e z ðx ef fp0003-6951/2015/107(12)/121110/5/$30.00 107, 121110-1VC 2015 AIP Publishing LLC
121110-2 Nasir et al. Appl. Phys. Lett. 107, 121110 (2015)diameter via post processing of the pores while keeping boththe inter-rod spacing and rod length constant.The optical properties of plasmonic nanorod metamaterialsare determined by the coupling of cylindrical surfaceplasmons supported by individual nanorods in the array. Onthe macroscopic level, this is well described by the Maxwell-Garnett type EMTs which can be validated through comparisonwith the experimental and microscopic numerical modelling.19 The effective permittivities in both the in-plane andout of plane directions are given byð1 þ pÞe Au þ ð1 pÞe he xy ¼ e hð1 pÞe Au þ ð1 þ pÞe he z ¼ pe Au þ ð1pÞe h ; (1)where p ¼ p(r/d) 2 is the nanorod concentration with r beingradius of the nanorods and d being distance between thenanorods, e Au and e h are the permittivities of Au and the hostmedium (Al 2 O 3 ), respectively. This model is valid awayfrom the Brillouin zone edge of the nanorod array withk 0 p d , where k 0 is the wavevector of the incident light inthe plane of the metamaterial slab. 11 The effective plasmafrequency of Au nanorod metamaterials, which is determinedby Re(e z ) ¼ 0, is affected by both the host medium propertiesand the nanorod concentration. 11 This frequency separatesthe elliptic dispersion regime, where the metamaterialbehaves as a strongly anisotropic dielectric, and the hyperbolicregime where the metamaterial supports bulk plasmonpolaritonsdue to its metallo-dielectric behaviour.In order to fabricate the metamaterial, Au nanorods areelectrodeposited in porous anodic alumina oxide (AAO) templates,synthesized by a two-step anodization process. An aluminiumfilm of 800 nm thickness was sputtered on amultilayer substrate comprised of a glass slide with a 10 nmthick adhesion layer of tantalum pentoxide and a 7 nm thickAu film acting as a weakly conducting layer. Tantalum pentoxideis deposited by sputtering tantalum using a 20% oxygen/80%argon mixture. The porous alumina structures weresynthesized by a two step-anodization in 0.3 M selenic acidat 40–48 V at 0 C. The temperature of the sample and electrolytewas controlled throughout the anodization process.After an initial anodization step, the porous layer formedwas removed by etching in a solution of H 3 PO 4 (3.5%) andCrO 3 (20 g l –1 )at70 C leaving an ordered, patterned surface.The samples were then subjected to a second anodizationstep under the same conditions as in the first step andsubsequently etched in 30 mM NaOH to tune the diametersof pores from 15 nm to 50 nm. Gold electrodeposition wasperformed using a three-electrode geometry and a noncyanidesolution. The length of the nanorods was controlledby the electrodeposition time.In general, the geometrical parameters of the porous aluminatemplate used for nanorod metamaterial fabrication aredetermined by the electrolyte used for anodization and bythe applied anodizing potential. The pore diameter and interporespacing of the porous alumina template is proportionalto the applied voltage with proportionality constantsk d ¼ 1.29 nm V 1 and k int ¼ 2.52 nm V 1 , respectively. 20–24Therefore, for a chosen electrolyte, the diameter of the porescan be increased in a pore widening process for a givenvoltage, resulting in an increase in the nanorod concentrationp. However, due to the limitations of currently used electrolytes,a different approach is required in order to provideENZ-related functionalities in the infrared, including at telecomwavelengths.Self-ordered porous alumina is typically obtained inacidic electrolytes, such as sulphuric, oxalic, phosphoric,malonic, and tartaric acids. 25–28 In these electrolytes, the diameterof pores is determined by the applied voltage with aproportionality constant equal to or greater than 1 nm/V andthe interpore distance determined by a proportionality constant2.52 nm V 1 . In selenic acid, 24 the pore diameter ofthe self-ordered porous alumina template has a weaker dependenceon the applied voltage (a proportionality constantis 0.3 nm/V) while the interpore distance has about thesame dependence (proportionality constant 2.33 nm V 1 ).Figure 1(b) shows the SEM image of the self-ordered porousalumina template formed in 0.3 M selenic acid at 48 V,keeping the temperature at 0 C. The regular pore arrayshave been observed within micrometre sized domains. Thediameter of the pores is 15 nm with an interpore separationof approximately 112 nm. The porosity, determining nanorodconcentration p, of the anodic alumina obtained in selenicacid is 3.4%, much lower than achievable with sulphuricacid (down to 12.6%). 22,29 It is very interesting to note thatboth sulphuric acid and selenic acid possess similar chemicalstructures (Fig. 1(c)) but totally different anodization behaviour.In the case of acids having similar chemical structure,acid strength decreases as the size of the central atomincreases. The atomic size of selenium is bigger than sulphur;therefore, selenic acid is less soluble than sulphuric acidunder the same anodization conditions. This low solubilityaccounts for the much smaller pore diameter observed here.With a decrease in anodization temperature, the solubilityof selenic acid decreases leading to the unusual selforderedporous alumina with smaller than expected porediameter, while the period remains unaffected. This uniqueporous alumina structure has enabled the metamaterials’plasma frequency to be tuned throughout the visible andnear-infrared spectral ranges. Additionally, since the geometryof the gold nanorods is determined by the porous aluminatemplate, the diameter of nanorods may be varied by changingthe pore diameter in a pore widening process while keepingthe inter-rod spacing the same. Alternatively, we canalso tune the pore diameter while keeping the interpore distanceconstant by raising the temperature during the anodizationprocess. Figure 1(e) shows the effect of varying thenanorod diameter while maintaining constant nanorod spacingand length. With a reduction in nanorod concentration, amonotonous long-wavelength shift of the dominating extinctionpeak is observed, accompanied by a broadening of thepeak due to the red-shift of the effective plasma frequency(as the concentration of metal becomes smaller the interactionbetween the nanorods becomes weaker).Metamaterials comprised of Au nanorods embedded inan AAO matrix were designed with the effective plasma frequencyin the infrared spectral range (Fig. 2). The extinctionspectra show the two typical dominating resonances, whichare well separated from each other spectrally. A short wavelengthresonance is associated with plasmonic excitations by
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