Optical Properties and Liquid Sensitivity of Au-SiO 2 -Au Nanobelt Structure

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1 DOI /s Opticl Properties nd Liquid Sensitivity of - - Nnoelt Structure Mehrdd Irnnejd 1 & Bo Cui 1 & Mustf Yvuz 1 Received: 16 Ferury 2015 /Accepted: 8 My 2015 # Springer Science+Business Medi New York 2015 Astrct Opticl properties of the - - nnoelt structure with qusiperiodicity long x-xis nd fixed height, width nd different spcer (dielectric) heights were numericlly investigted for sensing ppliction of liquids with refrctive indices close to the wter. Although oth propgted surfce plsmon mode nd loclized surfce plsmon mode were recorded in the single-lyer nd - - nnoelt structure, only the loclized surfce plsmon mode ws recorded in the - nnoelt structure. The effects of incresing the dielectric lyer (etween two nnoelts) height on the extinction spectrum were oserved s lue shift of whole extinction spectrum. It ws found tht the loclized surfce plsmon mode shift ws negligile while the propgted surfce plsmon mode ws significntly shifted towrds smller wvelength. The minimum resonnce linewidth ws clculted s 42 nm t refrctive index of 1.377, which is much smller thn tht in the single-lyer nd - nnoelt structures. Keywords Grting structure. Loclized surfce plsmon. Hyrid plsmonic mode. Nnoelt. Sensor Introduction Resonnt oscilltion of conduction electrons t the interfce etween metllic nd dielectric mterils (surfce plsmon) * Mehrdd Irnnejd mehrdd.irnnejd@uwterloo.c 1 Wterloo Institute for Nnotechnology, University of Wterloo, Wterloo, ON, Cnd with negtive nd positive permittivity hs gined much ttention in sensing pplictions especilly in chemicl nd iosensing pplictions in lst decdes [1, 2]. This rod interest is minly due to the ility of controlling the gps etween nnostructures to crete regions with high enhncement of electromgnetic (EM) field t the mterils interfce [3, 4]. Besides loclized surfce plsmon (LSPR), the resonnt oscilltion of electrons cn propgte perpendiculr to the incident wvevector (k z ), known s propgting surfce plsmon (PSP) [5]. In the lst decde, mny different nnostructures using nole metls such s gold nd silver hve een studied to mnipulte the opticl properties t the nnometre scle for novel pplictions including io-/chemicl sensors [6, 7], plsmonic wveguides [8], lser [9] nd drug delivery [10]. Nnostructures with suwvelength rectngulr crosssection were shown with strong plsmonic mode t the visile region nd thus could e considered s promising cndidtes for sensing pplictions due to their ility to tune the plsmonic resonnce modes to mtch the iologicl trnsprency window nd coincide with the light source. Recently, the opticl properties of gold nnoelt (nnowire with rectngulr cross-section) structures with different crosssectionl spect rtio (AR ¼ Width Height ) nd fixed length were studied y L.J.E. Anderson nd co-worker [4]. It ws found tht the plsmonic mode properties such s linewidth in such structure strongly depends on the x-ar, nd the resonnce wvelength cn e tuned over the visile region y chnging the x-ar. For exmple, nnoelt with low cross-sectionl spect rtio ws shown to hve plsmonic resonnce mode t green wvelength, while those with lrger cross-sectionl spect rtio offer plsmonic mode t red wvelength. In this pper, we numericlly study opticl properties of the sndwich nnoelt structure consisting of two gold nnoelts

2 with short length of 1 μm which were seprted y thin lyer of dielectric nnoelt spcer. The ility of such structures (single-lyer nnoelt grting nd the - - nnoelt grting) in sensing ppliction is lso investigted, nd the structurl ulk sensitivity nd its opticl properties re compred. Methodology The 3D full wvevector finite difference time domin (FDTD) method, which is powerful technique in solving Mxwell s equtions in complex geometries nd dispersive medi, ws employed to solve the Mxwell s equtions discretely using Yee cells [11] nd clculte the extinction spectrum of - - nnoelt in qusiperiodic structure long x-xis. The permittivity vlues of high refrctive index glss sustrte nd superstrte (i.e. trget mteril in sensing ppliction), ε(ω), were ssumed respectively s 6.25 (n=2.5)nd n 2, where n is the refrctive index of trget liquids. The nnoelt permittivity ws clculted using Lorntz-Drude model [12, 13] y fitting the empiricl dt of rel nd imginry prts of the gold permittivity over given wvelength region of interest s shown in Fig. 1. The FDTD method ws crried out using the commercil softwre pckge (FDTD Solution) from Lumericl Inc. The plne wve source, nnoelt structure, trnsmission nd reflection monitors were co-plnr with oundry conditions tht mde them effectively infinite. The incident light source ws plced in the sustrte region to reduce the incident light dispersion. Air ws considered s filling mteril etween ech grting nnoelt structure nd superstrte. In this study, we used plne wve of wvelength in the rnge of 300 nm to 1 μm with electric field mplitude of 1 V/m, different polriztion ngle in the rnge of 0 (TM-mode) to 90 (TE-mode) nd different incident ngle in the rnge of 0 to 30. The sensing liquids of thickness of 250 nm covered the nnoelt structure nd filled the spce etween them s schemticlly shown in Fig. 2, c. The periodic oundry condition ws used in x direction. While in the y nd z directions, the perfect mtching lyers (PML) were used to sor the reflected light ck to the nnoelt structure. The clcultion grid resolution ws s smll s 3 nm (grid point-to-point distnce) in the simultion cell, nd comintion of grding mesh nd conforml mesh methods ws used to clculte the electric nd mgnetic fields t the corner nd squre regions of the nnoelts ( html). The clcultion time ws set s 350 fs, nd the extinction (1-T) spectr were clculted using n x-y monitor t 250 nm wy from the nnoelt surfce. The plne wve source ws plced 200 nm underneth the structure inside the sustrte mteril s shown in Fig. 2c. Results nd Discussion Extinction Resonnce Spectrum (Norml Incidence, TM-Mode) The extinction resonnce spectr of single-lyer nnoelt with 20 nm height nd 100 nm width (x-ar=5), the - nd the - - nnoelt structures with 10 nm thick s spcer lyer (x-ar=5) re shown in Fig. 2d. Ascn e seen from this figure, for single-lyer nnoelt structure, there re two extinction peks t 675 nd 987 nm nd shoulder t wvelength of 938 nm, wheres for the - Fig. 1 Rel prt () nd imginry prt () of gold permittivity sed on Lorntz-Drude mode s function of wvelength in the rnge of 300 nm to 1 μm

3 Y c X Source Z Glss sustrte X Z Trnsmission monitor Reflection monitor Air/sensing liquid Fig. 2 The FDTD lyout of the - - nnoelt in XY view, perspective view nd c XZ view. d FDTD clculted extinction spectrum of qusiperiodic nnoelt structures in single-lyer, - nd - -. The nnoelt height nd width were fixed t 20 nd 100 nm, Y Extinction d trnsmission nm nm nm Wvelength ( m) Intrinsic of gold Sustrte respectively. The structurl periodicity long x-xis, spcer height nd the superstrte refrctive index were fixed t 300 nm (centre to centre), 10 nm nd , respectively nnoelt structure extinction resonnce pek t wvelength of 975 nm ws recorded. The propgted surfce plsmon resonnce (PSPR) long x direction t wvelength of 675 nm ws in greement with reported vlue with x-ar=5 nd linewidth of 444 nm [4]. From Fig. 2d, it is lso cler tht y dding 10 nm thick of (- ) the surfce plsmon resonnce t wvelength of 675 nm ws shifted to 643 nm with very low intensity, which could e ttriuted to phse velocity reduction of the propgting wve [14] s it is lso shown in Fig. 3d.Chnging the structure to the - - led to strong resonnce intensity nd lue shift in the resonnce position y 12 nd 8 nm, respectively. The resonnce peks of the new structure were recorded t wvelength of 975 nd 667 nm. Yet there ws no significnt chnge in the position of shoulder pek. The linewidth of the loclized surfce plsmon resonnce ws reduced from 296 to 128 nm s results of dding the spcer nd the second nnoelt. Furthermore, the linewidth of the propgting surfce plsmon resonnce mode ws incresed from 30 to 175 nm compred to the single-lyer nnoelt structure s cn e seen from Fig. 2d nd summrized in Tle 1. Yet it is smller thn the vlue reported y Anderson nd co-worker [4]. It cn lso e seen tht y dding spcer nnoelt on top of the nnoelt (red curve in Fig. 2d), there ws not ny vrition in the resonnce linewidth t wvelength of 975 nm compred to the one in the single-lyer nnoelt structure t wvelength of 987 nm s it is listed in Tle 1. Figure 3 shows the electric profile of the qusiperiodic single-lyer nnoelt, the - nnoelt nd the - - nnoelt structures. Both loclized surfce plsmon nd propgted surfce plsmon profile were oserved t the /glss sustrte interfce. It ws found tht y dding 10 nm thick, the resonnce pek t wvelength of 675 nm (Fig. 3) ws shifted y 32 nm nd ppers s resonnce pek with very low intensity t wvelength of 643 nm (Fig. 3d) s it ws lso shown in Fig. 2d. This lue shift results in electric field enhncement reduction t /glss sustrte interfce (Fig. 3d) which is in greement with the recorded trnsmission spectrum in Fig. 2d nd could e ttriuted to reduction of phse velocity of the propgting surfce plsmon mode (Fig. 4) ( oj_mesh_refinement.html). As it is cler from Figs. 3e nd 4, the resonnce mode t wvelength of 938 nm (shoulder pek) ws recorded s two loclized surfce plsmon mode with evnescent tils to the glss sustrte nd lyer, which is similr to the TEM 10 mode of n opticl wveguide with rectngulr cross-section [15, 16]. Therefore, electric field enhncement t resonnce wvelength of 938 nm ws reduced s it is evident y compring the Fig. 3e, f. Figures 3f nd 4c, show the electric field profile of the surfce plsmon resonnce mode t wvelength of 975 nm in the xz nd yz plnes, respectively. The electric field profile of the surfce plsmon t this wvelength ws similr to tht t resonnce wvelength of 643 nm with lrger propgtion length (decying til through the sustrtendsuperstrte). By chnging the structure from the - to the - - nnoelt structure, the electric field confinement ws oserved t the upper nnoelt nd nnoelt interfce with decying til in to the superstrte, s it is cler from Figs. 3g nd 4d. The oservtion of decying til could e due to the phse velocity reduction effects due to the presence of nnoelt [14]. As cn e seen from Fig. 4e, in ddition to the surfce plsmon resonnce mode t wvelength of 975 nm, four symmetric (rectngulr) hot zones were lso recorded t the / 2nd nnoelt interfce tht could e ttriuted to presence of grting orders [17]. It cn e concluded tht coupling etween the grting order resonnce modes nd surfce plsmon resonnce mode ws led to strong confined hyrid surfce plsmon mode t wvelength of 975 nm in the - - nnoelt structure compred to the - nnoelt with electric field enhncement s high s 21 with respect to the incident electric field.

4 λ=675 nm λ=938 nm c λ=987 nm Air Sustrte Glss d λ=643 nm e λ=938 nm f λ=975 nm Air Sustrte Glss g λ=667 nm λ=975 nm h Air Sustrte Glss Fig. 3 Top row,thee 2 profile of single-lyer nnoelt t resonnce wvelength of λ res =675 nm, λ res =938 nm nd c λ res =978 nm. Middle row,thee 2 profile of the - nnoelt t resonnce wvelength of d λ res =643 nm, e λ res =938 nm nd f λ res =975 nm. Bottom row, the The Effects of Vrying Polriztion Angle nd Incident Angle of Light Source The effects of vrying polriztion ngle (α) of the incident EM field (plne wve source) on the extinction spectrum of the - - structure were studied y vrying the polriztion ngle of incident EM field from 0 (TM-mode) to 90 (TE-mode) y increment of 10 o. Figure 5 shows extinction Tle 1 The resonnce wvelength position nd linewidth of different nnoelt structures Nnoelt structure Resonnce position (nm) Linewidth (nm) PSPR LSPR PSPR LSPR E 2 profile of the - - nnoelt t resonnce wvelength of g λ res =667 nm nd h λ res =975 nm. The incident electric field ws fixed t 1 V/m spectrum of the - - nnoelt structure with 10 nm thick which ws plced etween the two nnoelts of height nd width of 20 nd 100 nm, respectively. As it is cler from this figure, y incresing the polriztion ngle the extinction resonnce, pek intensity nd its width were incresed. It ws lso found tht the resonnce wvelength of LSPR mode ws shifted towrds smller wvelengths y 15 nm; however, the resonnce wvelength position of SPR mode ws shown red shift of 23 nm on incresing the polriztion ngle from 0 (TM-mode) to 90 (TE-mode) s it is evident in Fig. 5.Atpolriztionnglesof80 nd90 o (TEmode), there ws not ny SPR mode in the recorded extinction spectrum, wheres one extinction pek ws recorded t wvelength of 991 nm s it is evident in Fig. 5. The chnge in the extinction spectrum cn e explined y the physics of the surfce plsmon excittion. It is known tht surfce plsmon resonnce is excited only when the momentum conservtion is fulfilled etween the incident EM wve t TM-mode nd oscillting electron of the metllic surfce.

5 λ=643 nm λ=938 nm λ=975 nm c Source Sustrte Sustrte Sustrte d λ=667 nm e λ=975 nm SiO Grting orders 2 Sustrte Sustrte Fig. 4 Surfce plsmon resonnce profile long y-xis of the - ( c) nd the - - nnoelt structures (d, e) with qusiperiod of 300 nm, nnoelt width nd height of 100 nd 20 nm, respectively, nd 10 nm thick Hence, y chnging the polriztion ngle from 0 (TMmode) to 90 (TE-mode), momentum mismtch ws rised etween incident light wvevector, k z, nd surfce plsmon polriton wvevector, k spp,[3], which led to grting order resonnce peks insted of hyrid resonnce mode nd sence of the SPR mode t two polriztion ngles of 80 nd 90. The effects of vrying the incident ngle (θ) of the plne wve t TM-mode (α=0 o ) in the rnge of 0 to 30 on the extinction spectrum of the - - nnoelt structure Extinction Intrinsic trnsmission of gold film nm SPR nm LSPR Wvelength ( m) = 90 0,TE Fig. 5 The effects of vrying the polriztion ngle (α)nd incident ngle (θ) on the extinction spectrum of the - --nnoelt structurewith10nmthick, height nd width of 20 nd = 0 o,tm Extinction nm Gold trnsmission Wvelength ( m) =20 o =10 o 100 nm, respectively, structurl period of 300 nm nd mient medium refrctive index of The incident ngle (θ) nd the polriztion ngle (α) of the incident plne wve ws 0 in nd, respectively o o

6 were compred in Fig. 5. The extinction vlley t 478 nm which is independent of incident ngle (Fig. 5) nd polriztion ngle (Fig. 5) ws ssigned to the intrinsic trnsmission of gold film which ws discussed elsewhere [18]. As cn e seen from Fig. 5, the surfce plsmon resonnce intensity (extinction intensity) ws reduced y incresing the incident ngle. It is lso cler tht the resonnce linewidth ws incresed, nd the resonnce wvelength position offers lue shift especilly for LSPR mode s result of incresing the incident ngle. More interesting effects were oserved y using the EM field t incident ngle lrger thn 20. As it is cler from Fig. 5, the extinction spectrum of the - - nnoelt structure t incident ngle of θ=30 depicts two extinction peks t wvelengths of 607 nd 843 nm. The former shrp pek (mrked with ornge rrows) could e ttriuted to the Ryleigh Anomly [17], while the second one is the hyrid LSPR mode tht ws lue shifted y 132 nm compred to the extinction spectrum t incident ngle of 0. From Fig. 5, it cn e concluded tht the trnsmission intensity of the - - nnoelt t θ=30 ws dropped to 0 eyond the wvelength of 1000 nm, wheres the recorded trnsmission ws lrger thn 15 % eyond the LSPR mode t θ smller thn 30. Qusiperiodic Structure Vrition A series of numericl simultions were crried out to study the effects of vrying the structurl periodicity long x-xis on extinction spectrum of the - - nnoelt structure. In these studies, the nnoelt height nd width were fixed t 20 nd 100 nm, respectively, nd the height ws fixed s 10 nm. The nnoelt length ws chosen s 1 μm tovoid ny Fry-Perot resonnce effects [4, 19]. The effects of vrying the qusiperiodic structure long x-xis on the extinction spectrum t the ir/ interfce of the - - structure re shown in Fig. 6 for oth PSPR nd LSPR modes. As cn e seen from this figure, the linewidth of the loclized surfce plsmon mode ws incresed y incresing the structurl periodicity long x-xis. Therefore, it cn e concluded tht smller structurl periodicity offers nrrower linewidth compred to the lrger one. It ws lso found tht y incresing the structurl periodicity long x-xis, the resonnce wvelength of LSPR mode hs lue shift of 51 nm nd the extinction intensity ws reduced y ~35 % s it is shown in Fig. 6. The reduction in the resonnce extinction intensity could e ttriuted to weker coupling effects of LSPR mode of the - - structure with its closest neighours. The LSPR mode coupling etween two successive nnoelt structures ws reduced y incresing their seprtion distnce s the electric field mplitude of the surfce plsmon mode ws decying exponentilly [3]. Dielectric Height Vrition The effects of vrying the spcer height on extinction spectrum of the - - structure of 20 nm thick nd 100 nm wide were studied numericlly y vrying the height in the rnge of 10 to 100 nm. In ech numericl nlysis, the extinction dt ws recorded t 250 nm wy from the 2nd nnoelt (ir/ nnoelt interfce). Fig. 7 shows the recorded extinction spectrum of the nnoelt structure t different spcer heights. It ws found tht y incresing the spcer height from 10 to 100 nm, the LSPR wvelength ws shifted towrds smller wvelength nd resonnce wvelength ws recorded t wvelength of 935 nm t dielectric height of 100 nm s it is lso evident in Fig. 7. From Fig. 7, it cn lso e seen tht y incresing the spcer height from 10 to 40 nm, the linewidth of the LSPR mode ws incresed y 84 nm (green tringles), while y further incresing of the spcer height up to 100 nm, the Extinction Intrinsic trnsmission of gold film SPR LSPR Wvelength ( m) 300 nm 400 nm 500 nm 600 nm 700 nm 800 nm Fig. 6 The effects of vrying the qusiperiodic structure long x-xis on the extinction spectrum of PSPR nd LSPR modes nd resonnce position nd extinction resonnce intensity of LSPR mode. The height position (nm) Resonnce Resonnce position Intensity Period (nm) nd width were fixed s 20 nd 100 nm, respectively. The height, structure length long y-xis nd refrctive index of the superstrte lyer were fixed t 10 nm, 1 μm nd , respectively Intensity

7 Fig. 7 The effects of vrying the spcer height on the extinction nd resonnce wvelength nd linewidth of oth propgted nd loclized surfce plsmon modes. The height nd width were fixed t 20 nd 100 nm for ech nnoelt. The nnoelts length long y-xis ws fixed t 1 μm. The superstrte lyer ws considered s ir linewidth of the LSPR mode ws reduced y 29 nm. Furthermore, the linewidth of PSPR mode of the nnoelt structure ws reduced monoticlly from 175 to 77 nm y incresing the spcer height from 10 to 100 nm (lue tringles). The minimum linewidth vlues for PSPR nd LSPR modes were clculted s 77 nd 181 nm, respectively, t dielectric height of 100 nm. The min reson for chieving nrrower linewidth for PSPR modes could e ttriuted to more electric field confinement t lrger spcer height, lrger spect rtio of the structure nd smller phse velocity of the surfce plsmon resonnce wves [4, 14] which increses the SPR sorption y medium nd reduce its propgtion length. Surfce Sensing The effects of using liquids with different refrctive indices in the rnge of to on the sensing properties of singlelyer nnoelt structure nd the - - nnoelt structure with height nd width of 20 nd 100 nm, respectively, spcer height of 10 nm, nnoelt length of 1 μm long y-xis nd qusiperiodic of 300 nm long x-xis were studied numericlly. The surfce of the nnoelt structure ws covered y 250 nm thick liquids with refrctive indices of 1.333, 1.345, 1.356, nd Figure 8, shows the FDTD clculted extinction spectrum of the designed structures t different refrctive indices of the superstrte medium. Fig. 8 FDTD clculted extinction spectrum of liquids with different refrctive index s function of wvelength for single-lyer nnoelt nd the - - nnoelt structure. c Resonnce wvelength shift nd sensitivity of the - - nnoelt structure s function of refrctive index for two different reference points of wter nd ir. The height nd width of oth nnoelts nd the height were fixed s 20, 100 nd 10 nm, respectively. The nnoelt lengths long y-xis nd qusiperiodic structure long x-xis were fixed t 1 μm nd 300 nm, respectively c

8 As it is shown in Fig. 8, there ws not ny significnt shift in the resonnce extinction position t wvelength of 987 nm y incresing the liquid refrctive index from to 1.377; however, incresing the refrctive index from to resulted in 5 nm lue shift. It is lso cler tht y incresing the refrctive index of the superstrte eyond 1.000, the resonnce mode t wvelength of 675 nm disppered which ws lso reported in Fig. 2d nd discussed in Extinction Resonnce Spectrum (Norml Incidence, TM-Mode). The wvelength of hyrid surfce plsmon resonnce mode ws shown with red shift y incresing the refrctive index of the surrounding medium s it is shown in Fig. 8. It is cler tht on using liquids with refrctive index in the rnge of to 1.377, the resonnce wvelength ws shifted lmost linerly s it is lso evident in Fig. 8c. It is well known tht the resonnce wvelength position strongly depends on the effective dielectric constnt of the designted structure [20, 21], nd the FWHM of the resonnce wvelength ws decresed y incresing the refrctive index of the mient medium [22, 23]. Therefore, oserving the red shift in the resonnce wvelength of the - - nnoelt structure cn e ttriuted to the incresing of the mient refrctive index nd hence increses the effective dielectric constnt of the structure. From Fig. 8, c, it cn e concluded tht the mximum resonnce shift ws cquired, y using liquid with lrger refrctive index for oth ir nd wter reference points. Figure 8c lso depicts the resonnce ulk sensitivity, which is equl to the rtio of resonnce wvelength shift nd refrctive index chnge (Δλ/Δn), of the - - nnoelt structure which they were clculted with respect to the ir nd wter. It ws found tht lrger sensitivity ws clculted when it ws mesured with respect to the wter (n=1.333)compred to tht mesured with respect to the ir (n= ). By using wter s reference point, the mximum sensitivity of 768 nm/ RIU ws mesured t refrctive index chnge of s it is cler from Fig. 8c, wheres y using ir s the reference point, the mximum sensitivity of 313 nm/riu ws mesured t refrctive index chnge of It ws lso found tht y incresing the refrctive index, the linewidth of the LSPR mode ws reduced linerly s it is cler from Fig. 8 inset. The minimum linewidth of 42 nm ws clculted t refrctive index of while it ws clculted s high s 128 nm t refrctive index of By compring the linewidth of the hyrid SPR mode of oth single-lyer nd the - - nnoelt structures nd their sensitivity properties, it cn e concluded tht the - - nnoelt structure could e considered s cndidte for sensing chemicls with refrctive index close to the wter; however, this structure still cnnot compete with the nnohole rry structure [24, 25]. Conclusion Opticl properties of the nnoelt structures with qusiperiod long x-xis with optimum heights nd widths of the nnoelts for oth lyers nd dielectric spcer were investigted. It ws shown tht y incresing the dielectric lyer height, the extinction spectrum ws shifted towrds smller wvelength in the - - nnoelt structure. It ws found tht the loclized surfce plsmon mode ws shifted y 36 nm while the propgted surfce plsmon mode ws offered lue shift y 98 nm. 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