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A novel plasmonic resonance sensor based on an infrared perfect absorber

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A novel plasmonic resonance sensor based on an infrared perfect absorber
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  A novel plasmonic resonance sensor based on an infrared perfect absorber This article has been downloaded from IOPscience. Please scroll down to see the full text article.2012 J. Phys. D: Appl. Phys. 45 205102(http://iopscience.iop.org/0022-3727/45/20/205102)Download details:IP Address: 119.78.232.181The article was downloaded on 10/05/2012 at 02:57Please note that terms and conditions apply.View the table of contents for this issue, or go to the  journal homepage for more HomeSearchCollectionsJournalsAboutContact usMy IOPscience  IOP P UBLISHING  J OURNAL OF  P HYSICS  D: A PPLIED  P HYSICS J. Phys. D: Appl. Phys.  45  (2012) 205102 (5pp) doi:10.1088/0022-3727/45/20/205102 A novel plasmonic resonance sensor basedon an infrared perfect absorber Guanhai Li 1 , Xiaoshuang Chen 1 , Oupeng Li 2 , Chengxue Shao 2 ,Yuan Jiang 2 , Lujun Huang 1 , Bo Ni 1 , Weida Hu 1 and Wei Lu 1 1 National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, 200083 Shanghai, People’s Republic of China 2 Fundamental Science on EHF Laboratory, University of Electronic Science and Technology of China,611731 Chengdu, People’s Republic of ChinaE-mail: xschen@mail.sitp.ac.cn and luwei@mail.sitp.ac.cn Received 26 December 2011, in final form 13 March 2012Published 1 May 2012Online at stacks.iop.org/JPhysD/45/205102 Abstract We present an infrared perfect absorber model composed of gold nanobars and a photonicmicrocavity. The inevitable losses in metamaterials are taken as an advantage for highabsorbance efficiency. By adjusting the structural geometry, the device can be used forrefractive index sensing. In our calculation with a spacer thickness  H   =  90nm it can yieldmore than 99% absorbance in the near-infrared frequency region. The full-width athalf-maximum can be realized up to an extremely narrow value of 40.8nm and the figure of merit can be obtained as high as 357. For sensing applications with a perfect absorber, ourwork can serve as a model of coupling between the localized surface plasmon withinnanoparticles and the propagating surface plasmon along the planar metal layer. The novelconcept has great potential to maintain its performance of localized surface plasmon inpractical applications.(Some figures may appear in colour only in the online journal) 1. Introduction Localized surface plasmon resonance (LSPR) is attributed tothe strong interaction between the conduction electrons andincidentelectromagneticfieldwithinasubwavelengthmetallicnanostructure [1]. Contrary to the LSPR, when the interfaceof the metallic structure is large enough, the plasmon canpropagate in the form of an oscillating charge wave, whichis referred to as the propagating surface plasmon (PSP) [2]. The two plasmonic properties have been considered as thefoundation of many fascinating applications in biosensing[3–5], surface-enhanced Raman spectroscopy (SERS) [6,7] and second harmonic (SH) generation [8]. In combinationwith a thin metal layer, the subwavelength structure confinesthe electromagnetic energy and allows the hybridization of localized plasmon modes with the resonator modes in themicrocavity between them [9–13]. The LSPR strongly depends on the shape, composition,size, lightpolarizationandsurroundingdielectricenvironmentof the subwavelength metallic nanostructures. Particularly,the latter dependence opens a way towards refractive indexsensing,whichcanbeusedtodetectsmallquantitiespreferablydown to single molecules. Recently, different plasmonicstructures, such as nanoshells, nanospheres and so on, havebeen proposed to detect the large spectral shift for the changesin refractive index [14–17]. Moreover, some special physical mechanisms, such as the tunability of subradiant dipolar andFano-type plasmon resonances, the symmetry breaking inplasmonic nanocavities, and especially the classical analogueof electromagnetically induced transparency, are exploredfor more sensitive and prospective sensors [18–21]. No matter what the new configurations or mechanisms, theinevitable losses constrain the development of those sensorsin practical applications. Nevertheless, a creative idea basedon metamaterials turns the losses to be advantageous byintroducing the concept of a perfect absorber. Metamaterialsare subwavelength metallic nanostructures with unit cellsmuch smaller than the operating wavelength of incident light[22–24]. The basic principle of a perfect metamaterial absorber is to minimize the reflectance through impedance 0022-3727/12/205102+05$33.00  1  © 2012 IOP Publishing Ltd Printed in the UK & the USA  J. Phys. D: Appl. Phys.  45  (2012) 205102 G Li  et al matching or some other methods, and to simultaneouslyeliminate the transmittance by maximizing the metamateriallosses [10]. Alternatively, perfect absorption can also be achieved with microcavity, holes arrays and metallic surfaces.For plasmonic sensors, the sensitivity ( S  ) is commonlydefined in terms of the change or shift of a measurableparameter, typically the resonant wavelength, for detection of refractive index. The full-width at half-maximum (FWHM)is another factor determining the performance of plasmonicsensors. It is strongly dependent on the shape and size of the nanoparticles. Some results have indicated that linewidthnarrowing is potentially helpful in improving the performanceof LSPR sensors [13]. Generally, an overall performance parameter of the plasmonic sensor is defined as the figureof merit (FOM), FOM  =  S   /FWHM. This definition allowsnanoparticles to be judged against one another as sensingplatforms independent of shape and size [25]. However, this overall performance suffers from the restriction of Lorentzresonance peak shape, as well as the measuring device,such as the spectrometer. As an alternative, FOM ∗ , definedas FOM ∗ =  [ ( d I/ d n)/I  ] max , can be used to evaluate theperformanceoftheplasmonicsensorsincetheintensitychanged I   ismucheasiertodetectinanexperimentusing,forexample,a laser diode, at a fixed wavelength  λ 0  induced by a refractiveindex change d n  rather than that of the wavelength [26]. And it is appropriate to those resonant peaks which do not followthe Lorentz shape. Recently, considerable attention has beenpaid to the improvement of FWHM or FOM ∗ by optimizingthestructuredimensionsoradoptingnewmechanisms[26,27]. However, most of the studies just concentrate on only a singleparameter, such as FWHM or FOM ∗ , and ignore the otherparameters. It is well known that a new structure with simpleconfiguration and good performance, particularly narrowerFWHM and higher FOM ∗ , is still expected.In this paper, we present a novel plasmonic sensor withthe introduction of an infrared perfect absorber. The devicecan yield more than 99% absorbance in the calculated results.Our work has indicated that the FWHM is as narrow as40.8nm and the FOM ∗ is as high as 357. We also showthat the spacer thickness between the nanobars and the planarmetal layer is of particular importance to the performance of absorbance efficiency, and consequently plays a key role insensing qualities. The mechanism of LSP and PSP interactionis proposed to explain the effect of spacer thickness on theabsorbance efficiency. 2. Structures and the simulation method Figure 1 illustrates the schematic geometry of the infraredperfect absorber. It consists of three layers over the quartzsubstrate. The bottom is a large gold planar mirror and thetop an array of periodic gold nanobars. The unit cell of thenanobars is composed of two bars with the same parameters.The spacer between the two layers, namely the microcavity,is filled with SiO 2 . The test liquids or gases are channelledor diffused over the periodic gold nanobars. The structure hasdimensional parameters as follows: bar length ( L ), width ( W  ), Figure 1.  Schematic of the sensing structure and the coordinatesetup configuration. The whole structure is placed on the quartzsubstrate. Figure 2.  Calculated reflection and transmission spectra for H   =  90nm with the other parameters given in section 3. Thetransmission values are all less than 0.001 in the NIR regime. thickness( T  )andgapdistance( D ),thelatticeconstantinthe x -axis( P  1)andinthe y -axis( P  2),thedielectricspacerthickness( H  )andtheplanargoldlayerthickness( G ). Toinvestigatetheresonantbehaviouroftheabsorber,weemploythecommercialsoftware EastFDTD (from Dongjun Information TechnologyCo., Ltd), which is based on the 3D finite-difference time-domain method, for calculation. For the excitation of thestructure, a normal incident light with its polarization alongthe  x -axis is adopted. The refractive index of the dielectricspacer in the NIR region is taken as 1.4. The permittivity of bulk gold in the frequency range is described by the Lorentz–Drude model [28]. In our calculation, the damping constant of  the gold film is set to be higher than that of bulk gold owingto the grain boundary effects and the surface scattering in realthin films [29]. With an adjustment of the spacer thickness, the inevitable losses, which a traditional study would want toavoid, are desirable in our results.For the existence of bottom layer, the transmittance of the absorber is almost totally eliminated in the near-infrared(NIR) region, as shown in figure 2. The transmission valueis so small not only at the resonant wavelength but also inthe near-infrared regime that it can be looked as  T   =  0.In this way the absorbance can be calculated by 1  −  R ( R  is the resonant reflectance dip intensity). The perfect2  J. Phys. D: Appl. Phys.  45  (2012) 205102 G Li  et al Figure 3.  Simulated reflection spectrum of the sensing structure as afunction of the dielectric spacer thickness from  H   =  30nm to H   =  150nm for a 25% solution of glucose in water. Within the largethickness range the structure achieves high absorbance efficiency. Figure 4.  Calculated resonant wavelength and the dip intensity atdifferent spacer thicknesses. The resonant wavelength has ablueshift but the dip intensity has changed by less than 0.07. absorbercanbeachievedjustbyminimizingthereflectancedipintensity and fortunately it can be easily realized by adoptingdifferent nanoparticle shapes, changing the lattice constant orrationally designing the other structure dimensions. Amongthese choices, optimizing the spacer thickness offers a simpleand efficient method to reach nearly absolute absorbance.Here, special attention is paid to the optimization of dielectricspacerthicknessformaximumabsorbance. Insuchasituation,our results demonstrate the realization of the narrow FWHMand high FOM ∗ . 3. Results and discussion To have a direct look at the effects of spacer thickness onthe sensing qualities, we first show the reflection spectrum infigure2qualitatively. Theninfigures3and4,wequantitatively show the resonant dip position shift, the intensity change,and the FWHM and FOM ∗ , respectively. The test liquidover the nanobars is a 25% solution of glucose in water withrefractive index  n  =  1 . 3594 [30] for figures 2 and 3. For the determination of FOM ∗ , a measurement with water  n  = 1 . 3198 [31] is performed in figure 4. The other parameters of  the structure are preliminarily optimized and given as follows: L  =  380nm,  W   =  100nm,  T   =  70nm,  D  =  120nm, P  1  =  500nm,  P  2  =  500nm and  G  =  40nm.Asshowninfigure2,thereareaseriesofstrongreflectancedips. Here we emphasize that the shape of the spectrumbecomes more and more asymmetric as the spacer thicknessincreases. Theintensityoftheshortwavelengthpartdecreasesto be less than half of that of the long wavelength part.Considering that the FWHM values are obtained from theLorentz fitting of the plasmon resonant dips, the reflectionspectra are not carried out with thicknesses more than 150nm.Furthermore,itbecomessymmetricagainforthicknessesmorethan about 300nm, but the FWHM is more than twice widerthan that shown in figure 2. Based on the above discussions,we shall not take the thickness to be more than 150nm.To make a systematic discussion of the structure, wepresent the resonant wavelength, dip intensity, FWHM andFOM ∗ quantitatively in figures 3 and 4. It can be seen that over a wide range of spacer thicknesses the resonant dipintensities are all below 0.1 and change by less than 0.07.However, the maximum of the absorbance can reach morethan 99.5% for  H   =  110nm. As shown in figure 3, theabsorbance has a very small change and shows low sensitivityto the thickness. Depending on the low thickness sensitivityof absorbance efficiency, this structure offers more choicesof thickness and we can pay more attention to the FWHM andFOM ∗ . Moreover,wecanseethattheresonantwavelengthhasablueshift. Theresonantwavelengthislocatedat888.1nmfor H   =  30nm and undergoes a blueshift to 800.5nm when thethickness increases to 150nm. The blueshift of the resonantwavelength is attributed mainly to the plasmonic couplingbetweenLSPofthenanobarsandPSPofthebottomgoldlayer.Thedipoleresonanceinthebarsresultsinthecreationofimagechargesinthebottomlayer,effectivelyleadingtoaquadrupoleresonance. In figure 6 we discuss these interactions in detail.Figure 5 quantitatively shows the FWHM and FOM ∗ as afunction of the spacer thickness and elsewhere in the text. Thetwo parameters are highly sensitive to the thickness. It can beseen that the FWHM is 60.4nm at  H   =  30nm and has a bigdecrease up to  H   =  70nm. Then the FWHM remains nearlyconstant at a value of 40nm from  H   =  70 to 110nm. Afterthe stable step, it goes through a fast increase up to 60.7nmfrom  H   =  110 to 150nm. Notably, the FOM ∗ has a nearlyoppositetrendrelativetotheFWHM.From H   =  30to150nm,the FOM ∗ increases first, then approaches a value of 357 at H   =  90nm, and finally has a sharp fall up to  H   =  150nm.The variation range is more than 300. From figure 5 wecan see that the FWHM and FOM ∗ obviously depend on thespacer thickness. Fortunately, we get the optimization resultsof FWHM  =  40 . 8nm and FOM ∗ =  357 at  H   =  90nm andthe values satisfy the requirement of narrow FWHM and highFOM ∗ in the applications of plasmonic resonance sensing.To have a better look at the characteristic of the perfectabsorber, the distribution of magnetic field at resonant3  J. Phys. D: Appl. Phys.  45  (2012) 205102 G Li  et al Figure 5.  Calculated FWHM and FOM ∗ at different spacerthicknesses. The FWHM remains nearly unchanged around 40nmfrom  H   =  70nm to  H   =  110nm while the FOM ∗ has a peak of 357at  H   =  90nm. Figure 6.  Simulated magnetic field distribution of the  yz  plane atresonant frequency, where the nearly absolute absorbance occurs. frequency is depicted in figure 6. The circling field in thenanobars is perpendicular to the electric field. According tothe Maxwell equation, it is known that the surface currentsare excited antiparallel in the nanobars and bottom mirror,respectively. Generally, the circulating current is consideredto be a magnetic moment and it has a strong coupling withtheincidentmagneticcomponentoftheelectromagneticwave.Moreover, the accumulation of opposite charges at both endsof the bars forms an electric moment which has an interactionwith the incident electric field. Thus, the electromagneticenergy is mostly confined in the small volume of the dielectricspacer. Here, we know that the nanobar dimensions are muchsmaller than the resonant wavelength, so the simple quasi-static approximation is appropriate in such a situation. Fromanotherviewpoint,theenergyconfinementcanbeexplainedbythemodehybridizationsbetweenLSPinthenanobarsandPSPalong the bottom gold layer. The formed microcavity acts asa capacitor storing the energy and gives rise to the reflectanceresonant dip with nearly absolute absorbance in the spectrumas depicted in figure 2. 4. Conclusions In summary, we have presented a novel plasmonic sensorof perfect absorber in the near-infrared region. We take theadvantage of the metamaterial inevitable losses to design thenanobar arrays near a gold film with an SiO 2  spacer. It isnumerically indicated that the spacer thickness is of greatimportance to the FWHM and the FOM ∗ . The mechanismofLSPRandPSPinteractionisproposedtoexplaintheperfectabsorbanceandtheenergyconfinementinthedielectricspacer.Finally, the expected narrow FWHM of 40.8nm and highFOM ∗ of357isachieved. Ourworkhasgreatpotentialtoserveas a practical plasmonic resonance sensor by taking advantageof losses with a perfect absorber. Acknowledgments This work was supported in part by the State Key Programfor Basic Research of China Grant No 2007CB613206,the National Natural Science Foundation of China GrantNos 10725418, 10734090, 10990104 and 60976092, theFund of Shanghai Science and Technology FoundationGrant Nos 09DJ1400203, 09ZR1436100, 10JC1416100 and10510704700. References [1] Kreibig U and Vollmer M 1995  Optical Properties of MetalClusters  (Berlin: Springer)[2] Ameling R, Langguth L, Hentschel M, Mesch M, Braun P Vand Giessen H 2010 Cavity-enhanced localized plasmonresonance sensing  Appl. Phys. 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