Copyright © 2006 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Applied Physics Letters 88 (2006) and may be found at http://link.aip.org/link/?APPLAB/88/051109/1We show that the emission of light from a dye layer through an adjacent thin silver film is maximal for a silver thickness of approximately 50 nm. This effect is explained as the result of competition between enhancement of the electric field at the metal surface due to the excitation of a surface plasmon-polariton mode, the amount of power coupled to the surface plasmon-polariton mode, and the attenuation of the field transmitted through the silver, all three of which vary with metal thickness. We indicate how these findings may be of relevance in the design of some surface plasmon-polariton-based fluorescence biosensing schemes

G. Winter

Kretschmann E.

Raether H.

W. L. Barnes

Applied Physics Letters

Winter, G.

Barnes, William L.

Open Research Exeter

APPLIED PHYSICS LETTERS 88, 051109 2006Emission of light through thin silver films via near-field couplingto surface plasmon polaritonsG. Wintera and W. L. BarnesElectromagnetic Materials Group, University of Exeter, Exeter EX4 4QL, United Kingdom, EUReceived 27 June 2005; accepted 9 December 2005; published online 1 February 2006We show that the emission of light from a dye layer through an adjacent thin silver film is maximalfor a silver thickness of approximately 50 nm. This effect is explained as the result of competitionbetween enhancement of the electric field at the metal surface due to the excitation of a surfaceplasmon-polariton mode, the amount of power coupled to the surface plasmon-polariton mode, andthe attenuation of the field transmitted through the silver, all three of which vary with metalthickness. We indicate how these findings may be of relevance in the design of some surfaceplasmon-polariton-based fluorescence biosensing schemes. © 2006 American Institute of Physics.DOI: 10.1063/1.2170426The emission of light through thin metal films is of in-terest from a variety of applied perspectives, including top-emitting organic light-emitting diodes1 and fluorescence bio-sensors using surface plasmon polaritons SPPs.2 Thin silverfilms are also of topical interest for their possible use aslenses capable of beating the usual diffraction limit.3,4 Inboth lensing and SPP mediated emission of light, there ap-pears to be an optimum metal film thickness. Recently, theSPP mediated transmission of light through a thin silver filmusing a scheme in which the SPP modes were initially ex-cited via surface-roughness coupling was investigated.5,6More recently the same phenomenon was explored using themore controlled form of coupling to SPPs offered bywavelength-scale periodic gratings.7 In these studies, a silverfilm thickness of 50 nm was found to optimize the SPPmediated transmission of incident plane-wave light throughthe metal. Here, we explore a closely related phenomenon,SPP mediated emission of light through a thin metal film,i.e., the transmission of the near-field emission of light frommolecules through a metal film. One might expect, based onthe work on the transmission of incident plane-wave light,that a similar optimum film thickness would exist for SPPmediated emission through a metal film. However, the emis-sion process is complicated by the presence of the metal filmsince this film alters the decay properties of the emittingmolecules.8 Here, we specifically addressed the thickness de-pendence of SPP mediated emission through a thin silverfilm to explore this phenomenon further.We followed a well established technique that uses emis-sive dye molecules to excite the SPP modes of a thin metalfilm that was deposited on the base of a prism.9–12 Dye mol-ecules may excite SPP modes on the dye/silver interface, andthese SPP modes may in turn decay to produce light in theprism. Using this technique, we measured how the strengthof the SPP mediated emission into the prism varied withsilver film thickness. This technique has recently been calledsurface plasmon coupled emission2 and is a reversal of theusual Kretschmann–Raether attenuated total reflection ap-proach for the excitation of SPPs.13 In fact, this type of struc-ture Fig. 1 supports two SPP modes, one associated withthe dye-silver interface, the other associated with the silver-aElectronic mail: g.winter@exeter.ac.uk0003-6951/2006/885/051109/3/$23.00 88, 05110Downloaded 25 Jul 2008 to 144.173.6.22. Redistribution subject to Asilica interface. Only the SPP associated with the dye-silverinterface can couple to light in the prism, and it is this modewe investigate here. The SPP associated with the silver-silicainterface has too much momentum to couple to light in theprism and any power coupled from the dye to this mode iseventually lost as heat, we refer to this below as the lossySPP mode.A diagram of our experimental arrangement is shown inFig. 1. The samples consisted of a silica substrate onto whicha layer of silver, and then a layer of the dye aluminiumtris-8-hydroxyquinoline were evaporated. Before addingthe dye layer, the thickness and relative permittivity of thesilver were determined by fitting theoretical models to angledependent reflectivity data. This procedure was repeated af-ter the dye was added, thereby providing the thickness andrelative permittivity of the Alq3.To explore the SPP mediated emission of light, thesamples were index matched onto the base of a silica prismand the dye molecules were optically excited with 410 nmlight from a diode laser. SPP mediated emission through thesilver into the prism was collected with a lens and passed toa fiber-coupled spectrometer, spectra were recorded using acharge coupled device for a range of emission angles.Figure 2 shows four experimental spectra, all obtained atthe same polar emission angle. The spectra from the samplescontaining no silver i.e., dye deposited directly onto silicaFIG. 1. Color online Schematic of experimental system. The silica sub-strate supports a silver film 18–91 nm thick. 30 nm of Alq3 is evaporated ontop of the silver and the substrate is index matched to a silica prism.© 2006 American Institute of Physics9-1IP license or copyright; see http://apl.aip.org/apl/copyright.jsp051109-2 G. Winter and W. L. Barnes Appl. Phys. Lett. 88, 051109 2006for both transverse electric TE and transverse magneticTM polarizations show the expected characteristic emis-sion spectrum of Alq3. The addition of a 64.2 nm thick silverfilm leads to a significant decrease in both the TE and TMpolarized emission, except for the striking appearance of anew peak in the TM-polarized photoluminence. This peakarises from SPP mediated emission and remarkably is seen tobe stronger around the peak than the emission in the ab-sence of the metal. Theoretical simulations using a classicaldipole antenna model14 are indicated in Fig. 2 by lines andshow reasonable agreement with the experimental data.To confirm that the TM polarized peaks arise from SPPmediated emission, the PL spectra at specific polar emissionangles were compared with wavelength-dependent reflectiv-ity data acquired at the same angles. These data showed thatthe wavelength of the PL emission peak corresponds verywell with the wavelength of the reflectivity minimum, theposition of which followed the expected dispersion from theSPP mode supported by this structure.15Spectra similar to Fig. 2 were acquired for a wide rangeof emission angles. From these data, the TM polarized emis-sion as a function of silver film thickness could be deduced,and is shown in Fig. 3. Since the angle that offers best cou-pling between light and the SPP is a function of silver filmthickness, data for each silver film thickness were chosenfrom the angle that gave the strongest signal. There is clearlya peak in the transmission, occurring when the silver film is50 nm thick. The theoretical simulation, again based on theclassical radiating dipole model, calculated for the wave-length of 625 nm, shows good agreement with the experi-mental data.This peak in the emitted light as a function of silver filmthickness arises because of a competition between the en-hancement of the electric field16 at the silver-dye interfaceand attenuation through the metal film. The increasing elec-tric field enhancement arises from the decrease in the radia-tive decay rate of the SPP as the silver film thickness in-creases. In turn, the enhanced field strength leads to a greaterprobability of the dye molecules coupling to the SPP mode,FIG. 2. Color online Density of data points reduced for clarity. Experi-mental and theoretical TM and TE polarized luminescence spectra collectedthrough 64.2 nm silver, and emission spectrum of dye through no silver. Allspectra displayed were collected or simulated at a polar emission angle of=63.4°.owing to the associated increase in the density of opticalDownloaded 25 Jul 2008 to 144.173.6.22. Redistribution subject to Astates.8 The increasing probability of coupling to SPPs is incompetition with the rising levels of attenuation as the silverfilm is made thicker. For very thin silver films 20–50 nmthick, the increasing field enhancement is the stronger ef-fect, above 50 nm attenuation dominates.To explore one possible consequence of these results, weundertook simulations13 of the power transmitted through thesilver film from a system more suited to biomolecular sens-ing applications. For this simulation, isotropically alignedemissive dye molecules were positioned 5 nm, 15 nm, and30 nm from the planar silver surface, within a semi-infinitelayer of water. The silver separates the water from a semi-infinite layer of silica. Both the silver and silica are assumedto have similar properties to the materials used in the experi-ments described above Ag=−16+0.44i , Silica=2.126 at awavelength of 600 nm.Figure 4 shows the fraction of the total power emittedfrom the dye that is transmitted radiatively into the water andFIG. 3. Color online Experimental and theoretical maximum emissionthrough silver as a function of silver thickness. Data and simulation wereobtained at a wavelength of 625 nm, with the simulation using a silverpermittivity of=−17.8+0.49i.FIG. 4. Color online The fraction of the total power emitted from the dyethat is transmitted radiatively into the silica and the water. The emissivedipole is situated 5 nm, 15 nm, or 30 nm from the silver within a semi-infinite layer of water, and the silver has a permittivity of =−16+0.44i at awavelength of 600 nm. The quantum efficiency of the emitter was taken tobe unity.IP license or copyright; see http://apl.aip.org/apl/copyright.jsp051109-3 G. Winter and W. L. Barnes Appl. Phys. Lett. 88, 051109 2006into the silica. The power radiated into the water graduallyincreases and then saturates as the silver thickness is in-creased from 10 nm to 100 nm. The power radiated into thesilica varies markedly with the silver thickness and peakswhen the silver is approximately 50 nm thick, before decay-ing in an exponential manner. There is a marked reduction inpower radiated into both the water and the silica for silverfilms 4 nm thick. This arises because as the thickness ofthe silver film is reduced, the fraction of power coupled tothe lossy SPP associated with the silver-silica interface in-creases to such an extent that it is the dominant emissionpathway and leaves very little power radiated into either wa-ter or silica. As noted above, this mode cannot couple to lightin the prism; all power coupled to this mode is dissipatedwithin the silver.In the absence of any silver film, 70% of the poweremitted by the dye goes into the silica, the remaining 30%into the water. The higher fraction in the silica is due to thehigher refractive index of silica compared to water, thus giv-ing a greater density of optical states in the silica.8 Figure 4shows that with a silver film present and a dye-silver spacingof 30 nm, the power emitted into the silica through 40 nm ofsilver can be up to 30% of the total emission from the dye.Although this is less than the fraction of power that goes intothe silica in the absence of the metal film, the use of a metalfilm allows surface chemistry to be used for the molecule-specific binding required for biosensing. Further, only emis-sion originating close to the metal film contributes to SPPmediated emission, thus helping exclude backgroundfluorescence.Downloaded 25 Jul 2008 to 144.173.6.22. Redistribution subject to AIn summary, we have shown that the conditions leadingto an optimum metal thickness for SPP mediated transmis-sion of light through a metal film also occur for SPP medi-ated emission of light through a metal film. In both cases,this optimum thickness is approximately 50 nm. These re-sults have relevance for a variety of applications, includingthe development of fluorescence biosensors and top-emittingorganic light-emitting diodes.The authors would like to acknowledge financial supportfrom the Surface Plasmon Photonics Project NMP-CT-2003-505699 and the Royal Society, and would like to thankDr. I. R. Hooper for many productive discussions and Qine-tiq for assisting with sample fabrication.1M.-H. Lu, M. S. Weaver, T. X. Zhou, M. Rothman, R. C. Kwong, M.Hack, and J. J. Brown, Appl. Phys. Lett. 81, 3921 2002.2J. R. Lakowicz, Anal. Biochem. 324, 153 2004.3J. B. Pendry, Phys. Rev. Lett. 85, 3966 2000.4N. Fang, H. Lee, C. Sun, and X. Zhang, Science 308, 534 2005.5N. Fang, Z. W. Liu, T. J. Yen, and X. Zhang, Opt. Express 11, 682 2003.6Z. Liu, N. Fang, T. Yen, and X. Zhang, Appl. Phys. Lett. 83, 5184 2003.7A. Giannattasio, I. R. Hooper, and W. L. Barnes, Opt. Express 12, 58812004.8G. W. Ford and W. H. Weber, Phys. Rep. 113, 195 1984.9I. Pockrand, A. Brillante, and D. Möbius, Chem. Phys. Lett. 69, 4991980.10W. H. Weber and C. F. Eagen, Opt. Lett. 4, 236 1979.11H. J. Simon and J. K. Guha, Opt. Commun. 18, 391 1976.12R. E. Benner, R. Dornhaus, and R. K. Chang, Opt. Commun. 30, 1451979.13E. Kretschmann and H. Raether, Z. Naturforsch. A 23A, 2135 1968.14J. A. E. Wasey and W. L. Barnes, J. Mod. Opt. 47, 725 2000.15H. Raether, Surface Plasmons Springer, Berlin, 1988.16W. H. Weber and G. W. Ford, Opt. Lett. 6, 122 1981.IP license or copyright; see http://apl.aip.org/apl/copyright.jsp

Emission of light through thin silver films via near-field coupling to surface plasmon polaritons

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Emission of light through thin silver films via near-field coupling to surface plasmon polaritons

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