APPLIED PHYSICS LETTERS 93, 266102 共2008兲
Response to “Comment on ‘Surface plasmon coupled electroluminescent emission’” †Appl. Phys. Lett. 93, 266101 „2008…‡ D. M. Koller,1,2,a兲 A. Hohenau,1,2 H. Ditlbacher,1,2 N. Galler,1,2 F. R. Aussenegg,1,2 A. Leitner,1,2 J. R. Krenn,1,2 S. Eder,3,4 S. Sax,3,4 and E. J. W. List3,4 1
Institute of Physics, Karl-Franzens-University, A-8010 Graz, Austria Erwin Schrödinger Institute for Nanoscale Research, Karl-Franzens-University, A-8010 Graz, Austria 3 Christian Doppler Laboratory for Advanced Functional Materials, Institute of Solid State Physics, Technical University of Graz, A-8010 Graz, Austria 4 NanoTecCenter Weiz Forschungsgesellschaft mbH, A-8160 Weiz, Austria 2
共Received 29 October 2008; accepted 3 December 2008; published online 29 December 2008兲 关DOI: 10.1063/1.3056111兴 To clarify the questions raised in Ref. 1, we will address in the following the mode structure of our organic light emitting diode 共OLED兲 and the process of light emission to the substrate. 共Modes are named as in Ref. 2, which deviates from the notation in Ref. 1.兲 The investigated OLED consists of two metal and two dielectric thin layers, the surface plasmon 共SP兲 modes of the four different interfaces are coupled by their evanescent fields. The dispersion relations and field profiles of the different modes can only be calculated numerically by solving the mode equation of the complete system. Equation 共1兲 of the comment is an approximation, which is valid only if the layers are thick compared to the evanescent field decay of the SPs. To illustrate this we plot in Fig. 1共a兲 the calculated dispersion relation of the SP1 mode. The analysis of the field intensity 关Fig. 1共b兲兴 reveals a minimum in the Alq3 layer but maxima of similar strength at the Au-TPD and Al-Alq3 interfaces. Accordingly, SP1 is a coupled SP mode, governed mainly by the SPs at these two interfaces. The differences to the dispersion relations of SP modes on individual Au-TPD or Al-Alq3 interfaces 关Fig. 1共a兲, squares and triangles, respectively兴 can clearly be seen. SP1 is leaky with respect to the substrate. This mode can be readily excited and characterized by measuring either the spectrum of the reflectance at a fixed angle or the angular distribution of the reflectance at a fixed wavelength in attenuated total internal reflection 共ATR兲 geometry. In Fig. 2 of Ref. 2 we present the angular reflectance distribution of our OLED structure at 514 nm wavelength. For TM polarization we find a pronounced minimum in the reflectance at an angle of 59.7°, corresponding to a mode index of 1.48. This is in very good agreement with the mode index calculated above and the calculated reflectance spectra, which allows us to assign this minimum to the ATR excitation of the SP1 mode 关it should be noted that the ATR measurements were performed using high refractive index glass 共n = 1.72 at 550 nm light wavelength兲 as a substrate in order to extend the measurement range, whereas the simulations presented in Fig. 2 of Ref. 2 were performed with an imaginary substrate of n = 3. This, however, has no relevant effect on the mode index of the SP1, as our simulations indicate兴. a兲
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We now turn to the molecular emission process. Our OLED structure supports one leaky TE mode, one leaky TM mode 共SP1兲 and three bound TM modes 共SP2 as well as two modes that cannot be observed experimentally and were therefore not considered in Ref. 2兲 to which the molecular emission can couple. Additionally, our OLED could directly excite light modes in the substrate or superstrate, however very weakly since both metal electrodes are optically rather thick. In order to demonstrate that the observed emitted TM-light is indeed from the SP1 mode, one can reverse the ATR excitation scheme and detect 共a兲 the emission spectrum at a fixed angle or 共b兲 the angular emission distribution at a fixed wavelength. We opted for variant 共a兲. Consider the SP1 mode excited over the entire spectrum with equal intensity. If the propagation length of SP1 is at least several wavelengths, the spectral emission at a certain angle corresponds to the ATR absorption spectrum at that angle. In other words, a large intensity is observed in emission if the mode matching condition is fulfilled while deviations from that condition lead to decreased intensity. However, the strength of the SP mode excitation varies according to the molecular emission spectrum. One can then approximately predict the detected emission by convoluting the emission spectrum with the ATR absorption spectrum. In Ref. 2, we find for SP1 excellent agreement between this model and experiment, including the spectral shift of the
FIG. 1. 共Color online兲 Dispersion relations of the SP modes mentioned in the text 共a兲 and mode profile of the SP1 mode 共b兲. The inset in 共b兲 shows the OLED architecture. The SP1 dispersion is similar to the branch of an interface-SP above P.
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© 2008 American Institute of Physics
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Appl. Phys. Lett. 93, 266102 共2008兲
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emission maximum with varying angle. This corroborates that the detected TM polarized emission is indeed dominated by the leakage radiation of the SP1 mode and directly emitted light plays only a minor role.
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Z. Wu, S. Liang, B. Jiao, X. Zhao, and X. Hou, Appl. Phys. Lett. 93, 266101 共2008兲. 2 D. M. Koller, A. Hohenau, H. Ditlbacher, N. Galler, F. R. Aussenegg, A. Leitner, J. R. Krenn, S. Eder, S. Sax, and E. J. W. List, Appl. Phys. Lett. 92, 103304 共2008兲.
Downloaded 06 Jan 2009 to 117.32.153.167. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp
