[4] A. Drezet, A. Hohenau, J. R. Krenn, M. Brun, S. Huant, Micron 38 , (2007) 427. [5] D. W. Pohl, W. Denk, M. Lanz, Appl. Phys. Let. 44 , (1994) 651. [6] D. Courjon, Near-field microscopy and near-field optics ( Imperial College Press, London, 2003). [7] J. R. Krenn, A. Dereux, J. C. Weeber, E. Bourillot, Y. Lacroute, J. P. Goudonnet, G. Schider, W. Gotschy, A. Leitner, F. R. Aussenegg, C. Girard, Phys. Rev. Lett. 82 , (1999) 2590. [8] B. Hecht, H. Bielefeldt, L. Novotny, Y. Inouye, and D. W. Pohl, Phys. Rev. Lett. 77 , (1996) 1889. [9] A. Bouhelier, Th. Huser, H. Tamaru, H. -J. G¨ untherodt, D. W. Pohl, Fadi I. Baida and D. Van Labeke, Phys. Rev. B. 63 , (2001) 155404. [10] A. Stepanov, J. R. Krenn, H. Ditlbacher, A. Hohenau, A. Drezet, B. Steinberger, A. Leitner, and F. Aussenegg, Opt. Lett. 30 ,(2005) 1524. [11] J. J. Burke, G. I. Stegeman, T. Tamir, Phys. Rev. B 33 , (1986) 5186. [12] J. D. Jackson, Classical Electrodynamics (J. Wiley and Sons, New York, 1975). [13] G. N. ZhiZhin, M. A. Moskalova, E. V. Shomina and V. A. Yakovlev in Surface Polaritons , edited by V. Agranovich and D. L. Mills (North-Holland Publishing Company, Amsterdam, 1982) pp. 93-143. [14] A. V. Kats, S. Savel’ev, V. A. Yampol’skii, F. Nori, Phys. Rev. Lett. 98 , (2007) 073901. [15] P. B. Johnson and R. W. Christy, Phys. Rev. B 6 , (1972) 4370. [16] P. Halevi in Electromagnetic Surface modes edited by A. E. Boardman (John Wiley, New York, 1982) Chap. 7. [17] L. Novotny, B. Hecht, Principles of Nano-Optics (Cambridge Press, London 2006). [18] A. Hohenau, J. R. Krenn, A. L. Stepanov, A. Drezet, H. Ditlbacher, B. Steinberger, A. Leitner, F. R. Ausseneg, Opt. Lett. 30 , (2005) 893. [19] J. A. Dionne, L. A. Sweatlock, H. A. Atwater, A. Polman, Phys. Rev. B 72 , (2005) 075405. [20] H. J Simon, J. K. Guha, Opt. Comm. 18 , (1976) 391. [21] C. S¨ onnichsen, A. C. Duch, G. Steiniger, M. Koch, G. vonPlessen, J. Feldmann, Appl. Phys. Lett. 76 , (2000) 140. [22] M. Brun, A. Drezet, H. Mariette, N. Chevalier, J. C. Woehl, S. Huant, Europhys. Lett. 64 , (2003) 634. [23] M. Brun, S. Huant, J. C. Woehl, J.-F. Motte, L. Marsal, H. Mariette, Solid State Comm. 121 , 22