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[1]. SPP waves are subsequently emitted back into the glass substrate as leakage radiation (see Fig. 5). This light collected on a photographic plate forms a ring-like distribution 

<img src='content_image/340371.jpg'>

in agreement with Eqs. 11, 20, 21. The FWHM of the SPP wavevector distribution is in direct correspondence with the radial width of the ring [20]. Further progress was obtained recently with the development of near field scanning optical microscopy (NSOM) which allows the local optical excitation of evanescent waves in the vicinity of a metal surface. Such evanescent waves can carry a sufficient amount of momentum to couple to SPP waves. Direct observations have indeed confirmed this principle [8, 21, 22]. As an example we show in Fig. 6 an experiment in which the NSOM tip launches SPPs on an aluminum film which after interaction with a hole excites optically some quantum dots (QDs) located below. The collected signal shows a specific QD luminescence spectrum [23]. By scanning the sample around the NSOM tip one can realize SPP mapping since the hole acts a probe structure for the field emitted by the tip. Quantitative analysis of the total luninescence of the QDs 

