[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 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 12 FIG. 5: Observation of leakage radiation (LR) through a rough thin silver film (wavelength λ = 550 nm ). (A) SPPs are launched on the air/silver interface by using scattering by film rugosities to produce the amount of momentum necessary to match the SPP dispersion relation. SPPs are leaking through the glass substrate and detected using a photographic plate in the far-field. (B) Photography showing the leakage radiation cone. The photography is extracted from the work by H. J Simon and J. K. Guha [20] ( c ○ Opt. Comm., Elsevier, 1976).