all of which share the same two focal points, separated by 30 mm. For a SP wavelength of λ$_{SP}$ ≃ 750 nm, accessible to a tunable Ti:sapphire laser ( λ 0 ≃ 750 nm)[1], we set δa to 375 nm ( N = 1). The long half axes of the confocal ellipses measure from a$_{min}$ = 30 µ m to a$_{max}$ = 31 . 5 µ m and the number of protrusions per ellipse varies from n$_{min}$ = 832 to n$_{max}$ = 932 [compare Fig. 1(a)]. For local SP launch, a single protrusion (160 nm diameter, 80 nm height) is fabricated at the position of F 1 . The measurement of the SP fields is based on a leakage radiation (LR) microscope, sketched in Fig. 1(b). A linearly polarized laser beam is focused with a microscope objective O 1 $^{(50}$×, numerical aperture =0.7) onto the sample plane. By focusing on the single protrusion in F 1 a SP wave is launched[6] in the corral. Due to the presence of the glass substrate the SP is a leaky wave, giving rise to LR along the cone defined by the angle θ$_{LR}$ (which is always larger than the critical angle of total internal reflection), fulfilling the phase matching condition [1, 2, 7]. As the LR intensity is proportional to the SP intensity in any point of the plasmon carrying interface, an immersion objective ( O 2 $^{, 63}$×, numerical aperture= 1.25) and a charged-coupled-device (CCD) camera can be used to record leakage radiation images as maps of the SP intensity distribution. A SP wave generated by a subwavelength protrusion is characterized by a [cos ( θ )] 2 angular intensity distribution, where θ is the azimuthal angle between the polarization axis of the laser and the direction to the observation point. SP reflection at the corral gives rise to a corresponding interference pattern. By rotating the laser polarization with a half wave plate we must then observe modifications of the interferences in the elliptical corral. Fig- ures 2(a) and 2(b) show the interference patterns obtained for two polarization orientations (indicated by the double sided black arrows), while focusing the laser beam onto the single protrusion in F 1 . We see clearly the presence of fringes in the elliptical corral and a strong focalization of the SP beam at F 2 . We note that the grid of white spots in Figs. 2(a) and 2(b) is an artifact from the CCD camera, and that part of the images has been saturated to increase the visibility of the interference pattern. To theoretically model the SP wave we assume at every point on the sample plane the sum of a direct SP Ψ$_{I}$ launched from F 1 and of a reflected SP Ψ$_{R}$ coming from the individual protrusions of the mirror. The field is calculated in the first Born approximation [8] and takes into account the cos ( θ ) e ik SP r $_{/}$√$_{r}$ dependance [2, 6, 9] for the source term ( k$_{SP}$ is the complex SP in-plane wave vector and r the distance to the observation point). The response of each individual protrusion is sup- 3