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Three methods of coupling to surface plasmons. (a) Prism coupling (Kretschman); (b) grating coupling; (c) evanescent wave coupling (Otto). SP indicates the surface on which surface plasmons are excited.
Map of and . Contours of and on the plane representing the thickness of the air gap, , and the angle at which minimum (ideally zero) reflectance is obtained for thick samples. This map can be used to derive from experimental data. Points representing literature values (Refs. 10–12) for several metals are shown, together with our measured values. Al, Au, Cu, and In samples were prepared by vacuum evaporation (thickness ), and Cu and Ag samples from bulk ( purity).
Experimental setup. A typical image is shown on the left, and details of the tunnelling region are shown on the right.
Elliptical fringes observed for six samples. Fe does not exhibit SPR. All films have . The differences between the radii and the sharpness of the fringes can be seen clearly. In general, the larger and sharper the ellipse, the greater the SPR sensitivity of the sample.
Radial intensity profiles of images as a function of . The profiles are measured at the angle of incidence where highest contrast was obtained. The minimum intensity is not zero as predicted by the theory, probably because of surface scattering. For each profile, the intensity shown at a particular value of is an average of the values measured around an elliptical ring with the appropriate axis ratio.
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