Optical excitation of surface plasmons Table 1 Values for the permittivity of gold and silver obtained using surface plasmon excitation in the Kretschmann-Raether geometry Wavelength (nm) Gold 40 030 7 03 8 572 7979 345 124 14 147 90419 79797 159 135 80 -224 1-32 213 750 6 1·57 887878787878787 162 800 0842 20 2 have(in the absence of the prism) the same wavevector. This means that they couple together to give two coupled modes, one which is symmetric in surface charge and the other which is antisymmetric. The first of these modes has very weak electric fields in the metal and is the so-called long range surface plasmon(after Sarid[1on while the second is labelled the short range surface plasmon since it has large fields in the metal and is therefore strongly attenuated through Joule heating. One could choose to examine even more elaborate multi 0-00+ layered structures which support more complex coupled modes but this illustrates no particularly new physics and provides little extra potential for device development Figure 8. Surface plasmon resonance for film using the If we choose to have a non-planar interface then we Kretschmann-Raether geometry. Note this case the are not necessarily restricted to the prism-coupled critical angle is clearly visible at 34.5 olid line shows geometry. One possible technique, a somewhat un- the quality of fit which can be obtained to Fresnel theory satisfactory one, is to study a deliberately roughened interface. If we Fourier analyse the roughness there is 5. Experimental studies likely to be a component that supplies the extra momentum needed to couple radiation directly to the These two types of attenuated total reflection arrange- surface plasmon. While this may yield a broad band ments have formed the basis of most of the studies of optically excited surface plasmons over the past twenty response it is not a very satisfactory interface on which years although more intricate arrangements have also perform carefully controlled scientific experiments. A been devised. For example there are possible hybrid better and more systematic approach is to use a geometries in which a thin metal film is deposited on to diffraction grating with a well specified sinusoidal surface a dielectric spacer layer as illustrated in figure 4(c). This having known wavelength and groove depth. The grooves in the grating surface break the translational gives an Otto type plasmon on the first (dielectric invariance of the interface and allow k, of the outgoing spacer/metal) interface and a Kretschmann-Raether type wave to be different from that of the incoming wave plasmon on the second. If now we add a final overcoating Conservation of momentum now gives in the x direction of dielectric with the same constants as the first(both of course lower than the prism) then the two surface modes k, (outgoing)=k, (incoming)+ NG,(12)