Optical excitation of surface plasmons prsm 1000 1200 \\thin metal film Wavelength/nm Figure 3. This shows how the surface plasmon resonance width (oE /E varies with wavelength. Many other metals can support sharp resonances as the wavelength of the incident radiation is increased. Compiled from data in references [1] and [2. 4. Coupling to the surface (c) thin dielectric Before moving on to discuss some experimental results thin metal film we need finally to examine how best to couple radiation dielectric to the surface plasmon resonance given that we have Fi clearly established that its momentum is beyond that Figure 4. Geometries used uple photons into a surface available in the dielectric medium adjacent to the metal mode:(a)Otto,(b)Kretschmann-Raether, and(e)mixed hybrid arran Recall that, for our original two-dielectric system eyond the critical angle of incidence there will be an evanescent field in the second half space. This evanescent radiation at the prism/dielectric interface we vary the field does not propagate in the z-direction, but it has momentum in the x-direction and this allows for simple h the resonance. The fo form of the reflectivity is obvious that since sin 0;> sin 0.(= n2/n,), then curve for gold and silver at 632.8 nm is shown in figure n, hk sin 0:>nhk. Hence we have an enhancement 5, where we also show for comparison that for of the x-component of momentum in the second s-polarized light which is, of course, not capable of dielectric half space, above the limit value of nahk for creating the surface plasmon. The position of the a propagating wave. minimum of the resonance which is a measure of the This enhancement of momentum given by n, (sin 8: surface plasmon momentum, is no longer dictated simply sin Oe)hk may be used to couple radiation to a surface by the dielecric/metal boundary for it is additionally plasmon provided it is possible to place the metal/di- perturbed by the presence of the coupling prism electric interface which supports the surface plasmon Likewise the linewidth, which is a measure of damping close enough to the totally internally reflecting interface. is also perturbed by the presence of the prism. As the An obvious geometry to consider is that shown in figure coupling gap is increased so the perturbation by the Is conventionally called the Otto geometry, prism diminishes and the resonance moves to the after Otto who first demonstrated this coupling position corresponding to the two media surface technique in 1968[3]. An air gap (or a spacer of low plasmon and it also narrows. Of course in this index)less than a few radiation wavelengths thick (fo process, illustrated for gold in figure 6, the resonane visible< 2 um) provides the evanescent tunnel barrier progressively shallows. If we wish to achieve we c across which the radiation couples, from the totally coupling then for visible radiation the gap has to be of internally reflecting situation, to excite the surface the order of 0.5 um which for an air gap demands plasmon at the air (dielectric) metal interface. By extreme care in sample fabrication. This constraint is not e angle of incidence of the p-polarized so severe if we choose instead to work in the infra-red