Introduction

The interaction of light with the first few atomic layers of a solid is relevant to (at least) two rather different aspects of surface physics. First, the strength of the experimental signal for many of the spectroscopic tools at our disposal such as photoemission, electron scattering, Raman scattering, etc., depends crucially on the intensity of the electric and magnetic fields near the surface. One typically calculates these field amplitudes by use of the classical Fresnel formulae. It is important to inquire whether this approach is sufficient if one is interested in phenomena within an Ångström or two of the crystal surface. These considerations alone suggest that a thorough understanding of the nature of near-surface electromagnetic fields is of both practical and fundamental interest. A rather different motivation to study these fields comes from the realization that very long wavelength elementary excitations of the surface will couple directly to the ambient field. To account for this, we must generalize the results of the previous chapter beyond the static limit to include the finite propagation velocity of light. These excitations, known as surface polaritons, are coupled modes of the surface + electromagnetic field system. It is convenient to use the language of optics to discuss surface polaritons, although, in principle, no external driving field is needed to excite them.