Introduction

Abstract

The penetration depth of optical radiation into condensed matter is large, in general, which makes the isolation of a surface or interface contribution difficult (“interface”, from now on, is understood to include the surface, which is the condensed matter-ambient interface). Even in the near ultra-violet (UV) region, the interface contribution to the linear reflectivity from a semiconductor will only be a few percent. However, deeper understanding of the underlying physics of the optical response, combined with advances in instrumentation, have allowed the contribution from the interface to be identified. Particularly important has been the recognition that symmetry differences between the bulk and interface can be exploited, as can interface electronic and vibrational resonances [1]. The special efforts and techniques required to obtain this interface optical response, together with the increasing activity in the area, justify distinguishing it from more conventional optical studies. The term “epioptics” has been coined for this special area (from the Greek “epi” meaning “upon”) [2].