ABSTRACT
The pioneering work of Franken et al. [1.1] in 1961 and of Bloembergen et al. [1.2] in 1962 linked with the development of powerful lasers in the 60's opened a new and exciting field of research called nonlinear optics. The discovery of surface enhanced Raman scattering by Fleischman et al. [1.3] in 1974 brought an unexpected and wide problem to the physics community. This problem concerned explaining why a pyridine monolayer adsorbed over a rough silver electrode can present a Raman spectrum million times more intense than when dissolved in an electrolyte? A few years later, similar enhancements were detected with other nonlinear effects such as second harmonic generation [1.4] and two-photon fluorescence [1.5].
