Since the observation of the anomalous intensification of the Raman

signal from pyridine molecules adsorbed on a rough metal surface

[1] and the successive explanation of the phenomenon as the

amplification of the Raman scattering cross section due to the

presence of a nanostructured metal surface [2, 3], the research

in this field grew rapidly and it is still growing as witnessed

by about 2000 of papers published in this field just considering

the year 2012a. Two enhancement mechanisms of different origin

were proposed to account for the observed giant amplification of

the Raman signal: an electromagnetic and a chemical one [4, 5].

In the electromagnetic mechanism, the enhancement results from

the coupling of the incident electromagnetic field with artificially

metallic corrugated surface, in the chemical one, electronic states

generated by the interaction between the metal and the molecules

adsorbed on it are involved. In fact the order of magnitude of the

resulting enhancements is very different: theoretical estimates for

the enhancement factors point out for values up to 1010 [6, 7] for

the electromagnetic and up to 103 for the chemical one [8]. Both

mechanisms are still at the focus of intense research activity to

clarify the physical basis underlying the phenomenon.