  Since the concept of a surface collective excitation was first introduced by
Ritchie, surface plasmons have played a significant role in a variety of areas
of fundamental and applied research, from surface dynamics to surface-plasmon
microscopy, surface-plasmon resonance technology, and a wide range of photonic
applications. Here we review the basic concepts underlying the existence of
surface plasmons in metallic structures, and introduce a new low-energy surface
collective excitation that has been recently predicted to exist.
