In the presence of planar boundary, there is a new mode, called surface plasmon (SP), with broad spectrum of eigen frequencies from w = 0 to w = wp/ √_______ 1 + ed(ed-the dielectric constant of the medium adjacent to the metal) depending on the wave vector _›k . Introducing the concept “surface plasma” in addition to the plasma
concept extended the plasmon physics and opened the door for huge amount of studies in the last few decades. In an ideal semiinfinite medium, the dispersion relation of surface plasmon waves (SPW) lies to the right of the light line, which means that the SPs have longer wave vector than light waves of the same energy hw. Therefore, they are called “non-radiative” surface plasmons, i.e., they cannot decay by emitting a photon and, conversely, light incident on an ideal surface cannot excite SP.