From the various discussions on the difference between surface plasmon resonance (SPR) and surface plasmon polariton (SPP) I got the impression that there is some misunderstanding of what are SPR and SPP. It is important to understand what are plasmons. As pointed out by some one that plasmons are oscillations of the electrons in a plasma. Such oscillations are traveling waves with well defined frequency and wave vector. However, it should be noted that these plasma waves are longitudinal waves, ie the displacement of the electrons (relative to a uniform positive background formed by positive ion) are parallel to the direction of propagation of the wave. These plasma waves can exist in 2D (although the dispersion of the wave is different from that of 3D plasmon) or at the interface between two media (leading to surface plasmon). When plasmons are excited one creates plasmon resonances. The important thing to note now is that plasmons are longitudinal waves so they cannot be excited by a transverse wave like an electromagnetic wave. Usually the most effective way to excite a plasmon is to use electrons. For example, an electron beam passing through a thin metal foil will lose energy when it excites a plasmon. This loss in energy is exhibited by a peak in the electron energy loss spectrum (commonly known as EELS). Having said that plasmons (including SPR) cannot be excited by photons, then one can ask why there are SPP? A polariton, by definition, is a coupled dipole-electromagnetic wave. The best known example of polariton is the phonon-polariton in ionic crystals first proposed by the famous Chinese solid-state physicist: Kun HUANG in his classic paper:K. Huang, “Lattice Vi brations and Optical Waves in Ionic Crystals”,

Nature 16, 7779 (1951).This paper pointed out that the transverse optical (TO) phonon will couple to photon when their wave vectors and frequencies are the same. This coupling modifies the dispersions of both the optical phonon and photon. The resultant coupled TO phonon-photon mode was labeled by Huang as the polariton. The longitudinal optical (LO) phonon cannot couple to the photon since it is a longitudinal wave. This is the reason why plasmons in 3D cannot form polaritons. In case of an interface, the situation is different because a photon can be incident on the interface at an angle. For a photon polarized in the plane of incidence (p-polarized) the polarization can be decomposed into two components with one perpendicular to the interface and the other parallel to the interface. The parallel component can then excite a surface plasmon because this component is now a longitudinal wave. To form a SPP, the photon and SPR must have the same frequency and wave vector. This means that the component of the photon wave vector parallel to the interface has to be equal to that of the SPR. Thus to observe a SPP one has to monitor absorption of a incident EM wave of constant frequency as a function of the angle of incidence on the interface (by changing the angle of incidence one can tune the component of the photon wave vector parallel to the interface). The presence of a peak in the absorption will indicate the excitation of a SPP. By varying the incident photon frequency and determining the angle of incidence for observing the SPP (hence the wave vector of the SPP) one can determine the dispersion of the SPP.