The excitation of surface Plasmons by light is known as a Surface Plasmon Resonance or SPR. This is observed in nanometer-sized metallic structures. Surface Plasmons, are surface electromagnetic waves that propagate in a direction parallel to the metal or dielectric interface .This phenomenon is the basis of many standard tools for measuring adsorption of material onto planar metal (typically gold and silver) surfaces or onto the surface of metal nano particles.
Since the wave is on the boundary of the metal and the external medium, these oscillations are very sensitive to any change of this boundary, such as the adsorption of molecules to the metal surface. In order to excite surface Plasmon in a resonant manner, one can use an electron or light beam visible and infrared rays. The incoming beam has to match its impulse to that of the Plasmon .In the case of p-polarized light (polarization of light parallel to the plane of incidence), this is possible by passing the light through a block of glass to increase the wave number and achieve the resonance at a given wavelength and angle
When the surface Plasmon wave hits a local particle or irregularity -like on a rough surface-, part of the energy can be reemitted as light .This emitted light can be detected behind the metal film in various directions. Surface Plasmons have been used to enhance the surface sensitivity of several spectroscopic measurements including fluorescence, Raman scattering, and second harmonic generation .In their simplest form, SPR reflectivity measurements can be used to detect molecular adsorption, such as polymers, DNA or proteins, etc