Metal dielectric surface plasmons

Electromagnetic field of a surface plasmon is confined at the metal-dielectric interface and decreases exponentially into both media, Figure 6. 

Fig. 2 Surface plasmon resonance (SPR) principle. Surface plasmons are excited by the light energy at a critical angle (9) causing an oscillation and the generation of an evanescent wave. Under this condition a decrease in the reflected light intensity is observed. The angle 9 depends on the dielectric medium close to the metal surface and therefore is strongly affected by molecules directly adsorbed on the metal surface. This principle allows the direct detection of the interaction of the analyte and the antibody...

Surface plasmon-polaritons (SPP), also referred as to surface plasma waves, are special modes of electromagnetic field which can exist at the interface between a dielectric and a metal that behaves like a nearly-iree electron plasma. A surface plasmon is a transverse-magnetic mode (magnetic vector is perpendicular to the direction of propagation of the wave and parallel to the plane of interface) and is characterized by its propagation constant and field distribution. The propagation constant, P can be expressed as follows ... 

Assuming that the prism has only a minor influence on the propagation constant of the surface plasmon at the interface of a metal and a low refractive index dielectric, the coupling condition can be approximately expressed as follows ... 

TMM handles thin metallic films as well, as they are used in lO-sensors based on surface-plasmon-polaritons (SPP). SPPs appear at the dielectric-metal interface for TM polarization, exclusively. The sensor principle is to have a waveguide mode and the SPP close to resonance, and screen the resonance vs. angle or vs. wavelength to detect refractive index changes of the cladding. Figure 4 shows the resonance of the absorption vs. the... 

By means of this combination of the cross section for an ellipsoid with the Drude dielectric function we arrive at resonance absorption where there is no comparable structure in the bulk metal absorption. The absorption cross section is a maximum at co = ojs and falls to approximately one-half its maximum value at the frequencies = us y/2 (provided that v2 ). That is, the surface mode frequency is us or, in quantum-mechanical language, the surface plasmon energy is hcos. We have assumed that the dielectric function of the surrounding medium is constant or weakly dependent on frequency. 

Conduction electrons in a metal are nearly free to move within the metal in response to an applied electric field. A surface plasma wave, also called a surface plasmon, is an electromagnetic wave that propagates along the boundary between a metal and a dielectric (an electrical insulator). The electromagnetic field decreases exponentially into both layers but is concentrated in the dielectric layer. 

Thus the value of the dielectric constant at the sample/metal interface determines the shift of the resonance. When adsorption of molecules at the metal surface results in the change of the refractive index or of the local value of the dielectric constant, the change of reflectivity is observed. This phenomenon has been used as the mechanism for detection of gases (Fig. 9.18a) and of adsorbed biomolecules (Fig. 9.18b). The depth of penetration of the surface plasmon is comparable to that of the evanescent field, that is, 100-500 nm for the visible-near infrared range. 

---