Surface plasmon

Collective excitations of free electrons in metals to plasma oscillations against the ion cores are referred to in solid state physics as plasmons. Surface plasmons are surface acoustic wave ( evanescent wave ), in which the electronic longitudinal vibrations are excited in parallel to the surface of a metal. The resulting electric field strength is intensified in the space above the metal surface.

Suggestion

Surface plasmons can be excited under certain conditions with light. Even if the energy of the photons in the energy of surface plasmons, an incident beam of light can not excite the surface plasmon typically because first surface plasmons in metal phase have a lower speed than the speed of light. Therefore, the wave vector ( momentum) of the light and the surface plasmon does not match. Second, a surface plasmon evanescent wave, so it has a purely imaginary wave vector perpendicular to the surface. However, a coupling can only take place if all the components of the wave vector, both parallel and perpendicular to the surface are the same. In particular, therefore stimulating the shaft must be an evanescent wave itself. Current methods are the prism coupling according to Otto or Kretschmann. Both methods use total reflection, and the resulting evanescent wave as well as the differences in the speed of light in two dielectrics as well as the coupling grating, wherein a reciprocal lattice vector is added to the wave vector. The excitation of surface plasmons by light is less efficiently also to local defects in the metal surface or non- periodic structures (edges, line defects ) possible. The same methods also allow to couple light from surface plasmons.

Surface plasmons can also be excited by electrons; they can transfer energy and momentum to a surface plasmon.

Spread

Surface plasmons propagate along the metal surface, where its intensity decreases exponentially with the propagation length. For the attenuation of the propagation of the plasmon losses in the metal cable are responsible. With a light wavelength of 633 nm wide surface plasmons on gold about 9 microns ( 1 / e of the intensity), on silver about 60 microns from far. By suitable patterning of the metal surface, the direction of propagation of surface plasmons can be influenced. It can be mirrors, beam splitters and lenses for surface plasmons produce.

With a modified geometric approach succeeded in 2012 as a light beam to be coupled to a surface plasmon, that the light beam was focused at the exit point of 14-80 nm, and the intensity increased by 70%. The developed cuboid component is 2 microns long and tapers toward one end twice. The block consists of amorphous silicon dioxide and is coated with a 50 nm thick layer of gold. The specific geometry and the coupling of the surface plasmons resolves the problem of the diffraction limit, and thus enables the focusing.

Application

One application is the surface plasmon resonance ( SPR ) in biosensing. It makes use of use of the fact that the wavelength of the surface plasmons to changes in refractive index in the vicinity of the metal surface reacts strongly.

Furthermore, surface plasmons are currently the subject in the development of new storage technologies, as successor to the DVD or Blu -ray disc or for transmitting optical information in a highly integrated computer chips.

Surface plasmons and roughness

Surface plasmons can couple on rough surfaces without a denser medium. In contrast to the plasmons described above can also be coupled out again and thus produce a radiative transfer between two points on the surface. In highly conductive metals such as silver, the energy can be transported as up to 60 microns wide. This can occur at planar optical profilometers, such as white light interferometers lead to false Rauheitsmesswerten. Punctiform measuring systems - and that includes the imaging confocal technology - no interference experienced by surface plasmons, but only a reduced reflection.