Surface plasmon

from Wikipedia, the free encyclopedia

Collective excitations of free electrons in metals to plasma oscillations against the ion cores are called plasmons in solid-state physics . Surface plasmons are surface waves ( evanescent waves) in which the longitudinal electronic oscillations are excited parallel to the surface of a metal. The resulting electric field strength is increased in the space above the metallic surface.

Surface plasmon led for the first time Rufus Ritchie 1957, who reported with Eldridge 1962 on the emission of photons from irradiated metal foils, and surface plasmon polaritons (surface plasmon polariton, SPP) Andreas Otto 1968. The latter are quasi-particles from the surface plasmon and photon, which significantly contributed to the wide range of applications that surface plasmons found from the 1990s onwards, as they enable light to be manipulated in the nanoscale far below its diffraction limit.

Suggestion

Otto-schema.png
Otto arrangement
SPR-schema.png
Kretschmann arrangement


Surface plasmons can be excited with light under certain conditions. Even if the energy of the light quanta is in the range of the energies of surface plasmon, an incident light beam cannot normally excite surface plasmon, because firstly surface plasmons in metals have a lower phase velocity than the speed of light. Therefore, the wave vector ( momentum ) of the light and the surface plasmon do not match. Second, a surface plasmon is an evanescent wave , i.e. it has a purely imaginary wave vector perpendicular to the surface. However, coupling can only take place if all components of the wave vector, both parallel and perpendicular to the surface, are the same. In particular, the exciting wave itself must be an evanescent wave. Common methods are the prism coupling according to Andreas Otto and Kretschmann. Both methods use total reflection and the resulting evanescent waves, as well as the differences in the speed of light in two dielectrics and the grating coupling, in which a vector of the reciprocal grating is added to the wave vector. The excitation of surface plasmons by light is also possible with less efficiency at local defects of the metal surface or non-periodic structures (edges, line defects). The same methods also make it possible to couple light from surface plasmons.

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

Spread

Surface plasmons propagate along the metal surface, their intensity decreasing exponentially with the propagation length. Conduction losses in the metal are responsible for the dampening of plasmon propagation. At a light wavelength of 633 nm, surface plasmons spread about 9 µm ( 1 / e of the intensity) on gold and about 60 µm on silver. The direction of propagation of surface plasmons can be influenced by suitable structuring of the metal surface. Mirrors, beam splitters and lenses for surface plasmons can be produced.

With a modified geometric approach, it was possible in 2012 to couple a light beam to a surface plasmon in such a way that the light beam could be focused to 14–80 nm at the exit point and the intensity increased by 70%. The cuboid component developed is 2 µm long and tapers twice towards one end. The block consists of amorphous silicon dioxide and is coated with a 50 nm thick layer of gold. The special geometry and the coupling to the surface plasmons eliminates the problem of the diffraction limit and thus enables focusing.

application

One application is the surface plasmon resonance ( English surface plasmon resonance spectroscopy = SPRS) in the biosensor . This makes use of the fact that the wavelength of the surface plasmons reacts strongly to changes in the refractive index in the immediate vicinity of the metal surface. Surface plasmons are also used as an electromagnetic amplification effect in surface- amplified Raman spectroscopy .

Furthermore, surface plasmons are currently the subject of the development of new storage technologies, as successors to the DVD or Blu-ray Disc or for the transmission of optical information in highly integrated computer chips . Further applications are plasmonic solar cells , surface plasmon lasers and biosensors.

Surface plasmons and roughness

Surface plasmons can couple on rough surfaces without a denser medium. In contrast to the plasmons described above, they can also decouple again and thus generate radiation transport between two points on the surface. With highly conductive metals such as silver, the energy can be transported up to 60 µm. This can be done with planar optical profilometers , such as B. white light interferometers lead to incorrect roughness measurements. Systems measuring point-like - and this includes the imaging confocal technology - do not experience any interference from surface plasmons, but only a reduced reflection.

literature

  • Mark L. Brongersma: Surface plasmon nanophotonics. Springer, Dordrecht 2007, ISBN 978-1-4020-4349-9 .
  • Franz Aussenegg, Harald Ditlbacher: Plasmons as light transporters: nano-optics . In: Physics in Our Time . tape 37 , no. 5 , September 2006, p. 220–226 , doi : 10.1002 / piuz.200601102 .

Web links

  • Winspall ( Memento from March 19, 2011 in the Internet Archive ) - Analysis program for measurements of surface plasmons - Max Planck Institute for Polymer Research (English).

Individual evidence

  1. ↑ Pedro Echinque et al., Obituary in Physics Today, October 10, 2017 (published in Physics Today Volume 71, No. 4, 2018)
  2. ↑ A. Otto: Excitation of nonradiative surface plasma waves in silver by the method of frustrated total reflection. In: Journal of Physics . Volume 216, 1968, pp. 398-410, doi: 10.1007 / BF01391532 .
  3. ↑ E. Kretschmann: The determination of optical constants of metals by excitation of surface plasma oscillations. In: Journal of Physics. Volume 241, 1971, pp. 313-324, doi: 10.1007 / BF01395428 .
  4. ↑ Hyuck Choo include: Nanofocusing in a metal-insulator-metal plasmon waveguide gap with a three-dimensional linear taper . In: Nature Photonics . tape 6 , no. 12 , 2012, p. 838-844 , doi : 10.1038 / nphoton.2012.277 .
  5. ↑ Cutting- edge technology, 3D focusing of the light. ( Memento from January 6, 2013 in the web archive archive.today ) (3D focusing in the nanometer range, German).
  6. ^ Holger Dambeck: Nano gold particle technology. Monster DVD holds 2000 films . In: Spiegel online. May 21, 2009.
  7. ↑ Celeste Biever: Plasmonic computer chips move closer . In: New Scientist . March 17, 2005.
  8. ^ E. Kretschmann: The angular dependence and the polarization of light emitted by surface plasmons on metals due to roughness . In: Optics Communications . tape 5 , no. 5 , 1972, p. 331–336 , doi : 10.1016 / 0030-4018 (72) 90026-0 , bibcode : 1972OptCo ... 5..331K .