 Plasmonics

Plasmonics studies and uses the coupling of electromagnetic waves with collective electron motion, especially at material surfaces and in small metallic structures. A surface plasmon polariton propagates along an interface while its field decays away from it; a localized surface plasmon resonance confines an oscillating response near a finite structure. Such modes can concentrate fields on scales smaller than the free-space wavelength.

For an ideal flat interface between a lossless metal of relative permittivity ${\epsilon }_m$ and a dielectric of relative permittivity ${\epsilon }_d$, the bound transverse-magnetic mode hasWith ${\epsilon }_d>0$ and ${\epsilon }_m<-{\epsilon }_d$, its parallel wave number exceeds that of a propagating wave in the dielectric. This explains why direct illumination of a perfectly flat interface needs additional momentum coupling. A grating or nearby optical field can supply that coupling. Real metals have loss: stronger confinement does not automatically imply long propagation distances.

A striking experiment used periodic arrays of tiny holes in metal films. In 1998, T. W. Ebbesen and colleagues reported transmission peaks at wavelengths much larger than individual hole diameters. One silver sample had 150-nanometre holes, 900-nanometre spacing and 200-nanometre film thickness. A spectrophotometer measured the transmitted beam, and angle-dependent spectra supported coupling to surface electronic excitations. Efficiency normalized to hole area could exceed one; this compares with the small open area, not with all incident power.^[ref]

Plasmonics is a field of study that focuses on the interaction between electromagnetic fields and free electrons in metals, leading to the excitation of collective oscillations known as plasmons. These plasmons are quasiparticles resulting from the coupling of photons with the oscillations of electrons in a material, typically at the nanoscale.