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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]

T. W. Ebbesen is Thomas W. Ebbesen, a physical chemist whose research links optics with nanostructured materials. His institutional biography describes work in light–matter interactions at Strasbourg, and the Kavli Prize identifies his 2014 nanoscience award.^[ref][ref]

With collaborators, he reported unusually strong transmission through periodic arrays of holes smaller than the incident wavelength. They fabricated holes in metal films and measured transmitted spectra, including their dependence on illumination angle. The observations supported coupling between incident light and electronic excitations at the patterned surface, helping establish an experimental route into plasmonics.^[ref]

The normalization is essential to understanding the result. Transmission compared with the small total hole area can exceed the expectation for independent tiny apertures, or even exceed unity under that area normalization. This does not mean more optical energy emerges than was incident on the entire sample. The array and surrounding metal participate in collecting and coupling the electromagnetic field, so aperture area alone is an incomplete predictor.^[ref]