Plasmonen

Plasmonen are collective oscillations of the free electron gas in conducting materials, typically metals, that couple to an electromagnetic field. In metals, the conduction electrons can move collectively relative to the positively charged lattice, producing a plasmon. There are several forms, including bulk plasmons, surface plasmons, localized surface plasmons, and surface plasmon polaritons along metal–dielectric interfaces.

The concept arises from the plasma frequency, a characteristic energy set by the electron density. The plasma frequency ωp = sqrt(n e^2 /(ε0 m*)) determines the natural energy of the oscillation. In typical metals, ωp lies in the ultraviolet for bulk excitations, but nanoscale confinement and interfacial effects can yield visible to near-infrared resonances useful for plasmonics.

Bulk plasmons propagate inside materials; surface plasmons are bound to interfaces and have dispersion that depends on the dielectric environment. Localized surface plasmons are confined to nanoparticles and produce strong electromagnetic field enhancement and distinctive colors. Surface plasmon polaritons are traveling waves that propagate along metal–dielectric interfaces and enable guiding of light beyond the diffraction limit.

Excitation and detection of plasmon modes require momentum matching between photons and plasmons, achievable with prism coupling in configurations such as Kretschmann, gratings, or near-field tips. Electron energy loss spectroscopy (EELS) and scanning near-field optical microscopy (s-SNOM) are common methods to characterize plasmons at the nanoscale.

Applications of plasmonics include highly sensitive chemical and biological sensing (notably surface-enhanced Raman scattering, SERS), enhanced spectroscopy, nanoantennas, photothermal therapy, improved photovoltaics, and metamaterials. Damping, material losses, and quantum size effects set practical limits, especially for very small structures or gaps.