plasmonpolariteter

Plasmonpolariteter, commonly called plasmon polaritons or surface plasmon polaritons (SPPs) in English, are quasiparticles that arise from the strong coupling between photons and collective electron oscillations at a metal–dielectric interface. They form propagating electromagnetic waves that are bound to the interface and exhibit much shorter wavelengths than freely propagating light at the same frequency.

The physical picture is that an incident photon couples to a surface plasmon, a collective oscillation of conduction electrons in the metal. The resulting hybrid mode has a dispersion relation that lies to the right of the light line and can be written in simplified form as k_sp = (ω/c) sqrt[ ε_m(ω) ε_d / (ε_m(ω) + ε_d) ], where ε_m is the metal’s frequency-dependent permittivity and ε_d is the dielectric’s permittivity. Real parts of ε_m are negative over a broad range, enabling bound modes, while Im ε_m introduces losses. These modes are highly confined to the interface and have propagation lengths limited by material losses.

SPPs can be excited by overcoming momentum mismatch between light and the plasmon mode, using methods such as prism coupling in the Kretschmann or Otto configurations, grating couplers, or near-field probes. They occur at metal–dielectric interfaces (e.g., gold or silver in air or dielectric surroundings) and can also exist in thin-film geometries that support long-range variants.

Applications include chemical and biological sensing through refractive-index changes, subwavelength photonic components, nanoscale waveguides, and enhanced light–matter interactions in nanophotonics and quantum plasmonics. They are related but distinct from localized surface plasmon resonances, which pertain to non-propagating excitations in nanoparticles.