Plasmonresonans

Plasmonresonans, often written plasmon resonance, is the resonant oscillation of free conduction electrons at the interface between a metal and a dielectric when driven by light. In nanostructures this gives rise to localized surface plasmon resonances (LSPR) in nanoparticles and surface plasmon polaritons (SPP) on extended metal–dielectric surfaces. These resonances lead to strong optical absorption and scattering at specific wavelengths and to enhanced local electromagnetic fields.

In noble metals like gold and silver, conduction electrons collectively oscillate when the incident light frequency matches the natural frequency of the electron gas, modified by geometry and the surrounding medium. For small, spherical particles, Mie theory describes the resonance conditions; nonspherical shapes and nanoshells tune the resonance across the visible to near-infrared.

Resonance positions shift with particle size and shape, material, and the refractive index of the surrounding medium. Damping from electron-phonon scattering and surface scattering broadens the resonance and reduces intensity. Sensitivity to the local dielectric environment underpins many sensing applications.

Applications include biosensing and chemical sensing through refractive index changes, colorimetric assays based on plasmon coupling, surface-enhanced Raman scattering (SERS), bioimaging, photovoltaics, and photothermal therapies. Plasmonics also informs nanophotonic devices and metamaterials.

History and theory: early observations of anomalous coloration in colloidal metals led to the concept; Mie theory provided a quantitative framework; modern plasmonics combines nanofabrication with electrodynamics to exploit field enhancement and spectral tunability.