plasmons

A plasmon is a quantum of plasma oscillation: the collective, coherent oscillation of the free electron gas in a conducting material relative to the positively charged lattice. In solids, this can couple to electromagnetic fields and give rise to plasmons. The concept was introduced by David Pines and David Bohm in 1952.

There are several types. Bulk plasmons are volume oscillations with frequency the plasma frequency ωp = sqrt(n e^2/(ε0 m_e)). They lie in the ultraviolet for most metals and are difficult to excite with visible light. Surface plasmons are charge-density waves confined to a metal–dielectric interface; they can couple to light through momentum matching and give rise to surface plasmon polaritons (SPPs) when the electromagnetic field is involved. SPPs propagate along the interface and decay into the adjacent media, with properties that depend on the metal and dielectric.

In nanoscale structures, localized surface plasmon resonances occur in metal nanoparticles, producing strong local field enhancements. The resonance frequency depends on particle size, shape, and the surrounding medium, enabling tuning across the visible to near-infrared range for common metals like gold and silver.

Applications include plasmonic sensors (surface plasmon resonance), enhanced spectroscopy (SERS), subwavelength optics, photothermal therapies, and metamaterials. Limitations include optical losses due to damping in metals and quantum-size or nonlocal effects that become significant at small scales, which can shift resonances and complicate classical descriptions.