Plasmonics

Plasmonics is the branch of nanophotonics that studies the interaction between electromagnetic fields and the free electrons in metals, focusing on collective electron oscillations known as plasmons. The two most studied phenomena are surface plasmon polaritons, which propagate along metal–dielectric interfaces, and localized surface plasmons, which are resonant oscillations confined to metal nanoparticles. Through strong confinement of light to subwavelength scales, plasmonics enables enhanced optical fields and control of light beyond the diffraction limit.

In planar geometries, surface plasmon polaritons arise from coupling between electromagnetic waves and collective charge oscillations at the interface, with dispersion that lies to the light line and a finite propagation length determined by losses. In nanoparticles, localized surface plasmons depend on particle size, shape, and material, producing resonant absorption and scattering and producing intense near-field enhancement in the vicinity of the particle.

The optical response of metals is described by the Drude model, yielding a frequency-dependent complex permittivity with a negative real part at visible frequencies. Noble metals such as gold and silver are common plasmonic materials, although losses and chemical stability vary with wavelength and environment. Alternative materials, including doped semiconductors at infrared frequencies and graphene for two-dimensional plasmons, broaden spectral coverage.

Applications cover chemical and biological sensing via surface plasmon resonance, enhanced spectroscopy such as surface-enhanced Raman scattering, photovoltaics and photocatalysis, nanoantennas for controlling light at the nanoscale, and metasurfaces for wavefront shaping. Challenges include intrinsic ohmic losses, fabrication tolerances, and integration with photonic and electronic systems; ongoing research seeks materials, geometries, and hybrid approaches to improve efficiency and functionality.