Plasmonic

Plasmonics is the study and application of plasmons—collective oscillations of free electrons in conducting solids excited by light. In nanostructures these excitations can couple to photons as surface plasmon polaritons (SPPs), which travel along metal–dielectric interfaces, or as localized surface plasmon resonances (LSPRs) in metallic nanoparticles, yielding strong confinement of the electromagnetic field near the surface. Descriptions typically combine the Drude model with Maxwell’s equations. Plasmonic resonances depend on material properties, geometry, size, and the surrounding medium; smaller structures push resonances into the visible or near-infrared and enhance local fields.

Gold and silver are common plasmonic materials in the visible range; aluminum is used for ultraviolet applications. Other plasmonic materials include titanium nitride, copper, and doped oxides, while graphene supports infrared plasmons. Losses due to electron scattering limit propagation and quality factors, affecting long-range transport. Fabrication uses lithography or chemical synthesis to create shapes such as spheres, rods, and prisms that tailor LSPR frequencies for sensing, spectroscopy, and photonic devices.

Applications span plasmonic sensing, where refractive-index changes shift LSPR peaks; surface-enhanced Raman scattering and related spectroscopy; optical imaging and photothermal therapy; subwavelength waveguides and metasurfaces for nanophotonic circuits; and enhancements in photovoltaics and photocatalysis. The field seeks high field confinement with manageable losses and ongoing development of new materials and structures to extend operation ranges and enable integration with conventional optics.