Nanoplasmonics

Nanoplasmonics is the study of how light interacts with free electrons in nanoscale metallic structures, giving rise to plasmons—collective oscillations of conduction electrons. At optical frequencies, metallic nanoparticles can support localized surface plasmon resonances (LSPRs), which produce strong enhancements of the electromagnetic field near the particle surface. When particles are closely spaced, plasmon modes can couple and create intense, localized “hot spots.” Surface plasmon polaritons, which propagate along metal–dielectric interfaces, enable subwavelength guiding and confinement of light.

The behavior of these excitations is described by Maxwell's equations. For particles much smaller than the wavelength, classical models such as Mie theory and quasi-static approximations evaluate resonances using the Drude model for metal permittivity. Resonance conditions depend on particle size, shape, composition, and the surrounding dielectric environment, allowing tunability across the visible and near-infrared. At very small sizes, quantum effects such as nonlocal response and electron tunneling can modify resonances and broaden spectral features.

Fabrication and measurement in nanoplasmonics span multiple approaches. Colloidal synthesis and bottom‑up assembly yield nanoparticles and dimers, while lithographic and nanoimprinting techniques enable precise arrays and complex geometries. Characterization methods include ultraviolet–visible spectroscopy, electron energy loss spectroscopy, and near-field optical techniques to map local field enhancements.

Applications are centered on exploiting field enhancements and subwavelength confinement. Sensing relies on LSPR shifts from refractive-index changes or chemical binding; surface-enhanced Raman scattering provides sensitive molecular fingerprinting. Other uses include nanoscale imaging and spectroscopy, photothermal therapy, solar energy harvesting, and nonlinear optical processes, with ongoing integration into metasurfaces and nanophotonic devices.