Electrodynamics of nonspherical noble metal nanoparticles and nanoparticle aggregates

This article reviews recent advances in the theory of nanoparticle optical spectra using classical electrodynamics. Although Mie theory has dominated work in this field for nearly a century, this theory is limited to spherical particles and spherical shells, and thus it cannot describe many experiments of recent interest to the chemical and biological sensor community in which nonspherical particles, including particles with only C₁ symmetry, and particles in asymmetric environments (i.e. on surfaces and in nonspherical aggregates) are studied. There are now several approaches available for describing nonspherical particles, but the majority of recent applications to isolated particles have been done using the finite element discrete dipole approximation (DDA) theory. The review describes several applications of DDA to the determination of extinction spectra, including extensions to describe solvent and substrate effects. We also describe theories of nanoparticle aggregates based on approaches that explicitly describe electromagnetic coupling between the particles, and effective medium approaches for nonspherical aggregates.