Electrodynamics of noble metal nanoparticles and nanoparticle clusters

In this paper we examine the electrodynamics of silver nanoparticles and of clusters of nanoparticles, with an emphasis on extinction spectra and of e lectric fields near the particle surfaces that are important in determining surface-enhanced Raman (SER) intensities. The particles and clusters are c hosen to be representative of what has been studied in recent work on collo ids and with lithographically prepared particles. These include spheres, sp heroids, truncated tetrahedrons, and clusters of two or three of these part icles, with sizes that are too large to be described with simple electrosta tic approximations but small compared to the wavelength of light. The elect rodynamics calculations are mostly based on the discrete dipole approximati on (DDA), which is a coupled-finite element approach which produces exact o r nearly exact results for particles of arbitrary size and shape if fully c onverged. Mie theory results are used to study the validity of the DDA for spherical particles, and we also study the validity of the modified long wa velength approximation (MLWA), which is based on perturbative corrections t o the electrostatic limit, and of the single dipole per particle approximat ion (SDA). The results show how the dipole plasmon resonance properties and the electric field contours around the particle vary with particle shape a nd size for isolated particles. For clusters of particles, we study the eff ect of interparticle spacing on plasmon resonance characteristics. We also show that the quadrupole resonance is much less sensitive to particle shape and interparticle interactions than the dipole plasmon resonance. These re sults provide benchmarks that will be used in future comparisons with exper iment.