Optical properties of rodlike and bipyramidal gold nanoparticles from three-dimensional computations

Near- and far-field optical properties of several types of gold nanoparticle are investigated with rigorous three-dimensional computational electrodynamics. The primary focus is on the results obtained with the finite-difference time-domain method, although some results obtained with the discrete dipole approximation are also given. We first consider spheres and prolate spheroids, where analytical solutions are available for comparison. The spectra of gold nanorods and pentagonal bipyramids are then investigated and excellent agreement with recent experimental optical spectra is found. The local field enhancement $(\ensuremath{\mid}\mathbf{E}\ensuremath{\mid}∕\ensuremath{\mid}{\mathbf{E}}_{0}\ensuremath{\mid})$ is studied at the longitudinal plasmon resonance. Sharper structural features produce more significant enhancement and the largest enhancement of more than a factor of 200 is seen around the poles of the bipyramid. The fields within the nanoparticles are also studied. Whereas the field magnitude is nearly uniform within small spheres and spheroids, it can be nonuniform for nanorods and bipyramids. The field magnitude decreases from the center toward the poles in the case of nanorods, but increases rapidly in the case of bipyramids. A large internal field enhancement by more than a factor of 30 is seen for the bipyramids, which suggests that these particles will exhibit strong optical nonlinearities.