Generating narrow plasmon resonances from silver nanoparticle arrays: Influence of array pattern and particle spacing

The extinction spectra of silver nanoparticle arrays are studied using the couple dipole (CD) method, with emphasis on determined the array pattern and particle spacing which produces the narrowest plasmon resonances. All calculations refer to one and two dimensional arrays of spherical particles having radii 50 nm (or of nonspherical particles with the equivalent effective volume), and only particle spacings much larger than the particle radius are considered so that the dipole approximation is accurate. The narrowest lines in all cases occur when the incident wave vector is perpendicular to the plane of the array while the polarization vector is in the plane and along a symmetry axis which depends on the array structure. We find that the narrowest plasmon bands for square and hexagonal arrays have about the same width (about 100 meV), but that the array spacing for the square array where this occurs is smaller than that for the hexagonal array. The comparison at constant array density is closer. Much smaller widths (20 meV) occur for one dimensional arrays than for two dimensional arrays. For rectangular lattices, we find that the array spacings perpendicular to the polarization vector play a much more important role in determining the plasmon wavelength and width than do spacings parallel to the polarization vector. The evolution of spectra from a two dimensional array to a one dimensional chain is studied by considering rectangular arrays in which one spacing is very large. We find that when the large spacing is 5000 nm or more, the interactions between rows of particles is weak and extinction spectrum has a narrow peak that matches what is seen for the equivalent one dimensional chain.