Nanoparticle Optics:  The Importance of Radiative Dipole Coupling in Two-Dimensional Nanoparticle Arrays

In this paper, the electromagnetic interactions between noble metal nanoparticles are studied by measuring the extinction spectra of two-dimensional arrays of Au and Ag cylinders and trigonal prisms that have been fabricated with electron beam lithography. The nanoparticles are typically 200 nm in diameter and 35 nm in height; both hexagonal and square array patterns have been considered with lattice spacings that vary from 230 to 500 nm. The extinction spectra typically have a maximum in the 700−800 nm region of the spectrum, and this maximum blue shifts as lattice spacing is reduced, having typically a 40 nm decrease in λmax for a 100 nm decrease in lattice spacing. The results are similar for the different noble metals, array patterns, and nanoparticle shapes. The extinction spectra have been modeled using coupled dipole calculations, and the observed spectral variations are in good qualitative agreement with experimental data. Moreover, the computational analysis indicates that the blue shifts are due to radiative dipolar coupling between the nanoparticles and retardation effects. These effects result in a net depolarization of the dipole couplings for lattice spacings of 200−500 nm.