Metallic structures with sizes of the order of tens to hundreds of nanometers are known to support surface plasmons, collective oscillations of their conduction electrons, which result in very strong optical responses and confinement of the electromagnetic field in volumes less than the length of wave. Thanks to these properties, a tremendous effort has been made to design and investigate the optical response of sets of plasmonic nanostructures organized in different geometries. A particularly interesting example is the placement of metallic nanostructures in a two-dimensional periodic matrix, since it gives rise to collective responses known as lattice resonances. These excitations, which occur at wavelengths commensurate with the periodicity of the matrix, are the result of the coherent interaction between the localized plasmons of the nanoparticles that form the network. Due to their collective nature, lattice resonances produce strong optical responses, both in the far and near fields, with narrow spectral profiles and result in record quality factors for metallic systems. These extraordinary properties make periodic nanostructure matrices excellent candidates for a variety of applications such as optical sensing, nanoscale light emission, lenses, second harmonic generation, and color generation, and are ideal platforms for exploring new physical phenomena.
Recently, there has been a growing interest in investigating the optical response of periodic arrays of metallic nanostructures constructed from repeating unit cells containing more than one particle. As might be expected, these systems, being more complex than single-particle unit cell matrices, show much richer optical responses. One possible way to create a matrix with a multi-particle unit cell is to start with a single-particle matrix and remove some particles periodically. Since the resulting system is still periodic, it is expected to support network resonances. However, at that point, the following questions arise: What are the properties of these network resonances and how is their origin related to the presence of periodic vacancies?
In this article, in order to answer these questions, we employ a coupled dipole approximation to perform a comprehensive analysis of how particle size, matrix periodicity, and vacancy density determine the optical response of the matrix. In particular, we found that the lattice resonances of these systems appear further away from the corresponding Rayleigh anomalies and result in a smaller reflectance and larger absorbance than those of a single-particle matrix with the same periodicity. Finally, we analyze a possible implementation of these results using nanocylinder matrices embedded in a homogeneous dielectric medium.
The results of this work allow us to understand the effect of periodic vacancies on the response of periodic matrices of metallic nanoparticles and, therefore, serve to increase the potential of these systems for technological applications such as ultra-compact biosensors and nanoscale light sources.
Link to the paper
The work is a collaboration between the Institute of Optics and the Department of Physics and Astronomy at the University of New Mexico in Albuquerque, USA.
