Surface Plasmons in the Continuum

The interest to foster plasmonic applications at energies in the ultraviolet has escalated research initiatives in clusters of unconventional plasmonic materials like aluminum and indium, for which the surface-plasmon resonance appears above the ionization potential. Naturally, the quantum mechanical description calls for the incorporation of the ionization process, thereby making the ab initio calculations challenging. We present a robust approach within the time-evolution formalism of the time-dependent density-functional theory to calculate surface plasmon resonances in the continuum of metal clusters. Using the much studied Al13− as a system of reference, we show that accurate description of the continuum and of the ionization of the cluster allows to capture a broad surface-plasmon in the ultraviolet. Application of this approach in aluminum clusters has given the size-dependent evolution from discrete spectral features in Al6 to the surface-plasmon in larger clusters in the deep ultraviolet.