DOI: 10.25549/usctheses-c127-655913
title: Plasmonic excitations in nanostructures

description: The prevalent classical model of plasmonic calculations for nano-scale metallic clusters is based on the Mie solution. Which consists of solving Maxwell’s equations with the material being represented by a dielectric function on its spatial location. However, such a semi-empirical continuum description necessarily breaks down beyond a certain level of coarseness introduced by atomic length scales. Even the bulk based model used for the dielectric function fails by itself. This limitation of the Mie solution has been established by a quantum mechanical calculation with self-consistent treatment of the dielectric response. ❧ In order to understand better the plasmonic excitations at nearly atomic scale, we explored the collective electromagnetic response of atomic chains of various sizes and geometries, and we also computed plasmons in graphene in the presence of an impurity.  ❧ For the atomic chains, we calculated the plasmonic resonances as a function of the system shape, direction of the external applied field, electron filling and atomic separation. ❧ Their frequency, oscillator strength and spatial modulation of the induced charge density were analyzed. It was shown that longitudinal and transverse modes can be controlled in amplitude and frequency by the cluster size. It was also observed an abrupt dependence of the modes on the electronic filling. We also find that changes in atomic spacings have a very different impact on low-energy vs. high-energy modes. And it was seen that changing the position of a single atom in a nanostructure can completely alter its collective dielectric response. This strong sensitivity to small changes is the key to controlling the dielectric properties of atomic scale structures, and it can thus become the gateway to a new generation of quantum devices which effectively utilize quantum physics for new functionalities. ❧ For graphene it was shown that impurities induce the formation of nanoscale localized plasmonic excitations in graphene sheets. It was studied the dependence of these excitations on the magnitude and size of the impurity potential and electronic filling. It was shown that the impurity potential and doping can be used to tune the properties of nano-plasmonic excitations, demonstrating that graphene is an inherently plasmonic material. It was found that the chemical potential can be used to turn them on and off, but it does not affect their frequency. While their frequency and amplitude can be tuned by varying the strength of the impurity potential. The method employed for this calculation had not been seen before. In principle the results discussed can be tested experimentally by high-frequency optical probes or STM. ❧ These results showed that collective excitations in finite systems have properties different from their bulk correspondents. Since there is not a macroscopic number of electrons in the system, the variation of one single electron causes observable differences. The localized resonant modes are very sensitive to even small variations in the system, for example the position of a single atom. This makes it difficult to establish general rules about the properties of collective excitations in atomic structures. On the other hand it also provides a vast range of possibilities that can be explored for achieving new functionalities.