Nanoplasmonics

Field enhancement between a pair of sodium metallic nanowires from
Performance of non-local optics when applied plasmonic nanostructures

The goal of this line of research is the study of the way metallic nanostructures concentrate light and how to control this process. In fact, when metallic nanostructures are exposed to strong electromagnetic radiation (e.g., a strong laser field), their surface plasmons are excited and the induced field is dramatically enhanced in a very localised region of space. Such enhancement can be then exploited to increase the Raman cross section of a target molecule attached to the nanostructure in a well-established experimental technique know as Surface-Enhanced Raman Spectroscopy (SERS).

We model the metallic nanostructres using the Jellium approximation and their response through Time-Dependent Density-Functional Theory. In this way, one can investigate the optical response of systems which are beyond the possibilities of atomistic modelling (hundreds of atoms, and few nanometers) while keeping an acceptable level of accuracy. As a consequence, we can aim at comparing to the available experimental findings.

Another topic investigated is the nanoplasmonic response of graphene-like systems by TDDFT.
Indeed Plasmons, i.e., coherent oscillations of the electrons with respect to the ionic background, have been intensively investigated because of their wide range of possible applications in biosensing, photodetection, cancer therapy, photovoltaics, signal processiong, and quantum information. Recently, graphene has emerged as an alternative plasmonic material because it has some important advatages with respect to noble metals, e.g., the tunability of its plasmonic response via electrosta- tic doping and a longer plasmon lifetime.
However, there are experimental conditions — high magnetic fluxes, tunable lattice constants, precise minupulation of defects, edges and strain — which are relevant for applications, but very difficult or even impossible to reach employing native graphene. In order to overcome these limitations, artificial graphene-like structures with compatible plasmonic properties are being theoretically investigated (in collaboration with E. Räsänen, C.A. Rozzi).
With this project, we are going to present TDDFT calculations of the photoabsorption spectra and plas-
monic response of both graphene and graphene-like systems such as honeycomb superlattices of antidots on a h-BN monolayer. These results will be used to test the performance of a simple tight binding method that we implemented in order to hadle large scale graphene disks. Finally, our numerical results will be compared against previous theoretical and experimental findings to validate the domain of applicability of the single-pole approximation (Drude model).

Our current projects are in collaboration with:

• The group of Prof. D. Menzel at the Fritz Haber Institute. The goal of this project is to find a correlation between the geometry of a nanostructure and its photocatalythic action.
• Dr. P. Garcia Gonzalez and the group of F.-J. Garcia Vidal at the Universidad Autonoma de Madrid. The goal of this project is to quantify the effects of the electronic delocalisation on the nonlocal absorption of a pair of metallic nanopillars.
• For the nanoplasmonic response project with Dr. P. Garcia Gonzalez and Carlo A.Rozzi from CNR-Modena