Title: Probing plasmonic nanostructures with electron energy-loss spectroscopy
Supervisor
Prof. N. Asger Mortensen, DTU Fotonik
Co-supervisors
Assoc. Prof. Martijn Wubs, DTU Fotonik
Prof. Jakob Birkedal Wagner, DTU Center for Electron Nanoscopy
Evaluation Board
Prof. Jesper Mørk, DTU Fotonik
Prof. Dr. Martin Wegener, Karlsruhe Institute of Technology, Germany
Prof. Javier Garcia de Abajo, ICFO, The Institute of Photonic Sciences, Barcelona, Spain
Master of the Ceremony
Ass. Prof. Nicolas Stenger, DTU Fotonik
Abstract
This thesis presents theoretical and experimental results on plasmonic phenomena in nanosized metallic structures. The theoretical aspect concerns the extension of the local-response approximation, which leads to a description of metals based on the classical dielectric function, to account for nonlocal response. The experimental work comprises the use of electron energy-loss spectroscopy (EELS) to study both localized and propagating surface plasmons in metal structures.
Following a short introduction, we present the theoretical foundation to describe nonlocal response in Maxwell's equations for arbitrary geometries. We show that the key quantity which is modified by nonlocality is the induced charge in the metal. In particular, the induced surface charge is smeared over an Ångstrom length scale in contrast to the delta-function induced charge distribution in the local-response approximation. Irrespective of the microscopic origin, we find that nonlocal response modifies the electromagnetic wave equation by an additional Laplacian term. We go on to consider the implications of two nonlocal models, i.e., the hydrodynamic model and the generalized nonlocal optical response model, in the following plasmonic geometries: metal-insulator interface, nanosphere, dimer with nanometer-sized gaps, core-shell nanowire with ultrathin metal shell, and a thin metal film.
The application of EELS to study surface plasmons in nanosized metallic systems is then presented. We perform two separate EELS experiments and discuss their theoretical interpretations. The first experiment concerns the study of localized surface plasmon resonances of chemically prepared silver nanoparticles with diameter sizes down to 3.5 nm dispersed on a thin substrate. The second experiment is devoted to the investigation of propagating gap surface-plasmon modes in gold nanogrooves, which are experimentally observed to subsist in gaps of only 5 nm.