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Abstract
This thesis presents theoretical and experimental results on plasmonic phenomena in nanosized metallic structures. The theoretical aspect concerns the extension of the localresponse 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 energyloss spectroscopy (EELS) to excite and 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 deltafunction induced charge distribution in the localresponse approximation. Irrespective of the microscopic origin, we find that nonlocal response modifies the electromagnetic wave equation by an additional Laplacian term. The hydrodynamic model, which includes nonlocal response through the Thomas–Fermi pressure of a freeelectron gas, is discussed. We present also the generalized nonlocal optical response model, which expands the hydrodynamic model by taking into account the diffusion of free electrons in metals through Fick’s law. We go on to consider the implications of these two nonlocal models in the following plasmonic geometries: metalinsulator interface, nanosphere, dimer with nanometersized gaps, coreshell nanowire with ultrathin metal shell, and a thin metal film. In all cases we compare the nonlocal models with the localresponse approximation. Below the plasma frequency, we find that the distance between the induced positive and negative surface charges is the main indication for the importance of nonlocal response. Specifically, the mentioned distance in nanospheres translates into a sizedependent resonance energy and linewidth broadening of the surface plasmons, while in the dimer a gapdependent resonance energy and linewidth broadening is observed. Above the plasma frequency, resonant excitations are supported by nonlocal theory due to the inclusion of curlfree waves. The application of EELS to study surface plasmons in nanosized metallic systems is then presented. In particular, we discuss that EELS can provide important information on the optical response of plasmonic structures. 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 surfaceplasmon modes in gold nanogrooves, which are experimentally observed to subsist in gaps of only 5 nm.
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 deltafunction induced charge distribution in the localresponse approximation. Irrespective of the microscopic origin, we find that nonlocal response modifies the electromagnetic wave equation by an additional Laplacian term. The hydrodynamic model, which includes nonlocal response through the Thomas–Fermi pressure of a freeelectron gas, is discussed. We present also the generalized nonlocal optical response model, which expands the hydrodynamic model by taking into account the diffusion of free electrons in metals through Fick’s law. We go on to consider the implications of these two nonlocal models in the following plasmonic geometries: metalinsulator interface, nanosphere, dimer with nanometersized gaps, coreshell nanowire with ultrathin metal shell, and a thin metal film. In all cases we compare the nonlocal models with the localresponse approximation. Below the plasma frequency, we find that the distance between the induced positive and negative surface charges is the main indication for the importance of nonlocal response. Specifically, the mentioned distance in nanospheres translates into a sizedependent resonance energy and linewidth broadening of the surface plasmons, while in the dimer a gapdependent resonance energy and linewidth broadening is observed. Above the plasma frequency, resonant excitations are supported by nonlocal theory due to the inclusion of curlfree waves. The application of EELS to study surface plasmons in nanosized metallic systems is then presented. In particular, we discuss that EELS can provide important information on the optical response of plasmonic structures. 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 surfaceplasmon modes in gold nanogrooves, which are experimentally observed to subsist in gaps of only 5 nm.
Original language  English 

Place of Publication  Kgs. Lyngby 

Publisher  Technical University of Denmark 
Number of pages  203 
Publication status  Published  2014 
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Dive into the research topics of 'Probing Plasmonic Nanostructures with Electron Energy  Loss Spectroscopy'. Together they form a unique fingerprint.Projects
 1 Finished

Probing photonic nanostructures with electron energy loss spectroscopy
Raza, S., Mortensen, N. A., Wagner, J. B., Wubs, M., Mork, J., Abajo, J. G. D. & Wegener, M.
Technical University of Denmark
01/09/2011 → 15/11/2014
Project: PhD