Oberflächenplasmonen-artige Resonanzen auf metallischen Gittern

Die Geometrie einer Metalloberfläche bestimmt die Wechselwirkung zwischen der Oberflächenplasmonenresonanz und anderen Energieformen wie Photonen, anderen Oberflächenplasmonen und molekularen Anregungen. In dieser Arbeit wird der Einfluss dieser Effekte auf die optischen Eigenschaften von metallischen Reliefgittern untersucht.Aufgrund von Modellrechungen werden auf tiefen Gitterstrukturen Resonanzen erwartet, die sich als selbstgekoppelte Oberflächenplasmonen interpretieren lassen. Diese Resonanzen wurden aufgrund der Symmetrie des elektromagnetischen Feldes klassifiziert. Es wurden tiefe Gitter mit unterschiedlichen Profilformen hergestellt, die die experimentelle Beobachtung von drei gekoppelten Resonanzen erlauben. Variationen in der Tiefe und Asymmetrie der Gitter sowie in der experimentellen Geometrie zeigen die theoretisch vorausgesagten Effekte.Fluoreszenzfarbstoffe können mit elektromagnetischen Oberflächenresonanzen Energie austauschen. Die Lokalisierung des elektrischen Feldes von gekoppelten Resonanzen führt zu einer Ortsabhängigkeit der Kopplung, die bei 'freien' Oberflächenplasmonen nicht beobachtet wird. Ein theoretisches Modell ist in der Lage, die experimentellen Befunde weitgehend zu beschreiben. Die Ortsabhägigkeit der photochemischen Zerstörungsrate erlaubt die Anwendung dieses Effektes zur Messung von Diffusionsphänomenen in dünnen Filmen.Des weiteren wurde die Polarisationsabhängigkeit der Anregung von Oberflächenplasmonen wurde in konischer Reflexionsgeometrie und die Rolle der Oberflächenplasmonen in der thermisch induzierten Lichtemission untersucht. Ferner wurde eine vereinfachte Auswertungsroutine zur Anwendung von Gittern in der Untersuchung von dünnen dielektrischen Filmen entwickelt.

Fluorescence close to dielectric interfaces

The aim of the work was to study the correlation between the orientation and excited-state lifetimes of organic dyes close to dielectric interfaces. For this purpose, an experimental setup was designed and built, guiding the light through a prism in total internal reflection geometry. Fluorescence intensities and lifetimes for an ensemble of dye molecules were analyzed as a function of the excitation and detection polarizations. Working close to the total internal reflection angle, the differences between polarization combinations were enhanced. A classical electromagnetic model that assumes a chromophore as a couple of point-like electrical dipoles was developed. A numerical method to calculate the excitation and emission of dye molecules embedded in a multilayer system was implemented, by which full simulation of the time resolved fluorescence experiments was achieved. Free organic dyes and organic dyes covalently bound to polyelectrolyte chains were used. The polymer functionalization process avoided aggregation and provided control over the dyes position, within a few nanometers to the interface. Moreover, by varying the pH, the polymer chains could be deposited on different substrates with different conformations and the resulting fluorescence characteristics analyzed. Initially the fluorescence of organic dyes embedded in a polymer matrix was studied as a function of the distance between the fluorophores and the polymer-air interface. The non-radiative decay rate, vacuum decay rate and the relative angle between the excitation and emission dipoles of the chromophores could be determined. Different free organic dyes were deposited onto different dielectric spacers, as close as possible to the air-dielectric interface. Surprisingly, the fluorescence characteristics of dyes deposited onto polyelectrolyte layer were in good agreement with theoretical predictions of dyes in a polymer matrix, even when the layer was only 2 nm thick. When functionalized chains were deposited at low pH, on top of a polyelectrolyte spacer, the fluorescence had the characteristics of emitters embedded in a polymer matrix as well. Surface deposition at high pH showed an intermediate behaviour between emitters embedded in polymer and on top of the surface, in air. In general, for low pH values, the chains are deposited on a substrate in a train-like conformation. For high pH values, the chains are deposited in a loop-like conformation. As a consequence at low pH the functionalized polymer strongly interdigitates with the polyelectrolyte chains of the spacer, bringing most of the dyes inside the polymer. Thus, the fluorophores may experience the polymer as surrounding environment. On the other hand, for high pH values the dye-loaded chains adsorbed have a conformational arrangement of dense loops that extend away from the surface. Therefore many fluorophores experience the air as surrounding environment. Changing the spacer from polyelectrolyte to negatively charged silane produced contradictory results for lifetimes and intensities. The fluorescence intensities indicated the behaviour of emitters embedded in a polymer matrix, regardless of the pH value. On the other hand, for low pH values, the excited-state lifetimes showed that the emitters behaved as in air. For higher pH values, an intermediate behaviour between fluorophores located within and above of a dielectric film was observed. The poor agreement between theoretical and experimental data may be due to the simplified model utilized, by which the dipoles are assumed either in one side or in the other with respect to a geometrical air-dielectric interface. In the case when the dielectric film is constituted by the functionalized polymer chains themselves, reality is more complex and a different model may apply. Nevertheless, possible applications of the technique arise from a qualitative analysis.