Publikationen

The far-field reflectivity of metallic nanowire arrays designed to show strong surface-plasmon-polariton (SPP) resonances is studied numerically. The results of calculations in time and frequency space as well as the results of semi-analytic theories using different approximative boundary conditions at the metal surfaces are evaluated and compared. Good agreement between all different methods is obtained in most cases. The SPP-related features are superimposed on a strongly varying background. Combining FDTD simulations, finite element results, and semi-analytical calculations, the microscopic origin of the background contribution is identified. Resonant transmission through sub-wavelength slits leads to pronounced oscillations in the far-field reflectivity as a function of the height of the nanowires.

The influence of the nanowire shape on the excitation of surface plasmon polaritons at metallic nanowire arrays is studied numerically. For a system of silver nanowires housed on a polymer substrate, nanowires with rectangular and elliptical cross sections are compared. It was found that in the case of rectangular nanowires the excitation efficiency is higher for surface plasmons at the polymermetal interface than for surface plasmons at the airmetal interface. Conversely, in the case of elliptical nanowires the airmetal plasmon modes are stronger. Further, it is noted that the nanowire shape directly influences the position of the surface plasmon resonance.

The optimized geometries of bithiophene in the electronic ground and excited state have been calculated by time-dependent density functional theory using the hybrid functional B3-LYP. The dependence of the singlet excitation energies and transition dipoles on the torsion between the two thienyl groups has been determined and compared with the torsional potential curve previously found for biphenyl. Based on these data the torsional progressions in absorption and emission have been computed quantum-mechanically in order to show their contribution to the inhomogeneous broadening in the respective spectra of bithiophene.

Stimulated by the recent experimental report of optical properties of charged oligofluorenes [S. Fratiloiu et al., J. Phys. Chem. B 110, 5984-5993 (2006)], we have used time-dependent density functional theory (TD-DFT) to study the electronic structure of neutral and charged ter-fluorene with and without keto-defect. We have characterized the excited states by site- and space-based representations of the density matrix. For the defectless neutral species we found typical excitons, while the first excited states of the corresponding charged species represent long-range oscillations of the electron or hole along the whole oligomer. For the neutral ketonized ter-fluorene we found trapping of the excited electron. Similarly, the unpaired electron of the anion ground state is trapped. The excited state properties of the cation, however, are insensitive to the keto-defect.