Zeitschriftenaufsätze

In isolated conjugated polymers two explanations are in discussion for the redshift of the emission on a picosecond time scaleexciton energy transfer (EET) between conjugated segments along the chains and conformational changes of these segments themselves, i.e., torsional relaxation. In order to resolve this question we perform femtosecond time-resolved transient absorption measurements of the energy relaxation of poly[3-(2,5-dioctylphenyl)thiophene] in toluene solution. We show that torsional relaxation can be distinguished from EET by site-selectively exciting low-energy conjugated segments. We present a unified model that integrates EET and torsional dynamics. In particular, comparison to ultrafast depolarization measurements shows that torsional dynamics cannot be neglected when analyzing EET dynamics and furthermore reveals that the exciton extends itself by about 2 monomer units during torsional relaxation.

We discuss near-field wave function imaging, introducing a model for high spatial resolution photoluminescence imaging of semiconductor nanostructures. The model is applied to optically bright and dark exciton and biexciton states in different quantum dot systems, explicitly taking the experimental imaging configuration into account. Our results show that direct imaging of the exciton density is only possible in collection mode experiments with nonresonant excitation in the high-resolution limit. For other geometries and for biexcitonic states, the images reflect not only the size and shape of the wave function and the spatial resolution of the near-field probe but also in particular the inherent optical nonlinearity of the imaging process. Different examples for the effects of this nonlinearity are discussed, providing new insight into the interpretation of existing experiments, and guidelines for designing novel experiments.