Nanostructures afford the unique opportunity to tailor electronic wave functions and electro-magnetic modes through quantum and dielectric confinement effects. As a result, such systems can have fundamentally novel properties that are not achievable with bulk materials. Moreover, a wealth of new effects emerges from the combination of different nanostructures because of strong interactions at nanometer length scales.
In our research we are interested in using time-resolved and near-field optical spectroscopy techniques to explore new phenomena that distinguish nanoscale materials from their bulk counterparts and that emerge in nanoscale systems from the interaction of different nanomaterials. Optical techniques are particularly suited to study nanomaterials because their non-invasive nature eliminates direct contact. The spatial resolution provided by near-field optical microscopy allows one to resolve local optical properties, e.g. electro-magnetic mode structures, and to distinguish individual nanostructures in ensembles with large size and shape inhomogeneities.
Exciton-Plasmon Interactions in Hybrid Metal-Semiconductor Nanostructures
The high polarizability of metal nanostructures associated with free electron oscillations (surface plasmons) makes them very attractive for locally enhancing electric fields, thereby increasing absorption and radiative rate of dipoles that are located near the nanostructure. Such enhancement is attractive for increasing absorption cross-sections of photovoltaic devices, enhancing emission yields of light emitting devices, and fabricating more sensitive sensors. In order to exploit the advantages of hybrid exciton-plasmon devices, a fundamental understanding of all involved mechanisms in complex structures is necessary. We use advanced spectroscopy techniques such as angular, time, and spectrally resolved experiments to disentangle the various effects that lead to modified absorption and emission properties. Specifically, we want to understand how propagating and local plasmons interact with local dipoles with the goal to be able to design specific absorption and emission properties of the hybrid nanosystems.
Visualizing ultrafast surface plasmon pulses in metal nanostructures
Progress in most photonic devices aims at increased functionality, higher speed, and reduced dimensions. Metal nanostructures represent a novel approach for manipulating light on a sub-wavelength length scale. They allow waveguiding in the form of surface plasmons that significantly confine light. Moreover, they exhibit morphology controlled resonances that lead to strong field enhancements and non-linear effects. We are using a femtosecond photon scanning tunneling microscope for studying propagation phenomena of ultrafast plasmon pulses in metallic nanostructures with simultaneous femtosecond-scale time and nanometer-scale spatial resolution. Important insight into dispersion properties of both passive and active plasmonic devices will be gained.
Multiexciton Dynamics and Energies in Type-II Semiconductor NCs
Semiconductor nanocrystals (NCs) have size- and composition-tunable optical properties that make them attractive building blocks for optical devices. In addition, because of strong carrier confinement energies and their small sizes, NCs are ideal model systems for studying exciton-exciton interactions. Most prominent multiexciton phenomena include ultrafast Auger recombination and energy level shifts. We have studied these effects in conventional nanocrystals, in which excited electrons and holes are spatially occupying the same volume of the CdSe core. Contrarily, type-II NCs are designed such that opposite charge carriers are located in spatially distinct parts of the NCs. This is achieved in core/shell NCs by combining core/shell materials with different band offsets for electrons and holes. Exciton-exciton interactions are strongly modified in such type-II NCs allowing for optical gain with only a single exciton. The specific mechanism that is at the origin of single-exciton gain is repulsion between two excitons (negative biexciton binding energy). Another consequence of the spatial separated electrons and holes is a reduced radiative recombination rate and a modified Auger decay rate.
Ultrafast carrier dynamics in organic photovoltaic materials
In organic PV devices, it is a challenge to create materials that promote charge separation and, at the same time, provide high electron and hole conductivity. Conventional organic electron conductors and hole conductors mix well, which leads to efficient charge separation, but detrimental charge recombination at the interface and poor conductivity caused by the lack of a continuous phase. A possible solution is to assemble electron and hole conductors into nanoscale phase-segregated structures that have a high interface area for efficient charge separation and continuous phases that facilitate charge conductivity and avoid charge recombination. Important structural information can be gained from time-resolved spectroscopy techniques that allow measuring exciton lifetimes, dissociation dynamics, lifetimes of charge carriers after dissociation, and detrimental charge carrier surface trapping and recombination dynamics. Understanding these processes will allow us to provide feedback and guidelines for improving organic assemblies towards more efficient PV cells.