Picosecond and Femtosecond Coherent Multidimensional Spectroscopy of Colloidal PbSe Quantum Dot Structure and Dynamics

Book excerpt: Multiresonant coherent multidimensional spectroscopy (MR-CMDS) is a fully coherent mixed frequency/time domain technique that selectively characterizes vibrational and electronic energetic structure and dynamics. The work reported here describes technique development with 1-picosecond pulses on model vibrational systems, application of this technique to PbSe quantum dots, and recent adaptation to include use of 50-femtosecond pulses. An overview of the quantum mechanical theory behind experimental design and interpretation, with an emphasis on the perturbative limit, is included. A record of the instrumentation and optical component selection associated with both laser systems is provided, which is particularly important for the development of the femtosecond laser table. Initial experiments on the femtosecond system revealed environmental artifacts that have now been identified and characterized. Picosecond pulses are not a good match for studying PbSe quantum dots with MR-CMDS because the pulses are too long to isolate coherence lifetimes and fully coherent processes. Frequency domain results previously acquired on the picosecond system revealed coherence lifetimes near 50fs, which directed the selection of a more appropriate laser system. Experiments with femtosecond pulses are now confirmed to directly measure coherence lifetimes and selectively emphasize spectral signatures from fully coherent processes that were not visible with longer pulses. These confirmations reveal that the flexibility of MR-CMDS techniques, as developed on the model vibrational systems, is available for quantum-confined semiconductor structures. Future directions include laser system improvements and suggestions of samples that may better inform dye-sensitized solar cell core charge transfer events.