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Ultrafast Dynamics within the 1S Exciton Band of Colloidal PbSe Quantum Dots Using Multiresonant Coherent Multidimensional Spectroscopy

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journal contribution
posted on 06.03.2014, 00:00 by Daniel D. Kohler, Stephen B. Block, Schuyler Kain, Andrei V. Pakoulev, John C. Wright
The simple particle-in-a-sphere model of quantum dot excitons is the basis for understanding the excitonic peak positions, line widths, and relaxation dynamics in many spectroscopic experiments. Recent multiresonant coherent multidimensional spectroscopy (CMDS) with picosecond excitation pulses measured the two-dimensional spectra of PbSe quantum dots and successfully used this simple model of an inhomogeneous distribution of spherically confined exciton and biexciton states and rate constants to describe the dephasing and population relaxation dynamics. The long excitation pulses prevented resolution of faster dynamics. This work reports the development of multiresonant CMDS with femtosecond excitation pulses to resolve the spectra and dynamics associated with the 1S exciton line shape of PbSe quantum dots. The experiments use different combinations of excitation frequencies, excitation pulse time delays, and a monochromator to display and measure correlations between the spectral features and their dynamics. Line-narrowing of the inhomogeneous distribution occurs at short time delays where the excitation excites a subset of the quantum dots within the 1S line shape and the last pulse probes this subset. The line-narrowing disappears at longer delay times. Three pulse photon echo peak shifts (3PEPS) also occur when the line-narrowing is present, but the shifts disappear as the correlation between the first and last coherence frequencies disappears. Wigner plots reveal the spectral dynamics accompanying the peak shift and the disappearance of the line-narrowing. This work shows there is rapid relaxation dynamics occurring within the line profile of the quantum confined excitonic states that is not consistent with current understanding of the excitonic line broadening. The data suggest that the relaxation dynamics play a more dominant role in defining the excitonic line widths than the inhomogeneous broadening of the quantum dot size distribution. These observations are consistent with other spectroscopic experiments on CdSe and PbS quantum dots. The experiments also show the presence of a higher energy feature that lies outside the 1S line shape and undergoes very rapid relaxation.