An Overview: Femtosecond Spectroscopy at the Cutting Edge of Time Resolution

We report the first direct measurements of electronic dynamics in CdSe nanocrystals on a femtosecond time scale. Two pulse photon echo measurements show a rapid echo decay, on the order of 8 fs. Complimentary three pulse echo measurements indicate that the two pulse results are dominated by quantum beats associated with the 210 cm–1 LO phonon mode. Using a novel technique to cancel the effects of the quantum beat, we observe in the three pulse echo results evidence of two echo decay times, on the order of 8 fs and 18 fs respectively. In addition, time resolved spectral measurements show evidence of hole burning and dynamics on a femtosecond time scale. These results suggest that the electronic properties of these materials are more complicated than previously believed.