Ultrafast carrier dynamics and confined acoustic phonons in CdSe nanorods

We report on the observation of coherent confined acoustic phonons in CdSe core nanorods. We show that photobleaching and photoabsorption dynamics of nanocrystals, obtained with femtosecond pump-probe transient spectroscopy with a resonant pumping energy, exhibit evidence of oscillatory modulation, due to the modulation of the bandgap, despite the weak excitation regime (< mu J). In particular we found that the modulation frequency corresponds to the coherent radial breathing modes of the rods, suggesting that only the transverse ( radial) mode of coherent phonons is observed. No evidence of modulation is observed in the corresponding rod sample overcoated with a thin shell of CdS/ZnS, suggesting that the shell produces a strongly damped mechanism.