Prof. Andrea V. Bragas Profile

Prof. Andrea V. Bragas

at Univ de Buenos Aires

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Proceedings Article | 13 April 2005 Paper

Ultrafast generation of optical and acoustic phonons in nanocrystallites

Andrea Bragas, Cynthia Aku-Leh, Roberto Merlin

Proceedings Volume 5725, (2005) https://doi.org/10.1117/12.588718

KEYWORDS: Phonons, Acoustics, Raman spectroscopy, Laser energy, Absorption, Selenium, Semiconductors, Ultrafast phenomena, Glasses, Remote sensing

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We report on the impulsive generation of optical and acoustic phonons in CdTe_0.68Se_0.32 nanocrystallites embedded in glass, at room temperature. Using ultrafast laser pulses in a pump-probe configuration, we were able to generate coherent vibrations. The energy of our laser was tuned to the absorption edge of the nanocrystals so as to resonantly excite the quantum dots. We identified two longitudinal optical phonons, an optical mode of mixed longitudinal-transverse nature and a longitudinal-like acoustic mode. The frequency, amplitude, decay and phase as a function of excitation energy were determined for the optical modes. These results clearly identify impulsive stimulated Raman scattering as the underlying mechanism of the coherent field generation. The acoustic oscillations are associated with the lowest confined acoustic mode with pseudo angular momentum l=0. We find that the frequency of this mode increases as the laser central energy increases. Since the energy of the exciton at the fundamental gap depends strongly on the particle size, such a behavior is attributed to resonant size-selective excitation of the nanocrystallites. In contrast, spontaneous Raman measurements obtained from the same sample do not show size selectivity and, in addition, the resonant spectra show l=1 and l=2 modes, which are not seen in the pump-probe data. Possible explanations and comparison with other reports are discussed.

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