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Proceedings Paper

Ultrafast generation of optical and acoustic phonons in nanocrystallites
Author(s): Andrea V. Bragas; Cynthia Aku-Leh; Roberto Merlin
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Paper Abstract

We report on the impulsive generation of optical and acoustic phonons in CdTe0.68Se0.32 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.

Paper Details

Date Published: 13 April 2005
PDF: 10 pages
Proc. SPIE 5725, Ultrafast Phenomena in Semiconductors and Nanostructure Materials IX, (13 April 2005); doi: 10.1117/12.588718
Show Author Affiliations
Andrea V. Bragas, FCEyN, Univ. de Buenos Aires (Argentina)
Univ. of Michigan (United States)
Cynthia Aku-Leh, Univ. of Michigan (United States)
Roberto Merlin, Univ. of Michigan (United States)

Published in SPIE Proceedings Vol. 5725:
Ultrafast Phenomena in Semiconductors and Nanostructure Materials IX
Kong-Thon Tsen; Jin-Joo Song; Hongxing Jiang, Editor(s)

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