Raman and ultrafast optical spectroscopy of acoustic phonons in Cd Te0.68 Se0.32 quantum dots

Abstract

Acousticlike vibrations were observed in spontaneous Raman measurements on Cd Te0.68 Se0.32 quantum dots embedded in a borosilicate matrix and ascribed to the l=0, l=2, and l=1 modes of a spherical particle. Pump-probe stimulated Raman measurements on the same sample reveal only the symmetric l=0 mode, the frequency of which increases as the laser central energy moves to higher energies. Consistent with the strong dependence of the exciton energy on the particle size, such a behavior is attributed to resonant size-selective excitation of nanocrystallites whose absorption edge coincides with the central energy of the optical pulses. This selectivity, relying on the existence of double-resonant terms in the generation process, is not observed in spontaneous Raman scattering for which the resonances involve a single term. © 2006 The American Physical Society.