Raman and ultrafast optical spectroscopy of acoustic phonons inCdTe0.68Se0.32quantum dots

Abstract

Acousticlike vibrations were observed in spontaneous Raman measurements on $\mathrm{Cd}{\mathrm{Te}}_{0.68}{\mathrm{Se}}_{0.32}$ 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.