Ultrafast optical generation of coherent phonons in CdTe1-xSex quantum dots

TL;DR

This study demonstrates ultrafast generation of coherent phonons in CdTeSe quantum dots using femtosecond laser pulses, revealing mode coupling and impurity effects on phonon frequencies.

Contribution

It provides the first detailed analysis of coherent phonon generation and mode interactions in CdTeSe quantum dots via impulsive stimulated Raman scattering.

Findings

Identification of multiple phonon modes including mixed modes

Resonance behavior of phonon amplitude with laser energy

Impurity states influence phonon frequencies

Abstract

We report on the impulsive generation of coherent optical phonons in CdTe0.68Se0.32 nanocrystallites embedded in a glass matrix. Pump probe experiments using femtosecond laser pulses were performed by tuning the laser central energy to resonate with the absorption edge of the nanocrystals. We identify two longitudinal optical phonons, one longitudinal acoustic phonon and a fourth mode of a mixed longitudinal-transverse nature. The amplitude of the optical phonons as a function of the laser central energy exhibits a resonance that is well described by a model based on impulsive stimulated Raman scattering. The phases of the coherent phonons reveal coupling between different modes. At low power density excitations, the frequency of the optical coherent phonons deviates from values obtained from spontaneous Raman scattering. This behavior is ascribed to the presence of electronic impurity states which modify the nanocrystal dielectric function and, thereby, the frequency of the infrared-active phonons.