Spin--flip Limited Exciton Dephasing in CdSe/ZnS Colloidal Quantum Dots

TL;DR

This study measures the exciton dephasing times in CdSe/ZnS quantum dots, revealing phonon-assisted spin-flip as the main dephasing mechanism, with implications for understanding quantum dot coherence properties.

Contribution

It provides direct experimental evidence linking exciton dephasing to phonon-assisted spin-flip processes in colloidal quantum dots.

Findings

Zero-phonon line dephasing time of 109 ps at 5 K

Dephasing dominated by acoustic phonons and spin-flip processes

Dephasing much faster than exciton radiative lifetime

Abstract

The dephasing time of the lowest bright exciton in CdSe/ZnS wurtzite quantum dots is measured from 5 K to 170 K and compared with density dynamics within the exciton fine structure using a sensitive three-beam four-wave-mixing technique unaffected by spectral diffusion. Pure dephasing via acoustic phonons dominates the initial dynamics, followed by an exponential zero-phonon line dephasing of 109 ps at 5 K, much faster than the ~10 ns exciton radiative lifetime. The zero-phonon line dephasing is explained by phonon-assisted spin-flip from the lowest bright state to dark exciton states. This is confirmed by the temperature dependence of the exciton lifetime and by direct measurements of the bright-dark exciton relaxation. Our results give an unambiguous evidence of the physical origin of the exciton dephasing in these nanocrystals.