Quantum Beats Due to Excitonic Ground-State Splitting in Colloidal Quantum Dots

The dephasing of PbS quantum dots has been carefully measured using three pulse four-wave mixing and two-dimensional nonlinear optical spectroscopy. The temperature dependence of the homogeneous linewidth obtained from the two-dimensional spectra indicates significant scattering by acoustic phonons, whereas the excitation density dependence shows negligible excitation induced broadening in agreement with previous results. The rapid dephasing is attributed to elastic scattering by acoustic phonons. However, two dephasing components emerge, the short component that dominates the decay and a weaker longer decay, likely due to ''zero-phonon'' dephasing. Quantum beats originating from two separate states can be observed, possibly revealing an ∼23.6 meV splitting of the excitonic ground state. Finally, the emergence of biexcitonic effects enhanced by the high quantum confinement is discussed.