Femtosecond three-pulse photon echo and population grating studies of the optical properties of CdTe/ZnSe quantum dots

Femtosecond three-pulse two-color photon echo and population grating (PG) techniques have been used to characterize CdTe quantum dots (QDs) grown on ZnSe by molecular beam epitaxy. The time evolution of the PG signal exhibits a fast decay (2.5-3 ps), which is attributed to migration and tunneling of photoexcited carriers to neighboring QDs, followed by a slow decay (∼20 ps for small dots and >100 ps for large dots), which is ascribed to the lifetime of excitons in the quantum dots. The three-pulse photon echo intensity versus population time can be used to deduce values for the homogeneous broadening (Γh=0.8-1.2meV) and the exciton binding energy (∼13 meV). A weak dependence of these quantities on the detection wavelength is associated with a difference in dot sizes, which alters the coupling between the exciton and the acoustic phonons.