Time-and spectrally-resolved four-wave mixing in single CdTe/ZnTe quantum dots

We present transient four-wave mixing experiments on individual excitonic transitions in self-assembled CdTe∕ZnTe quantum dots. Using a two-dimensional femtosecond spectroscopy and heterodyne detection of the nonlinear signal we study the dephasing and mutual coherent coupling of single quantum dot states. For the homogeneous linewidth of the zero-phonon line (ZPL) values of 0.06–0.1meV (T2=13–20ps) are measured, and a ZPL weight in the total line shape of Z=0.9 at T=7K is estimated. We observe two linearly polarized fine-structure split exciton transitions with transition dipole moment ratios of 1.0–1.1 deduced from the four-wave mixing (FWM) amplitude, and splitting energies of 0.2–0.35meV deduced from the FWM spectral response or quantum beat period. Coherent coupling between excitonic states is identified by off-diagonal signals in the two-dimensional spectrally-resolved FWM. The presence of an inhomogeneous broadening caused by spectral diffusion in the time ensemble is evidenced by the formation of a photon echo in the time-resolved FWM from a single transition.