Time- and spectrally-resolved four-wave mixing in singleCdTe∕ZnTequantum dots

Abstract

We present transient four-wave mixing experiments on individual excitonic transitions in self-assembled $\mathrm{Cd}\mathrm{Te}∕\mathrm{Zn}\mathrm{Te}$ 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 $0.06--0.1\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ $({T}_{2}=13--20\phantom{\rule{0.3em}{0ex}}\mathrm{ps})$ are measured, and a ZPL weight in the total line shape of $Z=0.9$ at $T=7\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ 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 $0.2--0.35\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ 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.