Homogeneous linewidth and dephasing times of excitons in CuBr quantum dots

We study CuBr nanocrystals embedded in a borosilicate matrix. The excitonic absorption line is strongly inhomogeneously broadened by the size dispersion of the quantum dots. We explore two ways in order to study the dephasing processes of the transition. The homogeneous absorption linewidth is first determined by spectral hole-burning (SHB) measurements, using a nanosecond dye laser excitation. We then perform degenerate four-wave-mixing experiments (FWM), using 100 fs pulses from a frequency-doubled Ti:sapphire laser. Due to the inhomogeneous broadening, we observe a so-called photon-echo signal and we measure its dynamics. From SHB experiments, we obtain homogeneous linewidths larger than 0.3 meV. From the FWM experiment, we obtain values smaller than 0.2 meV. These discrepancies between SHB and FWM measurements can be explained if we take into account the high excitation intensity necessary to induce absorption saturation in SHB experiments. Moreover, during the 20 ns pulses used, spectral diffusion may occur due to interactions between the nanocrystals and their matrix. Spectrally resolved four-wave-mixing experiments (SR-FWM) are also performed. The oscillations with a 250 fs period we observe at short time delays could be interpreted in terms of quantum beats between two-exciton and one-exciton states.