Using a sensitive transient four-wave mixing technique in heterodyne detection, we measured the ground-state excitonic dephasing in PbS colloidal quantum dots of 4-6-nm diameter in the temperature range from 5 to 100 K. We observe an ultrafast (<100 fs) initial decay followed by a component of about 300 fs at 5 K, which becomes faster with decreasing dot size and increasing temperature. This dynamics is attributed to pure dephasing of phonon-assisted transitions in the peculiar band structure of PbS, which allows coupling to phonons with wave vectors both near the Brillouin zone center and near the (100) zone edge, the X point. We estimate an upper limit of 7% for the weight of the zero-phonon line with 2.6-ps spin-flip limited dephasing. The dephasing time inversely proportional to the homogeneous linewidth of an optical transition is a fundamental physical quantity in light-matter interaction. It was recognized about a decade ago that semiconductor quantum dots (QDs), in which carriers are confined in all three directions, exhibit a peculiar coupling of excitons with phonons as a result of the local lattice distortion associated with the excitonic excitation.1,2 This coupling gives rise to an excitonic homogeneous line shape at low temperatures consisting of a Lorentzian zero-phonon line (ZPL) superimposed onto a broad band of phonon-assisted transitions and, in turn, a strongly nonexponential dephasing.
2Among the various types of semiconductor quantum dots, PbX (X = S, Se) colloidal QDs (CQDs) have attracted much interest in recent literature due to their emission in the near infrared and their peculiar band structure. The effective masses of electrons and holes in these system are similar and relatively small, resulting in an exciton Bohr radius (46 nm for PbSe and 18 nm for PbS) that is large compared to the typical QD sizes. Hence PbX dots are considered a model system for QDs with strong confinement. Moreover, both the maximum of the valence band and the minimum of the conduction band are at the L point of the Brillouin zone, resulting in a 64-fold-degenerate ground-state exciton compared to the eightfold degeneracy in spherical QDs with a direct band gap at the point (e.g., InAs, InP, CdS, CdSe). 3 Furthermore the exciton radiative lifetime is in the microsecond range, 3 considerably longer than in other QDs, opening the prospect of ultralong radiative-lifetime limited dephasing times.While the homogeneous line shape of excitons in III-V and II-VI QDs, epitaxially grown and colloidal, has been measured and calculated extensively in the literature, 1,2,4,5 to date there have been only few works on PbX CQDs. Low-temperature ensemble photoluminescence (PL) gave an upper limit of 30 meV for the homogeneous linewidth in PbSe CQDs. Single-dot PL spectroscopy, 1 commonly used to determine the homogeneous linewidth, is limited by the small radiative emission rate and the high detector noise in the near-infrared wavelength region of PbX emission. 7 Moreover, spectral diffusion, a variation of the QD transition f...