Coherent quantum dynamics of excitons in semiconductor quantum dots (QDs) are of key interest, besides fundamental physics, for many applications ranging from quantum computing to advanced photonic devices. With the advances in colloidal synthesis, high-quality semiconductor nanocrystals can be fabricated at lower cost, and more flexibility in size, shapes and compositions. Despite its importance, measuring the exciton dephasing time in colloidal nanocrystals is technically challenging. Using three-beam transient resonant four-wave mixing technique in heterodyne detection not affected by spectral diffusion, we have measured the temperature-dependent ground-state exciton dephasing dynamics in CdSe/ZnS wurtzite QDs, CdSe/CdS spherical zincblende and rod-shape wurtzite QDs with variable core diameter and shell thickness / rod length, CdSe nanoplatelets, PbS QDs, InP/ZnSe QDs, and perovskite (CsPbBr₂Cl) QDs.

In these structures, the importance of phonon-assisted transitions, and the zero-phonon line (ZPL) dephasing by phonon-mediated spin-relaxation and radiative decay at low temperature are vastly varying. In PbS QDs, the peculiar band structure allows coupling with phonons at the zone edge (X-point), resulting in dominating phonon assisted transitions [1] even at low temperatures with sub-picosecond dephasing and a ZPL weight of less than 7%. In CdSe QDs [2,3] and dots in rods of similar size, the ZPL weight instead is above 50% since only zone-center phonons can couple, and the ZPL dephasing is limited by spin-relaxation into dark states on a 10-1000ps time scale, faster than the radiative lifetime of about 10ns. In InP QDs, a non-toxic alternative to CdSe, a similar behaviour is observed. In CdSe nano-platelets, the quasi two-dimensional confinement leads to quantum-well type behaviour, with a large exciton coherence area, such that the exciton dephasing is dominated by a fast radiative decay in the 1ps range [4]. In perovskite CsPbBr₂Cl QDs, the exciton ground state is bright, and a radiatively limited dephasing in the 10-100ps range is observed at low temperatures.  

[1] F. Masia et. al., Phys. Rev. B 83, 201309(R) (2011) DOI:10.1103/PhysRevB.83.201309
[2] F. Masia et al. Phys. Rev. Lett. 108, 087401 (2012) DOI:10.1103/PhysRevLett.108.087401
[3] N. Accanto et al., ACS Nano 6, 5227-5233 (2012) DOI:10.1021/nn300992a
[4] A. Naeem et al., Phys. Rev. B 91, 121302(R) (2015)  DOI 10.1103/PhysRevB.91.121302