Giant exciton oscillator strength and radiatively limited dephasing in two-dimensional platelets

We measured the intrinsic ground-state exciton dephasing and population dynamics in colloidal quasi-two-dimensional (2D) CdSe nanoplatelets at low temperature (5--50 K) using transient resonant four-wave mixing in heterodyne detection. Our results indicate that below 20 K the exciton dephasing is lifetime limited, with the exciton population lifetime being as fast as 1 ps. This is consistent with an exciton lifetime given by a fast radiative decay due to the large in-plane coherence area of the exciton center-of-mass motion in these quasi-2D systems compared to spherical nanocrystals. The radiative rate in such 2D platelet systems can be controlled by the platelet area over orders of magnitude.