Carrier Dynamics and Interactions for Bulklike Photoexcitation of Colloidal Indium Arsenide Quantum Dots

The remarkable photonic and photochemical properties of colloidal quantum dots (QD) depend critically on the dynamics of carrier interactions and relaxation. Despite their importance, a quantitative experimental evaluation of these processes has proven elusive due to the inherent challenge of exactly separating single-exciton and multiexciton dynamics, whose spectroscopic signatures overlap in time, spectrum, and excitation fluence. Here, we measure pump-fluence-dependent absolute pump-probe transients of indium arsenide QDs, refreshing the sample using beam scanning to limit repetitive excitation. Focusing on the low fluence limit near the onset of biexciton formation, excitation conditions were precisely controlled and characterized by averaging Poisson-distributed excitation statistics over all three spatial dimensions of the pump and probe beam spatial profiles to determine the average excitation probability. A saturation model is developed to uniquely decompose the pump-probe signal into single-exciton and biexciton signals. This method harnesses the distinct pump-fluence scaling of absolute pump-probe signals from singly and doubly excited QDs without any assumptions regarding the relative time scales or amplitudes of single-exciton and biexciton signals. Probing in the bulklike region of the QD absorption spectrum, the signal from biexcitons is found to be 1.8 times the signal from single excitons at T = 0, consistent with the conventionally assumed factor of 2 within the 95% confidence intervals. The biexciton signal contains the same hot-carrier relaxation dynamics as that from single excitons, but signal from a second exciton additionally exhibits a 26 ps exponential decay attributed to Auger recombination.