(Multi)exciton dynamics and exciton polatizability in colloidal InAs quantum dots
We report steady state and dynamic properties of (multi)excitons in InAs quantum dots using femtosecond time-resolved transient absorption and time-resolved terahertz spectroscopy. The polarizability of confined excitons in the ground state is found to be of the order of 104 Å3, increasing sharply with quantum dot radius as R3. The size-dependence of the exciton polarizability can be quantitatively reproduced by using multiband tight binding calculations. Following the generation of a single exciton with excess electronic energy, electron intraband relaxation from the 1Pe → 1Se level occurs on a time scale of 0.8 ps for particles of 2.2 nm radius, whereas hot hole cooling occurs faster than 150 fs. The intraband relaxation dynamics are consistent with an Auger process involving electron−hole energy transfer, indicating an electron−hole coupling time of 0.8 ps. Finally, when multiple excitons are generated through multiphoton absorption, multiexciton recombination dynamics occurs on 1−100 ps time scales. We illustrate the challenges associated with reliably extracting the time constants of multiexciton recombination from our data, which are helpful for elucidating the multiexciton recombination mechanism. Our results demonstrate that the exciton dynamics in InAs quantum dots are governed by the relatively low dielectric constant of InAs (resulting in strong carrier−carrier interactions) and the low energy spacing between valence levels (resulting from the relatively high value of the hole effective mass, and allowing for rapid phonon-mediated hole relaxation).