Spin pumping efficiency in room temperature CdSe nanocrystal quantum dots

To understand and optimize optical spin initialization in room temperature CdSe nanocrystal quantum dots (QDs) we studied the dependence of the pump energy $E$ on the time-resolved Faraday rotation signal in a series of QD samples with different sizes. In $6.1$-nm-diameter QDs, we observe two peaks in the spin signal vs. $E$. The first peak occurs on resonance at $1.95$eV, with the second peak $\sim 0.17$eV higher in energy before the spin signal falls off to near zero. We calculate the spin-dependent oscillator strengths of optical transitions using an 8-band effective mass model to understand these results. The observed $E$ dependence of the spin pumping efficiency (SPE) arises from the competition between the heavy hole (hh), light hole (lh) and split-off (so) band contributions to transitions to the conduction band. The two latter contributions lead to an electron spin polarization in the opposite direction from the former. At lower $E$ the transitions mainly involve the hh band, giving rise to the two main peaks. At higher $E$, the increasing contributions from the lh and so bands lead to a reduction in SPE. In smaller QDs, both peaks merge while moving to higher $E$, and the overall SPE is reduced.