Transient photoluminescence study on spin dynamics in InGaAs-based coupled nanostructures of quantum dots with quantum wells

We have made transient photoluminescence (PL) study on electron-spin dynamics in InGaAs-based coupled nanostructures of quantum dots (QDs) with quantum wells (QWs). Self-assembled InGaAs QDs were grown integrated with an InGaAs QW through a GaAs tunneling barrier or embedded in a GaAs QW. Time-resolved circularly polarized PL in the QDs was measured as a function of temperature after optical spin excitation selectively in the QW, reflecting electron-spin polarization injected from the QW into QDs. We show the spin injection dynamics induced by spin tunneling and subsequent energy relaxation from the QW into QDs in the former coupled QDs. Spin relaxation at excited states in the QDs after the dynamical spin injection is shown as a function of temperature. These coupled QD samples exhibit thermally persistent spin polarization up to 200 K, originating from ultrafast and thus efficient spin injection as well as longer spin-relaxation times compared to radiative decay times in the QDs after the injection.