Kazuki Takeishi

Interdot spin transfer dynamics in laterally coupled excited spin ensemble of high-density InGaAs quantum dots

Interdot spin transfer dynamics is studied in a laterally coupled excited spin ensemble of high-density InGaAs quantum dots (QDs). We observe a rise time of the photoluminescence intensity of ∼100 ps and a simultaneous increase in the spin polarization of the excited spin ensemble, indicating spin injection from higher-energy levels in smaller QDs. Moreover, this coupled ensemble exhibits decay properties of the spin polarization that vary with the excited spin density. This phenomenon can be quantitatively understood by considering interdot spin transfer into lower-energy levels of the surrounding QDs, where the transfer rate depends on the degree of state filling of each QD level.Interdot spin transfer dynamics is studied in a laterally coupled excited spin ensemble of high-density InGaAs quantum dots (QDs). We observe a rise time of the photoluminescence intensity of ∼100 ps and a simultaneous increase in the spin polarization of the excited spin ensemble, indicating spin injection from higher-energy levels in smaller QDs. Moreover, this coupled ensemble exhibits decay properties of the spin polarization that vary with the excited spin density. This phenomenon can be quantitatively understood by considering interdot spin transfer into lower-energy levels of the surrounding QDs, where the transfer rate depends on the degree of state filling of each QD level.

Growth optimization of spin-transport barriers used for spin-polarized light-emitting diodes based on InGaAs quantum dots

We have studied GaAs and AlGaAs barriers for the purpose of improving spin-transport performance in spin-polarized light-emitting diodes (LEDs) based on self-assembled quantum dots (QDs) of InGaAs. In the spin-LED utilizing a spin-functional optical active layer of In-based self-assembled QDs, growth temperatures of top barriers of GaAs and AlGaAs were reduced to suppress indium diffusion from the QDs into the barriers after forming the QDs. We show a significant improvement of spin-transport property as well as of carrier-transport one with increasing growth temperature of the Al0.1Ga0.9As barrier from 580 to 640 °C, while luminescent spectral energy and shape of the QDs are not markedly affected.

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.

Ultrahigh-density self-assembled quantum dots of InGaAs and suppression of optical state-filling effect

We have grown ultrahigh-density self-assembled quantum dots (QDs) of InGaAs with sheet densities up to 2.5×1011 cm-2 and lateral diameters down to 10 nm, where the dot density increases with increasing As pressure during dot growth under optimum growth conditions. A ground-state photoluminescence (PL) spectrum shows a spectral width of 47 meV for the highest-density sample. Optical excitation-density dependences of the PL intensity and time profile are studied. The PL intensity from QD excited states increases with increasing excitation power, originating from a state-filling effect in QDs, which is directly confirmed by a plateau-like behavior on the PL decay curve. We find that the filling effect is significantly suppressed in the above ultrahigh-density dot ensemble, which suggests potential applications to superior energy-saving lasing and spin-functional optical devices.

Electric-field control of spin states in InGaAs-based coupled nanostructures of quantum dots and well

Electric-field control of spin states has been studied in InGaAs-based coupled nanostructures composed of quantum dots (QDs) and wells (QWs), where electron spins optically excited in the QW can be transferred into the QDs via tunneling depending on the strength of applied electric field. The photoluminescence (PL) intensity in the QDs and the circular polarization, reflecting the electron-spin states after the injection, are found to be largely dependent on the electric field. The spin dynamics responsible for the field-dependent PL properties is discussed in terms of the potential-driven spin injection via tunneling and subsequent energy spin relaxation in the QDs.