Electron spin transfer of single quantum dots by using teraheltz and/or laser lights

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In this project, we aimed the development of the technique to transfer the spin between the Zeeman-split electron states in a single quantum dot (QD). Then, we tried the following two methods; use of teraheltz wave and two-photon process via half-localized state in a QD. As a consequence, by the latter method, we succeeded the control of the exciton spin state and the details were published in the academic journals. In the experiments, depending on the light polarizations of the co-linearly propagating delay-controlled double pulses, we observed the in-phase and out-of-phase oscillations of photoluminescence (PL) from the Zeeman-split two exciton states. In the case of the out-of-phase oscillations, the oscillation of the Zeeman splitting energy as well as the magnitude of the PLs was observed. This is because the nuclear spin polarization in a QD was formed and followed the modulation of the electron spin polarization. This result indicates that the half-localized state of QDs is available as the intermediate state in the Lambda-type transition in order to control of the electron and nuclear spins. In addition, we investigated the hyperfine interaction between electron and nuclear spins in a single QD, and the bistability and hysteretic response of the nuclear spin polarization were observed in the dependence on excitation power, polarization, and external magnetic field. By using this nuclear spin switching, we proved that the effective g factor of the QD electron could be tuned widely from negative to positive and be set to zero.