Spin Relaxation and Its Control of Change Tunable Quantum Dots

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Spin dephasing relaxation and nuclear spin effects on the electron spin polarization in single electron doped InP quantum dots (QDs) were studied. When an InP QD is doped with one electron on average, a narrow Lorentzian dip with half-width of 4.6 mT appeared in the Hanle curve. The half-width 4.6 mT was ascribed to spin-dephasing relaxation of doped electrons constituting negative trions. The corresponding spin coherence time at 5 K is 1.7 ns, which is determined by the frozen fluctuation of nuclear spins in the QDs. With increase of temperature, the spin-dephasing rate of the doped electrons increases. Pump probe photoluminescence measurements of electron spin relaxation were used to estimate the time period TN of the Larmor precession of nuclear spins in the hyperfine field of electrons. We found TN to be s$ at 5 K, under the vanishing external magnetic field Measurements of electron spin polarization in the longitudinal magnetic field at 5 K indicated that the Overhauser field induced by the dynamic nuclear polarization increases linearly with the excitation power. The effective magnetic field of the frozen fluctuations of nuclear spins was found to be 15 mT.Resonant spin orientation of excitons was observed under linearly polarized quasiresonant excitation in neutral InP QDs. Under the longitudinal and tilted magnetic fields of 1.5 and 2.5 T, two anticrossings of bright and dark excitons take place. At the anticrossing points, bright and dark excitons mix with each other and the wave function mixing induces the resonant spin orientation. Time-resolved circular polarization clearly showed the resonant spin orientation due to mixing of bright and dark excitons at the anticrossing points. Dark excitons can work as the carrier and spin reservoir because of their long lifetime. We demonstrated that magnetic field can control the spin entanglement and the resonant spin orientation of QDs.