Spin Relaxation and Its Control of Change Tunable Quantum Dots

研究了单电子掺杂InP量子点的自旋退相弛豫和核自旋效应对电子自旋极化的影响。当InP量子点平均掺杂一个电子时，Hanle曲线出现半宽为4.6mT的较窄的洛伦兹凹陷。半宽4.6mT归因于组成负三电子的掺杂电子的自旋退相弛豫。在5K下相应的自旋相干时间为1.7 ns，这是由量子点中核自旋的冻结涨落决定的。随着温度的升高，掺杂电子的自旋退相率增大。利用电子自旋弛豫的泵浦探测光致发光测量，估算了电子超精细场中核自旋Larmor进动的时间周期Tn。在外加磁场消失的情况下，对5K时纵向磁场中电子自旋极化的测量表明，动态核极化产生的Overhauser场随着激发功率的增加而线性增加。核自旋冻结涨落的有效磁场为15mT。在中性InP量子点中线极化准共振激发下，观察到激子的共振自旋取向。在1.5T和2.5T的纵磁场和斜磁场下，发生了亮激子和暗激子的两次反交叉。在反侵蚀点，亮激子和暗激子相互混合，波函数混合导致共振自旋取向。时间分辨圆偏振清晰地显示了由于反对角点上亮激子和暗激子的混合而产生的共振自旋取向。暗激子具有很长的寿命，可以作为载流子和自旋库。我们证明了磁场可以控制量子点的自旋纠缠和共振自旋取向。

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.