Spin Lifetime in Electron-Doped InP Quantum Dots

Spin relaxation of electrons doped in InP quantum dots was studied by means of luminescence pump-probe and Hanle measurements. Optical pumping makes spins of doped electrons to be oriented in parallel to the helicity of the circularly polarized excitation. The luminescence pump-probe showed the spin orientation of the doped electrons decay on a millisecond time-scale. Hanle measurement clarified the spin dephasing relaxation time of doped electrons is 1.7 ns which is explained by the frozen fluctuation of nuclear spins. Electron spins in quantum dots (QDs) are good candidates for the quantum memory in the quantum information technology, because transferring the quantum information from the photon polarization to the electron spin polarization is direct and one-to-one. Electrons in QDs are expected to have long spin lifetime and doped electrons in QDs have infinite lifetime. We investigated time-resolved optical orientation of charge tunable InP QDs and found that the spins of the doped electrons are oriented under the circularly polarized excitation and are preserved in part for up to sub-millisecond [1]. In this work, we report nanosecond dephasing time of the spins by means of Hanle measurement as well as up to millisecond preservation of the spin by means of photoluminescence (PL) pump-probe in the same QDs. The samples studied are charge tunable one-layer InP self-assembled QDs grown on n+-GaAs substrates. The excitation source was a cw Ti:sapphire laser and the PL was detected by a photomultiplier with a 2-channel gated photon counter (GPC). A photoelastic modulator (PEM) was used to produce right and left circular polarization alternatively at 42 kHz in the detection path or the excitation path. The number of electrons in the QDs was varied with the change of the electric bias applied across the quantum structure. Trionic quantum beats in PL showed that each InP QD contains one doped electron on an average at the electric bias of −0.1 ∼−0.2V [2]. Under the electric bias of −0.1V, the negative circular polarization (NCP) was observed in PL of InP QDs under the circularly polarized quasi-resonant excitation and the longitudinal magnetic field of 0.1T. Time-resolved study of the PL circular polarization showed that NCP started at 100ps and is preserved within the recombination lifetime [1]. The NCP is explained by considering the optical pumping of the spins of doped electrons under the circularly polarized excitation and the simultaneous spin Gate B ( PL) σ+ Probe ( ) σ+ ( ) σ+ ( PL) Gate A