We show that by illuminating an InGaAs/GaAs self-assembled quantum dot with circularly polarized light, the nuclei of atoms constituting the dot can be driven into a bistable regime, in which either a threshold-like enhancement or reduction of the local nuclear field by up to 3 Tesla can be generated by varying the intensity of light. The excitation power threshold for such a nuclear spin "switch" is found to depend on both external magnetic and electric fields. The switch is shown to arise from the strong feedback of the nuclear spin polarization on the dynamics of spin transfer from electrons to the nuclei of the dot.The hyperfine interaction in solids [1] arises from the coupling between the magnetic dipole moments of nuclear and electron spins. It produces two dynamical effects: (i) inelastic relaxation of electron spin via the "flip-flop" process ( Fig.1a) and (ii) the Overhauser shift of the electron energy [2]. Recently, the hyperfine interaction in semiconductor quantum dots (QDs) has attracted close attention [3,4,5,6,7,8,9,10,11,12,13,14] fuelled by proposals for QD implementation in quantum information applications [15]. The full quantization of the electron states in QDs is beneficial for removing decoherence mechanisms present in extended systems [16,17]. However, the electron localization results in a stronger (than in a bulk material) overlap of its wave-function with a large number of nuclei (N ∼ 10 4 in small selfassembled InGaAs/GaAs dots and up to 10 5 ÷ 10 6 in electrostatically-defined GaAs QDs), and the resulting hyperfine interaction with nuclear spins has been found to dominate the decoherence [3,4,5,12,13,14] and life-time [9] of the electron spin at low temperatures.In this Letter, we report the observation of a pronounced bistable behaviour of nuclear spin polarisation, S, in optically pumped self-assembled InGaAs/GaAs dots. In our experiments, spin-polarized electrons are introduced one-by-one into an individual InGaAs dot at a rate w x (see Fig.1b) by the circularly polarized optical excitation of electron-hole pairs 120 meV above the lowest QD energy states. Due to hole spin-flip during its energy relaxation, both bright and dark excitons can form in the dot ground state. The former will quickly recombine radiatively with a rate w rec ≈ 10 9 sec −1 , whereas the dark exciton can recombine with simultaneous spin transfer to a nucleus via a spin "flip-flop" process (as in Fig.1a) at the rate w rec N p hf [12,18]. Here N is the number of nuclei interacting with the electron and p hf is the probability of a "flip-flop" process, which from our perturbation theory treatment is given by:(1) Here γ is the exciton life-time broadening, h hf is the strength of the hyperfine interaction of the electron with a single nucleus and E eZ is the electron Zeeman splitting. E eZ is strongly dependent on the effective nuclear magnetic field B N generated by the nuclei. This provides a feedback mechanism between the spin transfer rate and the degree of nuclear polarization (B N ∝ S) in the dot [19]. Th...

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Nuclear polarization dynamics are measured in the nuclear spin bistability regime in a single optically pumped InGaAs/ GaAs quantum dot. The controlling role of nuclear spin diffusion from the dot into the surrounding material is revealed in pump-probe measurements of the nonlinear nuclear spin dynamics. We measure nuclear spin polarization decay times in the range of 0.2-5 s, strongly dependent on the optical pumping time. The long nuclear spin decay arises from polarization of the material surrounding the dot by spin diffusion for long ͑Ͼ5 s͒ pumping times. The time-resolved methods allow the detection of the unstable nuclear polarization state in the bistability regime otherwise undetectable in cw experiments. DOI: 10.1103/PhysRevB.77.125307 PACS number͑s͒: 72.25.Fe, 72.25.Rb, 72.80.Ey, 73.63.Kv The hyperfine interaction between the magnetic moments of the electron and nuclear spins 1 has been shown to limit the electron spin lifetime 2-7 and coherence 8-10 in semiconductor nanostructures. Optical excitation [11][12][13] and transport 5,9,10,14,15 of spin-polarized electrons in semiconductor quantum dots ͑QDs͒ have been found to lead to dynamic nuclear polarization: Overhauser magnetic fields up to a few Tesla have been detected in nanostructures [16][17][18][19] leading to strong modifications of the confined electron energy spectrum. [11][12][13][16][17][18][19] The control of the nuclear spin has been identified as one of the prerequisites for the coherent manipulation of the electron spin in semiconductor nanostructures. 20 In this context, extended nuclear spin polarization lifetimes in a dot are desirable and, as we show here, can be achieved by increasing the optical pumping time, during which not only the dot but also the surrounding material becomes polarized.In this work, we measure nuclear spin dynamics in the optically induced bistability regime in InGaAs/ GaAs selfassembled quantum dots in external magnetic fields B ext of 1-3 T. [16][17][18] We develop a sensitive time-resolved all-optical method based on the nonlinear behavior of the nuclear spin polarization in the bistability regime and measure the rise and decay time of the nuclear polarization in a single dot. Using this method, we find that the optical pumping time required to reach a nuclear spin polarization of S N Ͼ 0.3 in B ext = 2 T can be as short as 100 ms. Working in a pumpprobe mode, where the dot is free of electrons and holes during the "dark" pump-probe delay time, we find that the nuclear polarization decay in a single dot is strongly dependent on the pumping time: it can vary from 0.2 s for 100 ms pumping to 5 s for Ͼ3 s pumping. This observation indicates the importance of nuclear spin pumping outside the dot, which arises due to nuclear spin diffusion from the dot into the surrounding matrix. Finally, the pump-probe data allow us to reconstruct the whole nuclear spin bistability curve, including the unstable nuclear polarization state, which is otherwise undetectable in cw experiments.Previously, nuclear spin dynamics w...

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