The hyperfine interaction between electron and nuclei in InAs/GaAs quantum dot is supposed to produce a partial relaxation of electron spin in the nanosecond timescale. As a result, this should limit to30% the average circular polarization of positive trions X⁺ (consisting of a single electron associated with two holes). To address this issue, we have studied the optical orientation of X⁺ in charge-tunable quantum dots. Actually, we observe that under intra-dot
circularly-polarized excitation the X⁺ photoluminescence of individual quantum dots can be
polarized above 70%. We show that this effect is due to the dynamic polarization of nuclei in zero magnetic field produced by the hyperfine interaction itself. Indeed, analyzing the fine structure analysis of several X⁺ lines reveals a splitting (or Overhauser shift) of the X⁺ components by ≅12μeV which is clearly large enough to screen the nuclear field fluctuations
(g_e μ_B Δ_B ≅ 2 μeV) responsible for the spin relaxation. This effect is confirmed by
using a σ⁺/σ^- 50kHz-alternated circular polarization provided by a photo-elastic
modulator. Under such excitation conditions, the X⁺ polarization falls down to 30% as expected in the case of zero nuclear field, but can be progressively restored to 70% by applying a longitudinal magnetic field of 150mT which similarly screens the nuclear field
fluctuations. Coming back to the steady-state excitation, we show in addition that this magnetic-like nuclear field can be as well screened by an external longitudinal magnetic field. This is evidenced by a dip in the X⁺ polarization for a field of 70mT. Dynamic nuclear polarization and hyperfine-induced spin relaxation have to be treated self-consistently to reproduce the field dependence of X⁺ polarization

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