Asymmetric optical nuclear spin pumping in a single uncharged quantum dot

Highly asymmetric dynamic nuclear spin pumping is observed in a single self-assembled InGaAs quantum dot subject to resonant optical excitation of the neutral exciton transition. A large maximum polarization of 54% is observed and the effect is found to be much stronger upon pumping of the higher energy Zeeman level. Time-resolved measurements allow us to directly monitor the buildup of the nuclear spin polarization in real time and to quantitatively study the dynamics of the process. A strong dependence of the observed dynamic nuclear polarization on the applied magnetic field is found, with resonances in the pumping efficiency observed for particular magnetic fields. We develop a model that accounts for the observed behavior, where the pumping of the nuclear spin system is due to hyperfine-mediated spin-flip transitions between the states of the neutral exciton manifold.

Figure 1

(Color online) (a) Combined PL and PC measurements at $B_{\text{ext}}=0\text{T}$. Inset: schematic of the structure investigated consisting of a single layer of self-assembled QDs is embedded in the intrinsic region of a Schottky photodiode. (b) Both ${\text{X}}^0$ $s$-shell states measured in PC electric field sweeps at $B_{\text{ext}}=5\text{T}$. The individual curves were measured as described in the text.

Figure 2

(Color online) (a) Timing of electric field sequence and laser excitation of the sample used for time resolved measurements of ${\delta }_{\text{N}}$. (b) Center and right panel: time-dependent PC signal of $|E_1⟩$ for different initial detunings of the spin state from the laser $d_{\text{i}}$ (defined by $F_{\text{S}}$). Left panel: steady-state PC values of the time-dependent measurements as a function of $F_{\text{S}}$ [yellow/light gray symbols] in comparison with corresponding electric field sweep measurements [green trace, closed circles and blue trace, closed triangles; color and symbol coding identical to Fig. 1b]. (c) Left panel: ${\delta }_{\text{N}}^{\text{s}}$ as a function of time for different $d_{\text{i}}$. Right panel: time $t_{\text{equ}}$ until ${\delta }_{\text{N}}^{\text{s}}$ is reached as a function of $d_{\text{i}}$. The black solid line is a exponential fit, where an offset of 30 s is included. Identical symbols $\left(\Delta ,\nabla ,\dots \right)$ in different panels refer to the same $d_{\text{i}}$.

Figure 3

(Color online) (a) Calculated Breit-Rabi diagram for the bright $\left(|E_{1,2}⟩\right)$ and dark $\left(|E_{3,4}⟩\right)$ exciton states. (b) Steady-state Overhauser shift ${\delta }_{\text{N}}^{\text{s}}$ created by resonant excitation of either of the two ${\text{X}}^0$ states $|E_1⟩$ and $|E_2⟩$ as a function of $B_{\text{ext}}$ obtained from experiment (circles) and calculated using the model described in the text (solid line). The dashed vertical lines indicate the crossing of $|E_3⟩$ and $|E_{1,2}⟩$ in the absence of DNP, respectively.