Dynamics of Quantum Dot Nuclear Spin Polarization Controlled by a Single Electron

We present measurements of the buildup and decay of nuclear spin polarization in a single semiconductor quantum dot. Our experiment shows that we polarize the nuclei in a few milliseconds, while their decay dynamics depends drastically on external parameters. We show that a single electron can very efficiently depolarize nuclear spins in milliseconds whereas in the absence of the electron the nuclear spin lifetime is on the scale of seconds. This lifetime is further enhanced by 1–2 orders of magnitude by quenching the nonsecular nuclear dipole-dipole interactions with a magnetic field of 1 mT.

Figure 1

(a) Schematic of the pulse sequences used in the buildup and decay time measurements of DNSP. An acousto-optical modulator (AOM) deflects the excitation beam on and off the sample, serving as a fast switch (o and c denote the open and closed state, respectively). The AOM creates pump (probe) pulses of respective lengths ${\tau }_{\mathrm{pump}}$ (${\tau }_{\mathrm{probe}}$), separated by a waiting time ${\tau }_{\mathrm{wait}}$. A mechanical shutter blocks the pump pulse from reaching the spectrometer, while letting the probe pulse pass. (b) DNSP buildup curves obtained by varying ${\tau }_{\mathrm{pump}}$ at fixed ${\tau }_{\mathrm{wait}}$ (0.5 ms) and ${\tau }_{\mathrm{probe}}$ (0.2 ms). The red (black) data points correspond to QD excitation with light of positive (negative) helicity. The blue (gray) line is an exponential fit, yielding a buildup time of ${\tau }_{\mathrm{buildup}}=9.4\mathrm{ms}$. (c) DNSP decay curves obtained by varying ${\tau }_{\mathrm{wait}}$, at fixed ${\tau }_{\mathrm{pump}}$ (50 ms) and ${\tau }_{\mathrm{probe}}$ (0.5 ms). The color coding is identical to (a). The exponential fit reveals a decay time of ${\tau }_{\mathrm{decay}}=1.9\mathrm{ms}$.

Figure 2

(a) Timing diagram for the gate voltage switching experiment: During the period ${\tau }_{\mathrm{wait}}$, the QD gate voltage is switched to a value where the neutral exciton is the stable QD charge complex. Using transient pulses, the switching time is $30\mu \mathrm{s}$. Ejecting the residual QD electron removes its effect on DNSP depolarization. This is demonstrated in (b), which shows DNSP decay time measurements in the absence of the residual QD electron. The red (black) data points represent DNSP decay under ${\sigma }^{+}$ (${\sigma }^{-}$) excitation. For comparison, the blue (gray) curve shows the mean of the data presented in Fig. 1b. (c) Same measurement as in (b), but over a longer time scale. The exponential fit (blue or gray) indicates a decay time constant of ${\tau }_{\mathrm{decay}}\sim 2.3\mathrm{s}$.

Figure 3

Measurements of buildup and decay of DNSP in an external magnetic field $B_{\mathrm{ext}}\sim -220\mathrm{mT}$: (a) Buildup of DNSP. In the presence of $B_{\mathrm{ext}}$, it is more efficient and thus faster to produce a nuclear magnetic field compensating the latter (black, ${\sigma }^{-}$ excitation) than one that enforces it (red, ${\sigma }^{+}$ excitation) [13]. (b) If DNSP decay is mediated through the residual QD electron, it is again more efficient to depolarize the nuclei if the total effective magnetic field seen by the electron is minimized. The color coding is the same as in (a). Solid curves in (a) and (b) show exponential fits to the data, the resulting buildup- and decay times are given in the figures. (c) Decay of DNSP in the absence of the QD electron. Compared to the zero-field case [Fig. 2c], DNSP decay time is prolonged to ${\tau }_{\mathrm{decay}}\sim 60\mathrm{s}$. The inset shows OS after a waiting time of $1\mathrm{s}$ as a function of external magnetic field. DNSP decay is suppressed on a magnetic field scale of $\sim 1\mathrm{mT}$, indicative of DNSP decay mediated by nuclear dipole-dipole interactions. (d) shows the respective directions of the external magnetic field and the nuclear fields $B_{\mathrm{nuc}}^{{\sigma }^{+}}$ ($B_{\mathrm{nuc}}^{{\sigma }^{-}}$) induced by QD excitation with ${\sigma }^{+}$ (${\sigma }^{-}$) polarized light.