Optical Pumping of the Electronic and Nuclear Spin of Single Charge-Tunable Quantum Dots

We present a comprehensive examination of optical pumping of spins in individual GaAs quantum dots as we change the net charge from positive to neutral to negative with a charge-tunable heterostructure. Negative photoluminescence polarization memory is enhanced by optical pumping of ground state electron spins, which we prove with the first measurements of the Hanle effect on an individual quantum dot. We use the Overhauser effect in a high longitudinal magnetic field to demonstrate efficient optical pumping of nuclear spins for all three charge states of the quantum dot.

Figure 1

(a) PL intensity (gray scale) for the neutral exciton ($X^0$), negative trion ($X^{-}$), and positive trion ($X^{+}$) in a single QD as a function of emitted photon energy and applied bias. The PL energy scale is relative to the $X^0$ peak at 3 V bias ($E_{X^0}=1.663\mathrm{e}\mathrm{V}$). The excitation photon energy was 1.691 eV ($E_{X^0}+28\mathrm{m}\mathrm{e}\mathrm{V}$). (b) PL polarization memory for spectra in (a) (solid symbols). Open circles: 15 times higher excitation intensity. Dotted line: excitation energy $E_{X^0}+15\mathrm{m}\mathrm{e}\mathrm{V}$. (c) QD band profiles with spin configurations.

Figure 2

(a) Hanle effect measurements for all three charge states. (b) Experimental geometry: optically created spins (antiparallel to $Z$) precess in the $Y\mathrm{\text{−}}Z$ plane about the magnetic field $B_X$. (c) Bias dependence of $X^{-}$ polarization for two laser intensities and two magnetic fields. The thickness of the shaded regions corresponds to the depth of the $X^{-}$ Hanle peak.

Figure 3

(a) PL spectra of $X^0$, $X^{-}$, and $X^{+}$ in a longitudinal magnetic field (5 T) for both polarizations of the laser. (b) Raw polarizations calculated from peak intensities in each Zeeman doublet, as a function of laser polarization. (c) Spin splitting ($\mathrm{\text{Zeeman}}\mathrm{\text{splitting}}+\mathrm{\text{Overhauser}}\mathrm{\text{shift}}$) as a function of laser polarization (retardance).

Figure 4

(a) PL intensity (gray scale) for a single quantum dot as a function of the emitted photon energy and applied bias at 5 T and ${\sigma }^{-}$ laser polarization. (b) Polarization memory calculated from peak intensities in (a) and in the analogous spectrum for ${\sigma }^{+}$ laser polarization. (c) Nuclear spin polarization (proportional to Overhauser shift) obtained from doublet splittings in (a) and in the analogous spectrum for ${\sigma }^{+}$ laser polarization.