Light-polarization-independent nuclear spin alignment in a quantum dot

We study experimentally nuclear spin-pumping mechanisms in neutral InP/GaInP quantum dots under nonresonant optical excitation. We find two distinct regimes of dynamic nuclear polarization. At low optical powers when the dot is populated with “dark” excitons we observe nuclear spin polarization up to $~10\%$ with direction insensitive to polarization and wavelength of light. Measurements of photoluminescence of both “dark” and “bright” excitons in single dots reveal that at low optical power nuclear spin pumping occurs via a virtual spin-flip transition between these states accompanied by photon emission. Under these conditions the sign of the nuclear spin polarization is determined by asymmetry in the exciton energy spectrum rather than by the sign of the exciton spin polarization. By contrast at high optical powers resulting in saturation of the quantum dot and suppression of exciton photoluminescence we detect nuclear spin polarization with direction and degree (up to $~50\%$) determined by the polarization of light.

Figure 1

(a) Magnetic-field dependence of exciton PL energies in QD1 (symbols); lines show fitting (see text). (b) PL spectra of the exciton in a neutral quantum dot detected in $\sigma {}^{+}$($\blacksquare$) and $\sigma {}^{-}$ ($◯$) polarizations under $\sigma {}^{+}$ (top) and $\sigma {}^{-}$ (bottom) excitation with photon energy 1.88 eV at $B_z=6.0$ T. (c) Energy diagram of exciton and biexciton states in high magnetic field $B_z>B_z^{\mathrm{cr}}\approx 2.2$ T. Heavy holes $\Uparrow$($\Downarrow$) and electrons $\uparrow$($\downarrow$) with spin parallel (antiparallel) to external field form optically allowed ($\Uparrow \downarrow$, $\Downarrow \uparrow$) and “dark” ($\Uparrow \uparrow$, $\Downarrow \downarrow$) excitons with total spin $J_z=\pm 1$ and $\pm 2$, respectively. Thick arrows show circularly polarized recombination of biexciton and “bright” excitons; thin arrows correspond to weak recombination of “dark” exciton states. Zigzag lines show electon-nuclear ($e$-$N$) spin-flips induced by the hyperfine interaction.

Figure 2

Results of PL power dependence in a neutral dot QD1 at $B_z=6$ T under $\sigma {}^{+}$ (open symbols) and $\sigma {}^{-}$ (solid symbols) excitation with photon energy 1.88 eV. Photoluminescence intensities (left scale) for all four exciton states and for the total intensity of biexciton emission are plotted. Spectral splitting (right scale) of the two optically allowed exciton states ($E_{|+1\rangle }-E_{|-1\rangle }$) is shown with diamonds. Lines show fitting obtained using the model presented in text. Additional scale on the right shows nuclear field $B_N$ on the dot deduced from the splitting.

Figure 3

Magnetic-field dependence of the maximum nuclear field $B_N$ induced at low power ($P_{\mathrm{exc}}<10$ $\mu$W) of $\sigma {}^{+}$ ($◯$) and $\sigma {}^{-}$($\blacksquare$) polarized excitation with photon energy $E_{\mathrm{exc}}=1.88$ eV in QD1.

Figure 4

Power dependence of the bright excitons spectral splitting ($E_{|+1\rangle }-E_{|-1\rangle }$) in two neutral quantum dots QD2 (a) and QD3 (b) at $B_z=6$ T under $\sigma {}^{+}$ (open symbols) and $\sigma {}^{-}$ (solid symbols) excitation with photon energy 1.88 eV.