Overhauser effect in individual $\mathrm{In}\mathrm{P}∕{\mathrm{Ga}}_x{\mathrm{In}}_{1-x}\mathrm{P}$ dots

Sizable nuclear spin polarization is pumped in individual electron-charged $\mathrm{In}\mathrm{P}∕\mathrm{Ga}\mathrm{In}\mathrm{P}$ dots in a wide range of external magnetic fields $B_Z=0–5\mathrm{T}$ by circularly polarized optical excitation. We observe nuclear polarization of up to $\approx 40\%$ at $B_Z=1.5\mathrm{T}$ corresponding to an Overhauser field of $\approx 1.2\mathrm{T}$. We find a strong feedback of the nuclear spin on the spin pumping efficiency. This feedback, which is produced by the Overhauser field, leads to nuclear spin bi-stability at low magnetic fields of $B_Z\approx 0.3-1\mathrm{T}$. We find that the splitting in magnetic field between the trion radiative recombination peaks markedly increases, when the Overhauser field in the dot cancels the external field. This counterintuitive result is shown to arise from the opposite contribution of the electron and hole Zeeman splittings to the optical transition energies.

Figure 1

(Color online) (a) A typical PL spectrum of ensemble of $\mathrm{In}\mathrm{P}∕\mathrm{Ga}\mathrm{In}\mathrm{P}$ dot measured at $T=15\mathrm{K}$. Emission from the GaAs substrate is observed around $1.51\mathrm{eV}$. (b) Trion PL spectra measured at $B_Z=5\mathrm{T}$ for a single $\mathrm{In}\mathrm{P}∕\mathrm{Ga}\mathrm{In}\mathrm{P}$ dot using linearly polarized excitation and ${\sigma }^{+}$ (gray) and ${\sigma }^{-}$ (black) detection. (c) Trion PL spectra measured at in-plane field $B_{X^{\prime }}=5\mathrm{T}$ for a single $\mathrm{In}\mathrm{P}∕\mathrm{Ga}\mathrm{In}\mathrm{P}$ dot using linearly polarized detection. (d) Trion peak energies versus external magnetic field in the Voigt and Faraday configurations. (c) Schematic diagram of allowed optical transitions in a negatively charged InP dot in Voigt and Faraday configurations. Electrons (holes) are shown with small (large) arrows schematically representing spin-up and spin-down states. After emission of a ${\sigma }^{+}\left({\sigma }^{-}\right)$-polarized photon, an electron with spin up (down) is left on the dot.

Figure 2

Power dependences of the splitting $E_{xZ}$ [see Fig. 1d] measured for a negatively charged exciton at (a) $B_Z=1.5$ and (b) $5\mathrm{T}$ for ${\sigma }^{-}$ (diamonds), ${\sigma }^{+}$ (circles), and linearly (triangles) polarized excitation. The inset in (b) shows $E_{xZ}$ power dependence measured at $B_Z=0\mathrm{T}$.

Figure 3

Power dependence of the splitting $E_{xZ}$ (as defined in Fig. 1d) measured for a single InP dot trion at $B_Z=0.5\mathrm{T}$ for ${\sigma }^{+}$ circularly polarized excitation. Arrows show the direction in which the power was scanned.