Direct Observation of the Electron Spin Relaxation Induced by Nuclei in Quantum Dots

We have studied the electron spin relaxation in semiconductor $\mathrm{InAs}/\mathrm{GaAs}$ quantum dots by time-resolved optical spectroscopy. The average spin polarization of the electrons in an ensemble of $p$-doped quantum dots decays down to $1/3$ of its initial value with a characteristic time $T_{\Delta }\approx 500\mathrm{ps}$, which is attributed to the hyperfine interaction with randomly oriented nuclear spins. We show that this efficient electron spin relaxation mechanism can be suppressed by an external magnetic field as small as 100 mT.

Figure 1

(a) cw photoluminescence spectrum of the QD. (b) Circular polarization of the QD ground state luminescence for (●) $B_z=0$ and (△) $B_z=100\mathrm{mT}$. (c) Scheme of a positively charged exciton $X^{+}$ formed by a spin polarized electron and two holes with opposite angular momentum projection.

Figure 2

Circular polarization dynamics of the QD luminescence after a circularly polarized $\sigma +$ laser excitation. Inset: Photoluminescence intensity copolarized $I^{+}$ and counterpolarized $I^{-}$ with the laser (semilogarithmic scale). The detection energy is centered at $E_{\det }=1.11\mathrm{eV}$.

Figure 3

Circular polarization dynamics of the QD ground state luminescence (semilogarithmic scale) for $B_z=0$, $B_z=100\mathrm{mT}$, and $B_z=400\mathrm{mT}$. The inset displays the calculated time dependence of the average electron spin $⟨\mathbf{S}(t)⟩/{\mathbf{S}}_0$ (see text).