Fast exciton spin relaxation in single quantum dots

Exciton spin relaxation is investigated in single epitaxially grown semiconductor quantum dots in order to test the expected spin relaxation quenching in this system. We study the polarization anisotropy of the photoluminescence signal emitted by isolated quantum dots under steady-state or pulsed nonresonant excitation. We find that the longitudinal exciton spin relaxation time is strikingly short $(⩽100\mathrm{ps})$ even at low temperature. This result breaks down the picture of a frozen exciton spin in quantum dots.

Figure 1

Energy level diagram of a QD showing the two bright exciton states $\mid X⟩$ and $\mid Y⟩$ and the system ground state $\mid g⟩$. (a) The system dynamics is determined by an exciton photogeneration rate $G$ and two oscillator strengths ${\gamma }_X$ and ${\gamma }_Y$ accounting for radiative recombination. An additional relaxation channel is considered: nonradiative recombination (b) or longitudinal exciton spin relaxation (c).

Figure 2

Normalized differential photoluminescence spectra $I_X-I_Y$ for three quantum dots at $10\mathrm{K}$, where $I_X$ and $I_Y$ are the photoluminescence spectra recorded for the $X$ and $Y$ analysis axes, respectively. Fits (solid line) are performed with two orthogonal linearly polarized lines with $I_{\mid X⟩}∕I_{\mid Y⟩}$ equal to 1.2 (a), 3 (b), and 10 (c) and an energy splitting $\Delta \sim 35\mu \mathrm{eV}$ (a), $\Delta \sim 105\mu \mathrm{eV}$ (b), and $\Delta <40\mu \mathrm{eV}$ (c).

Figure 3

(a) Time-resolved photoluminescence intensity at $5\mathrm{K}$, for the $X$ (top curve) and $Y$ (bottom curve) polarization analysis axes, for the QD of Fig. 2c. Inset: schematic diagram of the exciton ground-state fine structure. (b) Time-resolved traces for the $X$ analysis axis at 5, 20, and $40\mathrm{K}$ on a semilogarithmic scale. Fits (solid line) are solutions of the rate equation model sketched in the inset.