Spin flip from dark to bright states in InP quantum dots

We report measurements of the time for spin flip from dark (nonlight emitting) exciton states in quantum dots to bright (light emitting) exciton states in InP quantum dots. Dark excitons are created by two-photon excitation by an ultrafast laser. The time for spin flip between dark and bright states is found to be at most $200\mathrm{ps}$, independent of density and temperature, below $70\mathrm{K}$. This is much shorter than observed in other quantum dot systems.

Figure 1

Total luminescence intensity from the dots as a function of the excitation laser wavelength. Squares: $120\mathrm{mW}$ average laser power. Circles: $70\mathrm{mW}$ average laser power.

Figure 2

Total luminescence intensity as a function of average laser power, for two different wavelengths. Circles: $1465\mathrm{nm}$. Triangles: $1340\mathrm{nm}$.

Figure 3

Total luminescence intensity from the dots as a function of time, following two-photon excitation by a laser pulse at $t=0$. Heavy line: fit to the theory discussed in the text.