Optical Blocking of Electron Tunneling into a Single Self-Assembled Quantum Dot

Time-resolved resonance fluorescence (RF) is used to analyze electron tunneling between a single self-assembled quantum dot (QD) and an electron reservoir. In equilibrium, the RF intensity reflects the average electron occupation of the QD and exhibits a gate voltage dependence that is given by the Fermi distribution in the reservoir. In the time-resolved signal, however, we find that the relaxation rate for electron tunneling is, surprisingly, independent of the occupation in the charge reservoir—in contrast to results from all-electrical transport measurements. Using a master equation approach, which includes both the electron tunneling and the optical excitation or recombination, we are able to explain the experimental data by optical blocking, which also reduces the electron tunneling rate when the QD is occupied by an exciton.

Figure 1

Resonance fluorescence (RF) scan of the exciton ($X$) (with a fine structure splitting of about $8\mu \mathrm{eV}$) and trion ($X^{-}$) for different laser excitation energies and gate voltages.

Figure 2

(a) Measured time-resolved RF signal of the exciton transition (trion out of resonance) for three different charging voltages $V_2$ (red lines are exponential fits, used to obtain relaxation rates). (b) Normalized RF signal at $t=40\mu \mathrm{s}$ as a function of gate voltage.

Figure 3

(a) Bare tunneling rates ${\gamma }_{\text{In}}$ and ${\gamma }_{\text{Out}}$ (data points) and fits with Fermi functions (solid lines) versus gate voltage together with the measured relaxation rate ${\gamma }_m$ (black rectangles) and the transport relaxation rate (green dashed line). (b) Energy scheme of the fluorescent and nonfluorescent states around $V_2=0.265\mathrm{V}$, where the energy of the 0 state is aligned with the energy of the $e^{-}$ state. Arrows indicate optical and transport processes with their respective rates $\gamma$.

Figure 4

(a) Time-resolved RF signal from the $X^{-}$ transition, for different gate voltages when the second electron can tunnel out of the QD. (b) Plot of the experimental relaxation rates versus gate voltage with a fitted Fermi function. (c) Energy scheme of the fluorescent and nonfluorescent states around $V_2=0.352\mathrm{V}$, where the energy of the $e^{-}$ state is aligned with the energy of the $2e^{-}$ state. Arrows indicate optical and transport processes with their respective rates $\gamma$.