Radiatively limited dephasing in InAs quantum dots

We measure the dephasing time of the exciton ground-state transition in ${\mathrm{In}}_{1-x}{\mathrm{Ga}}_x\mathrm{As}$ quantum dots using a sensitive four-wave mixing technique. We find experimental evidence that the dephasing time is given only by the radiative lifetime at low temperatures. We demonstrate the tunability of the radiatively limited dephasing time from $400\mathrm{ps}$ up to $2\mathrm{ns}$ in a series of annealed ${\mathrm{In}}_{1-x}{\mathrm{Ga}}_x\mathrm{As}$ quantum dots with increasing quantum-confinement energy from $69\mathrm{meV}\text{to}330\mathrm{meV}$.

Figure 1

(Color online) Nonresonantly excited PL spectra of the investigated QD samples at room temperature. The nominal annealing temperature is given for the spectra, which are vertically displaced for clarity. The wetting-layer (WL) and the QD ground-state transitions are indicated. The expected QD shapes are sketched.

Figure 2

Time-integrated FWM amplitude of the unannealed sample versus delay time $\tau$ at $T=5\mathrm{K}$ for different polarization configurations. Top: circular polarization $\left({\sigma }^{+}\right)$. Inset: probability distribution of the fine-structure splitting ${\delta }_1$ in the ensemble. Bottom: Linear polarization along $\left[1\overline{1}0\right]$, [110], and [100], as indicated. The sketch displays the FWM geometry.

Figure 3

Exciton levels and transitions in the QD which are important for the FWM resonant to the exciton ground state. QD ground state $\left(\mid 0⟩\right)$, single exciton states of circular $(\mid {\sigma }^{+}⟩,\mid {\sigma }^{-}⟩)$ or linear $(\mid x⟩,\mid y⟩)$ basis, biexciton state $\left(\mid B⟩\right)$. Left: cylindrical symmetry $\left(D_{2d}\right)$; Right: lower symmetry $(<D_{2d})$.

Figure 4

Polarization decay rates ${\gamma }_x$ at $T=5\mathrm{K}$ for QD’s of different annealing temperature versus their confinement energy $E_c$. A $1∕\sqrt{E_c}$ dependence (dotted line) is shown for comparison. The inset shows the temperature dependence of ${\gamma }_x$ for the unannealed sample. A simplified sketch of the in-plane confinement potential is also shown.