Direct Observation of Controlled Coupling in an Individual Quantum Dot Molecule

We report the direct observation of quantum coupling in individual quantum dot molecules and its manipulation using static electric fields. A pronounced anticrossing of different excitonic transitions is observed as the electric field is tuned. A comparison of our experimental results with theory shows that the observed anticrossing occurs between excitons with predominant spatially direct and indirect character and reveals a field driven transition of the nature of the molecular ground state exciton wave function. Finally, the interdot quantum coupling strength is deduced optically and its dependence on the interdot separation is calculated.

Figure 1

Main panel: Photoluminescence of a single QDM as a function of the applied electric field ranging from 15 to $25\mathrm{k}\mathrm{V}/\mathrm{c}\mathrm{m}$. The two excitons, A and B, show a clear anticrossing with a minimum splitting of $\Delta E=1.4\mathrm{m}\mathrm{e}\mathrm{V}$ at $\sim 18.8\mathrm{k}\mathrm{V}/\mathrm{c}\mathrm{m}$. Inset: Schematic of the investigated n-i Schottky diodes. The QDMs are embedded in the intrinsic GaAs layer with the electric field ($F$) oriented along the stack axis, parallel to the growth direction ([001]).

Figure 2

(a) Calculated and experimentally obtained exciton energies of state A and B as a function of the electric field for model QDMs as described in the text. (b) Energy splitting between A and B and (c) Relative intensities $I_{\mathbf{A}/\mathbf{B}}/(I_{\mathbf{A}}+I_{\mathbf{B}})$. The vertical line indicates the field coincidence of minimum $\Delta E$ and equal relative intensities, illustrating the clear anticrossing.

Figure 3

(a) Exciton energy as a function of dot separation ($d$) for identical dots with the structural parameters described in the text. The direct (black) and indirect (orange, gray) branches are nondegenerate, representing states with the hole in the upper (solid line) and lower (dashed line) QD. (b) Stark shift for excitonic states with the hole localized in the upper QD for $d=11\mathrm{n}\mathrm{m}$. As discussed in the text the ground state undergoes a transition from direct to indirect character at $F\sim 15\mathrm{k}\mathrm{V}/\mathrm{c}\mathrm{m}$ and shows an anticrossing with the initially indirect exciton. The dashed line is the calculated QCSE of a single dot, for comparison.

Figure 4

Calculated anticrossing energy as a function of the dot separation (squares). The full line shows a fit to the calculated points, describing the exponential nature of the dot-dot coupling strength. The horizontal shaded bar indicates the standard deviation of the experimental values. The vertical bar represents the dot separation derived from the experimental data (circles) and the calculated values of $\Delta E$.