Excited state quantum couplings and optical switching of an artificial molecule

We optically probe the spectrum of ground and excited state transitions of an individual, electrically tunable self-assembled quantum dot molecule. Photocurrent absorption measurements show that the spatially direct neutral exciton transitions in the upper and lower dots are energetically separated by only $\sim$2 meV. Excited state transitions $\sim$8–16 meV to higher energy exhibit pronounced anticrossings as the electric field is tuned due to the formation of hybridized electron states. We show that the observed excited state transitions occur between these hybridized electronic states and different hole states in the upper dot. By simultaneously pumping two different excited states with independent laser fields we demonstrate a strong ($88\%$ on-off contrast) laser-induced switching of the optical response. The results represent an electrically tunable, discrete coupled quantum system with a conditional optical response.

Figure 1

Combined result of an electric-field-dependent PL and PC measurement: The PL of the QDM as a function of the applied electric field is shown as a contour plot from 0 to 130 cps from white to black on a logarithmic scale. The dashed red lines indicate the uncoupled neutral exciton in the upper dot and the indirect exciton as depicted schematically in the inset. The PC resonances are shown as red squares and blue triangles for the neutral exciton transitions in the upper and lower dots, respectively.

Figure 2

(a) Typical PLE scan detecting on $X_{\mathrm{B}}$. The inset compares PLE recorded at $F=23.2$ kV/cm detecting either $X_{\mathrm{B}}$ (black) or $X_{\mathrm{AB}}$ (red), respectively. (b) Spectrum of QDM transitions determined by PL (black closed symbols) and PLE (black, red open symbols). States with bonding (antibonding) character are plotted as triangles (circles). (c) Energy separation ($\Delta E$) between corresponding anticrossing states, fits using Eq. (1).

Figure 3

(a) PL intensity of $X_{\mathrm{ud}}$ for the excitation of a resonance with direct character (pump, red), indirect character (block, blue), and both lasers (black). PL from the excitation of the direct resonance is quenched due to the presence of the block laser. (b) Intensity of $X_{\mathrm{ud}}$ as a function of the block laser energy. Whenever the block laser hits an excited state with indirect character, the PL from the upper dot is decreased.

Figure 4

PL of the upper and lower quantum dot as a function of the blocking laser power. Resonant excitation of an exciton in the upper dot is blocked by the presence of an exciton in the lower dot due to the Coulomb interaction. (b) Intensity of the PL of the upper and lower dot and (c) suppression ratio $\rho$ as a function of the blocking laser power.