Excited-state spectroscopy of single lateral self-assembled InGaAs quantum dot molecules

We perform excited-state spectroscopy of single self-assembled lateral InGaAs quantum dot molecules embedded in a planar microcavity structure, which are grown using a combination of metal-organic vapor phase and solid-source molecular beam epitaxy. The individual quantum dot molecules (QDMs) consist of two single dots that are coupled along the $\left[1\overline{1}0\right]$ crystal direction via electron tunneling. By microphotoluminescence excitation spectroscopy, we investigate the presence and structure of the excited states confined in the system using a wide-band tunable Ti:sapphire laser source. We show that the excited states are very similar even for different molecules. We attribute this to the presence of a potential basin below the quantum dots that levels the differences among various QDMs.

Figure 1

(a) AFM image of a low-density lateral QDM sample. The labels L and R highlight the QDM orientation for the later discussion. (b) Electric field-dependent PL of a single QDM under nonresonant WL excitation at 1433 meV at around 35 W cm${}^{-2}$. The applied voltages range between 0 and $-$3 V, displaying the typical seamless, reversible switching behavior of the QDMs under lateral electric fields along the coupling axis. (c) Cross sections displaying the PL intensities as a function of the emission energy for the three different voltages marked by the dashed lines in (b).

Figure 2

$\mu$-PLE measurements for two different molecules denominated as QDM 1 (a) and QDM 2 (b). Very similar groups of higher shell resonances can be found for both molecules as highlighted by the gray and green boxes. The blue spectra on the left side show the PL at the same voltages chosen for the $\mu$-PLE measurements. The light gray boxes indicate the exciton selected for detection in the respective PLE measurements. The red lines are fits of the PLE peaks with Lorentzian line shapes. Possible optical phonon resonances are marked by PH. The green box indicates the resonances for which detailed voltage- and power-dependent PL measurements were performed under quasiresonant excitation.

Figure 3

(a) Schematic of the morphology of a QDM consisting of two single QDs above the basin. The Indium content is indicated by the color gradient from the bottom, where the Indium content is at its minimum, to the maximum at the top. (b) Schematic band-structure of the QDM along the [1-10] crystal axis and sketch of ground and excited states. The ground states are strongly localized in the respective QDs of a QDM; the excited states are localized in the basin.

Figure 4

Voltage-dependent PLE measurements on QDM 1 for (a) X1 and (b) X2. The excitation power has been chosen around 30 W cm${}^{-2}$ and can vary over the scan of the excitation laser.

Figure 5

(a) PL from QDM 1 as a function of the emission energy and the applied lateral electric field (voltage steps 0.05 V) under quasiresonant excitation at 1358.5 meV and 17 W cm${}^{-2}$. (b) Excitation power-dependent PL spectrum of QDM 1 at zero-field using the same quasiresonant excitation (1358.5 meV) as in (a) and cross sections of the power-dependent PL spectrum at the positions of the dashed lines at (c) 7 and (d) 26 W cm${}^{-2}$.