Direct observation of metastable hot trions in an individual quantum dot

Magnetophotoluminescence and excitation spectroscopy are used to probe the excited-state spectrum of negatively charged trions in an InGaAs quantum dot. A single dot optical charging device allows us to selectively prepare a specific few ($1e$, $2e$) electron states and stabilize hot trions against decay via electron tunneling from excited orbital states. The spin structure of the excited state results in the formation of metastable trions with strong optical activity that are directly observed in luminescence. Excitation spectroscopy is employed to map the excited singlet and triplet states of the two-electron wave function, and fine-structure splittings are measured for the lowest-lying and excited orbital states. Magneto-optical measurements allow us to compare the $g$ factors and diamagnetic response of different trion states.

Figure 1

Typical image plot of PL spectra obtained for various voltage biases applied in the growth direction.

Figure 2

(a) Evolution of the PL spectrum for fixed readout voltage $V_{\mathrm{read}}=0.86$ V ($F_{\mathrm{read}}=0$ kV/cm) and reducing (increasing) reset voltage (electric-field) pulse with a duration of 500 ns. (b) PL intensity of the individual excitons from (a) as a function of the reset voltage.

Figure 3

(a) Photoluminescence excitation spectra of the neutral exciton $X^0$, the singly charged exciton $X^{-1}$, and the two emission lines of $X_{\text{T}}^{-1}$. The emission at the higher (lower) energy of $X_{\text{T}}^{-1}$ is labeled as $X_{\text{TH}}^{-1}$ ($X_{\text{TL}}^{-1}$). The electric-field sequence applied on the device while obtaining the $X^0$ spectrum is shown in (b). The corresponding electric-field sequence for $X^{-1}$, $X_{\text{TH}}^{-1}$, and $X_{\text{TL}}^{-1}$ is shown in (c). (d) PL spectra of $X^{-1}$, $X_{\text{TH}}^{-1}$, and $X^0$ under resonant excitation in higher orbital states with the electric-field sequence (c) applied.

Figure 4

(a) Photoluminescence excitation spectra of the energetically lowest resonances of $X_{\text{TH}}^{-1}$ and $X_{\text{TL}}^{-1}$ and (b) their photoluminescence spectrum under WL excitation. (c) Energy-level diagram of two electrons and one hole in a QD. The optical transitions are indicated by the wavy arrows, and the nonradiative transitions are indicated with straight broken lines.

Figure 5

(a) Polarization-resolved PL spectra of $X_{\text{T}}^{-2}$, $X^{-1}$, the two lines of $X_{\text{T}}^{-1}$, and $X^0$ as a function of magnetic field applied in the growth direction. (b) Magnetic-field dependence of the energy gap between the $X_{\text{T}}^{-1}$ and $X^{-1}$ transitions.