Confinement and Interaction of Single Indirect Excitons in a Voltage-Controlled Trap Formed Inside Double InGaAs Quantum Wells

Voltage-tunable quantum traps confining individual spatially indirect and long-living excitons are realized by providing a coupled double quantum well with nanoscale gates. This enables us to study the transition from confined multiexcitons down to a single, electrostatically trapped indirect exciton. In the few exciton regime, we observe discrete emission lines identified as resulting from a single dipolar exciton, a biexciton, and a triexciton, respectively. Their energetic splitting is well described by Wigner-like molecular structures reflecting the interplay of dipolar interexcitonic repulsion and spatial quantization.

Figure 1

(a) Scheme of the DQW band gap within a field-effect device. $E_{\mathrm{C}}$ and $E_{\mathrm{V}}$ denote the conduction and valence band edge, respectively. The electron and hole wave function of the ground states $h{h}_1^{\mathrm{QW}2}$ and $e_1^{\mathrm{QW}1}$ are indicated. The energetic tilt of the DQW is tuned by the gate voltages $V_{\mathrm{T},\mathrm{G}}$ with respect to the flatband voltage $V_{\mathrm{FB}}\sim 0.75$ V. (b) In-plane variation of the $h{h}_1^{\mathrm{QW}2}$ and $e_1^{\mathrm{QW}1}$ ground state and the resulting confinement potential $V_{\mathrm{IX}}$. (c) Displays a scanning electron micrograph of the gate pattern. (d) Logarithmically displays the PL intensity vs energy and position $y$ at $x=0$ [$V_{\mathrm{G}}=0.75\mathrm{V}$, $V_{\mathrm{T}}=0.25\mathrm{V}$, $T_{\mathrm{Lattice}}=242\mathrm{mK}$, $P_{\mathrm{Laser}}=2.8n\mathrm{W}$, $E_{\mathrm{Laser}}=1.494\mathrm{eV}$].

Figure 2

(a) PL spectra for different occupations of the trap nonlinearly changed by laser power between 55 pW and 10 nW. In comparison to the emission from individual self-assembled InGaAs quantum dots, PL counts in the single exciton limit are typically found to be 1000-fold lower, reflecting the correspondingly increased radiative lifetime of IX. The corresponding integrated PL intensity and PL blueshift are shown in (b) and (c) [$V_{\mathrm{G}}=0.65\mathrm{V}$, $V_{\mathrm{T}}=0.30\mathrm{V}$, $T_{\mathrm{Lattice}}=245\mathrm{mK}$, $E_{\mathrm{Laser}}=1.494\mathrm{eV}$].

Figure 3

(a) Sketches an excitonic Wigner molecule of three equidistantly spaced dipolar excitons confined in parabolic trap potential as displayed in (b). (c) PL energy and intensity (logarithmically plotted) vs trap gate voltage. Below 0.35 V we observe the splitting displayed in (d) [$V_{\mathrm{G}}=0.55\mathrm{V}$, $T_{\mathrm{Lattice}}=244\mathrm{mK}$, $P_{\mathrm{Laser}}=2.8\mathrm{nW}$, $E_{\mathrm{Laser}}=1.494\mathrm{eV}$]. (e) Characteristic spectra for different magnetic fields in equidistant steps from 0 T (bottom) to 2.5 T (top). From the quadratic fit (solid line) to the PL energies (symbols) in (f) we extract the effective Bohr radii $r_e$ ($r_e^{1\mathrm{IX}}=18.3\mathrm{nm}$, $r_e^{2\mathrm{IX}}=18.4\mathrm{nm}$, and $r_e^{3\mathrm{IX}}=21.7\mathrm{nm}$) [$V_{\mathrm{G}}=0.75\mathrm{V}$, $V_{\mathrm{T}}=0.25\mathrm{V}$, $T_{\mathrm{Lattice}}=242\mathrm{mK}$, $P_{\mathrm{Laser}}=2.8\mathrm{nW}$, $E_{\mathrm{Laser}}=1.494\mathrm{eV}$].