heterostructure containing three sets of DQWs grown on a $n^+$-doped GaAs (001) wafer by molecular beam epitaxy. Each DQW consists of two GaAs QWs with thickness of 14 and 17 nm (QW$_{1}$ and QW$_{2}$, respectively - cf. lower right inset in Fig. 1 (a)) separated by a 4-nm thick Al$_{0.33}$Ga$_{0.67}$As tunnelling barrier. The DQWs were positioned at a depth of 700 nm below the sample surface within an undoped 2.5-$\mu$m-thick GaAs/(Al,Ga)As superlattice (SL). The electric field ${F_\mathrm{Z}}$ is induced by a reverse bias ${V_\mathrm{BIAS}}$ applied across the Schottky diode formed between a 10-nm-thick semitransparent titanium electrode (STE) on the sample surface and the n$^+$-doped substrate, which serves as the common ground \[cf. Fig. \[Fig1\](a)\]. The lower left inset shows the bending of the conduction (CB) and valence (VB) band edges along the growth (z) direction induced by ${F_\mathrm{Z}}$. The excitons are locally excited underneath the STE area by a 780 nm pulsed diode laser (average power density of approx. 12 J/cm$^2$). The use of SLs reduces the leakage photo-currents to values below 1 nA (for bias between -2 and 2 V and laser excitation power in the range up to a few tens of $\mu$W), thus ensuring that IXs are 