Single-electron charging of quantum-dot atoms

Arrays of field-effect-confined quantum dots with diameters smaller than 100 nm have been prepared starting from ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As-GaAs heterostructures. In far-infrared spectroscopy, we induce transitions between the 2-meV-separated quantum levels. We observe discrete steps in the gate-voltage dependence of the integrated absorption strength indicating directly the incremental occupation of each dot with N=1, 2, 3, and 4 electrons. From the gate-voltage dependence, we can estimate a Coulomb charging energy of about 15 meV. On a very fine scale, we also observe a spectral fine structure for the excitation of the quantum-dot atoms.