Excitation of three-dimensional quantum dots

Arrays of field-effect-confined quantum dots with diameters down to 100 nm have been prepared from ${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As-GaAs heterostructures. In far-infrared spectroscopy, transitions are induced between the 2–3-meV separated quantum levels of the lateral confinement. In tilted magnetic fields we observe a resonant coupling to states arising from the confinement in the growth direction. The level spacing in this z direction is found to be only a few meV in our samples and thus comparable to the lateral confinement. We observed in tilted magnetic fields that the excitation spectrum is determined by the total magnetic field B. Thus the quantum dots are essentially three-dimensional objects.