Far-infrared spectroscopy of single quantum dots in high magnetic fields

Excitation spectra of single $\mathrm{GaAs}/{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ quantum dots (QD’s) are studied by using a technique of far-infrared single-photon detection. The spectra consist of a single resonance line, the resonance frequency of which is by several percent larger than the cyclotron-resonance frequency but is substantially smaller than the magnetoplasma frequencies of mesa-etched QD’s earlier found through the standard far-infrared transmission spectroscopy. The resonance frequency is found to be in substantial agreement with the characteristic frequency of the parabolic bare confinement potential of the QD’s, and is ascribed to the excitation of the upper branch of the Kohn-mode collective plasma oscillations.