Excitation spectra of single GaAs/Al x Ga 1Ϫx As quantum dots ͑QD's͒ 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

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... tial 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.