Quantum dots or rings are artificial nanometre-sized clusters that confine electrons in all three directions. They can be fabricated in a semiconductor system by embedding an island of low-bandgap material in a sea of material with a higher bandgap. Quantum dots are often referred to as artificial atoms because, when filled sequentially with electrons, the charging energies are pronounced for particular electron numbers; this is analogous to Hund's rules in atomic physics. But semiconductors also have a valence band with strong optical transitions to the conduction band. These transitions are the basis for the application of quantum dots as laser emitters, storage devices and fluorescence markers. Here we report how the optical emission (photoluminescence) of a single quantum ring changes as electrons are added one-by-one. We find that the emission energy changes abruptly whenever an electron is added to the artificial atom, and that the sizes of the jumps reveal a shell structure.

Excitonic interband optical transitions within single InAs self-assembled quantum dots have been directly observed in a transmission experiment at 4.2 K. Using Stark shift, the excitonic energy levels of a single quantum dot are tuned into resonance with a narrow-band laser line. The Stark shift is also modulated at low frequencies. Relative changes in transmission can be detected this way down to one part per million. The oscillator strength as well the homogeneous linewidth of the transition is obtained.

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