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We report on the observation of photoluminescence from positive, neutral andnegative charge states of single semiconductor quantum dots. For this purposewe designed a structure enabling optical injection of a controlled unequalnumber of negative electrons and positive holes into an isolated InGaAs quantumdot embedded in a GaAs matrix. Thereby, we optically produced the charge states-3, -2, -1, 0, +1 and +2. The injected carriers form confined collective'artificial atoms and molecules' states in the quantum dot. We resolvespectrally and temporally the photoluminescence from an optically excitedquantum dot and use it to identify collective states, which contain charge ofone type, coupled to few charges of the other type. These states can be viewedas the artificial analog of charged atoms such as H$^{-}$, H$^{-2}$, H$^{-3}$,and charged molecules such as H$_{2}^{+}$ and H$_{3}^{+2}$. Unlike higherdimensionality systems, where negative or positive charging always results inreduction of the emission energy due to electron-hole pair recombination, inour dots, negative charging reduces the emission energy, relative to thecharge-neutral case, while positive charging increases it. Pseudopotentialmodel calculations reveal that the enhanced spatial localization of thehole-wavefunction, relative to that of the electron in these dots, is thereason for this effect.Comment: 5 figure

Optical spectroscopy of single quantum dots at tunable positive, neutral, and negative charge states