Optical spectroscopy on single quantum dots

Quantum dots, often referred to as artificial atoms, open the field of highly resolved spectroscopy to semiconductor physics. In the current contribution we report about optical experiments on single self-assembled quantum dots. In power dependent low temperature magneto-photoluminescence experiments we have analysed the emission spectra of single dots for increasing exciton occupation numbers. Decays from different configurations (up to 4 excitons) lead to a renormalization of the emission lines in the region of the s- and p-shell of the dot. Photoluminescence excitation spectroscopy further allows us to explore the absorption properties of a single quantum dot. Both interband absorption and comparably strong phonon assisted absorption via InGaAs and GaAs LO phonons are observed, as well as sequential phonon assisted biexciton generation followed by sequential biexciton decay. Working towards applications of single quantum dots, local charge injection/extraction into/from dots is performed on special p-i-n structures via a STM-tip. STM-induced luminescence from a single dot results in single line emission from the quantum dot ground state under the condition of low injection currents. Reverse operation of such a single quantum dot LED allows in addition for spectrally resolved photocurrent experiments. Characteristic sharp peaks in the STM tip current versus excitation energy are attributed to resonant quantum dot interband absorption processes.