Search

In a single self-assembled InGaAs quantum dot, the one exciton ground state transition defines a two-level system, which appears as an extremely narrow resonance of only a few μeV width. The resonant interaction of this two-level system with cw laser fields can be studied in detail by photocurrent spectroscopy, revealing the fine structure of the excitonic ground state as well as the effects of nonlinear absorption and power broadening. For the case of pulsed laser fields and in the absence of decoherence, the two-level system represents a qubit. Excitations with ps laser pulses result in qubit rotations, which appear as Rabi oscillations in photocurrent experiments. Double pulse experiments further allow us to infer the decoherence time and to perform coherent control on a two-level system. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Deliberate control of the electric field in quantum well structures can be achieved by designing n+-i-n+ heterostructures. The active part in such structures is the i-region. We have designed different layer sequence for the i-region in order to induce specific physical properties in our structures. These properties are the optically induced charge accumulation, the intrinsic optoelectronic bistability, and the electric-field-induced Γ-X transition. [ABSTRACT FROM AUTHOR]