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978-3-8439-1421-5, Reihe Physik
Matthias Michael Heldmaier
Optical and electronic properties of single laterally coupled (In,Ga)As/GaAs quantum dot molecules
252 Seiten, Dissertation Universität Stuttgart (2013), Softcover, A5
In this thesis optical investigations on laterally coupled self-assembled (In,Ga)As/GaAs quantum dot molecules (QDMs) are presented under various excitation conditions. Sample growth is performed using a unique combination of metal-organic vapor-phase epitaxy, molecular beam epitaxy and in-situ selective etching. This combined growth approach leads to the formation of lateral quantum dot bi-molecules with a deterministic alignment of the quantum dots along the [1-10] crystal direction. For enhancing the luminescence extraction efficiency, the QDMs are placed in the center of a low-Q cavity. Electrical fields can be applied to the QDMs by metal electrodes lithographically processed on the sample surface. Investigations are performed at low temperatures around 4 Kelvin in a Helium-flow cryostat.
Study of the sample under non-resonant excitation in comparison with preceding works enables for an unambiguous identification of the molecular luminescence by analysis of the polarization, power and voltage dependence of the emitted light. First and unambiguous demonstration of quantum mechanical coupling in the QDM system was provided by photon-cross-correlation in works preceding the actual investigations in this thesis.
The thorough initial characterization under non-resonant excitation is the basis for the subsequent investigations of excited states by photoluminescence excitation spectroscopy. A detailed analysis of the excited state structure, again in combination with the application of electric fields, is performed leading to an understanding of the excited states mostly being localized in an Indium-rich basin below the QDM which originates from the specific growth process utilizing Arsenic bromide etching.
A new excitation scheme is then developed in order to enable resonant excitation of QDMs in a reflective confocal microscopy setup, exploiting polarization properties of the photons as well as geometrical stray light suppression, namely darkfield microscopy. Such investigations for the first time enable a direct observation of incoherent phonon-assisted tunneling in this lateral QDM system. Simultaneously, the spectroscopic approach paves the way to gain coherent control over the QDMs and the implementation of a quantum gate.