Thesis defense of Janina Schindler

Excitons are fundamental electronic excitations in semiconductors. They may couple to their crystal environment, namely to phonons which are elementary vibrational excitations of the crystal lattice. The coupling between excitons and phonons is essential, since it ultimately determines the radiative features of semiconducting materials and their relevance for diverse applications in optoelectronics, magneto-optics, spin- and, e.g., valleytronics. Understanding and exploiting the exciton-phonon interactions are therefore crucial for gaining insight into the physics of low-dimensional semiconductors. The focus of this thesis lies on investigating exciton-phonon interactions in self-assembled quantum dots and uncapped and hBN-encapsulated transition metal dichalcogenide monolayers by photoluminescence and inelastic laser-light scattering spectroscopy. The first part of the thesis deals with a novel Fano-type quantum interference in InGaAs/GaAs quantum dots between the bright and dark exciton states and a continuum composed of two orthogonally linear-polarized acoustic phonons. It is sensitive to external factors such as magnetic field strength and direction and optical pumping intensity. The Fano interaction, observed between the excitonic spin transition and the acoustic phonon continuum, provides a valuable method for probing weak couplings in two-level quantum systems and offers insights into previously hidden optically inactive states in semiconductor nanostructures. In the second part, different kinds of charge carrier-phonon interactions in van der Waals heterostructures are studied. In a WSe2 monolayer, the interlayer electron-phonon interaction leads to a significant increase in the excitonic emission intensity which is attributed to a double resonance phenomenon. Additionally, phonon-polariton anticrossings at the neutral and negatively charged exciton resonances are revealed, as well as an upconversion of a dark intervalley exciton into a bright intravalley exciton. The energy gain associated to this upconversion is described by a cooling of the resident electrons or by an exciton scattering with ⇤- or K-valley phonons. Moreover, through tuning the electron doping levels via the hBN thickness, the fine structure of excitonic complexes in MoS2 heterostructures is explored, and remarkably enhanced g-factors are obtained in the ternary alloy MoWSe2. The control and manipulation of excitonic properties and interactions, such as the excitonic g-factor and their intricate couplings to phonons, provide opportunities for further advancements in spintronics and quantum information processing. The results also highlight that the interactions between excitons and phonons must be studied thoroughly in order to exploit the full potential of these semiconductor materials and to allow for tailoring their functional and structural properties.

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