Haller, Anika: Laser control and electron dynamics of ultrafast energy transfer processes in quantum dots. , 2019
Freie Universität Berlin
The inter-Coulombic decay (ICD) is an ultrafast energy transfer process between an electronically excited species that relaxes and a neighboring one that is simultaneously ionized. This Coulomb interaction mediated mechanism has been recently discovered likewise in atomic, molecular, bio- logical, and nanostructured systems, whereas the latter ones require more sophisticated theoretical interpretations as well as an experimental proof. In this thesis, ICD is theoretically studied focusing on the role of the efficient decay of a two-electron resonance excited state in a pair of negatively charged quantum dots (QD). The outcome is an electron in the continuum and the second one in the ground level of the formerly excited QD. To this end, electron dynamics calculations are performed with the antisymmetrized multiconfiguration time-dependent Hartree method. The initiation and control of ICD are carried out by means of a resonant infrared laser pulse of picosecond length. The process is optimized by studying competing excitation processes under a variation of the focus and the intensity of the exciting laser. Those competing processes are direct ionizations and multi-photon excitations, which prevent the QDs from undergoing ICD and shall therefore be avoided. Moreover, the impact of the laser polarization on the electron emission direction is studied. Two types of QDs are examined. In the first type the QDs are embedded inside a nanowire, such that emitted electrons can only move along a single continuum direction. For reasons of experimental feasibility and in view of potential device applications, the second type of QD conformation is considered, which is generally more widely investigated. In this case, the QDs are arranged on a two-dimensional wetting layer and the electrons can escape from the dots into the continuous surface. Consequently, the highest ICD efficiency is achieved for excitations by resonant π-pulses. Moreover, the laser should be focused on the specific dot that shall be brought to an excited level as part of the resonance state. Additional efficiency gain is obtained when allowing for a two-dimensional continuum as in laterally arranged dots. For these QDs the laser polarization has an impact on the decaying state, which is a mixture of resonance states of both directions. The QD-ICD in the presented systems is an efficient and promising process for future QD infrared photodetectors. This is due to its sensitivity to weak and low-frequency light. Besides, the energy conversion into an electric current is enhanced through the intermediate ICD process, in comparison to already existing devices that function via direct QD ionization.