Ph.D Thesis

Ph.D StudentMarderfeld Ilia
SubjectSubpicosecond Dynamics of Photoexcitations in Semiconductor
Quantum Structures
DepartmentDepartment of Physics
Supervisors PROFESSOR EMERITUS David Gershoni
Full Thesis textFull thesis text - English Version


In this work we used short pulse and continuous wave optical spectroscopy to investigate semiconductor quantum structures.  Two types of quantum structures were investigated. The first type was periodic structures of quantum wells (QW) and quantum well superlattices (SL). These structures were chiefly investigated by means of time resolved photoinduced intersubband absorption excitation spectroscopy. The second type was single self-assembled semiconductor quantum dots embedded within a P-I-N junction. These structures were studied by means of confocal micro photoluminescence set up with diffraction limited spatial resolution. For these goals, we built two experimental setups. The first is based on a 500 femtosecond pulsed tunable interband laser pulse, followed by 800 fs infrared probe pulse, tuned to the intersubband resonances of either the quantum wells or the superlattices. The second is a high temporal and spatial resolutions, polarization sensitive dual beam setup, for pump and probe spectroscopy of single quantum dots. In the first system the interband, pump laser pulse, photogenerates carriers in the quantum structure's quantized levels, while the infrared probe pulse induces intersubband optical transitions of the photogenerated electrons. In QWs, where the dispersion along the growth axis is zero, we show that this method provides means for measuring the intrinsic radiative lifetime of the zero momentum excitons. In SLs, this method provides a sensitive tool for probing the temporal evolution of the electronic momentum component along the SL symmetry axis. In the second system single quantum dots were studied. Full polarization analysis of each spectral line in the photoluminescence spectrum was performed. This proved to form an efficient tool for identifying optical transitions between states involving multiple confined charge carriers. In particular, we used this tool to demonstrate circular negative polarization memory. This observation is explained as due to resonant excitation to states containing light holes. Surprisingly, the negative circular polarization memory was observed only for the doubly and triply negatively charged exciton lines (X-2 and X-3 respectively). While, at the same time, the negative charged exciton line (X-1) does not show any appreciable circular positive memory, as it shows for other excitations at energies lower than that of the wetting layer band-gap.  Using our pulsed spectroscopy we measured excitonic lifetimes of various optical transitions and various charge states. We also demonstrated Rabi Oscillations between two levels of single QDs and used our observation to estimate the quantum dots' dipole moment.