|Ph.D Student||Regelman Vadim|
|Subject||Near Field Optical Spectroscopy of Semiconductor|
|Department||Department of Physics||Supervisor||Professor David Gershoni|
This thesis reports on the optical studies of lower dimensionality semiconductor quantum structures, namely, quantum wires and quantum dots.
The studied quantum dots were prepared by self assembly of nanometer size In(Ga)As islands on GaAs substrate. We resolved the photoluminescence emission from single dots spatially, spectroscopically, and temporally, in order to obtain information about the dynamics of photoexcited carriers within those dots . In addititon, we studied the statistical properties of the emitted light from these dots.
Spatial isolation of a single dot was made possible using low temperature diffraction limited scanning confocal microscope and/or near field optical microscope. We use optical excitation in order to populate few electron-hole pairs within the isolated quantum dot. The number of pairs was controlled by the optical excitation power. The excited electron-hole pairs quickly diffuse into the quantum dot and then radiatively recombine, giving rise to a rich photoluminescence spectrum. We measured the evolution of this spectrum with the increase in both steady state and pulse excitation powers. The temporal evolution of the emission spectrum after pulsed excitation was also measured at various ambient temperatures. A multi-exciton model was used for calculating the temporal and excitation intensity dependencies of the measured spectra. The quantitative agreement between the measured and calculated spectra provide a determination of the radiative lifetime of a single quantum dot exciton. In addition we showed that this lifetime is temperature independent . By imbedding the quantum dots within a system of coupled quantum wells we were able to separate between the optically excited electron-hole pair, thereby to effectively charge the quantum dots. Thus, we were able to observe and identify photoluminescence from positive and negative charge states from the same single semiconductor quantum dots .
Since radiative recombination of single electron-hole pair within a single quantum dot results in the emission of a single photon, a quantum dot can be used as a source of single photons. We investigate the properties of these single photons by measuring the second order temporal coherence function of the photons emitted at various wavelengths. This function was measured as a function of the excitation power. We show both experimentally and theoretically, for the first time, that quantum dot is a source of correlated non-classical multicolor photons with tunable intensity correlation properties. We found that the emitted photon statistics can be varied by the excitation rate from a sub-Poissonian one, where the photons are temporally antibunched, to super-Poissonian, where they are temporally bunched .
In the last part of this work, we present the design, the study and the optical characterization of strain induced cleaved edge overgrown quantum wires. We achieved strong quantum confinement by utilizing optimizations of the structure geometry and its composition. Low temperature confocal microscope was applied to optically study the photoluminescence of quantum wires which were fabricated according to our optimization scheme. The confinement energies were found to be lower than the predicted ones, however, they were still the highest confinement energies ever demonstrated in this type of one dimensional quantum structures .