In my work I report on experimental
studies of optical excitations of semiconductor quantum dots using two
resonant, temporally delayed, picosecond laser pulses. The experiments we
performed included the preparation of a coherent superposition of a quantum-dot
confined exciton eigenstates, and readout of the exciton state after an
adjustable time delay. The preparation and readout were done using polarized
laser-pulses tuned to the exciton and to the biexciton resonances,
respectively. Since the photo-generated exciton state is a superposition of the
two non-degenerate exciton eigenstates, their relative phase evolves with time.
The time difference between the preparation and the readout pulses is used to
follow this evolution. The absorption of the second, readout pulse generates
two-excitons (biexciton) state. The absorption of the second pulse depends on
the nature of the biexciton resonance and the time difference between the two
pulses, which determine the relative spin polarization between the first
exciton to the second one. The absorption into the biexciton resonance results
eventually in photon emission from one of the quantum dot spectral lines. By
monitoring this emission, one can measure the magnitude of the time dependent
absorption into the biexciton resonance. We measured, this way, oscillations in
the biexciton spectral lines as a function of the delay time between the pulses
and thus determined the temporal evolution of the exciton's spin. At the same
time we measured in addition, oscillations in the exciton spectral lines as
well. These oscillations resulted from the effect of the probe pulse on the
exciton part which was not converted into a biexciton. In my work I present and
discuss the construction of the experimental setup and preliminary measurements
which demonstrated these effects, for the first time.
