We describe quantum information schemes involving photon polarization and the spin of a single electron trapped
in a self-assembled quantum dot. Such schemes are based on spin-selective reflection in the weak-coupling regime
of cavity quantum electrodynamics. We discuss their practical implementation in oxide-apertured micropillar
cavities. We introduce a technique, based on the creation of small surface defects by means of a focused intense
laser beam, to permanently tune the optical properties of the microcavity without damaging the cavity quality.
This technique allows low-temperature polarization-selective tuning of the frequencies of the cavity modes and
the quantum dot optical transitions.
We demonstrate direct control over the level of lateral quantum coupling between two self-assembled InGaAs/GaAs
quantum dots. This coupled system, which we also refer to as a lateral quantum dot molecule, was produced using a
unique technique which combines molecular beam epitaxy and in-situ atomic layer etching. Atomic force microscopy
measurements show that each molecule consists of two structurally distinct dots, which are aligned along the [1-10]
direction. Each molecule exhibits a characteristic photoluminescence spectrum primarily consisting of two neutral
excitonic and two biexcitonic transitions. The various transitions have been investigated using micro-photoluminescence
measurements as a function of excitation power density, time, and applied electric field. Photon statistics experiments
between the excitonic emission lines display strong antibunching in the second-order cross-correlation function which
confirms that the two dots are quantum coupled. Cascaded emission between corresponding biexcitonic and excitonic
emission has also been observed. Using a parallel electric field we can control the quantum coupling between the dots.
This control manifests itself as an ability to reversibly switch the relative intensities of the two neutral excitonic
transitions. Furthermore, detailed studies of the emission energies of the two neutral excitonic transitions as a function of
parallel lateral electric field show a clear anomalous Stark shift which further demonstrates the presence of quantum
coupling between the dots. In addition, this shift allows for a reasonable estimate of the coupling energy. Finally, a
simple one-dimensional model, which assumes that the coupling is due to electron tunneling, is used to qualitatively
describe the observed effects.
Process monitoring is a vital part of industrial laser applications that enables intelligent control of processes by observing acoustic, optical, thermal and other emissions. By monitoring these emission during laser processing, it is possible to ascertain characteristics that help diagnose features of the laser processed material and hence to optimize the technique. An experimental set up of observing plasmas during laser spot welding is described here. A pulsed Nd:YAG laser was used to spot-weld a variety of materials of different thickness, the plasmas generated during welding were monitored by a number of techniques, and the data obtained was used to characterize the welds. In the study photodiodes were set at different angles and observed the intensity and generation of the plasmas during the laser spot-welding process thereby giving a weld 'signature.' A portable spectrometer was used off-axis to obtain spectra of the emissions from the plasmas. Post process analysis was performed on the materials by mechanical polishing and chemical etching and observations of weld penetration depth and weld quality were correlated with the data collected on the plasmas. Different cover gases were also used during laser welding and the results of the effects of the various gases on the plasma are shown. The results indicate the relationship between laser weld generated plasma characteristics and weld features such as penetration depth. A direct correlation between the intensities of the photodiode and portable spectrometer signals was observed with weld penetration depth.