A tunable microcavity for semiconductor quantum dots

Semiconductor optical microcavities that combine high Q-factors with a small mode volume play a vital role in modifying the interaction between light and matter. Several interesting phenomena arise when an emitter is introduced into such a cavity. These include enhancement or suppression of the spontaneous emission rate (weak coupling) and normal mode splitting (strong coupling). In order to fully exploit the high Q and low mode volume of current microcavities, it is crucial for the emitter to be resonant with the cavity mode. Furthermore the emitter must be located at the exact antinode of the cavity electric eld. Wavelength tuning in semiconductor monolithic microcavities is challenging and is traditionally achieved by altering the temperature or using the Stark shift to alter the emitter's wavelength. Spatially matching the emitter to the electric eld antinode in monolithic cavities is even more challenging. The work in this thesis addresses these challenges. The realisation of a miniaturised, fully tunable Fabry-P erot type microcavity for semiconductor quantum dot experiments is presented. The cavity has a high Q-factor and low mode volume. The cavity wavelength is tuned by altering the air gap between the mirrors to vary the cavity length. This allows a much broader tuning range than is possible using monolithic cavities. In this work the cavity modes are characterised using a xed wavelength laser technique and varying the cavity length. A high nesse is obtained by using a miniaturised concave mirror which laterally con nes the optical mode. Unprecedented in situ control over a single InAs/GaAs quantum dot within the cavity mode is demonstrated at 4 K. The Purcell e ect is demonstrated for a single quantum dot, spatially positioned at the exact antinode of the electric eld. The cavity beam waist at the dot layer is experimentally measured and shown to be in good agreement with the theoretical value. The cavity mode volume is calculated from the measured beam waist. The e ect of weakly coupling a single dot to the cavity transverse modes is also investigated. Photoluminesence data from a high dot density sample within the cavity is presented. An anti-crossing behaviour between an ensemble of dots and the cavity mode is demonstrated. Finally, a dot dependent Fano e ect in the absorption lineshapes in a chargetunable sample is presented. A subtle interaction with an electronic continuum is revealed. The Fano e ect is shown to become more pronounced with increasing excitation power

Patterns of larval distribution and settlement in a river-dominated estuary

Several species of crabs utilize the estuary as adults, with larval development occurring in the plankton either within the estuary or offshore. Habitat selectivity necessitates postlarval movement within the estuary in order for suitable settlement sites to be reached. Postlarval dispersal and settlement are critical factors in determining adult abundances. Data on such dispersal and settlement are particularly sparse for the more poorly studied small, riverdominated systems prevalent along the southeastern coast of the United States. Night plankton tows were taken during flood tides to examine issues of postlarval distribution and transport to upper portions of the Cape Fear River Estuary (NC, USA) for several brachyuran species commonly found in the estuary. The blue crab Callinectes sapidus was used as a model organism to compare planktonic availability, settlement (utilizing passive larval collectors) and juvenile abundances over a broad estuarine gradient. Sampling occurred in five-day periods around both the new and full moons from July to mid-October 2001 to target peak recruitment. The four groups of megalopae present in sufficient numbers for statistical analysis: 1) Uca spp., 2) xanthids, 3) Sesarma cinereum, and 4) Callinectes spp. all demonstrated a high degree of spatial and within-period variability. When the abundance of Callinectes megalopae in plankton tows was low, settlement occurred primarily at sites closer to the mouth of the estuary, however, in mid-September when abundances in the plankton rose, the settlement pattern shifted to the upper portion of the estuary. Patterns of early juvenile abundances appeared to reflect late period settlement of Callinectes megalopae in the upper regions of the estuary. Callinectes megalopae appear to follow a mixed model settlement pattern that varies seasonally and this may have consequences for megalopal survival