Professor Robert Taylor

Abstract:

We present an investigation of free-carrier screening in coupled asymmetric GaN quantum discs with embedded AlGaN barriers using time-integrated and time-resolved micro-photoluminescence measurements, supported by three-dimensional multi-band k.p computational modeling. We observe that with increasing optical excitation the carrier lifetime decreases and emission energy blue-shifts. This originates from the screening of built-in piezo- and pyroelectric fields in the quantum discs by photo-generated free-carriers. Due to non-resonant tunneling of carriers from the smaller disc to the larger disc, free carrier screening is enhanced in the larger disc. The non-resonant tunneling was found to have a significant role in samples with a thin barrier, as the screening decreased with barrier thickness (i.e. decreased tunneling). Computational modeling was in good agreement with the experimental results.

Abstract:

We report studies of the modification of the band structure of single-walled carbon nanotubes through encapsulation of the inorganic material manganese ditelluride (Mn Te2). Using photoluminescence excitation mapping we show that this leads to a global reduction of their first and second band gap energies by a similar percentage (up to 3.8%). We interpret this as due to the additional screening which causes a change in the internal dielectric constant and a possible lowering of the carbon-carbon transfer integrals. The shifts increase with increasing tube diameter due to the increased quantity of filled materials. © 2006 The American Physical Society.

Abstract:

By using two-photon excitation spectroscopy we take advantage of the increased spatial resolution to perform time-integrated and time-resolved photoluminescence measurements on several discrete stacks of GaN quantum discs. The stack structure consisted of coupled asymmetric GaN quantum discs with embedded AlGaN barriers, which were grown at the tip of a GaN nanocolumn. We observed that with increasing optical excitation power the carrier lifetime decreased due to free-carrier screening, with an enhancement of the screening effect in the larger quantum disc due to carrier tunnelling from the smaller quantum disc. © 2006 IOP Publishing Ltd.

Abstract:

Carrier localization in InGaNGaN multiple-quantum wells (MQWs) with three different well thicknesses was investigated optically using time-integrated and time-resolved microphotoluminescence spectroscopy. An anomalous temperature dependence of the photoluminescence peak energy was observed, as a consequence of local potential fluctuations. The carrier localization was more prominent in the case of MQWs with wide well thickness. The results indicate that the degree of potential fluctuation increases with increasing well thickness. Emission from quantum-dot-like states only became apparent in MQWs with wide well thickness, which supports the assertion that carrier localization in InGaNGaN MQWs is due to the formation of quantum dots. © 2006 American Institute of Physics.

Abstract:

Er3N@C80 exhibits sharp optical emission lines in the near-infrared attributed to fluorescence from the Er3+ ion. Here we demonstrate that high magnetic fields cause this spectrum to split, corresponding to transitions from the lowest field-split Kramers doublet of the 4I13/2 manifold to the four lowest field-split levels of the 4I15/2 manifold. The internal structure of these fullerenes can be spatially aligned with a preferred orientation under high magnetic field; the effect of alignment is to reduce the broadening associated with the isotropic spatial averaging characteristic of powder or frozen-solution spectra. Using a tunable 1.5 μm laser, we directly observe non-cage-mediated optical interactions with the Er3 ion. This spectroscopic method provides the opportunity to map the energy level structure of the incarcerated ion and to coherently control its quantum state. These qualities suggest that rare-earth endohedral fullerenes have characteristics that could be employed as a readout pathway for fullerene-based quantum information processing. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA.