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Abstract:

We report a detailed study of the magnetophotoluminescence of single-wall carbon nanotubes at various temperatures in fields up to 58 T. We give direct experimental evidence of the diameter dependence of the Aharanov-Bohm phase-induced band gap shifts. Large increases in intensity are produced by the magnetic field at low temperatures which are also significantly chiral index [(n,m)] dependent. These increases are attributed to the magnetic field induced mixing of the wave functions of the exciton states. A study of the emission from nanotubes aligned perpendicular to the applied magnetic field shows even larger field-induced photoluminescence intensity enhancements and unexpectedly large redshifts in band gap energies, not predicted theoretically. © 2007 The American Physical Society.

Abstract:

The great success that GaN-based structures have enjoyed in implementing efficient optoelectronic devices has fostered a rapidly expanding interest amongst researchers aimed at understanding their underlying physics. There has been an active debate on the mechanisms that give rise to efficient luminescence in these materials. In this paper we approach these questions through optical studies of single InGaN/GaN quantum dots in the context of the available experimental and theoretical understanding of InGaN structures in general, and of three-dimensional localization in this material in particular. We will also show how it is possible to exploit the various unique properties that nitride-based materials offer, such as the strong inbuilt electric field, in a controlled manner. Such control may in the future prove essential for the implementation of single quantum dot devices in applications such as quantum information processing. We also show how nonlinear spectroscopy provides an invaluable tool in suppressing background luminescence effects inherent in this material.

Abstract:

Builiding on earlier studies of single InGaN quantum dots (QDs), we are considering their potential for use in blue- and green-emitting single photon sources. Envisaging a device based on a resonant cavity light emitting diode, we have studied the effect of growing QDs on an underlying AlN/GaN distributed Bragg reflector, and have shown that enhanced single QD emission may be obtained. Additionally, we have studied the effect of the growth and activation of a p-type cap on an underlying QD layer and have shown that the QDs survive the anneal process. © 2007 IOP Publishing Ltd.

Abstract:

Photoluminescence (PL) properties of a single nanorod containing multiple GaN quantum disks separated by AlGaN potential barriers are investigated using micro-PL spectroscopy. Previous studies reported ensemble spectra from many nanorods. The PL spectra show different features depending on the region of the nanorod excited by the laser, including a sharp feature originating from the quantum disk region. The distinct differences between the PL from the different regions are discussed. The results imply that excitons are strongly confined in the quantum disks, and the authors suggest that small quantum disks can be regarded as quantum dots having a discrete density of states. © 2007 American Institute of Physics.

Abstract:

The saturation of exciton resonances with increasing carrier density has been measured in the case of resonant and non-resonant excitations by using a pump-probe technique. We observed that the bleaching density for resonantly excited excitons is considerably higher than that for non-resonantly excited excitons and that with increasing excitation energy, the exciton bleaching density converges to the Mott density of free carrier model. This results suggest that resonant excitons show weaker screening than an unbound electron-hole plasma mixed with excitons and that the transient optical nonlinearities associated with screening and bandgap renormalization are governed for the ratio of excitons to free carriers with different interactions. Also, we observed that the non-resonantly excited excitons had longer lifetimes than the resonantly excited excitons, which we attributed to the relaxation of free carriers and to the polariton bottle-neck effect.