Prof Michael Johnston

Abstract:

The dynamics of free electron-hole pairs and excitons in GaAs-AlGaAs-GaAs core-shell-skin nanowires is investigated using femtosecond transient photoluminescence spectroscopy at 10 K. Following nonresonant excitation, a bimolecular interconversion of the initially generated electron-hole plasma into an exciton population is observed. This conducting-to-insulating transition appears to occur gradually over electron-hole charge pair densities of 2-4 × 10(16) cm(-3) . The smoothness of the Mott transition is attributed to the slow carrier-cooling during the bimolecular interconversion of free charge carriers into excitons and to the presence of chemical-potential fluctuations leading to inhomogeneous spectral characteristics. These results demonstrate that high-quality nanowires are model systems for investigating fundamental scientific effects in 1D heterostructures.

Abstract:

Quantitative measurement based on THz absorption spectrum is of great importance in THz applications. Several researchers have worked on it and gained some achievements, but most of them explored pure component or no more than 2-component s samples. In this paper, a mixture sample consisting of Glutamine, Histidine and Threonine is investigated in the frequency range from 0.3 to 2.6 THz. The quantitative measurement principle is the Lambert-Beer's Law which have been accepted in infrared and visible spectra. Our experiments show the validation of the law in THz region. A Least-Mean-Square algorithm is adopted and measurement errors of Glutamine, Histidine and Threonine are 17.60%, 4.44% and 2.59%.© 2012 SPIE.

Abstract:

Graphene is a single layer of covalently bonded carbon atoms, which was discovered only 8 years ago and yet has already attracted intense research and commercial interest. Initial research focused on its remarkable electronic properties, such as the observation of massless Dirac fermions and the half-integer quantum Hall effect. Now graphene is finding application in touch-screen displays, as channels in high-frequency transistors and in graphene-based integrated circuits. The potential for using the unique properties of graphene in terahertz-frequency electronics is particularly exciting; however, initial experiments probing the terahertz-frequency response of graphene are only just emerging. Here we show that the photoconductivity of graphene at terahertz frequencies is dramatically altered by the adsorption of atmospheric gases, such as nitrogen and oxygen. Furthermore, we observe the signature of terahertz stimulated emission from gas-adsorbed graphene. Our findings highlight the importance of environmental conditions on the design and fabrication of high-speed, graphene-based devices.