Prof Michael Johnston

Abstract:

We have investigated the ultrafast far-infrared transmission of isolated single-walled carbon nanotubes using optical-pump THz-probe spectroscopy. The THz dielectric response is dominated by excitons with an initial, rapid decay due to Auger recombination followed by a slow decay of isolated single excitons. Frequencydependent analysis of the photomduced dielectric function suggest an internal excitonic excitation at ∼11 meV with further low-frequency (∼0.6 and 1.4 THz) absorption features at high densities ascribed to exciton complexes. A featureless conductivity bleaching is attributed to an exciton-induced reduction in the mobility of free carriers caused by phase-space filling. © 2009 American Chemical Society.

Abstract:

The anisotropic complex dielectric properties of materials in the terahertz band is a topic that has attracted considerable attention recently in the fields of physics, chemistry and biochemistry. The mathematical formalism for analysing polarization-resolved terahertz time-domain data is presented, and particular cases including birefringence, optical activity and circular dichroism are discussed. © 2009 IOP Publishing Ltd.

Abstract:

We have used transient terahertz photoconductivity measurements to assess the efficacy of two-temperature growth and core-shell encapsulation techniques on the electronic properties of GaAs nanowires. We demonstrate that two-temperature growth of the GaAs core leads to an almost doubling in charge-carrier mobility and a tripling of carrier lifetime. In addition, overcoating the GaAs core with a larger-bandgap material is shown to reduce the density of surface traps by 82%, thereby enhancing the charge conductivity.