Michael B Johnston

Publications (2)3.4 Total impact

• Source

  Available from: Edmund Harold Linfield

  Article: Excitation-density-dependent generation of broadband terahertz radiation in an asymmetrically excited photoconductive antenna.

  Prashanth C Upadhya, Wenhui Fan, Andrew Burnett, John Cunningham, A Giles Davies, Edmund H Linfield, James Lloyd-Hughes, Enrique Castro-Camus, Michael B Johnston, Harvey Beere
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  ABSTRACT: The generation of terahertz (THz) transients in photoconductive emitters has been studied by varying the spatial extent and density of the optically excited photocarriers in asymmetrically excited, biased low-temperature-grown GaAs antenna structures. We find a pronounced dependence of the THz pulse intensity and broadband (>6.0 THz) spectral distribution on the pump excitation density and simulate this with a three-dimensional carrier dynamics model. We attribute the observed variation in THz emission to changes in the strength of the screening field.
  Optics Letters 08/2007; 32(16):2297-9. · 3.40 Impact Factor
• Source

  Available from: Edmund Harold Linfield

  Article: Emission of collimated THz pulses from photo-excited semiconductors

  Michael B Johnston, Annette Dowd, Robert Driver, Edmund H Linfield, A Giles Davies, David M Whittaker
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  ABSTRACT: It is shown experimentally that surface-field terahertz (THz) emitters can produce well-collimated beams of THz radiation, making them useful devices for time-domain spectroscopy applications. Simulations of the carrier-dynamics are used to explain the mechanism of THz generation in InAs and GaAs, and it is shown that inter-valley scattering of electrons must be considered in order to fully describe THz emission from InAs.