Publications by Michael Johnston


Polarisation-sensitive terahertz detectors

(2005) 582-583

MB Johnston, E Castro-Camus, J Lloyd-Hughes, MD Fraser, HH Tan, C Jagadish

We have developed a detector of coherent terahertz (THz) radiation that can recover the full polarisation state of a THz transient. The device is a three-contact photoconductive receiver, which is capable of recording two time-varying electric field components of a THz pulse simultaneously. Our receiver was fabricated on Fe+ implanted InP and showed a cross-polarised extinction ratio greater than 100:1. The detector will be useful for spectroscopy of birefringent and optically active materials.


Polarization-sensitive terahertz detection by multicontact photoconductive receivers

Applied Physics Letters 86 (2005) 254102 3pp-

MB Johnston, E. Castro-Camus, J. Lloyd-Hughes, M. D. Fraser


Emission of collimated THz pulses from photo-excited semiconductors

Semiconductor Science and Technology 19 (2004)

MB Johnston, A Dowd, R Driver, EH Linfield, AG Davies, DM Whittaker

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.


Carrier dynamics in ion-implanted GaAs studied by simulation and observation of terahertz emission

Physical Review B - Condensed Matter and Materials Physics 70 (2004) 1-6

J Lloyd-Hughes, E Castro-Camus, MD Fraser, C Jagadish, MB Johnston

We have studied terahertz (THz) emission from arsenic-ion implanted GaAs both experimentally and using a three-dimensional carrier dynamics simulation. A uniform density of vacancies was formed over the optical absorption depth of bulk GaAs samples by performing multienergy implantations of arsenic ions (1 and 2.4 MeV) and subsequent thermal annealing. In a series of THz emission experiments the frequency of peak THz power was found to increase significantly from 1.4 to 2.2 THz when the ion implantation dose was increased from 10 13 to 1016 cm-3. We used a semiclassical Monte Carlo simulation of ultrafast carrier dynamics to reproduce and explain these results. The effect of the ion-induced damage was included in the simulation by considering carrier scattering at neutral and charged impurities, as well as carrier trapping at defect sites. Higher vacancy concentrations and shorter carrier trapping times both contributed to shorter simulated THz pulses, the latter being more important over experimentally realistic parameter ranges.


Simulation and optimization of arsenic-implanted THz emitters

(2004) 577-578

MB Johnston, J Lloyd-Hughes, E Casto-Camus, MD Fraser, C Jagadish

We have used a three-dimensional pseudo-classical Monte Carlo simulation to investigate the effects of As+ ion-implantation on pulsed terahertz radiation emitters. Devices based on surface-field emitters and photoconductive switches have been modelled. Two implantations of As+ ions at 1.0 MeV and 2.4 MeV were found to produce a uniform distribution of vacancies over the volume of GaAs contributing to THz generation in these devices. We calculate that ionimplantation increases the THz bandwidth of the devices with the cost of decreasing the spectral intensity at lower THz frequencies.


Emission of collimated THz pulses from photo-excited semiconductors

SEMICOND SCI TECH 19 (2004) S449-S451

MB Johnston, A Dowd, R Driver, EH Linfield, AG Davies, DM Whittaker

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.


Selective dielectrophoretic manipulation of surface-immobilized DNA molecules

Nanotechnology 14 (2003) 896-902

WA Germishuizen, C Wälti, R Wirtz, MB Johnston, M Pepper, AG Davies, APJ Middelberg

The fabrication of nanoscale molecular devices is becoming increasingly important and research into their fabrication has intensified over the last few years. In particular, the attachment of molecular objects onto various surfaces has attracted considerable attention. Here, we report a multistep surface immobilization procedure, which allows the specific and controlled attachment of very long DNA molecules onto gold electrodes. Further, we report the effect of dielectrophoresis on these surface-bound DNA molecules with respect to amplitude and frequency, and we show that selected surface-immobilized DNA molecules can be manipulated by dielectrophoresis. Finally, we investigated the use of dielectrophoresis in conjunction with the multistep surface immobilization of fluorescently labelled, surface-bound γ-DNA in a basic data-storage device.


Low-energy vibrational modes in phenylene oligomers studied by THz time domain spectroscopy

Chemical Physics Letters 377 (2003) 256-262

MB Johnston, L. M. Herz, A. L. T. Khan, A. Köhler


Enhancement of THz emission from semiconductor devices

COMMAD 2002 PROCEEDINGS (2002) 281-284

A Dowd, MB Johnston, DM Whittaker, AG Davies, EH Linfield


Thermally stimulated luminescence in ion-implanted GaAs

JOURNAL OF LUMINESCENCE 96 (2002) PII S0022-2313(01)00219-8

M Gal, LV Dao, E Kraft, MB Johnston, C Carmody, HH Tan, C Jagadish


Generation of high-power terahertz pulses in a prism

Optics Letters 27 (2002) 1935-1937

MB Johnston, DM Whittaker, A Dowd, AG Davies, EH Linfield, X Li, DA Ritchie

A compact, high-power emitter of half-cycle terahertz (THz) radiation is demonstrated. The device consists of an epitaxial InAs emitter upon a GaAs prism and produces THz pulses that are 20 times more powerful than those from conventional planar InAs emitters. This improvement is a direct result of reorienting the transient THz dipole such that its axis is not perpendicular to the emitting surface. © 2002 Optical Society of America.


Theory of magnetic-field enhancement of surface-field terahertz emission

Journal of Applied Physics 91 (2002) 2104-2106

MB Johnston, DM Whittaker, A Corchia, A G. Davies, EH Linfield

We present a theoretical treatment of surface-field THz generation in semiconductors, which explains the power enhancement observed when a magnetic field is applied. Our model consists of two parts: a Monte Carlo simulation of the dynamics of carriers generated by a subpicosecond optical pulse, and a calculation of the resulting THz radiation emitted through the semiconductor surface. The magnetic field deflects the motion of the carriers, producing a component of the THz dipole parallel to the surface. This causes the power transmitted through the surface to be increased by more than one order of magnitude. © 2002 American Institute of Physics.


Theory of magnetic-field enhancement of surface-field terahertz emission

Journal of Applied Physics 91 (2002) 2104-2106

MB Johnston, DM Whittaker, A Corchia, A G. Davies, EH Linfield

We present a theoretical treatment of surface-field THz generation in semiconductors, which explains the power enhancement observed when a magnetic field is applied. Our model consists of two parts: a Monte Carlo simulation of the dynamics of carriers generated by a subpicosecond optical pulse, and a calculation of the resulting THz radiation emitted through the semiconductor surface. The magnetic field deflects the motion of the carriers, producing a component of the THz dipole parallel to the surface. This causes the power transmitted through the surface to be increased by more than one order of magnitude. © 2002 American Institute of Physics.


Optimisation of growth parameters for photonic crystal structures

MBE 2002 - 2002 12th International Conference on Molecular Beam Epitaxy (2002) 153-154

P Atkinson, MB Johnston, HE Beere, DA Ritchie

© 2002 IEEE. The preferential facetting that occurs during regrowth on patterned substrates by MBE, together with the different adatom migration lengths on different facets has been successfully used to provide good 1D confinement following overgrowth on mesas. However, in some instances it is preferable that the initial surface profile remains unmodified during successive growth stages. An example of this is for photonic crystal structures which can be created by patterning the surface with an array of mesas, and then regrowing a sequence of alternating layers of different refractive indices with spatially varying periodicity of order of hundreds of nanometers in both the lateral and growth directions.


Magnetic-field-induced enhancement of terahertz emission from III-V semiconductor surfaces

Physica E: Low-Dimensional Systems and Nanostructures 13 (2002) 896-899

MB Johnston, A Corchia, A Dowd, EH Linfield, AG Davies, R McLaughlin, DD Arnone, M Pepper

We discuss the origins of the magnetic-field-induced enhancement of terahertz (THz) emission from bulk semiconductor surfaces. The principal effect of the magnetic field is to rotate the THz dipole and hence dramatically increase the THz power radiated through the semiconductor surface. It also significantly affects the ability of the photo-created carriers to screen surface electric fields. The sensitivity of THz emission to the motion of photo-created carriers makes this an ideal probe of hot carrier dynamics both in bulk semiconductors and sophisticated heterostructures. © 2002 Elsevier Science Ltd. All rights reserved.


The development of terahertz sources and their applications.

Phys Med Biol 47 (2002) 3679-3689

AG Davies, EH Linfield, MB Johnston

The terahertz region of the electromagnetic spectrum spans the frequency range between the mid-infrared and the millimetre/microwave. This region has not been exploited fully to date owing to the limited number of suitable (in particular, coherent) radiation sources and detectors. Recent demonstrations, using pulsed near-infrared femtosecond laser systems, of the viability of THz medical imaging and spectroscopy have sparked international interest; yet much research still needs to be undertaken to optimize both the power and bandwidth in such THz systems. In this paper, we review how femtosecond near-infrared laser pulses can be converted into broad band THz radiation using semiconductor crystals, and discuss in depth the optimization of one specific generation mechanism based on ultra-fast transport of electrons and holes at a semiconductor surface. We also outline a few of the opportunities for a technology that can address a diverse range of challenges spanning the physical and biological sciences, and note the continuing need for the development of solid state, continuous wave, THz sources which operate at room temperature.


Enhancement of terahertz emission from semiconductor surfaces

THZ 2002: IEEE TENTH INTERNATIONAL CONFERENCE ON TERAHERTZ ELECTRONICS PROCEEDINGS (2002) 48-51

MB Johnston, A Dowd, DM Whittaker, A Corchia, AG Davies, EH Linfield


Simulation of terahertz generation at semiconductor surfaces

Physical Review B: Condensed Matter and Materials Physics 65 (2002) 165301 6pp-

MB Johnston, D. M. Whittaker, A. Corchia, A. G. Davies


Effects of magnetic field and optical fluence on terahertz emission in gallium arsenide

Physical Review B - Condensed Matter and Materials Physics 64 (2001) 2052041-2052045

A Corchia, R McLaughlin, MB Johnston, DM Whittaker, DD Arnone, EH Linfield, AG Davies, M Pepper

The excitation density dependence of magnetic-field-enhanced terahertz (THz = 1012 Hz) emission from (100) GaAs is studied. It is found that THz power saturates at a higher optical-excitation density, when a magnetic field is applied. This observation explains the different magnetic field enhancements that have been reported recently. At low excitation densities the results are shown to be consistent with a simple model of carrier-carrier scattering, whilst at higher densities surface field screening becomes important.


Effects of magnetic field and optical fluence on terahertz emission in gallium arsenide

Physical Review B - Condensed Matter and Materials Physics 64 (2001) 2052041-2052045

MB Johnston, DM Whittaker, DD Arnone, EH Linfield, AG Davies

The excitation density dependence of magnetic-field-enhanced terahertz (THz = 1012 Hz) emission from (100) GaAs is studied. It is found that THz power saturates at a higher optical-excitation density, when a magnetic field is applied. This observation explains the different magnetic field enhancements that have been reported recently. At low excitation densities the results are shown to be consistent with a simple model of carrier-carrier scattering, whilst at higher densities surface field screening becomes important.

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