Publications by Michael Johnston


Enhancement of ultrafast conductivity in surface-passivated GaAs

Optics InfoBase Conference Papers (2007)

J Lloyd-Hughes, SKE Merchant, L Fu, HH Tan, C Jagadish, E Castro-Camus, MB Johnston

Optical-pump/terahertz-probe spectroscopy and terahertz emission spectroscopy were used to measure the conductivity and surface electric field change resulting from passivating the surface of GaAs. An enhanced terahertz radiation generation from passivated photoconductive antenna was observed. © 2006 Optical Society of America.


Terahertz-frequency conductivity of charge stripes in the antiferromagnet La5/3Sr1/3NiO4

(2007) 852-853

J Lloyd-Hughes, D Prabhakaran, E Castro-Camus, AT Boothroyd, MB Johnston

We report the complex refractive index of La5/3Sr1/3NiO4 over the terahertz frequency range, obtained using time-domain spectroscopy. Negligible change in the complex refractive index with magnetic flux densities up to 6T was seen, while changes were observed as the lattice temperature was increased from 1.5 K to the charge-ordering temperature at 220 K. The terahertz frequency response therefore originates from the dielectric function rather than the magnetic permeability.


Superfocusing of terahertz waves

NATURE PHOTONICS 1 (2007) 14-15

MB Johnston


Transmission and emission terahertz time-domain spectroscopy with polarisation-sensitive photoconductive receivers

IRMMW-THz2007 - Conference Digest of the Joint 32nd International Conference on Infrared and Millimetre Waves, and 15th International Conference on Terahertz Electronics (2007) 210-211

E Castro-Camus, J Lloyd-Hughes, L Fu, HH Tan, C Jagadish, MB Johnston

In this contribution a detailed characterisation of the performance of three-contact polarisation-sensitive terahertz detectors will be discussed. Furthermore the appropriate mathematical formalism required for the analysis of polarisation-resolved THz measurements will be presented. In addition, measurements of the polarisation-resolved transmission of quartz and polarisation-resolved emission of (110) ZnTe will be shown in order to illustrate the relevance of polarisation in time domain measurements. Finally perspectives on experiments that require polarisation sensitivity will be discussed.


Conductivity of nanoporous InP membranes investigated using terahertz spectroscopy

IRMMW-THz2007 - Conference Digest of the Joint 32nd International Conference on Infrared and Millimetre Waves, and 15th International Conference on Terahertz Electronics (2007) 224-225

SKE Merchant, J Lloyd-Hughes, L Sirbu, IM Tiginyanu, P Parkinson, LM Herz, MB Johnston

We have investigated the conductivity of equilibrium and photoexcited electrons in nanoporous indium phosphide (InP) of various porosities and of two orientations: (100) and (111). We observed an enhanced transmission through the nanoporous samples compared with bulk InP, resulting from a suppression of the conductivity by the pores. The frequency-dependent conductivity was extracted numerically from the transmission data. We examined the dynamical conductivity of photoexcited carriers using optical-pump THz-probe spectroscopy. After the rapid photoexcitation of electrons, the timeresolved conductivity was observed to decay slowly, with carrier recombination lifetimes exceeding 1 ns for all (100)- and (111)-oriented samples.


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

Proceedings of SPIE - The International Society for Optical Engineering 6118 (2006)

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

We have experimentally measured the terahertz radiation from a series of ion-implanted semiconductors, both from the bare semiconductor surface and from photoconductive switches fabricated on them. GaAs was implanted with As + ions, and InGaAs and InP with Fe+ ions, and all samples were annealed post implantation. An increase in emission power is observed at high frequencies, which we attribute to the ultrafast trapping of carriers. We use a three-dimensional carrier dynamics simulation to model the emission process. The simulation accurately predicts the experimentally observed bandwidth increase, without resorting to any fitting parameters. Additionally, we discuss intervalley scattering, the influence of space-charge fields, and the relative performance of InP, GaAs and In As based photoconductive emitters.


Terahertz emission and lifetime measurements of ionimplanted semiconductors: Experiment and simulation

Optics InfoBase Conference Papers (2006)

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

The spectral width of terahertz emission from ion-implanted terahertz emitters increases with ion damage, owing to ultrafast carrier capture. Carrier dynamics simulations reinforce these findings. Optical-pump, terahertz-probe experiments confirm the subpicosecond lifetimes of these materials. © 2006 Optical Society of America.


Charge trapping in polymer transistors probed by terahertz spectroscopy and scanning probe potentiometry

Applied Physics Letters 89 (2006) 112101 pp3-

MB Johnston, J. Lloyd-Hughes, T. Richards, H. Sirringhaus


Detecting the full polarization state of terahertz transients - art. no. 61200Q

P SOC PHOTO-OPT INS 6120 (2006) Q1200-Q1200

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

We have developed a detector which records the full polarization state of a terahertz (THz) pulse propagating in free space. The three-electrode photoconductive receiver simultaneously records the electric field of an electromagnetic pulse in two orthogonal directions as a function of time. A prototype device fabricated on Fe+ ion implanted InP exhibited a cross polarized extinction ratio better than 390:1. The design and optimization of this device are discussed along with its significance for the development of new forms of polarization sensitive time domain spectroscopy, including THz circular dichroism spectroscopy.


Polymer transistor performance monitored by terahertz spectroscopy

(2006) 203-203

J Lloyd-Hughes, T Richards, E Castro-Camus, H Sirringhaus, LM Herz, MB Johnston

Research on polymer-based transistors is leading to the development of flexible, printable circuitry, which will be extremely cost effective to manufacture. However, the longterm performance of state-of-the-art polymer field effect transistors (pFETs) is limited by device degradation. We show that terahertz spectroscopy is an ideal tool to probe polymer device performance. Specifically we have monitored charge carrier trapping at the polymer-insulator boundary of a pFET. From these results we show that device degradation is primarily caused by a trapping of holes in the channel of the pFET, rather than by a change in hole mobility.


Terahertz emission and lifetime measurements of ion-implanted semiconductors: Experiment and simulation

Conference on Lasers and Electro-Optics and 2006 Quantum Electronics and Laser Science Conference, CLEO/QELS 2006 (2006)

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

The spectral width of terahertz emission from ion-implanted terahertz emitters increases with ion damage, owing to ultrafast carrier capture. Carrier dynamics simulations reinforce these findings. Optical-pump, terahertz-probe experiments confirm the subpicosecond lifetimes of these materials. © 2006 Optical Society of America.


Polarization sensitive terahertz time domain spectroscopy

Conference on Lasers and Electro-Optics and 2006 Quantum Electronics and Laser Science Conference, CLEO/QELS 2006 (2006)

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

We present a terahertz radiation detector that measures both transverse components of a terahertz single cycle's electric field, allowing the study of polarization dependent properties of materials. Measurements of birefringence in quartz are presented. © 2006 Optical Society of America.


Carrier dynamics in ion-implanted semiconductors studied by simulation and observation of terahertz emission - art. no. 61180K

P SOC PHOTO-OPT INS 6118 (2006) K1180-K1180

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

We have experimentally measured the terahertz radiation from a series of ion-implanted semiconductors, both from the bare semiconductor surface and from photoconductive switches fabricated on them. GaAs was implanted with As+ ions, and InGaAs and InP with Fe+ ions, and all samples were annealed post implantation. An increase in emission power is observed at high frequencies, which we attribute to the ultrafast trapping of carriers. We use a three-dimensional carrier dynamics simulation to model the emission process. The simulation accurately predicts the experimentally observed bandwidth increase, without resorting to any fitting parameters. Additionally, we discuss intervalley scattering, the influence of space-charge fields, and the relative performance of InP, GaAs and InAs based photoconductive emitters.


Broadband terahertz emission from ion-implanted semiconductors

SPRINGER PROC PHYS 110 (2006) 77-80

J Lloyd-Hughes, E Castro-Camus, MD Fraser, HH Tang, C Ja-gadish, MB Johnston

The terahertz radiation emitted from Fe+ ion-implanted InGaAs surface emitters and InP photoconductive switches was measured. We experimentally observe an increase in the spectral width of terahertz radiation at greater ion damage, which we attribute to the ultrafast capture of photoexcited carriers. Results from a three-dimensional carrier dynamics simulation support this explanation.


Light induced recovery of polymer field effect transistors

(2006) 482-482

J Lloyd-Hughes, T Richards, E Castro-Camus, H Sirringhaus, MB Johnston, LM Herz

We have used differential terahertz spectroscopy to monitor performance degradation in state-of-the-art polymer field effect transistors (pFETs) based on poly[(9,9-dioetylfluorene-2,7-diyl)-co-(bithiophene)] (F8T2). After extended periods of operation holes are trapped in the polymer, increasing the device's threshold voltage. We monitor the trapped charge density using THz spectroscopy, and investigate the device's recovery as trapped holes are thermally removed. Illuminating the devices for a period with above-bandgap photons leads to a change in the terahertz transmission through the device, which is short lived after switching the light off.


THz emitters and detectors based on ion implanted III-V semiconductors

(2006) 149-149

J Lloyd-Hughes, L Fu, E Castro-Camus, S Merchant, HH Tan, C Jagadish, MB Johnston

Ultrafast charge carrier dynamics in serniconductoring materials ultimately determine the performance of photoconductive terahertz (THz) emitters and receivers. Ion implantation of III-V semiconductors allows carrier dynamics to be tailored for a particular application, and thus the technique is increasingly being applied to the development of advanced materials for terahertz photonics. In this talk I will briefly introduce the technique of ion implantation and review some recent applications in THz photonics. I will then present time resolved conductivity studies of GaAs:As+, InGaAs:Fe+, InP:O+ and InP:Fe+ and relate these results to improved terahertz emitter and detector performance.


Influence of surface passivation on ultrafast carrier dynamics and terahertz radiation generation in GaAs

Applied Physics Letters 89 (2006)

J Lloyd-Hughes, SKE Merchant, L Fu, HH Tan, C Jagadish, E Castro-Camus, MB Johnston

The carrier dynamics of photoexcited electrons in the vicinity of the surface of (N H4) 2 S -passivated GaAs were studied via terahertz emission spectroscopy and optical-pump terahertz-probe spectroscopy. Terahertz emission spectroscopy measurements, coupled with Monte Carlo simulations of terahertz emission, revealed that the surface electric field of GaAs reverses after passivation. The conductivity of photoexcited electrons was determined via optical-pump terahertz-probe spectroscopy and was found to double after passivation. These experiments demonstrate that passivation significantly reduces the surface state density and surface recombination velocity of GaAs. Finally, it was demonstrated that passivation leads to an enhancement in the power radiated by photoconductive switch terahertz emitters, thereby showing the important influence of surface chemistry on the performance of ultrafast terahertz photonic devices. © 2006 American Institute of Physics.


Longitudinal electron bunch profile diagnostics at 45 MeV using coherent Smith-Purcell radiation

Physical Review Special Topics - Accelerators and Beams 9 (2006)

G Doucas, V Blackmore, B Ottewell, C Perry, PG Huggard, E Castro-Camus, MB Johnston, JL Hughes, MF Kimmitt, B Redlich, A Van Der Meer

We have used coherent Smith-Purcell radiation in order to investigate the longitudinal (temporal) profile of the electron bunch at the FELIX facility. Detection of the far-infrared radiation was achieved by a simple and compact experimental arrangement, consisting of an array of 11 room-temperature pyroelectric detectors. Accurate determination of the background radiation, use of high quality optical filters, and an efficient light collection system are essential for this type of experiment. The radiated power is in good agreement with the predictions of the surface current description of this process. It is concluded that 90% of the bunch particles are contained within 5.5 ps, with a temporal profile that could be approximately triangular in shape. © 2006 The American Physical Society.


Simulation and optimisation of terahertz emission from InGaAs and InP photoconductive switches

Solid State Communications 136 (2005) 595-600

J Lloyd-Hughes, E Castro-Camus, MB Johnston

We simulate the terahertz emission from laterally biased InGaAs and InP using a three-dimensional carrier dynamics model in order to optimise the semiconductor material. Incident pump-pulse parameters of current Ti:Sapphire and Er:fibre lasers are chosen, and the simulation models the semiconductor's bandstructure using parabolic Γ, L and X valleys, and heavy holes. The emitted terahertz radiation is propagated within the semiconductor and into free space using a model based on the Drude-Lorentz dielectric function. As the InGaAs alloy approaches InAs an increase in the emitted power is observed, and this is attributed to a greater electron mobility. Additionally, low-temperature grown and ion-implanted InGaAs are modelled using a finite carrier trapping time. At sub-picosecond trapping times the terahertz bandwidth is found to increase significantly at the cost of a reduced emission power. © 2005 Elsevier Ltd. All rights reserved.


Three-dimensional carrier-dynamics simulation of terahertz emission from photoconductive switches

Physical Review B - Condensed Matter and Materials Physics 71 (2005)

E Castro-Camus, J Lloyd-Hughes, MB Johnston

A semi-classical Monte Carlo model for studying three-dimensional carrier dynamics in photoconductive switches is presented. The model was used to simulate the process of photoexcitation in GaAs-based photoconductive antennas illuminated with pulses typical of mode-locked Ti:Sapphire lasers. We analyzed the power and frequency bandwidth of THz radiation emitted from these devices as a function of bias voltage, pump pulse duration and pump pulse location. We show that the mechanisms limiting the THz power emitted from photoconductive switches fall into two regimes: when illuminated with short duration (<40 fs) laser pulses the energy distribution of the Gaussian pulses constrains the emitted power, while for long (>40 fs) pulses, screening is the primary power-limiting mechanism. A discussion of the dynamics of bias field screening in the gap region is presented. The emitted terahertz power was found to be enhanced when the exciting laser pulse was in close proximity to the anode of the photoconductive emitter, in agreement with experimental results. We show that this enhancement arises from the electric field distribution within the emitter combined with a difference in the mobilities of electrons and holes. © 2005 The American Physical Society.

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