Publications by Michael Johnston


Photoconductive response correction for detectors of terahertz radiation

Journal of Applied Physics 104 (2008)

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

Photoconductive detectors are convenient devices for detecting pulsed terahertz radiation. We have optimized Fe+ ion-damaged InP materials for photoconductive detector signal to noise performance using dual-energy doses in the range from 2.5× 1012 to 1.0× 1016 cm-2. Ion implantation allowed the production of semiconducting materials with free-carrier lifetimes between 0.5 and 2.1 ps, which were measured by optical pump terahertz probe spectroscopy. The time resolved photoconductivity of the detector substrates was acquired as a function of time after excitation by 2 nJ pulses from a laser oscillator. These data, when combined with a deconvolution algorithm, provide an excellent spectral response correction to the raw photocurrent signal recorded by the photoconductive detectors. © 2008 American Institute of Physics.


Efficient generation of charges via below-gap photoexcitation of polymer-fullerene blend films investigated by terahertz spectroscopy

Physical Review B - Condensed Matter and Materials Physics 78 (2008)

P Parkinson, J Lloyd-Hughes, MB Johnston, LM Herz

Using optical-pump terahertz-probe spectroscopy, we have investigated the time-resolved conductivity dynamics of photoexcited polymer-fullerene bulk heterojunction blends for two model polymers: poly[3-hexylthiophene] (P3HT) and poly[2-methoxy-5- (3,7 -dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) blended with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). The observed terahertz-frequency conductivity is characteristic of dispersive charge transport for photoexcitation both at the π-π* absorption peak (560 nm for P3HT) and significantly below it (800 nm). The photoconductivity at 800 nm is unexpectedly high, which we attribute to the presence of a charge-transfer complex. We report the excitation-fluence dependence of the photoconductivity over more than four orders of magnitude, obtained by utilizing a terahertz spectrometer based upon on either a laser oscillator or an amplifier source. The time-averaged photoconductivity of the P3HT:PCBM blend is over 20 times larger than that of P3HT, indicating that long-lived hole polarons are responsible for the high photovoltaic efficiency of polymer:fullerene blends. At early times (∼ps) the linear dependence of photoconductivity upon fluence indicates that interfacial charge transfer dominates as an exciton decay pathway, generating charges with mobility of at least ∼0.1 cm2 V-1 s -1. At later times, a sublinear relationship shows that carrier-carrier recombination effects influence the conductivity on a longer time scale (>1 μs) with a bimolecular charge annihilation constant for the blends that is approximately two to three orders of magnitude smaller than that typical for neat polymer films. © 2008 The American Physical Society.


Conductivity of nanoporous InP membranes investigated using terahertz spectroscopy.

Nanotechnology 19 (2008) 395704-

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

We have investigated the terahertz conductivity of extrinsic and photoexcited electrons in nanoporous indium phosphide (InP) at different pore densities and orientations. The form of electronic transport in the film was found to differ significantly from that for bulk InP. While photo-generated electrons showed Drude-like transport, the behaviour for extrinsic electrons deviated significantly from the Drude model. Time-resolved photoconductivity measurements found that carrier recombination was slow, with lifetimes exceeding 1 ns for all porosities and orientations. When considered together, these findings suggest that the surfaces created by the nanopores strongly alter the dynamics of both extrinsic and photoexcited electrons.


Exciton dissociation in polymer field-effect transistors studied using terahertz spectroscopy

Physical Review B - Condensed Matter and Materials Physics 77 (2008)

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

We have used terahertz time-domain spectroscopy to investigate photoinduced charge generation in conjugated polymer field-effect transistors. Our measurements show that excitons dissociate in the accumulation layer under the application of a gate voltage, with a quantum efficiency of ∼0.1 for an average gate field of ∼1× 108 V m-1. The transistor history is found to affect the exciton dissociation efficiency, which decreases as holes are increasingly trapped in the accumulation layer. The quantum efficiency of charge formation from excitons is compared with the two contrasting models proposed by Onsager and Arkhipov based on the assumption that field-induced exciton dissociation is assisted by the Brownian diffusive motion or an initial excess energy supplied by excited vibrational modes, respectively. © 2008 The American Physical Society.


Conformational changes of photoactive yellow protein monitored by terahertz spectroscopy

Chemical Physics Letters 455 (2008) 289-292

E Castro-Camus, MB Johnston

Observing the structural dynamics of proteins under conditions as close as possible to those in a living organism is essential for understanding the biological functions of proteins accurately. Here we demonstrate that terahertz spectroscopy is a convenient probe of conformational changes in proteins suspended in physiological buffer solution. We have observed that the partial unfolding of photoactive yellow protein leads to a clear increase in absorption at terahertz frequencies. Using normal mode and molecular dynamics simulations we show that this increase in absorption is related to an increase in the density of delocalised vibrational modes in the more flexible partially unfolded state. © 2008 Elsevier B.V. All rights reserved.


Terahertz photoconductivity of mobile electrons in nanoporous InP honeycombs

Physical Review B - Condensed Matter and Materials Physics 78 (2008)

J Lloyd-Hughes, SKE Merchant, L Sirbu, IM Tiginyanu, MB Johnston

Nanostructured semiconductors with favorable optoelectronic properties can be created by electrochemical etching, a fabrication process that is scalable for mass market applications. Using terahertz photoconductivity measurements, we demonstrate that nanoporous InP has an unusually long carrier recombination lifetime that exceeds 100 ns at low temperatures and low carrier density, and an electron mobility half that of bulk InP. Modeling confirms that these observations result from band bending with holes confined to the surface and electrons away from the pores. © 2008 The American Physical Society.


Terahertz magnetoconductivity of excitons and electrons in quantum cascade structures

Physical Review B - Condensed Matter and Materials Physics 77 (2008)

J Lloyd-Hughes, HE Beere, DA Ritchie, MB Johnston

We examined the quasiparticles formed by the photoexcitation of GaAsAlGaAs terahertz quantum cascade structures using terahertz time-domain spectroscopy. At low temperature and excitation density the measured conductivity was excitonic, with a 1s-2p transition energy indicative of three-dimensional excitons correlated across the quantum well barriers. Free electrons increasingly dominated the conductive response at higher lattice temperatures and excitation densities. Under an external magnetic field transitions from the 1s level into 2p states with different magnetic quantum number were observed, while at high excitation densities the electron cyclotron resonance became more prominent. © 2008 The American Physical Society.


Low-energy collective dynamics of charge stripes in the doped nickelate La2-x Srx Ni O4+δ observed with optical conductivity measurements

Physical Review B - Condensed Matter and Materials Physics 77 (2008)

J Lloyd-Hughes, D Prabhakaran, AT Boothroyd, MB Johnston

We have investigated charge dynamics in the static stripe ordered phase of La2-x Srx Ni O4+δ at lattice temperatures below the charge ordering transition, via optical conductivity measurements at low energies (1-10 meV). The thermally activated dynamic response of the charge stripes is found to be characteristic of a collective mode such as a pinned charge density wave. At incommensurate doping levels, the pinning energy is reduced, owing to the presence of real-space defects in the stripe order, and a pronounced increase in the oscillator strength is seen. The results provide compelling evidence for the existence of low-energy collective modes of the charge stripes. © 2008 The American Physical Society.


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

Opt Lett 32 (2007) 2297-2299

PC Upadhya, W Fan, A Burnett, J Cunningham, AG Davies, EH Linfield, J Lloyd-Hughes, E Castro-Camus, MB Johnston, H Beere

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.


Compound semiconductors for terahertz photonics

(2007) 409-410

MB Johnston, E Castro-Camus, J Lloyd-Hughes, SKE Merchant, P Parkinson, YJ Wang, LM Herz, L Fu, Q Gao, HH Tan, C Jagadish

Terahertz photonics is a rapidly growing field. The ultra-fast dynamics of charge carriers in semiconductors is closely linked to the performance of THz photonic devices. We study a range of ion-implanted and nano-scale semiconductors for application in novel terahertz devices.


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.


Terahertz probe of carrier trapping in polymer transistors

Optics InfoBase Conference Papers (2007)

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

The trapped charge density at the polymer-insulator boundary of polymer transistors was monitored by terahertz time-domain spectroscopy. Additionally, the thermal removal of trapped holes and the light-induced transmission change were studied. © 2006 Optical Society of America.


Plasmonics: Superfocusing of terahertz waves

Nature Photonics 1 (2007) 14-15

MB Johnston


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.


An ion-implanted InP receiver for polarization resolved terahertz spectroscopy.

Opt Express 15 (2007) 7047-7057

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

We report on the construction, optical alignment and performance of a receiver which is capable of recording the full polarization state of coherent terahertz radiation. The photoconductive detector was fabricated on InP which had been implanted with Fe(+) ions. The device operated successfully when it was gated with near infrared femtosecond pulses from either a Ti:sapphire laser oscillator or a 1 kHz regenerative laser amplifier. When illuminated with terahertz radiation from a typical photoconductive source, the optimized device had a signal to noise figure of 100:1 with a usable spectral bandwidth of up to 4 THz. The device was shown to be very sensitive to terahertz polarization, being able to resolve changes in polarization of 0.34 degrees. Additionally, we have demonstrated the usefulness of this device for (i) polarization sensitive terahertz spectroscopy, by measuring the birefringence of quartz and (ii) terahertz emission experiments, by measuring the polarization dependence of radiation generated by optical rectification in (110)-ZnTe.


Transient terahertz conductivity of GaAs nanowires

Nano Letters 7 (2007) 2162-2165

P Parkinson, J Lloyd-Hughes, Q Gao, HH Tan, C Jagadish, MB Johnston, LM Herz

The time-resolved conductivity of isolated GaAs nanowires is investigated by optical-pump terahertz-probe time-domain spectroscopy. The electronic response exhibits a pronounced surface plasmon mode that forms within 300 fs before decaying within 10 ps as a result of charge trapping at the nanowire surface. The mobility is extracted using the Drude model for a plasmon and found to be remarkably high, being roughly one-third of that typical for bulk GaAs at room temperature. © 2007 American Chemical Society.


Terahertz-frequency conductivity of charge stripes in the antiferromagnet La<inf>5/3</inf>Sr<inf>1/3</inf>NiO<inf>4</inf>

IRMMW-THz2007 - Conference Digest of the Joint 32nd International Conference on Infrared and Millimetre Waves, and 15th International Conference on Terahertz Electronics (2007) 869-870

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

We report the complex refractive index of La5/3Sr 1/3NiO4 over the terahertz frequency range, obtained using time-domain spectroscopy. Negligible change in the complex refractive index with magnetic flux densities up to 6 T was seen, while changes were observed as the lattice temperature was increased from 1.5K to the charge-ordering temperature at 220 K. The terahertz frequency response therefore originates from the dielectric function rather than the magnetic permeability.

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