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My research interests are:

- Improvement of ultra-broad-bandwidth single cycle terahertz sources and detectors.

- Development of circularly polarized single cycle THz emitters and detectors.

- Photonic crystals, meta-materials and other devices for applications in the terahertz region

- Terahertz spectroscopy of biological systems

- Articles: 12

- Proceedings: 10

- Conference presentations: 35

- Cites: 108

Publications

1. Modifying the polarization state of terahertz radiation using anisotropic twin-domains in {LaAlO$_3$},
   J Lloyd-Hughes, SPP Jones, E Castro-Camus, KI Doig, JL MacManus-Driscoll
   Opt. Lett., 39:1121--1124 (2014) [ abstract | pdf | doi:10.1364/OL.39.001121 ]
   Polarization-resolved terahertz (THz) time-domain spectroscopy was utilized to examine the complex refractive index of lanthanum aluminate (LaAlO3), a rhombohedrally distorted perovskite that exhibits crystallographic twin domains. The uniaxial anisotropy of the refractive index was quantified. The ellipticity of THz radiation pulses after transmission through single domains indicated that LaAlO3 can be used as a quarter- or half-wave plate. The effective anisotropy of [001]-oriented LaAlO3 was found to be reduced when the material exhibited multiple, narrow twin domains.
2. Simulation of fluence-dependent photocurrent in terahertz photoconductive receivers,
   E Castro-Camus, MB Johnston, J Lloyd-Hughes
   Semicond. Sci. Technol., 27:115011 (2012) [ abstract | pdf | doi:10.1088/0268-1242/27/11/115011 ]
   A semi-classical Monte Carlo simulation of carrier dynamics in photoconductive detectors of terahertz (THz) radiation is presented. We have used this simulation to elucidate the importance of carrier trapping in the operation of photoconductive detectors. Simulations of the detection of single-cycle THz pulses by photoconductive antennas based on GaAs with trap densities between 2 × 10 17 and 2 × 10 18 cm −3 are presented. We show that the high frequency (>1 THz) spectral response of photoconductive devices decreases with increasing excitation fluence. Our simulations reveal that this effect is a direct consequence of the saturation of trapping centres.
3. Polarization-resolved terahertz time-domain spectroscopy,
   E Castro-Camus
   J. Infrared Millim. Terahertz Waves, 33:418-430 (2012) [ abstract | pdf | doi:10.1007/s10762-011-9856-8 ]
   Measuring the full polarization state of radiation in terahertz time-domain spectroscopy has allowed scientists to study a number of complex dielectric anisotropic properties of materials that could not be easily measured before. Novel polarization sensitive photoconductive detectors have simplified this task and their development has been a significant challenge. In this review I will present some of these devices and will also discuss some of the most recent studies that involve the use of polarization resolved terahertz spectroscopy.
4. Extraction of the anisotropic dielectric properties of materials from polarization-resolved terahertz time-domain spectra,
   E Castro-Camus, MB Johnston
   J. Opt. A-Pure Appl. Opt., 11:105206 (2009) [ abstract | pdf | doi:10.1088/1464-4258/11/10/105206 ]
   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.
5. Photoconductive response correction for detectors of terahertz radiation,
   E Castro-Camus, L Fu, J Lloyd-Hughes, HH Tan, C Jagadish, MB Johnston
   J. Appl. Phys., 104:053113 (2008) [ abstract | pdf | doi:10.1063/1.2969035 ]
   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.
6. Conformational changes of photoactive yellow protein monitored by terahertz spectroscopy,
   E Castro-Camus, MB Johnston
   Chem. Phys. Lett., 455:289-292 (2008) [ abstract | pdf | doi:10.1016/j.cplett.2008.02.084 ]
   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.
7. Excitation-density-dependent generation of broadband terahertz radiation in an asymmetrically excited photoconductive antenna,
   PC Upadhya, W Fan, A Burnett, J Cunningham, AG Davies, EH Linfield, J Lloyd-Hughes, E Castro-Camus, MB Johnston, H Beere
   Opt. Lett., 32:2297 (2007) [ abstract | pdf | doi:10.1364/OL.32.002297 ]
   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.
8. An ion-implanted InP receiver for polarization resolved terahertz spectroscopy,
   E Castro-Camus, J Lloyd-Hughes, L Fu, HH Tan, C Jagadish, MB Johnston
   Opt. Express, 15:7047-7057 (2007) [ abstract | pdf | doi:10.1364/OE.15.007047 ]
   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.
9. Influence of surface passivation on ultrafast carrier dynamics and terahertz radiation generation in GaAs,
   J Lloyd-Hughes, SKE Merchant, L Fu, HH Tan, C Jagadish, E Castro-Camus, MB Johnston
   Appl. Phys. Lett., 89:232102 (2006) [ abstract | pdf | doi:10.1063/1.2398915 ]
   The carrier dynamics of photoexcited electrons in the vicinity of the surface of (NH4)(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. (c) 2006 American Institute of Physics.
10. Charge trapping in polymer transistors probed by terahertz spectroscopy and scanning probe potentiometry,
    J Lloyd-Hughes, T Richards, H Sirringhaus, E Castro-Camus, LM Herz, MB Johnston
    Appl. Phys. Lett., 89:112101 (2006) [ abstract | pdf | doi:10.1063/1.2340057 ]
    Terahertz time-domain spectroscopy and scanning probe potentiometry were used to investigate charge trapping in polymer field-effect transistors fabricated on a silicon gate. The hole density in the transistor channel was determined from the reduction in the transmitted terahertz radiation under an applied gate voltage. Prolonged device operation creates an exponential decay in the differential terahertz transmission, compatible with an increase in the density of trapped holes in the polymer channel. Taken in combination with scanning probe potentiometry measurements, these results indicate that device degradation is largely a consequence of hole trapping, rather than of changes to the mobility of free holes in the polymer.
11. Longitudinal electron bunch profile diagnostics at 45MeV using coherent Smith-Purcell radiation,
    G Doucas, V Blackmore, B Ottewell, C Perry, PG Huggard, E Castro-Camus, MB Johnston, J Lloyd-Hughes, MF Kimmitt, B Redlich, A van der Meer
    Phys. Rev. Spec. Top.-Accel. Beams, 9:092801 (2006) [ abstract | pdf | doi:10.1103/PhysRevSTAB.9.092801 ]
    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.
12. Simulation and optimisation of terahertz emission from InGaAs and InP photoconductive switches,
    J Lloyd-Hughes, E Castro-Camus, MB Johnston
    Solid State Commun., 136:595-600 (2005) [ abstract | pdf | doi:10.1016/j.ssc.2005.09.037 ]
    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 T, 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.
13. Polarization-sensitive terahertz detection by multicontact photoconductive receivers,
    E Castro-Camus, J Lloyd-Hughes, MB Johnston, MD Fraser, HH Tan, C Jagadish
    Appl. Phys. Lett., 86:254102 (2005) [ abstract | pdf | doi:10.1063/1.1951051 ]
    We have developed a terahertz radiation detector that measures both the amplitude and polarization of the electric field as a function of time. The device is a three-contact photoconductive receiver designed so that two orthogonal electric-field components of an arbitrary polarized electromagnetic wave may be detected simultaneously. The detector was fabricated on Fe+ ion-implanted InP. Polarization-sensitive detection is demonstrated with an extinction ratio better than 100:1. This type of device will have immediate application in studies of birefringent and optically active materials in the far-infrared region of the spectrum. (c) 2005 American Institute of Physics.
14. Three-dimensional carrier-dynamics simulation of terahertz emission from photoconductive switches,
    E Castro-Camus, J Lloyd-Hughes, MB Johnston
    Phys. Rev. B, 71:195301 (2005) [ abstract | pdf | doi:10.1103/PhysRevB.71.195301 ]
    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.
15. Carrier dynamics in ion-implanted GaAs studied by simulation and observation of terahertz emission,
    J Lloyd-Hughes, E Castro-Camus, MD Fraser, C Jagadish, MB Johnston
    Phys. Rev. B, 70:235330 (2004) [ abstract | pdf | doi:10.1103/PhysRevB.70.235330 ]
    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 10(16) 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.
16. Prediction of the physical response for the two-photon photorefractive effect,
    E Castro-Camus, LF Magana
    Opt. Lett., 28:1129-1131 (2003) [ abstract | pdf | doi:10.1364/OL.28.001129 ]
    We present a new model for the two-photon photorefractive recording process. We solved the resulting set of nonlinear coupled partial differential equations of the model within a linear approximation of the steady state. We found very good agreement with experimental results. (C) 2003 Optical Society of America.

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