Older list of papers (JLH only)

Papers from 2016

Terahertz quantum Hall effect for spin-split heavy-hole gases in strained Ge quantum wells

M. Failla, J. Keller, G. Scalari, C. Maissen, J. Faist, C. Reichl, W. Wegscheider, O. J. Newell, D. R. Leadley, M. Myronov, J. Lloyd-Hughes New J. Phys. 18:113036 (Nov 2016) [ pdf ][ ref ] Spin-split heavy-hole gases in strained germanium quantum wells were characterized by polarisation-resolved terahertz time-domain spectroscopy. Effective masses, carrier densities, g-factors, transport lifetimes, mobilities and Rashba spin-splitting energies were evaluated, giving quantitative insights into the influence of strain. The Rashba coefficient was found to lower for samples with higher biaxial compressive strain, while heavy-hole mobilities were enhanced to over 1.5x106cm2V−1s−1 at 3 K. This high mobility enabled the observation of the optical quantum Hall effect at terahertz frequencies for spin-split two-dimensional heavy-holes, evidenced as plateaux in the transverse magnetoconductivity at even and odd filling factors.

Papers from 2015

Narrow heavy-hole cyclotron resonances split by the cubic Rashba spin-orbit interaction in strained germanium quantum wells

M. Failla, M. Myronov, C. Morrison, D. R. Leadley, and J. Lloyd-Hughes Physical Review B, 92:045303 (July 2015) [ pdf ][ supplemental info ][ ref ] The spin-orbit interaction was found to split the cyclotron resonance of heavy holes confined in high-mobility, compressively strained germanium quantum wells. The interference between coherent spin-split cyclotron resonances was tracked on picosecond time scales using terahertz time-domain spectroscopy. Analysis in the time domain, or using a time-frequency decomposition based on the Gabor-Morlet wavelet, was necessary when the difference between cyclotron frequencies was comparable to the linewidth. The cubic Rashba spin-orbit coefficient β was determined via two methods: (i) the magnetic-field dependence of the cyclotron frequencies, and (ii) the spin-resolved subband densities. An enhanced β and spin polarization was created by tailoring the strain to enhance the spin-orbit interaction. The amplitude modulation of the narrow, interfering cyclotron resonances is a signature of spin coherences persisting for more than 10 ps.

Interfacial and bulk polaron masses in Zn_1-xMg_xO/ZnO heterostructures examined by terahertz time-domain cyclotron spectroscopy

J. Lloyd-Hughes, M. Failla, J. Ye, S.P.P. Jones, K.L. Teo, and C. Jagadish Appl. Phys. Lett. 106:202103 (May 2015) [ pdf ][ ref ] The cyclotron resonance of polarons in Zn1-xMgxO/ZnO heterostructures (with 0.15<x<0.22) was studied by terahertz time-domain spectroscopy. Low-temperature magnetoconductivity spectra of the 2D electron gas at the Zn1-xMgxO/ZnO interface determined the polaron density, mass, and scattering rate. The cyclotron mass of 2D polarons was found to increase significantly with magnetic field B from 0.24me at B=2T to 0.37me at B=7.5T. A nonlinear cyclotron frequency with B was also observed for 3D polarons in ZnO. The findings are discussed in the context of polaron mass renormalization driven by the electron-LO-phonon and electron-acoustic phonon interactions.

Structural, optical and vibrational properties of self-assembled Pb_n+1(Ti_1-xFe_x)_nO_3n+1-δ Ruddlesden-Popper superstructures

K. I. Doig, J. J. P. Peters, S. Nawaz, D. Walker, M. Walker, M. R. Lees, R. Beanland, A. M. Sanchez, C. F. McConville, V. R. Palkar, J. Lloyd-Hughes Scientific Reports, 5:7719 (Jan 2015) [ pdf ][ Supp. Info. ][ ref ] Bulk crystals and thin films of PbTi1−xFexO3−δ (PTFO) are multiferroic, exhibiting ferroelectricity and ferromagnetism at room temperature. Here we report that the Ruddlesden-Popper phase Pbn+1(Ti1-xFex)nO3n+1-δ forms spontaneously during pulsed laser deposition of PTFO on LaAlO3 substrates. High-resolution transmission electron microscopy, x-ray diffraction and x-ray photoemission spectroscopy were utilised to perform a structural and compositional analysis, demonstrating that n~8 and x~0.5. The complex dielectric function of the films was determined from far-infrared to ultraviolet energies using a combination of terahertz time-domain spectroscopy, Fourier transform spectroscopy, and spectroscopic ellipsometry. The simultaneous Raman and infrared activity of phonon modes and the observation of second harmonic generation establishes a non-centrosymmetric point group for Pbn+1(Ti0.5Fe0.5)nO3n+1−δ, a prerequisite for (but not proof of) ferroelectricity. No evidence of macroscopic ferromagnetism was found in SQUID magnetometry. The ultrafast optical response exhibited coherent magnon oscillations compatible with local magnetic order, and additionally was used to study photocarrier cooling on picosecond timescales. An optical gap smaller than that of BiFeO3 and long photocarrier lifetimes may make this system interesting as a ferroelectric photovoltaic.

Papers from 2014

Terahertz spectroscopy of quantum 2D electron systems

J. Lloyd-Hughes J. Phys. D: Appl. Phys., 47:374006 (Sept 2014) [ pdf ][ ref ] Terahertz time-domain spectroscopy permits the coherent motion of charges to be examined in a diverse range of two-dimensional semiconductor heterostructures. Studies of the THz conductivity and magnetoconductivity of two-dimensional quantum systems are reviewed, including cyclotron resonance spectroscopy and the transverse conductivity in the Hall and quantum Hall regimes. Experiments are described that demonstrate quantum phenomena at THz frequencies, principally coherent control and enhanced light–matter coupling in electromagnetic cavities.

Influence of nonmagnetic Zn substitution on the lattice and magnetoelectric dynamical properties of the multiferroic material CuO

S.P.P. Jones, N.C. Wurz, M. Failla, D. Prabhakaran, C.F. McConville, J. Lloyd-Hughes Phys. Rev. B, 90:064405 (Aug 2014) [ pdf ][ ref ] Electromagnons were observed in the high-temperature multiferroic CuO even after Zn substitution disturbed the spin structure. The results demonstrate that electromagnons and dynamic magnetoelectric coupling can be maintained in disordered spin systems. Further, the dynamic lattice response of CuZnO was investigated by Raman and Fourier-transform spectroscopy, and was used to show strong spin-phonon coupling in both the antiferromagnetic low-temperature phase and the intermediate-temperature multiferroic phase.

High-temperature electromagnons in the magnetically induced multiferroic cupric oxide driven by intersublattice exchange

S.P.P. Jones, S.M. Gaw, K.I. Doig, D. Prabhakaran, E.M. Hétroy Wheeler, A.T. Boothroyd, J. Lloyd-Hughes Nat. Commun., 5:3787 (Apr 2014) [ pdf with Supp. Info. ][ ref ] Magnetically induced ferroelectric multiferroics present an exciting new paradigm in the design of multifunctional materials, by intimately coupling magnetic and polar order. Magnetoelectricity creates a novel quasiparticle excitation - the electromagnon - at terahertz frequencies, with spectral signatures that unveil important spin interactions. To date, electromagnons have been discovered at low temperature (<70K) and predominantly in rare-earth compounds such as RMnO3. Here we demonstrate using terahertz time-domain spectroscopy that intersublattice exchange in the improper multiferroic cupric oxide (CuO) creates electromagnons at substantially elevated temperatures (213-230K). Dynamic magnetoelectric coupling can therefore be achieved in materials, such as CuO, that exhibit minimal static cross-coupling. The electromagnon strength and energy track the static polarization, highlighting the importance of the underlying cycloidal spin structure. Polarized neutron scattering and terahertz spectroscopy identify a magnon in the antiferromagnetic ground state, with a temperature dependence that suggests a significant role for biquadratic exchange.

Modifying the polarization state of terahertz radiation using anisotropic twin-domains in LaAlO₃

J. Lloyd-Hughes, S.P.P. Jones, E. Castro-Camus, K.I. Doig, J.L. Macmanus-Driscoll Optics Lett., 39 1121 (Mar 2014) [ pdf ][ ref ] 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.

Papers from 2013

Coherent magnon and acoustic phonon dynamics in tetragonal and rare-earth-doped BiFeO$_{3}$ multiferroic thin films

K.I. Doig, F. Aguesse, A.K. Axelsson, N.M. Alford, S. Nawaz, V.R. Palkar, S.P.P. Jones, R.D. Johnson, R.A. Synowicki, J. Lloyd-Hughes Phys. Rev. B, 88:094425 (Sep 2013) [ pdf ][ ref ] Coherent magnons and acoustic phonons were impulsively excited and probed in thin films of the room temperature multiferroic Bi_{1-x-y}Dy_xLa_yFeO3 using femtosecond laser pulses. The elastic moduli of rhombohedral, tetragonal, and rare-earth doped BiFeO3 were determined from acoustic-mode frequencies in conjunction with spectroscopic ellipsometry. A weak ferromagnetic order, induced alternately by magnetization in the growth direction or by tetragonality, created a magnon oscillation at 75 GHz, indicative of a Dzyaloshinskii-Moriya interaction energy of 0.31 meV.

Direct Observation of Charge-Carrier Heating at WZ–ZB InP Nanowire Heterojunctions

C.K. Yong, J. Wong-Leung, H.J. Joyce, J. Lloyd-Hughes, Q. Gao, H.H. Tan, C. Jagadish, M.B. Johnston, L.M. Herz Nano Lett., 13:1 (Aug 2013) [ pdf ][ ref ] We have investigated the dynamics of hot charge carriers in InP nanowire ensembles containing a range of densities of zinc-blende inclusions along the otherwise wurtzite nanowires. From time-dependent photoluminescence spectra, we extract the temperature of the charge carriers as a function of time after nonresonant excitation. We find that charge-carrier temperature initially decreases rapidly with time in accordance with efficient heat transfer to lattice vibrations. However, cooling rates are subsequently slowed and are significantly lower for nanowires containing a higher density of stacking faults. We conclude that the transfer of charges across the type II interface is followed by release of additional energy to the lattice, which raises the phonon bath temperature above equilibrium and impedes the carrier cooling occurring through interaction with such phonons. These results demonstrate that type II heterointerfaces in semiconductor nanowires can sustain a hot charge-carrier distribution over an extended time period. In photovoltaic applications, such heterointerfaces may hence both reduce recombination rates and limit energy losses by allowing hot-carrier harvesting.

Electronic properties of GaAs, InAs and InP nanowires studied by terahertz spectroscopy

H.J. Joyce, C.J. Docherty, Q. Gao, H.H. Tan, C. Jagadish, J. Lloyd-Hughes, L.M. Herz, M.B. Johnston Nanotechnology, 24:214006 (May 2013) [ pdf ][ ref ] We have performed a comparative study of ultrafast charge carrier dynamics in a range of III–V nanowires using optical pump–terahertz probe spectroscopy. This versatile technique allows measurement of important parameters for device applications, including carrier lifetimes, surface recombination velocities, carrier mobilities and donor doping levels. GaAs, InAs and InP nanowires of varying diameters were measured. For all samples, the electronic response was dominated by a pronounced surface plasmon mode. Of the three nanowire materials, InAs nanowires exhibited the highest electron mobilities of 6000 cm^2/V/s , which highlights their potential for high mobility applications, such as field effect transistors. InP nanowires exhibited the longest carrier lifetimes and the lowest surface recombination velocity of 170 cm/s. This very low surface recombination velocity makes InP nanowires suitable for applications where carrier lifetime is crucial, such as in photovoltaics. In contrast, the carrier lifetimes in GaAs nanowires were extremely short, of the order of picoseconds, due to the high surface recombination velocity, which was measured as 5.4 × 10^5 cm/s. These findings will assist in the choice of nanowires for different applications, and identify the challenges in producing nanowires suitable for future electronic and optoelectronic devices.

Papers from 2012

Ultra-low Surface Recombination Velocity in InP Nanowires Probed by Terahertz Spectroscopy

H.J. Joyce, J. Wong-Leung, C. Yong, C.J. Docherty, S. Paiman, Q. Gao, H.H. Tan, C. Jagadish, J. Lloyd-Hughes, L.M. Herz, M.B. Johnston Nano Lett., 12:5325-–5330 (Oct 2012) [ pdf ][ ref ] Using transient terahertz photoconductivity measurements, we have made non-contact, room temperature measurements of the ultrafast charge carrier dynamics in InP nanowires. InP nanowires exhibited a very long photoconductivity lifetime of over 1ns, and carrier lifetimes were remarkably insensitive to surface states despite the large nanowire surface area-to-volume ratio. An exceptionally low surface recombination velocity (170cm/s) was recorded at room temperature. These results suggest that InP nanowires are prime candidates for optoelectronic devices, particularly photovoltaic devices, without the need for surface passivation. We found that the carrier mobility is not limited by nanowire diameter, but is strongly limited by the presence of planar crystallographic defects such as stacking faults in these predominantly wurtzite nanowires. These findings show the great potential of very narrow InP nanowires for electronic devices, but indicate that improvements in the crystallographic uniformity of InP nanowires will be critical for future nanowire device engineering.

Simulation of fluence-dependent photocurrent in terahertz photoconductive receivers

E. Castro-Camus, M.B. Johnston, J. Lloyd-Hughes Semicond. Sci. Technol., 27:115011 (Sep 2012) [ pdf ][ ref ] 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.

Investigation of coherent acoustic phonons in terahertz quantum cascade laser structures using femtosecond pump-probe spectroscopy

A. Bruchhausen, J. Lloyd-Hughes, M. Hettich, R. Gebs, M. Grossmann, O. Ristow, A. Bartels, M. Fischer, M. Beck, G. Scalari, J. Faist, A. Rudra, P. Gallo, E. Kapon, T. Dekorsy J. Appl. Phys., 112:033517 (Aug 2012) [ pdf ][ ref ] The dynamics of acoustic vibrations in terahertz quantum cascade laser structures (THz-QCLs) is studied by means of femtosecond pump-probe spectroscopy. The phonon modes are characterized by the folding of the acoustic dispersion into an effective reduced Brillouin zone. An accurate identification of this dispersion allows the sample structure and periodicity to be determined with high precision on the order of 0.1%. By temperature tuning the energy of the electronic levels of the system and performing wavelength dependent measurements, we are able to study the impulsive resonant generation and detection of coherent acoustic phonon modes. These results are supported by simulations of the electronic system that well explain the experimental observations. The effects of interface (IF) roughness on coherent acoustic phonon spectra are clearly observed for equal nominal THz-QCL structures but with different interface qualities.

Ultrafast Dynamics of Exciton Formation in Semiconductor Nanowires

C.K. Yong, H.J. Joyce, J. Lloyd-Hughes, Q. Gao, H.H. Tan, C. Jagadish, M.B. Johnston, L.M. Herz Small, 8:1725--1731 (Jun 2012) [ pdf ][ ref ] The dynamics of free electron–hole pairs and excitons in GaAs–AlGaAs–GaAs core–shell–skin nanowires is investigated using femtosecond transient photoluminescence spectroscopy at 10 K. Following nonresonant excitation, a bimolecular interconversion of the initially generated electron–hole plasma into an exciton population is observed. This conducting-to-insulating transition appears to occur gradually over electron–hole charge pair densities of 2–4 × 10^16 cm-3. The smoothness of the Mott transition is attributed to the slow carrier-cooling during the bimolecular interconversion of free charge carriers into excitons and to the presence of chemical-potential fluctuations leading to inhomogeneous spectral characteristics. These results demonstrate that high-quality nanowires are model systems for investigating fundamental scientific effects in 1D heterostructures.

A Review of the Terahertz Conductivity of Bulk and Nano-Materials

J. Lloyd-Hughes, T.I. Jeon J Infrared Milli Terahz Waves, 33:871 (Sep 2012) [ pdf ][ ref ] We review pioneering and recent studies of the conductivity of solid state systems at terahertz frequencies. A variety of theoretical formalisms that describe the terahertz conductivity of bulk, mesoscopic and nanoscale materials are outlined, and their validity and limitations are given. Experimental highlights are discussed from studies of inorganic semiconductors, organic materials (such as graphene, carbon nanotubes and polymers), metallic films and strongly correlated electron systems including superconductors.

Generalized conductivity model for polar semiconductors at terahertz frequencies

J. Lloyd-Hughes Appl. Phys. Lett., 100:122103 (Mar 2012) [ pdf ][ ref ] A theoretical framework is presented that calculates the conductivity of polar semiconductors at terahertz frequencies without resorting to phenomenological fit parameters, using an expression derived from the Boltzmann transport equation. The time-dependent photoconductivity of InAs and the temperature dependent conductivity of n-doped GaAs are found experimentally by terahertz time-domain spectroscopy. The observed deviation from the Drude-Lorentz conductivity in these model systems is accounted for by including an energy-dependent electron scattering time.

Photoinduced modification of surface states in nanoporous InP

J. Lloyd-Hughes, S. Mueller, G. Scalari, H. Bishop, A. Crossley, M. Enachi, L. Sirbu, I.M. Tiginyanu Appl. Phys. Lett., 100:132106 (Mar 2012) [ pdf ][ ref ] Porous honeycombs of n-type InP were investigated by terahertz time-domain and x-ray photoemission spectroscopies. After photoexcitation the dark conductivity was found to increase quasi-irreversibly, recovering only after several hours in air. The calculated electron density for different surface pinning energies suggests that photoexcitation may reduce the density of surface states.

Papers from 2011

Observation of zone-folded acoustic phonons in terahertz quantum cascade lasers using picosecond ultrasonics

A. Bruchhausen, M. Hettich, J. Lloyd-Hughes, M. Fischer, M. Beck, G. Scalari, J. Faist, T. Dekorsy Chin. J. Phys., 49:41-48 (Feb 2011) [ pdf ][ ref ] We have investigated the time-resolved vibrational properties of terahertz quantum cascade lasers by means of ultra-fast laser spectroscopy. By observing the acoustic folded branches, and by analyzing the involved phonon modes, it is possible to extract accurate structural information about these devices, which is essential for their design and performance.

Improved performance of {GaAs}-based terahertz emitters via surface passivation and silicon nitride encapsulation

C. Headley, L. Fu, P. Parkinson, X.L. Xu, J. Lloyd-Hughes, C. Jagadish, M.B. Johnston IEEE J. Sel. Top. Quantum Electron., 17:17-21 (Jan 2011) [ pdf ][ ref ] We have improved the stability and performance of terahertz (THz) photoconductive (Auston) switches using a combination of (NH4)(2)S surface passivation (SP) and silicon nitride (Si3N4) encapsulation. The influences of SP and encapsulation on the ultrafast electron dynamics in GaAs were examined using THz emission spectroscopy and optical pump-THz probe spectroscopy. The power of THz radiation from the surface of photoexcited GaAs increased by a factor of 5 after passivation and encapsulation, while the process lengthened the trapping time for photoexcited charge carriers. By fabricating and assessing the performance of photoconductive switches, we found that passivation and encapsulation increased the average THz power generated fourfold.

Papers from 2010

Thermo-optic detection of terahertz radiation from a quantum cascade laser

A. van Kolck, M. Amanti, M. Fischer, M. Beck, J. Faist, J. Lloyd-Hughes Appl. Phys. Lett., 97:251103 (Dec 2010) [ pdf ][ ref ] We investigate the asynchronous detection of terahertz radiation from a quantum cascade laser using an electro-optic sampling apparatus. The signal does not vary substantially upon rotating the detection crystal, while a characteristic angle dependence is exhibited for synchronized time-domain pulses from a photoconductive emitter. Upon increasing the electrical modulation frequency of the cascade laser the unsynchronized signal decreases in good agreement with a thermal detector. Rather than being electro-optic in origin, we therefore ascribe the unsynchronized signal to a thermo-optic modulation of the refractive index. A simple model is in good agreement with the effect’s frequency dependence.

Magnetically assisted quantum cascade laser emitting from 740 GHz to 1.4 THz

G. Scalari, D. Turcinkova, J. Lloyd-Hughes, M.I. Amanti, M. Fischer, M. Beck, J. Faist Appl. Phys. Lett., 97:081110 (Sep 2010) [ pdf ][ ref ] In this paper we show that by applying a perpendicular magnetic field to a quantum cascade structure it is possible to enhance the gain of different optical transitions. The combination of magnetic confinement with a broadband, cutoff-free optical resonator allows the demonstration of laser action over a large bandwidth, from 733 GHz to 1.38 THz together with the emission at 3.2 THz. A different lasing scheme is revealed that does not rely on resonant tunneling as the main injection mechanism. In combination with the magnetically enhanced gain laser emission at 1 THz is observed up to a temperature of 115 K, which corresponds to a ratio kBT/hν = 2.3 between the lattice thermal energy and the laser photon energy.

Papers from 2009

Spectroscopic determination of the doping and mobility of terahertz quantum cascade structures

J. Lloyd-Hughes, Y.L. Delley, G. Scalari, M. Fischer, V. Liverini, M. Beck, J. Faist J. Appl. Phys., 106:093104 (Nov 2009) [ pdf ][ ref ] Terahertz time-domain spectroscopy is shown to provide a convenient and rapid means to measure the conductivity of individual layers in semiconductor heterostructures such as terahertz quantum cascade lasers. By modeling the complex transmission at terahertz frequencies, the electron density and the in-plane momentum scattering time of the active regions and doped contact layers were determined for both GaAs/AlGaAs and InGaAs/InAlAs epilayers. The measured temperature dependence of the electron scattering rate revealed the significance of impurity and LO phonon scattering. The implications for laser operation at room temperature are discussed by considering the changes in absorption and resonant tunneling current with temperature.

Coupling terahertz radiation between sub-wavelength metal-metal waveguides and free space using monolithically integrated horn antennae

J. Lloyd-Hughes, G. Scalari, A. van Kolck, M. Fischer, M. Beck, J. Faist Opt. Express, 17:18387--18393 (Sep 2009) [ pdf ][ ref ] Broadband horn antennae are presented that efficiently couple terahertz radiation between sub-wavelength metal-metal waveguides and free space. Sub-picosecond terahertz pulses were coupled into and out from sub-wavelength parallel-plate waveguides by using the horn antennae in a terahertz time-domain spectrometer. Monolithic antennae were fabricated at the facets of metal-metal terahertz quantum cascade lasers, and laser action was observed for devices emitting at 1.4 THz, 2.3 THz and 3.2 THz. A good far-field laser radiation pattern (FWHM less than 11degrees) is obtained as a result of the significant expansion of the optical mode by the antenna.

Papers from 2008

Photoconductive response correction for detectors of terahertz radiation

E. Castro-Camus, L. Fu, J. Lloyd-Hughes, H.H. Tan, C. Jagadish, M.B. Johnston J. Appl. Phys., 104:053113 (Sep 2008) [ pdf ][ ref ] 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.

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

P. Parkinson, J. Lloyd-Hughes, M.B. Johnston, L.M. Herz Phys. Rev. B, 78:115321 (Sep 2008) [ pdf ][ ref ] 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-(-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 −* 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.

Conductivity of nanoporous {InP} membranes investigated using terahertz spectroscopy

S.K.E. Merchant, J. Lloyd-Hughes, L. Sirbu, I.M. Tiginyanu, P. Parkinson, L.M. Herz, M.B. Johnston Nanotechnology, 19:395704 (Aug 2008) [ pdf ][ ref ] 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.

Terahertz photoconductivity of mobile electrons in nanoporous {InP} honeycombs

J. Lloyd-Hughes, S.K.E. Merchant, L. Sirbu, I.M. Tiginyanu, M.B. Johnston Phys. Rev. B, 78:085320 (Aug 2008) [ pdf ][ ref ] 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.

Low-energy collective dynamics of charge stripes in the doped nickelate {La}$_{2-x}${Sr}$_{x}${NiO}$_{4+\delta}$ observed with optical conductivity measurements

J. Lloyd-Hughes, D. Prabhakaran, A.T. Boothroyd, M.B. Johnston Phys. Rev. B, 77:195114 (May 2008) [ pdf ][ ref ] We have investigated charge dynamics in the static stripe ordered phase of La_{2-x}Sr_{x}NiO_{4+\\delta} 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.

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

J. Lloyd-Hughes, T. Richards, H. Sirringhaus, M.B. Johnston, L.M. Herz Phys. Rev. B, 77:125203 (Mar 2008) [ pdf ][ ref ] 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 ~1×10^8 Vm-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.

Terahertz magnetoconductivity of excitons and electrons in quantum cascade structures

J. Lloyd-Hughes, H.E. Beere, D.A. Ritchie, M.B. Johnston Phys. Rev. B, 77:125322 (Mar 2008) [ pdf ][ ref ] We examined the quasiparticles formed by the photoexcitation of GaAs/AlGaAs 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.

Papers from 2007

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

P.C. Upadhya, W. Fan, A. Burnett, J. Cunningham, A.G. Davies, E.H. Linfield, J. Lloyd-Hughes, E. Castro-Camus, M.B. Johnston, H. Beere Opt. Lett., 32:2297 (Aug 2007) [ pdf ][ ref ] 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.

Transient terahertz conductivity of {GaAs} nanowires

P. Parkinson, J. Lloyd-Hughes, Q. Gao, H.H. Tan, C. Jagadish, M.B. Johnston, L.M. Herz Nano Lett., 7:2162-2165 (Jul 2007) [ pdf ][ ref ] 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.

An ion-implanted InP receiver for polarization resolved terahertz spectroscopy

E. Castro-Camus, J. Lloyd-Hughes, L. Fu, H.H. Tan, C. Jagadish, M.B. Johnston Opt. Express, 15:7047-7057 (May 2007) [ pdf ][ ref ] 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.

Papers from 2006

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

J. Lloyd-Hughes, S.K.E. Merchant, L. Fu, H.H. Tan, C. Jagadish, E. Castro-Camus, M.B. Johnston Appl. Phys. Lett., 89:232102 (Dec 2006) [ pdf ][ ref ] 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.

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

J. Lloyd-Hughes, T. Richards, H. Sirringhaus, E. Castro-Camus, L.M. Herz, M.B. Johnston Appl. Phys. Lett., 89:112101 (Sep 2006) [ pdf ][ ref ] 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.

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

G. Doucas, V. Blackmore, B. Ottewell, C. Perry, P.G. Huggard, E. Castro-Camus, M.B. Johnston, J. Lloyd-Hughes, M.F. Kimmitt, B. Redlich, A. van der Meer Phys. Rev. Spec. Top.-Accel. Beams, 9:092801 (Sep 2006) [ pdf ][ ref ] 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.

Papers from 2005

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

J. Lloyd-Hughes, E. Castro-Camus, M.B. Johnston Solid State Commun., 136:595-600 (Dec 2005) [ pdf ][ ref ] 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.

Polarization-sensitive terahertz detection by multicontact photoconductive receivers

E. Castro-Camus, J. Lloyd-Hughes, M.B. Johnston, M.D. Fraser, H.H. Tan, C. Jagadish Appl. Phys. Lett., 86:254102 (Jun 2005) [ pdf ][ ref ] 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.

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

E. Castro-Camus, J. Lloyd-Hughes, M.B. Johnston Phys. Rev. B, 71:195301 (May 2005) [ pdf ][ ref ] 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.

Papers from 2004

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

J. Lloyd-Hughes, E. Castro-Camus, M.D. Fraser, C. Jagadish, M.B. Johnston Phys. Rev. B, 70:235330 (Dec 2004) [ pdf ][ ref ] 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.