ORCID Profile
0000-0002-1805-9743
Current Organisation
University of Leeds
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Publisher: IOP Publishing
Date: 21-06-2007
Publisher: American Physical Society (APS)
Date: 31-10-2011
Publisher: IEEE
Date: 09-2008
Publisher: AIP Publishing
Date: 21-03-2013
DOI: 10.1063/1.4795606
Abstract: We report three-well, resonant-phonon depopulation terahertz quantum cascade lasers with semi-insulating surface-plasmon waveguides and reduced active region (AR) thicknesses. Devices with thicknesses of 10, 7.5, 6, and 5 μm are compared in terms of threshold current density, maximum operating temperature, output power, and AR temperature. Thinner ARs are technologically less demanding for epitaxial growth and result in reduced electrical heating of devices. However, it is found that 7.5-μm-thick devices give the lowest electrical power densities at threshold, as they represent the optimal trade-off between low electrical resistance and low threshold gain.
Publisher: AIP Publishing
Date: 11-04-2011
DOI: 10.1063/1.3579258
Abstract: We report a polarization-sensitive terahertz time-domain spectroscopy system, which allows the simultaneous measurement of orthogonal components of the polarization of a free-space propagating terahertz beam using a dual electro-optic detection scheme. We demonstrate the operation of our system by measuring the birefringence of lithium niobate, simultaneously obtaining terahertz spectra from two orthogonal crystallographic directions.
Publisher: AIP Publishing
Date: 07-12-2009
DOI: 10.1063/1.3271030
Abstract: We report the development of a terahertz pulsed spectroscopic imaging system based on the concept of compressive sensing. A single-point terahertz detector, together with a set of 40 optimized two-dimensional binary masks, was used to measure the terahertz waveforms transmitted through a s le. Terahertz time- and frequency-domain images of the s le comprising 20×20 pixels were subsequently reconstructed. We demonstrate that both the spatial distribution and the spectral characteristics of a s le can be obtained by this means. Compared with conventional terahertz pulsed imaging, no raster scanning of the object is required, and ten times fewer terahertz spectra need be taken. It is therefore ideal for real-time imaging applications.
Publisher: IEEE
Date: 07-2019
Publisher: Optica Publishing Group
Date: 13-04-2021
DOI: 10.1364/OE.414178
Abstract: We report on both experiments and theory of low-terahertz frequency range (up to 400 GHz) magnetoplasmons in a gated two-dimensional electron gas at low ( K) temperatures. The evolution of magnetoplasmon resonances was observed as a function of magnetic field at frequencies up to ∼400 GHz. Full-wave 3D simulations of the system predicted the spatial distribution of plasmon modes in the 2D channel, along with their frequency response, allowing us to distinguish those resonances caused by bulk and edge magnetoplasmons in the experiments. Our methodology is anticipated to be applicable to the low temperature ( K) on-chip terahertz measurements of a wide range of other low-dimensional mesoscopic systems.
Publisher: Wiley
Date: 10-01-2012
Publisher: IEEE
Date: 09-2010
Publisher: The Optical Society
Date: 25-06-2019
DOI: 10.1364/OL.44.003314
Publisher: Optica Publishing Group
Date: 26-10-2009
DOI: 10.1364/OE.17.020631
Publisher: AIP Publishing
Date: 03-04-2006
DOI: 10.1063/1.2191423
Abstract: We report on measurements of the excitation, propagation, and detection of picosecond duration electrical pulses at cryogenic (∼4K) temperatures in a microstrip circuit. A reduction is observed in the measured excitation and propagating pulse widths at low temperatures, compared with room temperature. The results indicate both that the electrical properties of low-temperature-grown GaAs make it suitable for photoconductive excitation and detection in cryogenic optoelectric circuits and that an organic polymer is a good transmissive medium for terahertz (THZ) frequency range excitations at low temperatures. This work will prove an invaluable starting point for future guided wave terahertz experiments at cryogenic temperatures.
Publisher: AIP Publishing
Date: 31-08-2009
DOI: 10.1063/1.3216579
Abstract: We have measured the picosecond time-domain response of Goubau-line waveguides, formed on quartz substrates, by integrating regions of low-temperature-grown gallium arsenide into the waveguides to act both as pulsed current emitters and detectors. Using one pair of photoconductive switches for excitation and a second pair for detection, pulsed signal propagation of a low dispersion electric field mode was demonstrated in the Goubau-lines, with the signal bandwidth extending beyond 800 GHz. Furthermore, it was demonstrated that terahertz bandstop filters can be integrated into a Goubau-line for removal of specific frequencies from the transmitted pulses.
Publisher: IOP Publishing
Date: 05-08-2013
Publisher: AIP Publishing
Date: 06-2021
DOI: 10.1063/5.0046186
Abstract: Near-field microscopy techniques operating in the terahertz (THz) frequency band offer the tantalizing possibility of visualizing with nanometric resolution the localized THz fields supported by in idual resonators, micro-structured surfaces, and metamaterials. Such capabilities promise to underpin the future development and characterization of a wide range of devices, including THz emitters, detectors, optoelectronic modulators, sensors, and novel optical components. In this work, we report scattering-type scanning near-field optical microscopy using a THz-frequency quantum cascade laser (QCL) to probe coherently the localized field supported by in idual micro-resonator structures. Our technique demonstrates deep sub-wavelength mapping of the field distribution associated with in-plane resonator modes in plasmonic dipole antennas and split ring resonator structures. By exploiting electronic tuning of the QCL in conjunction with the coherent self-mixing effect in these lasers, we are able to resolve both the magnitude and the phase of the out-of-plane field. We, furthermore, show that the elliptically polarized state of the QCL field can be exploited for the simultaneous excitation and measurement of plasmonic resonances in these structures while suppressing the otherwise dominant signal arising from the local material permittivity.
Publisher: IEEE
Date: 09-2006
Publisher: The Optical Society
Date: 30-07-2019
DOI: 10.1364/OE.27.023164
Publisher: Springer Science and Business Media LLC
Date: 11-02-2020
DOI: 10.1038/S41467-020-14662-W
Abstract: The fast modulation of lasers is a fundamental requirement for applications in optical communications, high-resolution spectroscopy and metrology. In the terahertz-frequency range, the quantum-cascade laser (QCL) is a high-power source with the potential for high-frequency modulation. However, conventional electronic modulation is limited fundamentally by parasitic device impedance, and so alternative physical processes must be exploited to modulate the QCL gain on ultrafast timescales. Here, we demonstrate an alternative mechanism to modulate the emission from a QCL device, whereby optically-generated acoustic phonon pulses are used to perturb the QCL bandstructure, enabling fast litude modulation that can be controlled using the QCL drive current or strain pulse litude, to a maximum modulation depth of 6% in our experiment. We show that this modulation can be explained using perturbation theory analysis. While the modulation rise-time was limited to ~800 ps by our measurement system, theoretical considerations suggest considerably faster modulation could be possible.
Publisher: Springer Science and Business Media LLC
Date: 30-11-2018
DOI: 10.1038/S41467-018-07629-5
Abstract: The original version of this Article contained an error in the Acknowledgements, which incorrectly omitted the following: ‘We also acknowledge support from the Australian Research Council’s Discovery Projects Funding Scheme (Grant DP 160 103910).’ This has been corrected in both the PDF and HTML versions of the Article.
Publisher: IEEE
Date: 08-2017
Publisher: IEEE
Date: 09-2009
Publisher: AIP Publishing
Date: 30-04-2018
DOI: 10.1063/1.5027202
Abstract: Tapered coplanar waveguides with integrated photoconductors were designed, fabricated, and measured, with pulsed transmission results comparing well with High Frequency Structure Simulator simulations which predict increased confinement and electric field concentration in the tapered region. Devices made with titanium/gold metallisation were used to demonstrate transmission and confinement, while the magnetoresistive properties of devices with cobalt/copper multilayers were used to demonstrate the field concentration. In the latter case, a mathematical framework was developed to understand the relationship between tapering effects and the picosecond magnetoresistance response.
Publisher: IEEE
Date: 05-2007
Publisher: IEEE
Date: 06-2012
Publisher: Optica Publishing Group
Date: 02-08-2007
DOI: 10.1364/OL.32.002297
Abstract: 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.
Publisher: The Optical Society
Date: 27-06-2014
DOI: 10.1364/OE.22.016595
Publisher: Springer Science and Business Media LLC
Date: 06-08-2018
DOI: 10.1038/S41467-018-05601-X
Abstract: Single-mode frequency-tuneable semiconductor lasers based on monolithic integration of multiple cavity sections are important components, widely used in optical communications, photonic integrated circuits and other optical technologies. To date, investigations of the ultrafast switching processes in such lasers, essential to reduce frequency cross-talk, have been restricted to the observation of intensity switching over nanosecond-timescales. Here, we report coherent measurements of the ultrafast switch-on dynamics, mode competition and frequency selection in a monolithic frequency-tuneable laser using coherent time-domain s ling of the laser emission. This approach allows us to observe hopping between lasing modes on picosecond-timescales and the temporal evolution of transient multi-mode emission into steady-state single mode emission. The underlying physics is explained through a full multi-mode, temperature-dependent carrier and photon transport model. Our results show that the fundamental limit on the timescales of frequency-switching between competing modes varies with the underlying Vernier alignment of the laser cavity.
Publisher: AIP Publishing
Date: 05-2012
DOI: 10.1063/1.4711101
Abstract: We demonstrate that aqueous suspensions of microbubbles, formed into arrays using standing surface acoustic waves (SSAWs), can be transported by controlled modulation of the SSAW frequency. The array is repeatedly captured at a sequence of spatial positions along the acoustic beam path and long-range transportation is achieved by periodic cycling of the applied frequency across the transducer bandwidth. We also demonstrate that controllable alignment and transport can be achieved in a detachable microfluidic device, where the microfluidic channel, in which particle transport occurs, is separated from the piezoelectric substrate by an acoustic coupling gel. Proof-of-concept transport is first discussed using a test system of latex particles before the non-invasive manipulation technique is applied to arrays of microbubbles. We explore the role of acoustic radiation forces in the spatial control of particles by analysing the dynamics of particle manipulation by SSAWs. Our results highlight the exquisite control which we have over the position and transport of particles and we anticipate that this technique could find wide applications for the accurate and programmable, non-invasive ordering and transport of biological s les in microfluidic systems.
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 07-2016
Publisher: IEEE
Date: 09-2013
Publisher: IEEE
Date: 08-2017
Publisher: IEEE
Date: 08-2015
Publisher: SPIE
Date: 09-02-2006
DOI: 10.1117/12.660716
Publisher: IEEE
Date: 10-2012
Publisher: The Optical Society
Date: 11-11-2016
DOI: 10.1364/OE.24.026986
Publisher: IEEE
Date: 09-2006
Publisher: Wiley
Date: 24-07-2009
DOI: 10.1002/JRS.2352
Publisher: AIP Publishing
Date: 02-02-2009
DOI: 10.1063/1.3076127
Abstract: The two-dimensional concentration and manipulation of micron-scale particles by orthogonal, surface acoustic, standing waves is demonstrated. The particles are organized by liquid pressure waves in a microfluidic system over a piezoelectric substrate and form a uniform two-dimensional array with a spacing governed by the mechanical nodes of the two orthogonal, surface acoustic, standing waves. The nodal spacing can be controlled in each orthogonal direction independently by adjustment of the radio frequency applied to the separate acoustic wave transducers. This technique could be used to enhance the particle concentrations at sensing locations in DNA or protein array detectors.
Publisher: AIP Publishing
Date: 19-04-2016
DOI: 10.1063/1.4946845
Abstract: We demonstrate the generation of continuous wave terahertz (THz) frequency radiation from photomixers fabricated on both Fe-doped InGaAs and Fe-doped InGaAsP, grown by metal-organic chemical vapor deposition. The photomixers were excited using a pair of distributed Bragg reflector lasers with emission around 1550 nm, and THz radiation was emitted over a bandwidth of greater than 2.4 THz. Two InGaAs and four InGaAsP wafers with different Fe doping concentrations were investigated, with the InGaAs material found to outperform the InGaAsP in terms of emitted THz power. The dependencies of the emitted power on the photomixer applied bias, incident laser power, and material doping level were also studied.
Publisher: American Chemical Society (ACS)
Date: 26-03-2021
Publisher: IOP Publishing
Date: 05-04-2023
Abstract: Terahertz (THz) radiation encompasses a wide spectral range within the electromagnetic spectrum that extends from microwaves to the far infrared (100 GHz–∼30 THz). Within its frequency boundaries exist a broad variety of scientific disciplines that have presented, and continue to present, technical challenges to researchers. During the past 50 years, for instance, the demands of the scientific community have substantially evolved and with a need for advanced instrumentation to support radio astronomy, Earth observation, weather forecasting, security imaging, telecommunications, non-destructive device testing and much more. Furthermore, applications have required an emergence of technology from the laboratory environment to production-scale supply and in-the-field deployments ranging from harsh ground-based locations to deep space. In addressing these requirements, the research and development community has advanced related technology and bridged the transition between electronics and photonics that high frequency operation demands. The multidisciplinary nature of THz work was our stimulus for creating the 2017 THz Science and Technology Roadmap (Dhillon et al 2017 J. Phys. D: Appl. Phys. 50 043001). As one might envisage, though, there remains much to explore both scientifically and technically and the field has continued to develop and expand rapidly. It is timely, therefore, to revise our previous roadmap and in this 2023 version we both provide an update on key developments in established technical areas that have important scientific and public benefit, and highlight new and emerging areas that show particular promise. The developments that we describe thus span from fundamental scientific research, such as THz astronomy and the emergent area of THz quantum optics, to highly applied and commercially and societally impactful subjects that include 6G THz communications, medical imaging, and climate monitoring and prediction. Our Roadmap vision draws upon the expertise and perspective of multiple international specialists that together provide an overview of past developments and the likely challenges facing the field of THz science and technology in future decades. The document is written in a form that is accessible to policy makers who wish to gain an overview of the current state of the THz art, and for the non-specialist and curious who wish to understand available technology and challenges. A such, our experts deliver a ‘snapshot’ introduction to the current status of the field and provide suggestions for exciting future technical development directions. Ultimately, we intend the Roadmap to portray the advantages and benefits of the THz domain and to stimulate further exploration of the field in support of scientific research and commercial realisation.
Publisher: The Optical Society
Date: 04-11-2019
DOI: 10.1364/OE.27.033768
Publisher: IEEE
Date: 09-2009
Publisher: AIP Publishing
Date: 26-03-2012
DOI: 10.1063/1.3697403
Abstract: We demonstrate the directed control of charge carriers in graphene using the electric field that accompanies the propagation of surface acoustic waves (SAWs) on a piezoelectric surface. Graphene grown by chemical vapor deposition was transferred to the surface of lithium niobate, allowing its direct integration with interdigital transducers used for SAW generation and detection. Radio frequency (RF) signal applied to the transducers at their resonant frequency was found to generate a direct current flow by the transport of p-type charge carriers. The acoustically induced current scales linearly with the applied RF power and can be observed even in presence of a counter-flow current induced by an applied bias.
Publisher: IEEE
Date: 09-2006
Publisher: Springer Science and Business Media LLC
Date: 04-2006
Publisher: IEEE
Date: 08-2017
Publisher: IOP Publishing
Date: 04-01-2017
Publisher: Wiley
Date: 22-01-2008
Publisher: SPIE
Date: 29-11-2007
DOI: 10.1117/12.785002
Publisher: American Chemical Society (ACS)
Date: 20-07-2017
DOI: 10.1021/ACS.ANALCHEM.7B01235
Abstract: Terahertz-frequency-range measurements can offer potential insight into the picosecond dynamics, and therefore function, of many chemical systems. There is a need to develop technologies capable of performing such measurements in aqueous and polar environments, particularly when it is necessary to maintain the full functionality of biological s les. In this study, we present a proof-of-concept technology comprising an on-chip planar Goubau line, integrated with a microfluidic channel, which is capable of low-loss, terahertz-frequency-range spectroscopic measurements of liquids. We also introduce a mathematical model that accounts for changes in the electric field distribution around the waveguide, allowing accurate, frequency-dependent liquid parameters to be extracted. We demonstrate the sensitivity of this technique by measuring a homologous alcohol series across the 0.1-0.8 THz frequency range.
Publisher: American Chemical Society (ACS)
Date: 06-08-2013
DOI: 10.1021/AC401657R
Abstract: The absorption coefficient and refractive index have been measured for a homologous series of tetraalkylammonium bromides over the frequency range 0.3-5.5 THz. Spectral features are found to shift to lower frequencies as the molecular mass is increased, as expected. However, to understand the detailed structure of the observed spectral features, density functional perturbation theory calculations have been performed on the first four crystalline compounds in the series. From these calculations, we find that each spectrum is dominated by three translatory modes involving asymmetric motion of the ammonium cation and bromine counterion, although the overall number of active modes increases with increasing molecular size. The experimentally observed absorption is not completely described by the infrared active phonon modes alone. We show that it is also necessary to include the coupling of the phonon modes with the macroscopic field generated by the collective displacement of the vibrating ions, and we have applied an effective medium theory, which accounts for particle shape to allow for this effect in the calculation of the terahertz spectra.
Publisher: AIP Publishing
Date: 21-01-2008
DOI: 10.1063/1.2835705
Abstract: We demonstrate an evanescent field modality for terahertz frequency time-domain measurements, based on the interaction between a s le and the evanescent field extending above lithographically defined terahertz waveguides. We quantify this interaction using freely positionable dielectric s les (GaAs) moved in close proximity to the waveguide (a terahertz microstrip line), finding a reduction in the microstrip-propagating pulse litude and an increase in its time delay when the dielectric is brought into the microstrip evanescent field. We also show that the frequency response of resonant passive circuit elements (stub band-stop filters), integrated into the microstrip line, can be used to determine the terahertz frequency properties of scanned s les, opening the way for a terahertz subwavelength imaging modality, the resolution of which is limited by lithographic constraints, rather than by free-space diffraction.
Publisher: AIP Publishing
Date: 17-05-2005
DOI: 10.1063/1.1938255
Abstract: We report the operation of band-stop filters in the terahertz (THz) frequency range, working at a center frequency of 600GHz. The filters were characterized by embedding them in a microstrip line attached to photoconductive switches which act as THz emitters and detectors. The filters have applications in sensing and detection. The chosen filter design allows cascading of several filters along the same microstrip line, which is of particular importance for a proposed molecular sensing array.
Publisher: SPIE
Date: 28-09-2200
DOI: 10.1117/12.689389
Publisher: SPIE
Date: 27-04-2007
DOI: 10.1117/12.719283
Publisher: Royal Society of Chemistry (RSC)
Date: 2009
DOI: 10.1039/B817839A
Abstract: Terahertz frequency time-domain spectroscopy has been used to analyse a wide range of s les containing cocaine hydrochloride, heroin and ecstasy--common drugs-of-abuse. We investigated real-world s les seized by law enforcement agencies, together with pure drugs-of-abuse, and pure drugs-of-abuse systematically adulterated in the laboratory to emulate real-world s les. In order to investigate the feasibility of automatic spectral recognition of such illicit materials by terahertz spectroscopy, principal component analysis was employed to cluster spectra of similar compounds.
Publisher: International Union of Crystallography (IUCr)
Date: 14-12-2010
DOI: 10.1107/S0021889810043372
Abstract: The application of terahertz time-domain spectroscopy and imaging to the study of proteins in crystalline form is demonstrated. Terahertz time-domain spectroscopy is particularly sensitive to the long-range ordering of molecules, with proven utility for the spectroscopy of crystalline biological small molecules. Here, the terahertz time-domain absorption response of a macromolecular protein single crystal is investigated for the first time. In particular, the effect of dehydration on the terahertz absorption coefficient of tetragonal hen egg white lysozyme crystals is reported.
Publisher: IEEE
Date: 09-2009
Publisher: IEEE
Date: 09-2010
Publisher: AIP Publishing
Date: 08-2013
DOI: 10.1063/1.4816736
Abstract: We present the self-aligned fabrication of on-chip devices in which waveguides, incorporating integrated photoconductive switches, are combined with two-dimensional electron systems to allow probing of the ultrafast (terahertz frequency range) properties of confined semiconductor systems, both at cryogenic temperatures and in high magnetic fields. We demonstrate the direct injection of on-chip terahertz pulses into the mesoscopic system by femtosecond, near infra-red laser excitation of in-plane photoconductive switches formed on an epitaxially grown, low-temperature GaAs layer, which is integrated monolithically with a GaAs/AlGaAs heterostructure containing a two-dimensional electron system. Both the input and output terahertz signals of an on-chip waveguide are s led by altering dynamically the photoconductive excitation/detection arrangement in situ on a single device. We also demonstrate a new method for sub-Kelvin excitation and detection of on-chip terahertz frequency radiation in a 3He/4He dilution refrigerator that allows the photocurrent and detected terahertz transient to be mapped as function of the near-infrared excitation position at the emitter and the detector, respectively. Furthermore, we demonstrate transmission of terahertz transients through a two-dimensional electron system in a coplanar waveguide under magnetic field at temperatures as low as 200 mK.
Publisher: Royal Society of Chemistry (RSC)
Date: 2013
DOI: 10.1039/C3LC50485A
Abstract: We investigate the effect of substrate thickness on the transmission bandwidth of on-chip terahertz-frequency-range planar Goubau lines both experimentally and theoretically. The bandwidth and frequency resolution are improved through substrate thinning and geometry modifications (reducing reflections arising from the THz photoconductive generators and detectors). We demonstrate that the enhanced bandwidth (2 THz) and resolution (3.75 GHz) allows this type of on-chip waveguide to be used for spectroscopic measurements of polycrystalline materials from cryogenic (4 K) to room temperature (292 K) by recording vibrational absorption spectra from overlaid s les of lactose monohydrate.
Publisher: SAGE Publications
Date: 06-2007
DOI: 10.1366/000370207781269701
Abstract: Broadband terahertz time-domain spectroscopy (THz-TDS) has been used to measure the far-infrared (FIR) vibrational spectra of several commonly used pure explosives, including 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), 1,3-dinitrato-2,2-bis(nitratomethyl)-propane (PETN), and two types of plastic explosive, SEMTEX and SX2. A number of distinct absorption peaks, originating from FIR-active vibrational modes of these polycrystalline energetic materials, were observed in the frequency range 0.3–7.5 THz (10–250 cm −1 ). In addition, the temperature-dependent FIR vibrational spectra of PETN were measured between 4 K and 296 K with several well-resolved absorption peaks observed across this temperature range. We find that as the temperature is reduced, the observed absorption peaks resolve into narrower features and shift towards higher frequencies. The temperature dependence of the spectra is explained in terms of the anharmonicity of the vibrational potentials of crystalline compounds, and an empirical fit is given to describe the peak shift with temperature.
Publisher: Wiley
Date: 26-01-2010
Abstract: The terahertz frequency spectrum of pentaerythritol tetranitrate (PETN) is calculated using Discover with the COMPASS force field, CASTEP and PWscf. The calculations are compared to each other and to terahertz spectra (0.3-3 THz) of crystalline PETN recorded at 4 K. A number of analysis methods are used to characterise the calculated normal modes.
Publisher: IEEE
Date: 05-2007
Publisher: Elsevier BV
Date: 03-2008
Publisher: IEEE
Date: 08-2017
Publisher: IOP Publishing
Date: 29-10-2014
DOI: 10.1088/0953-8984/26/47/475801
Abstract: We have investigated experimentally the scaling behaviour of quantum Hall transitions in GaAs/AlGaAs heterostructures of a range of mobility, carrier concentration, and spacer layer width. All three critical scaling exponents γ, κ and p were determined independently for each s le. We measure the localization length exponent to be γ ≈ 2.3, in good agreement with expected predictions from scaling theory, but κ and p are found to possess non-universal values. Results obtained for κ range from κ = 0.16 ± 0.02 to κ = 0.67 ± 0.02, and are found to be Landau level (LL) dependent, whereas p is found to decrease with increasing s le mobility. Our results demonstrate the existence of two transport regimes in the LL conductivity peak universality is found within the quantum coherent transport regime present in the tails of the conductivity peak, but is absent within the classical transport regime found close to the critical point at the centre of the conductivity peak. We explain these results using a percolation model and show that the critical scaling exponent depends on certain important length scales that correspond to the microscopic description of electron transport in the bulk of a two-dimensional electron system.
Publisher: IEEE
Date: 09-2013
Publisher: IOP Publishing
Date: 14-03-2019
Publisher: IEEE
Date: 09-2008
Publisher: IEEE
Date: 08-2015
Publisher: IEEE
Date: 09-2008
Publisher: AIP Publishing
Date: 05-04-2011
DOI: 10.1063/1.3575201
Abstract: We report terahertz (THz) diffuse reflectance measurements of bulk powdered s les at a frequency of 2.83 THz using a narrowband quantum cascade laser. S les studied comprise polydisperse powders with absorption coefficients extending over two orders of magnitude from ∼3 cm−1 to & cm−1. Diffuse reflectance measurements are used to obtain the effective absorption coefficient of these s les from the backscattering cross-section, predicted under the quasi-crystalline approximation (QCA) in the T-matrix formulation and in conjunction with the Percus-Yevick pair distribution function. Results are compared with effective absorption coefficients obtained from THz time-domain spectroscopy measurements on pressed pellet s les, and show good agreement over the range of effective absorption coefficients studied. We observe that the backscattering cross-section predicted under the QCA is strongly dependent on both the real and imaginary components of the complex permittivity of the s le, and we show that reliable determination of the absorption coefficient from diffuse reflectance measurements therefore requires knowledge of the s le's refractive index. This work demonstrates the applicability of diffuse reflectance measurements, using a THz frequency quantum cascade laser, to the high-resolution spectroscopic analysis of bulk powdered s les at THz frequencies.
Publisher: Springer Science and Business Media LLC
Date: 22-12-2015
Publisher: SPIE
Date: 09-02-2006
DOI: 10.1117/12.659662
Publisher: Springer Science and Business Media LLC
Date: 28-09-2018
DOI: 10.1038/S42005-018-0059-7
Abstract: Acoustoelectric devices convert acoustic energy to electrical energy and vice versa. Devices working at much higher acoustic frequencies than those currently available have potential scientific and technological applications, for ex le, as detectors in phononics experiments and as transducers in bulk acoustic wave filters at terahertz (THz) frequencies. Here we demonstrated an active acoustoelectronic device based on a GaAs heterostructure: an acoustically gated transistor or phonotransistor. Instead of being controlled in the conventional manner by an electrical signal applied to a metallic or semiconductor gate as in a high electron mobility transistor (HEMT), the drain-source current was controlled by a bulk sub-THz acoustic wave passing through the channel in a direction perpendicular to the current flow.
Publisher: IOP Publishing
Date: 22-08-2012
Publisher: IEEE
Date: 09-2013
Publisher: AIP Publishing
Date: 03-11-2008
DOI: 10.1063/1.3013349
Abstract: We demonstrate that terahertz microstrip-line waveguides can be used to measure absorption spectra of polycrystalline materials with a high frequency resolution (∼2 GHz) and with a spatial resolution that is determined by the microstrip-line dimensions, rather than the free-space wavelength. The evanescent terahertz-bandwidth electric field extending above the microstrip line interacts with, and is modified by, overlaid dielectric s les, thus enabling the characteristic vibrational absorption resonances in the s le to be probed. As an ex le, the terahertz absorption spectrum of polycrystalline lactose monohydrate was investigated the lowest lying mode was observed at 534(±2) GHz, in excellent agreement with free-space measurements. This microstrip technique offers both a higher spatial and frequency resolution than free-space terahertz time-domain spectroscopy and requires no contact between the waveguide and s le.
Publisher: IEEE
Date: 09-2010
Publisher: IEEE
Date: 09-2010
Publisher: IEEE
Date: 09-2012
Publisher: IEEE
Date: 09-2013
Publisher: IEEE
Date: 06-2007
Publisher: IEEE
Date: 09-2013
Publisher: AIP Publishing
Date: 10-05-2010
DOI: 10.1063/1.3427191
Abstract: We demonstrate the generation of broadband terahertz (THz) frequency radiation from photoconductive emitters formed from Fe-doped InGaAs (Fe:InGaAs), grown by metal-organic chemical vapor deposition, following pulsed (femtosecond) laser excitation at wavelengths ranging from 830 nm to 1.55 μm. The Fe is incorporated epitaxially during growth, giving precise control over the doping level. Using both single-crystal ZnTe and GaP electro-optic detectors over the same wavelength range, the emission spectra from several Fe:InGaAs wafers with different Fe content were measured, with THz emission from all wafers showing bandwidths in excess of 2.0 THz. The THz output power was found to be strongly dependant on the Fe content, the thickness of the Fe:InGaAs layer, and the excitation wavelength.
Publisher: The Optical Society
Date: 06-01-2017
DOI: 10.1364/OE.25.000486
Publisher: AIP Publishing
Date: 21-03-2011
DOI: 10.1063/1.3571289
Abstract: We report the detection of terahertz frequency radiation using photoconductive antennas fabricated from Fe-doped InGaAs, grown by metal-organic chemical vapor deposition. Coherent photoconductive detection is demonstrated using femtosecond laser pulses centered at either an 800 or a 1550 nm wavelength. The InGaAs resistivity and the sensitivity of photoconductive detection are both found to depend on the Fe-doping level. We investigate a wide range of probe laser powers, finding a peak in detected signal for ∼5 mW probe power, followed by a reduction at larger powers, attributed to screening of the detected THz field by photo-generated carriers in the material. The measured signal from Fe:InGaAs photoconductive detectors excited at 800 nm is four times greater than that from a low-temperature-grown GaAs photodetector with identical antenna design, despite the use of a ten times smaller probe power.
Publisher: AIP Publishing
Date: 15-09-2006
DOI: 10.1063/1.2335601
Abstract: We report on a numerical study of the sensitivity of electrical terahertz (THz) frequency range passive filters under conditions of dielectric loading. We investigate band-pass filters, ring resonators, and band-stop filters, all of which have been proposed as devices capable of measuring the hybridization state of overlaid DNA films, thereby acting as genetic sensors. The transmission characteristics of these filters are examined when loaded with s les of different relative permittivities and thicknesses, and these results are discussed in the context of recent experimental works. A comparative analysis of the three filter types is performed, and conclusions are drawn on the suitability of the various filters for the analysis of DNA films during genetic testing and other dielectric materials.
Publisher: International Union of Crystallography (IUCr)
Date: 25-12-2013
DOI: 10.1107/S1600576713029506
Abstract: The effect of temperature on the terahertz-frequency-range material properties of lyophilized and single-crystal hen egg-white lysozyme has been measured using terahertz time-domain spectroscopy, with the results presented and discussed in the context of protein and solvent dynamical and glass transitions. Lyophilized hen egg-white lysozyme was measured over a temperature range from 4 to 290 K, and a change in the dynamical behaviour of the s le at around 100 K was observed through a change in the terahertz absorption spectrum. Additionally, the effect of cryoprotectants on the temperature-dependent absorption coefficient is studied, and it is demonstrated that terahertz time-domain spectroscopy is capable of resolving the true glass transition temperature of single-crystal hen egg-white lysozyme at ∼150 K, which is in agreement with literature values measured using differential scanning calorimetry.
Publisher: IEEE
Date: 10-2011
Publisher: AIP Publishing
Date: 28-01-2008
DOI: 10.1063/1.2838748
Abstract: We report on the use of standing surface acoustic waves, formed on a single-crystal piezoelectric substrate, to organize micron-scale latex particles into an array comprising a series of lines in an adjacent microfluidic system. The lines of particles are formed parallel to the substrate surface and perpendicular to the surface acoustic wave vector. They extend across the width of the acoustic beam aperture, with a periodicity of one-half the surface acoustic wavelength. The position and spacing of the particle arrays can be altered by adjusting the acoustic wave frequency within the device passband. We discuss the mechanism responsible for the formation of the lines, which could be widely applicable to the alignment of microscopic objects held in suspension.
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
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Funder: Engineering and Physical Sciences Research Council
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Funder: Engineering and Physical Sciences Research Council
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Funder: Engineering and Physical Sciences Research Council
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Funder: Engineering and Physical Sciences Research Council
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Funder: Engineering and Physical Sciences Research Council
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