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Woodward RM, Wallace VP, Arnone DD, Linfield EH, Pepper M. Terahertz pulsed imaging of skin cancer in the time and frequency domain. J Biol Phys 2013; 29:257-9. [PMID: 23345842 DOI: 10.1023/a:1024409329416] [Citation(s) in RCA: 228] [Impact Index Per Article: 20.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Terahertz Pulsed Imaging(TPI) is a new medical imaging modality forthe detection of epithelial cancers. Overthe last two years this technique has beenapplied to the study of in vitrobasal cell carcinoma (BCC). Usingtime-domain analysis the contrast betweendiseased and normal tissue has been shownto be statistically significant, andregions of increased terahertz (THz)absorption correlated well with thelocation of the tumour sites in histology.Understanding the source of this contrastthrough frequency-domain analysis mayfacilitate the diagnosis of skin cancer andrelated skin conditions using TPI. Wepresent the first frequency-domain analysisof basal cell carcinoma in vitro,with the raw power spectrum giving aninsight into the surface features of theskin. Further data manipulation is requiredto determine whether spectral informationcan be extrapolated at depth. These resultshighlight the complexity of working inreflection geometry.
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Shen YC, Davies AG, Linfield EH, Taday PF, Arnone DD, Elsey TS. Determination of Glucose Concentration in Whole Blood using Fourier-Transform Infrared Spectroscopy. J Biol Phys 2013; 29:129-33. [PMID: 23345828 DOI: 10.1023/a:1024480423056] [Citation(s) in RCA: 34] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Fourier-transform infrared(FTIR) transmission spectroscopy has beenused for the determination of glucoseconcentrations in whole blood samples fromtwenty-eight patients. A four-vectorpartial least squares calibration model,using the spectral range 950-1200 cm(-1),yielded a standard error of prediction of0.59 mM for an independent test set. Forblood samples from a single patient, wefound that the glucose concentration wasproportional to the difference between thevalues of the second derivative spectrum at1082 cm(-1) and 1093 cm(-1), suggestingthat these two specific wavelengths can beused for determining glucose concentrationsin blood.
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Maysonnave J, Maussang K, Freeman JR, Jukam N, Madéo J, Cavalié P, Rungsawang R, Khanna SP, Linfield EH, Davies AG, Beere HE, Ritchie DA, Dhillon SS, Tignon J. Mode-locking of a terahertz laser by direct phase synchronization. OPTICS EXPRESS 2012; 20:20855-20862. [PMID: 23037209 DOI: 10.1364/oe.20.020855] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/01/2023]
Abstract
A novel scheme to achieve mode-locking of a multimode laser is demonstrated. Traditional methods to produce ultrashort laser pulses are based on modulating the cavity gain or losses at the cavity roundtrip frequency, favoring the pulsed emission. Here, we rather directly act on the phases of the modes, resulting in constructive interference for the appropriated phase relationship. This was performed on a terahertz quantum cascade laser by multimode injection seeding with an external terahertz pulse, resulting in phase mode-locked terahertz laser pulses of 9 ps duration, characterized unambiguously in the time domain.
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Maysonnave J, Jukam N, Ibrahim MSM, Maussang K, Madéo J, Cavalié P, Dean P, Khanna SP, Steenson DP, Linfield EH, Davies AG, Tignon J, Dhillon SS. Integrated injection seeded terahertz source and amplifier for time-domain spectroscopy. OPTICS LETTERS 2012; 37:731-733. [PMID: 22344163 DOI: 10.1364/ol.37.000731] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/31/2023]
Abstract
We used a terahertz (THz) quantum cascade laser (QCL) as an integrated injection seeded source and amplifier for THz time-domain spectroscopy. A THz input pulse is generated inside a QCL by illuminating the laser facet with a near-IR pulse from a femtosecond laser and amplified using gain switching. The THz output from the QCL is found to saturate upon increasing the amplitude of the THz input power, which indicates that the QCL is operating in an injection seeded regime.
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Ravaro M, Manquest C, Sirtori C, Barbieri S, Santarelli G, Blary K, Lampin JF, Khanna SP, Linfield EH. Phase-locking of a 2.5 THz quantum cascade laser to a frequency comb using a GaAs photomixer. OPTICS LETTERS 2011; 36:3969-3971. [PMID: 22002355 DOI: 10.1364/ol.36.003969] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/31/2023]
Abstract
We report the heterodyne detection and phase locking of a 2.5 THz quantum cascade laser (QCL) using a terahertz frequency comb generated in a GaAs photomixer using a femtosecond fiber laser. With 10 mW emitted by the QCL, the phase-locked signal at the intermediate frequency yields 80 dB of signal-to-noise ratio in a bandwidth of 1 Hz.
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Dean P, Burnett AD, Tych K, Khanna SP, Lachab M, Cunningham JE, Linfield EH, Davies AG. Measurement and analysis of the diffuse reflectance of powdered samples at terahertz frequencies using a quantum cascade laser. J Chem Phys 2011; 134:134304. [DOI: 10.1063/1.3575201] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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32
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Lever L, Hinchcliffe NM, Khanna SP, Dean P, Ikonic Z, Evans CA, Davies AG, Harrison P, Linfield EH, Kelsall RW. Terahertz ambipolar dual-wavelength quantum cascade laser. OPTICS EXPRESS 2009; 17:19926-19932. [PMID: 19997216 DOI: 10.1364/oe.17.019926] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/28/2023]
Abstract
Terahertz frequency quantum cascade lasers (THz QCLs) are compact solid-state sources of terahertz radiation that were first demonstrated in 2002. They have a broad range of potential applications ranging from gas sensing and non-destructive testing, through to security and medical imaging, with many polycrystalline compounds having distinct fingerprint spectra in the terahertz frequency range. In this article, we demonstrate an electrically-switchable dual-wavelength THz QCL which will enable spectroscopic information to be obtained within a THz QCL-based imaging system. The device uses the same active region for both emission wavelengths: in forward bias, the laser emits at 2.3 THz; in reverse bias, it emits at 4 THz. The corresponding threshold current densities are 490 A/cm(2) and 330 A/cm(2), respectively, with maximum operating temperatures of 98K and 120 K.
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Chassagneux Y, Colombelli R, Maineults W, Barbieri S, Khanna SP, Linfield EH, Davies AG. Predictable surface emission patterns in terahertz photonic-crystal quantum cascade lasers. OPTICS EXPRESS 2009; 17:9491-9502. [PMID: 19506596 DOI: 10.1364/oe.17.009491] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/27/2023]
Abstract
We demonstrate a framework to understand and predict the far-field emission in terahertz frequency photonic-crystal quantum cascade lasers. The devices, which employ a high-performance three-well active region, are lithographically tunable and emit in the 104-120 microm wavelength range. A peak output power of 7 mW in pulsed mode is obtained at 10 K, and the typical device maximum operating temperature is 136 K. We identify the photonic-crystal band-edge states involved in the lasing process as originating from the hexapole and monopole modes at the G point of the photonic band structure, as designed. The theoretical far-field patterns, obtained via finite-difference time-domain simulations, are in excellent agreement with experiment. Polarization measurements further support the theory, and the role of the bonding wires in the emission process is elucidated.
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Shaukat MU, Dean P, Khanna SP, Lachab M, Chakraborty S, Linfield EH, Davies AG. Generation of Bessel beams using a terahertz quantum cascade laser. OPTICS LETTERS 2009; 34:1030-1032. [PMID: 19340209 DOI: 10.1364/ol.34.001030] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/27/2023]
Abstract
We report the generation of Bessel beams using polytetrafluoroethene conical lenses and a quantum cascade laser emitting at 2.8 THz. The formation of a central beam spot that retains its size over distances exceeding the characteristic Rayleigh range is demonstrated, and the power transport properties of these beams are compared with those obtained using parabolic reflectors. These lenses could provide an attractive alternative to parabolic reflectors for terahertz imaging applications where a large depth of focus and/or efficient and controllable coupling of radiation onto a small target are desirable.
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Fan WH, Burnett A, Upadhya PC, Cunningham J, Linfield EH, Davies AG. Far-infrared spectroscopic characterization of explosives for security applications using broadband terahertz time-domain spectroscopy. APPLIED SPECTROSCOPY 2007; 61:638-43. [PMID: 17650376 DOI: 10.1366/000370207781269701] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/16/2023]
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.
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Wood C, Cunningham J, Hunter IC, Tosch P, Linfield EH, Davies AG. On-chip pulsed terahertz systems and their applications. ACTA ACUST UNITED AC 2007. [DOI: 10.1007/s10762-006-9107-6] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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37
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Barbieri S, Alton J, Beere HE, Linfield EH, Ritchie DA, Withington S, Scalari G, Ajili L, Faist J. Heterodyne mixing of two far-infrared quantum cascade lasers by use of a point-contact Schottky diode. OPTICS LETTERS 2004; 29:1632-1634. [PMID: 15309842 DOI: 10.1364/ol.29.001632] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/24/2023]
Abstract
We demonstrate heterodyne mixing of two free-running, multimode, 3.3-THz quantum cascade lasers by use of a point-contact Schottky diode. By temperature tuning the emission wavelength of one laser, a difference frequency signal spanning the 2-4-GHz range is obtained, with a signal-to-noise ratio of 30 dB. The frequency of the heterodyne signal is subject to random fluctuations of a few megahertz, principally from instabilities in the temperatures of the devices. From single-shot measurements we derive an instantaneous linewidth for a single Fabry-Perot mode of 20 kHz, corresponding to an integration time of 3.6 ms.
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Giazotto F, Grove-Rasmussen K, Fazio R, Beltram F, Linfield EH, Ritchie DA. Josephson Current in Nb/InAs/Nb Highly Transmissive Ballistic Junctions. ACTA ACUST UNITED AC 2004. [DOI: 10.1023/b:josc.0000021231.01260.94] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
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39
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Barkan A, Tittel FK, Mittleman DM, Dengler R, Siegel PH, Scalari G, Ajili L, Faist J, Beere HE, Linfield EH, Davies AG, Ritchie DA. Linewidth and tuning characteristics of terahertz quantum cascade lasers. OPTICS LETTERS 2004; 29:575-577. [PMID: 15035475 DOI: 10.1364/ol.29.000575] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/24/2023]
Abstract
We have measured the spectral linewidths of three continuous-wave quantum cascade lasers operating at terahertz frequencies by heterodyning the free-running quantum cascade laser with two far-infrared gas lasers. Beat notes are detected with a GaAs diode mixer and a microwave spectrum analyzer, permitting very precise frequency measurements and giving instantaneous linewidths of less than -30 kHz. Characteristics are also reported for frequency tuning as the injection current is varied.
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Safonov SS, Savchenko AK, Bagrets DA, Jouravlev ON, Nazarov YV, Linfield EH, Ritchie DA. Enhanced shot noise in resonant tunneling via interacting localized states. PHYSICAL REVIEW LETTERS 2003; 91:136801. [PMID: 14525327 DOI: 10.1103/physrevlett.91.136801] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/03/2003] [Indexed: 05/24/2023]
Abstract
In a variety of mesoscopic systems shot noise is seen to be suppressed in comparison with its Poisson value. In this work we observe a considerable enhancement of shot noise in the case of resonant tunneling via localized states. We present a model of correlated transport through two localized states which provides both a qualitative and a quantitative description of this effect.
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Li L, Proskuryakov YY, Savchenko AK, Linfield EH, Ritchie DA. Magnetoresistance of a 2D electron gas caused by electron interactions in the transition from the diffusive to the ballistic regime. PHYSICAL REVIEW LETTERS 2003; 90:076802. [PMID: 12633258 DOI: 10.1103/physrevlett.90.076802] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/29/2002] [Indexed: 05/24/2023]
Abstract
On a high-mobility 2D electron gas we have observed, in strong magnetic fields (omega(c)tau>1), a parabolic negative magnetoresistance caused by electron-electron interactions in the regime of k(B)Ttau/ variant Planck's over 2pi approximately 1, which is the transition from the diffusive to the ballistic regime. From the temperature dependence of this magnetoresistance the interaction correction to the conductivity deltasigma(ee)(xx)(T) is obtained in the situation of a long-range fluctuation potential and strong magnetic field. The results are compared with predictions of the new theory of interaction-induced magnetoresistance.
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Abstract
The terahertz region of the electromagnetic spectrum spans the frequency range between the mid-infrared and the millimetre/microwave. This region has not been exploited fully to date owing to the limited number of suitable (in particular, coherent) radiation sources and detectors. Recent demonstrations, using pulsed near-infrared femtosecond laser systems, of the viability of THz medical imaging and spectroscopy have sparked international interest; yet much research still needs to be undertaken to optimize both the power and bandwidth in such THz systems. In this paper, we review how femtosecond near-infrared laser pulses can be converted into broad band THz radiation using semiconductor crystals, and discuss in depth the optimization of one specific generation mechanism based on ultra-fast transport of electrons and holes at a semiconductor surface. We also outline a few of the opportunities for a technology that can address a diverse range of challenges spanning the physical and biological sciences, and note the continuing need for the development of solid state, continuous wave, THz sources which operate at room temperature.
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43
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Johnston MB, Whittaker DM, Dowd A, Davies AG, Linfield EH, Li X, Ritchie DA. Generation of high-power terahertz pulses in a prism. OPTICS LETTERS 2002; 27:1935-1937. [PMID: 18033407 DOI: 10.1364/ol.27.001935] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/25/2023]
Abstract
A compact, high-power emitter of half-cycle terahertz (THz) radiation is demonstrated. The device consists of an epitaxial InAs emitter upon a GaAs prism and produces THz pulses that are 20 times more powerful than those from conventional planar InAs emitters. This improvement is a direct result of reorienting the transient THz dipole such that its axis is not perpendicular to the emitting surface.
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Zolleis KR, Ford CJB, Kardynal B, Ritchie DA, Linfield EH, Rose PD, Jones GAC. Tunneling spectroscopy of a two-dimensionally periodic electron system. PHYSICAL REVIEW LETTERS 2002; 89:146803. [PMID: 12366065 DOI: 10.1103/physrevlett.89.146803] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/27/2001] [Indexed: 05/23/2023]
Abstract
The tunneling current between an electron gas with a periodic potential in two dimensions and a plain two-dimensional electron system (2DES) has been studied. The strength of the periodic potential, the subband energy of the plain 2DES, and an applied in-plane magnetic field were varied, mapping the Fourier transform of the periodic wave function. Periodic peaks were observed and explained by translations in the reciprocal lattice. When the potential was strongly modulated to form an array of antidots, commensurability peaks were seen in lateral transport, but, as expected, not in tunneling.
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Hamilton AR, Simmons MY, Pepper M, Linfield EH, Ritchie DA. Metallic behavior in dilute two-dimensional hole systems. PHYSICAL REVIEW LETTERS 2001; 87:126802. [PMID: 11580536 DOI: 10.1103/physrevlett.87.126802] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/06/2000] [Indexed: 05/23/2023]
Abstract
We have studied the metallic behavior in low-density two-dimensional p-GaAs systems, close to the apparent metal-insulator transition. Two observations are made concerning the origins of the metallic-like behavior. Within a given sample the strength of the metallic behavior is almost independent of the asymmetry of the confining potential, and is predominantly determined by the low-temperature resistivity (i.e., by k(F)l). In all our samples we find that at low densities, close to the transition from insulating to metallic behavior, the fractional decrease in conductivity with increasing temperature scales as T/T(F).
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Micolich AP, Taylor RP, Davies AG, Bird JP, Newbury R, Fromhold TM, Ehlert A, Linke H, Macks LD, Tribe WR, Linfield EH, Ritchie DA, Cooper J, Aoyagi Y, Wilkinson PB. Evolution of fractal patterns during a classical-quantum transition. PHYSICAL REVIEW LETTERS 2001; 87:036802. [PMID: 11461579 DOI: 10.1103/physrevlett.87.036802] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/14/2000] [Indexed: 05/23/2023]
Abstract
We investigate how fractals evolve into nonfractal behavior as the generation process is gradually suppressed. Fractals observed in the conductance of semiconductor billiards are of particular interest because the generation process is semiclassical and can be suppressed by transitions towards either fully classical or fully quantum-mechanical conduction. Investigating a range of billiards, we identify a "universal" behavior in the changeover from fractal to nonfractal conductance, which is described by a smooth evolution rather than deterioration in the fractal scaling properties.
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Simmons MY, Hamilton AR, Pepper M, Linfield EH, Rose PD, Ritchie DA. Weak localization, hole-hole interactions, and the "Metal"-insulator transition in two dimensions. PHYSICAL REVIEW LETTERS 2000; 84:2489-2492. [PMID: 11018917 DOI: 10.1103/physrevlett.84.2489] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/22/1999] [Indexed: 05/23/2023]
Abstract
A detailed investigation of the metallic behavior in high-quality GaAs-AlGaAs two-dimensional hole systems reveals the presence of quantum corrections to the resistivity at low temperatures. Despite the low density ( r(s)>10) and high quality of these systems, both weak localization (observed via negative magnetoresistance) and weak hole-hole interactions (giving a correction to the Hall constant) are present in the so-called metallic phase where the resistivity decreases with decreasing temperature. If these quantum corrections persist down to T = 0, the results suggest that even at high r(s) there is no metallic phase in two dimensions.
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Shapira S, Linfield EH, Lambert CJ, Seviour R, Volkov AF, Zaitsev AV. Anomalous behavior of superconducting-normal mesoscopic structures near T(c). PHYSICAL REVIEW LETTERS 2000; 84:159-162. [PMID: 11015859 DOI: 10.1103/physrevlett.84.159] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/21/1999] [Indexed: 05/23/2023]
Abstract
We observe a maximum in the conductance of Al/n-GaAs junctions at temperatures 20 mK lower than the superconducting transition temperature (T(c)). This is the first observation of a peak in the conductance near the superconducting transition in superconducting-normal (S/N) junctions. To accommodate this effect we calculate the full temperature dependence of the conductance of these structures, invoking quasiclassical Green's functions in the diffusive limit. In addition to the well-known low-temperature peak at temperatures on the order of the Thouless energy, we find a maximum near T(c). This peak has the same origin as the subgap conductance observed in S/N junctions at low temperatures.
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Turner N, Nicholls JT, Linfield EH, Brown KM, Jones GA, Ritchie DA. Tunneling between parallel two-dimensional electron gases. PHYSICAL REVIEW. B, CONDENSED MATTER 1996; 54:10614-10624. [PMID: 9984858 DOI: 10.1103/physrevb.54.10614] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/12/2023]
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50
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Hughes RJ, Savchenko AK, Frost JE, Linfield EH, Nicholls JT, Pepper M, Kogan E, Kaveh M. Distribution-function analysis of mesoscopic hopping conductance fluctuations. PHYSICAL REVIEW. B, CONDENSED MATTER 1996; 54:2091-2100. [PMID: 9986060 DOI: 10.1103/physrevb.54.2091] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/12/2023]
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