H. E. Beere

Publications (114)191.79 Total impact

• Article: Mode-locking of a terahertz laser by direct phase synchronization.

  J Maysonnave, K Maussang, J R Freeman, N Jukam, J Madéo, P Cavalié, R Rungsawang, S P Khanna, E H Linfield, A G Davies, H E Beere, D A Ritchie, S S Dhillon, J Tignon
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  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 9ps duration, characterized unambiguously in the time domain.
  Optics Express 09/2012; 20(19):20855-62. · 3.59 Impact Factor
• Source

  Available from: ArXiv

  Article: Ultra-shallow quantum dots in an undoped GaAs/AlGaAs 2DEG

  W. Y. Mak, F. Sfigakis, K. Das Gupta, O. Klochan, H. E. Beere, I. Farrer, J. P. Griffiths, G. A. C. Jones, A. R. Hamilton, D. A. Ritchie
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  ABSTRACT: We report quantum dots fabricated on very shallow 2-dimensional electron gases (2DEG), only 30nm below the surface, in undoped GaAs/AlGaAs heterostuctures grown by molecular beam epitaxy. Due to the absence of dopants, an improvement of up to two orders of magnitude in mobility with respect to doped heterostructures with similar depths is observed. These undoped wafers can easily be gated with surface metallic gates patterned by e-beam lithography, as demonstrated here from single-level transport through a quantum dot showing large charging energies (up to 1.75 meV) and excited state energies (up to 0.5 meV).
  12/2011;
• Article: Broad gain in a bound-to-continuum quantum cascade laser with heterogeneous active region

  J. R. Freeman, A. Brewer, J. Madéo, P. Cavalié, S. S. Dhillon, J. Tignon, H. E. Beere, D. A. Ritchie
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  ABSTRACT: We demonstrate the operation of heterogeneous terahertz quantum cascade lasers with broadened gain by optimising the sub-stacks to align at the same field. In single plasmon waveguides, we find two-colour operation for nearly the entire dynamic range of the lasers with similar performance to homogeneous lasers. Time domain spectroscopy measurements confirm that a flat gain spectrum is present and the sub-stacks align at the same time. When incorporated into metal-metal waveguides, we find that performance is consistent with the constituent sub-stacks and there is broadband operation over 380 GHz.
  Applied Physics Letters 12/2011; 99:241108. · 3.84 Impact Factor
• Source

  Available from: ArXiv

  Article: Linear non-hysteretic gating of a very high density 2DEG in an undoped metal-semiconductor-metal sandwich structure

  K. Das Gupta, A. F. Croxall, W. Y. Mak, H. E. Beere, C. A. Nicoll, I. Farrer, F. Sfigakis, D. A. Ritchie
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  ABSTRACT: Modulation doped GaAs-AlGaAs quantum well based structures are usually used to achieve very high mobility 2-dimensional electron (or hole) gases. Usually high mobilities ($>10^{7}{\rm{cm}^{2}\rm{V}^{-1}\rm{s}^{-1}}$) are achieved at high densities. A loss of linear gateability is often associated with the highest mobilites, on account of a some residual hopping or parallel conduction in the doped regions. We have developed a method of using fully undoped GaAs-AlGaAs quantum wells, where densities $\approx{6\times10^{11}\rm{cm}^{-2}}$ can be achieved while maintaining fully linear and non-hysteretic gateability. We use these devices to understand the possible mobility limiting mechanisms at very high densities.
  11/2011;
• Source

  Available from: ArXiv

  Article: Stabilization of single-electron pumps by high magnetic fields

  J. D. Fletcher, M. Kataoka, S. P. Giblin, Sunghun Park, H. -S. Sim, P. See, T. J. B. M. Janssen, J. P. Griffiths, G. A. C. Jones, H. E. Beere, D. A. Ritchie
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  ABSTRACT: We study the effect of perpendicular magnetic fields on a single-electron system with a strongly time-dependent electrostatic potential. Continuous improvements to the current quantization in these electron pumps are revealed by high-resolution measurements. Simulations show that the sensitivity of tunnel rates to the barrier potential is enhanced, stabilizing particular charge states. Nonadiabatic excitations are also suppressed due to a reduced sensitivity of the Fock-Darwin states to electrostatic potential. The combination of these effects leads to significantly more accurate current quantization.
  07/2011;
• Article: Gain enhancement in a terahertz quantum cascade laser with parylene antireflection coatings

  R. Rungsawang, N. Jukam, J. Maysonnave, P. Cavalié, J. Madéo, D. Oustinov, S. S. Dhillon, J. Tignon, P. Gellie, C. Sirtori, S. Barbieri, H. E. Beere, D. A. Ritchie
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  ABSTRACT: We study the effect of parylene antireflection coatings on the gain of a 2.8 THz quantum cascade laser using terahertz time-domain spectroscopy. With antireflection coatings the threshold current increases as the mirror losses are increased, and the gain clamps at 16 cm−1, compared to 10 cm−1 for an uncoated device. These values are consistent with a drop in reflectivity from 0.320 to 0.053 as a consequence of the coating deposition. Further improvements could reveal the bare cavity gain and permit the quantum cascade laser to be used as an efficient terahertz amplifier.
  Applied Physics Letters 03/2011; 98(10):101102-101102-3. · 3.84 Impact Factor
• Source

  Available from: ArXiv

  Article: Enhancement of edge channel transport by a low frequency irradiation

  A. D. Chepelianskii, J. Laidet, I. Farrer, H. E. Beere, D. A. Ritchie, H. Bouchiat
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  ABSTRACT: The magnetotransport properties of high mobility two dimensional electron gas have recently attracted a significant interest due to the discovery of microwave induced zero resistance states. Here we show experimentally that microwave irradiation with a photon energy much smaller than the spacing between Landau levels can induce a strong decrease in the four terminal resistance. This effect is not predicted by the bulk transport models introduced to explain zero resistance states, but can be naturally explained by an edge transport model. This highlights the importance of edge channels for zero resistance state physics that was proposed recently.
  02/2011;
• Source

  Available from: hindawi.com

  Article: Experimental Progress towards Probing the Ground State of an Electron-Hole Bilayer by Low-Temperature Transport

  K. Das Gupta, A. F. Croxall, Waldie J, C. A. Nicoll, H. E. Beere, Farrer I, D. A. Ritchie, Pepper M
  Advances in Condensed Matter Physics. 01/2011;
• Source

  Available from: ArXiv

  Article: Distinguishing impurity concentrations in GaAs and AlGaAs, using very shallow undoped heterostructures

  W. Y. Mak, K. Das Gupta, H. E. Beere, I. Farrer, F. Sfigakis, D. A. Ritchie
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  ABSTRACT: We demonstrate a method of making a very shallow, gateable, undoped 2-dimensional electron gas. We have developed a method of making very low resistivity contacts to these structures and systematically studied the evolution of the mobility as a function of the depth of the 2DEG (from 300nm to 30nm). We demonstrate a way of extracting quantitative information about the background impurity concentration in GaAs and AlGaAs, the interface roughness and the charge in the surface states from the data. This information is very useful from the perspective of molecular beam epitaxy (MBE) growth. It is difficult to fabricate such shallow high-mobility 2DEGs using modulation doping due to the need to have a large enough spacer layer to reduce scattering and switching noise from remote ionsied dopants. Comment: 4 pages, 5 eps figures
  07/2010;
• Source

  Available from: 144.206.159.178

  Article: Pulsed terahertz time domain spectroscopy of vertically structured photoconductive antennas

  R. Faulks, S. Rihani, H. E. Beere, M. J. Evans, D. A. Ritchie, M. Pepper
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  ABSTRACT: We present a terahertz (THz) photoconductive emitter structure, which employs a n-doped layer underneath a low-temperature-grown GaAs region to enable the THz transient to couple vertically through a defined mesa. A nonlinear bias dependence is observed, yielding an order in magnitude improvement in power for a mesa device with a 100 μm2 area over a conventional planar control reference device at 32 V and 5 mW illumination power. We relate the bias dependence of the THz signal to the breakdown voltage observed in the current-voltage characteristic. Reducing the antenna gap size through reducing the thickness of the low temperature-GaAs region below 1 μm shows a large improvement in the bandwidth of the device, with an enhancement of the normalized intensity between 0.2 to 2 THz for a bow-tie antenna geometry.
  Applied Physics Letters 02/2010; 96(8):081106-081106-3. · 3.84 Impact Factor
• Source

  Available from: Isabelle Sagnes

  Article: Microwave modulation of terahertz quantum cascade lasers: a transmission-line approach

  W. Maineult, L. Ding, P. Gellie, P. Filloux, C. Sirtori, S. Barbieri, T. Akalin, J.-F. Lampin, I. Sagnes, H. E. Beere, D. A. Ritchie
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  ABSTRACT: We report on microwave impedance measurements of metal-metal ridge-waveguide terahertz quantum cascade lasers. Experimental data, recorded at 4 K in the 100 MHz–55 GHz range, are well reproduced by distributed-parameter transmission-line simulations, showing that the modulation cutoff is limited by the propagation losses that increase for higher microwave frequencies, yielding a 3 dB modulation bandwidth of ∼ 70 GHz for a 1 mm-long ridge. By using a shunt-stub matching we demonstrate amplitude modulation of a 2.3 THz QCL up to 24 GHz.
  Applied Physics Letters 01/2010; 96(2):021108-021108-3. · 3.84 Impact Factor
• Conference Proceeding: Terahertz plasmonic antennas for Quantum Cascade Lasers

  T. Akalin, W. Maineult, P. Gellie, E. Peytavit, A. Andronico, P. Filloux, G. Leo, J.-F. Lampin, C. Sirtori, H.E. Beere, D.A. Ritchie, S. Barbieri
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  ABSTRACT: Quantum Cascade Lasers (QCL) are probably the most promising sources for Terahertz wave generation. The key issue especially for double metal QCLs is the extraction and the beam shaping of the emitted power. We propose a solution with a TEM-Horn Antenna and we compare it to the most recently proposed solutions: plasmonic array, Silicon-lens etc. New Terahertz antenna designs will also be presented.
  Microwave Conference, 2009. APMC 2009. Asia Pacific; 01/2010
• Article: Quasi-periodic distributed feedback laser

  L.~Mahler, A.~Tredicucci, F.~Beltram, C.~Walther, J.~Faist, H.~E.~Beere, D.~A.~Ritchie, D.~S.~Wiersma
  Nature Photonics. 01/2010; 4:165-169.
• Article: Differential near-field scanning optical microscopy with THz quantum cascade laser sources.

  R Degl'Innocenti, M Montinaro, J Xu, V Piazza, P Pingue, A Tredicucci, F Beltram, H E Beere, D A Ritchie
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  ABSTRACT: We have realized a differential Near-field Scanning Optical Microscope (NSOM) working with subwavelength resolution in the THz spectral region. The system employs a quantum cascade laser emitting at lambda approximately 105 microm as source, and the method, differently from conventional NSOM, involves diffracting apertures with size comparable to the wavelength. This concept ensures a higher signal-to-noise level at the expense of an additional computational step. In the implementation here reported lambda/10 resolution has been achieved; present limiting factors are investigated through finite difference time domain simulations.
  Optics Express 12/2009; 17(26):23785-92. · 3.59 Impact Factor
• Article: Wide dynamic range terahertz detector pixel for active spectroscopic imaging with quantum cascade lasers

  S. Cibella, M. Ortolani, R. Leoni, G. Torrioli, L. Mahler, Ji-Hua Xu, A. Tredicucci, H. E. Beere, D. A. Ritchie
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  ABSTRACT: A superconducting bolometer with an on-chip lithographic terahertz antenna has been illuminated by two quantum cascade lasers operating at 2.5 and 4.4 THz. The detector displays a 1.2 μs time constant, a noise equivalent power of 20 fW/Hz1/2 and a 60 dB dynamic range. We fabricated a monolithic prototype detector array of five elements. This scalable detector is a suitable candidate for terahertz spectroscopic imaging systems, as it can measure both full illuminator power and strongly attenuated or diffuse reflected signals in subsequent frames.
  Applied Physics Letters 11/2009; 95(21):213501-213501-3. · 3.84 Impact Factor
• Source

  Available from: Jihua Xu

  Article: Spectral behavior of a terahertz quantum-cascade laser.

  J M Hensley, Juan Montoya, M G Allen, J Xu, L Mahler, A Tredicucci, H E Beere, D A Ritchie
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  ABSTRACT: In this paper, the spectral behavior of two terahertz (THz) quantum cascade lasers (QCLs) operating both pulsed and cw is characterized using a heterodyne technique. Both lasers emitting around 2.5 THz are combined onto a whisker contact Schottky diode mixer mounted in a corner cube reflector. The resulting difference frequency beatnote is recorded in both the time and frequency domain. From the frequency domain data, we measure the effective laser linewidth and the tuning rates as a function of both temperature and injection current and show that the current tuning behavior cannot be explained by temperature tuning mechanisms alone. From the time domain data, we characterize the intrapulse frequency tuning behavior, which limits the effective linewidth to approximately 5 MHz.
  Optics Express 10/2009; 17(22):20476-83. · 3.59 Impact Factor
• Article: Molecular beam epitaxy of high mobility In0.75Ga0.25As for electron spin transport applications

  P. J. Simmonds, S. N. Holmes, H. E. Beere, I. Farrer, F. Sfigakis, D. A. Ritchie, M. Pepper
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  ABSTRACT: The authors describe the molecular beam epitaxy of relaxed, nominally undoped In _0.75 Ga _0.25 As – In _0.75 Al _0.25 As quantum well structures grown on InP substrates. The maximum two-dimensional electron density is 2×10¹¹ cm ^-2 , with a peak mobility of 2.2×10⁵ cm ² V ^-1 s ^-1 at 1.5 K . In high magnetic field, the electron g -factor was shown to have a magnitude of 9.1±0.1 at Landau-level filling factor of 4. The Rashba coefficient, determined from the analysis of the magnetoresistance at high Landau-level filling factor (≫12) , is 1×10^-11 eV m . The mobility is sufficiently high in these two-dimensional electron gases that spin-orbit effects are observed up to 4.2 K . The interface asymmetry, defined as the difference between the wavefunction penetration into the upper and lower In _0.75 Al _0.25 As quantum barriers, makes no contribution to the Rashba spin-orbit coupling parameter in this system. Quantum wires defined in these two-dimensional electron gases using insulated, split surface gates show clear quarter-integer quantized conductance plateaux at exactly 0.25(2e²/h) and 0.75(2e²/h) in nonequilibrium transport. In _0.75 Ga _0.25 As may have important application as an- alternative field effect transistor channel to silicon, and the large electronic g -factor and Rashba spin-orbit coupling parameter make this material combination suitable for exploring spin related phenomena in one-dimensional systems.
  Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 08/2009; · 1.34 Impact Factor
• Source

  Available from: ArXiv

  Article: Nuclear spin coherence in a quantum wire

  A. Corcoles, C. J. B. Ford, M. Pepper, G. A. C. Jones, H. E. Beere, D. A. Ritchie
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  ABSTRACT: We have observed millisecond-long coherent evolution of nuclear spins in a quantum wire at 1.2 K. Local, all-electrical manipulation of nuclear spins is achieved by dynamic nuclear polarization in the breakdown regime of the Integer Quantum Hall Effect combined with pulsed Nuclear Magnetic Resonance. The excitation thresholds for the breakdown are significantly smaller than what would be expected for our sample and the direction of the nuclear polarization can be controlled by the voltage bias. As a four-level spin system, the device is equivalent to two qubits. Comment: 5 pages, 5 figures
  06/2009;
• Article: Low temperature transport in undoped mesoscopic structures

  S. Sarkozy, K. Das Gupta, C. Siegert, A. Ghosh, M. Pepper, I. Farrer, H. E. Beere, D. A. Ritchie, G. A. C. Jones
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  ABSTRACT: Undoped GaAs/AlGaAs heterostructures in which carriers are attracted from the Ohmic contacts by a voltage bias on an insulated top gate allows higher mobilities to be obtained at lower electron densities than is possible with modulation doped heterostructures. However a two level gating scheme and an Ohmic contacting process that maximizes lateral diffusion are necessary to fully exploit the advantages of the undoped system for fabricating lower dimensional mesoscopic structures. Ionized background impurities (at low densities) and interface roughness (at high densities) are found to be the dominant sources of scattering. An approximate length scale set by the number of impurities the interfacial wave function intersects is observed in the magnetoconductance of two-dimensional mesoscopic regions.
  Applied Physics Letters 05/2009; · 3.84 Impact Factor
• Article: SPIE Defense, Security, and Sensing

  H. Richter, A. D. Semenov, S. G. Pavlov, L. Mahler, A. Tredicucci, H. E. Beere, D. A. Ritchie, M. Ortolani, U. Schade, K. S. Il'in, M. Siegel, H.-W. Hübers
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  ABSTRACT: The terahertz (THz) portion of the electromagnetic spectrum provides specific spectroscopic information for substance identification. It has been shown that the spectral features of explosive materials might be used for detection and identification at stand-off distances. We report on the development of a THz spectrometer for explosive detection and identification. The system is based on THz quantum cascade lasers working at different frequencies. These are used for illumination of the object under test. The reflected and backscattered radiation from the object under test is detected with a sensitive heterodyne receiver. As a first step a single frequency, liquid-cryogen free heterodyne receiver operating at 2.5 THz has been developed. In order to realize maximum sensitivity a phonon-cooled NbN hot electron bolometric mixer with a quantum cascade laser as local oscillator were chosen. The concept of the system and first results will be presented.© (2009) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
  04/2009;