# School of Physics and Astronomy

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## Recent PublicationsView all

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##### Article: Chemically-specific dual/differential CARS micro-spectroscopy of saturated and unsaturated lipid droplets
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ABSTRACT: We have investigated the ability of dual-frequency Coherent Antistokes Raman Scattering (D-CARS) micro-spectroscopy, based on femtosecond pulses (100 fs or 5 fs) spectrally focussed by glass dispersion, to distinguish the chemical composition of micron-sized lipid droplets consisting of different triglycerides types (poly-unsaturated glyceryl trilinolenate, mono-unsaturated glyceryl trioleate and saturated glyceryl tricaprylate and glyceryl tristearate) in a rapid and label-free way. A systematic comparison of Raman spectra with CARS and D-CARS spectra was used to identify D-CARS spectral signatures which distinguish the disordered poly-unsaturated lipids from the more ordered saturated ones both in the CH-stretch vibration region and in the fingerprint region, without the need for lengthy CARS multiplex acquisition and analysis. D-CARS images of the lipid droplets at few selected wavenumbers clearly resolved the lipid composition differences, and exemplify the potential of this technique for label-free chemically selective rapid imaging of cytosolic lipid droplets in living cells. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim).
Journal of Biophotonics 01/2014; 7(1-2). DOI:10.1002/jbio.201200197
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##### Article: EFM data mapped into 2D images of tip-sample contact potential difference and capacitance second derivative
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ABSTRACT: We report a simple technique for mapping Electrostatic Force Microscopy (EFM) bias sweep data into 2D images. The method allows simultaneous probing, in the same scanning area, of the contact potential difference and the second derivative of the capacitance between tip and sample, along with the height information. The only required equipment consists of a microscope with lift-mode EFM capable of phase shift detection. We designate this approach as Scanning Probe Potential Electrostatic Force Microscopy (SPP-EFM). An open-source MATLAB Graphical User Interface (GUI) for images acquisition, processing and analysis has been developed. The technique is tested with Indium Tin Oxide (ITO) and with poly(3-hexylthiophene) (P3HT) nanowires for organic transistor applications.
Scientific Reports 11/2013; 3:3352. DOI:10.1038/srep03352
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##### Article: Measuring the Lamellarity of Giant Lipid Vesicles with Differential Interference Contrast Microscopy
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ABSTRACT: Giant unilamellar vesicles are a widely utilized model membrane system, providing free-standing bilayers unaffected by support-induced artifacts. To measure the lamellarity of such vesicles, fluorescence microscopy is one commonly utilized technique, but it has the inherent disadvantages of requiring lipid staining, thereby affecting the intrinsic physical and chemical properties of the vesicles, and it requires a calibration by statistical analysis of a vesicle ensemble. Herein we present what we believe to be a novel label-free optical method to determine the lamellarity of giant vesicles based on quantitative differential interference contrast (qDIC) microscopy. The method is validated by comparison with fluorescence microscopy on a statistically significant number of vesicles, showing correlated quantization of the lamellarity. Importantly, qDIC requires neither sample-dependent calibration nor sample staining, and thus can measure the lamellarity of any giant vesicle without additional preparation or interference with subsequent investigations. Furthermore, qDIC requires only a microscope equipped with differential interference contrast and a digital camera.
Biophysical Journal 09/2013; 105(6):1414-20. DOI:10.1016/j.bpj.2013.07.048
• ##### Article: Chain length and temperature dependence of alkanedithiol molecular conductance under ultra high vacuum
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ABSTRACT: We report scanning tunnelling microscope (STM) measurements of the single molecule conductance of α,ω-alkanedithiols for a large range of molecular chain lengths (N = 3-10) and temperatures (180-390 K) under ultra high vacuum. Two STM-based measurement techniques were employed on molecules trapped between tip and substrate: (i) the well established current-distance or I(z) technique and (ii) a new I(V,z) technique in which the current-voltage characteristics are determined as the tip-substrate distance z is varied. Low, medium, and high conductance groups were observed for each molecular length, which were temperature independent over the range examined, consistent with off-resonance tunnelling. For N > 4 the current-voltage characteristics and conductance trend with chain length is well described using a simple rectangular tunnel barrier model with parameters in excellent agreement with previously reported values. However, both 1,3-propanedithiol (N = 3) and 1,4-butanedithiol (N = 4) show an anomalous behaviour which is qualitatively similar to, but much less pronounced than, that reported by Haiss et al. (Phys. Chem. Chem. Phys., 2009, 11, 10831) for measurements performed under air and nitrogen gas.
Nanoscale 08/2013; 5(19). DOI:10.1039/c3nr03682k
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##### Article: Non-circular beam correction to the CMB power spectrum
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ABSTRACT: In the era of high precision CMB measurements, systematic effects are beginning to limit the ability to extract subtler cosmological information. The non-circularity of the experimental beam has become progressively important as CMB experiments strive to attain higher angular resolution and sensitivity. The effect of non-circular beam on the power spectrum is important at multipoles larger than the beam-width. For recent experiments with high angular resolution, optimal methods of power spectrum estimation are computationally prohibitive and sub-optimal approaches, such as the Pseudo-Cl method are used. We provide an analytic framework for correcting the power spectrum for the effect of beam non-circularity and non-uniform sky coverage (including incomplete/masked sky maps). The approach is perturbative in the distortion of the beam from non-circularity allowing for rapid computations when the beam is mildly non-circular. We advocate that when the non-circular beams are important, it is computationally advantageous to employ ‘soft’ azimuthally apodized masks whose spherical harmonic transforms die down fast with m.
New Astronomy Reviews 07/2013; DOI:10.1016/j.newar.2006.09.011
• ##### Article: Dual/differential coherent anti-Stokes Raman scattering module for multiphoton microscopes with a femtosecond Ti:sapphire oscillator
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ABSTRACT: In the last decade, coherent anti-Stokes Raman scattering (CARS) microscopy has emerged as a powerful multiphoton imaging technique offering label-free chemical sensitivity and high three-dimensional resolution. However, its widespread application in the life sciences has been hampered by the use of costly pulsed lasers, the existence of a nonresonant background requiring involved technical solutions for its efficient suppression, and the limited acquisition speed of multiplex techniques addressing several vibrational resonances, if improved chemical specificity is needed. We have recently reported a differential CARS technique (D-CARS), which simultaneously measures two vibrational frequencies, enhancing the chemical selectivity and sensitivity without introducing costly hardware, while maintaining fast acquisition. In this study, we demonstrate a compact, fully automated, cost-effective module, which integrates on hardware and software level with a commercial multiphoton microscope based on a single 100 fs Ti:Sapphire oscillator and enables D-CARS microscopy in a user-friendly format for applications in the life sciences.
Journal of Biomedical Optics 06/2013; 18(6):66004. DOI:10.1117/1.JBO.18.6.066004
• ##### Article: Masia et al. Reply:
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ABSTRACT: A Reply to the Comment by M. J. Fernée, et al..
Physical Review Letters 11/2012; 109(22):229702. DOI:10.1103/PhysRevLett.109.229702
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##### Article: Drift-Diffusion Model of the Fragmentation of the External Ring Structure in the Photoluminescence Pattern Emitted by Indirect Excitons in Coupled Quantum Wells
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ABSTRACT: Under optical excitation, coupled quantum wells are known to reveal fascinating features in the photoluminescence pattern originating from dipole orientated indirect excitons. The appearance of an external ring has been attributed to macroscopic charge separation in the quantum well plane. We present a classical model of non-linear diffusion to account for the observed fragmentation of the external ring into a periodic array of islands. The model incorporates the Coulomb interactions between electrons, holes and indirect excitons. At low temperatures, these interactions lead to pattern formation similar to the experimentally observed ring fragmentation. The fragmentation is found to persist to temperatures above the quantum degeneracy temperature of indirect excitons.
Physical Review Letters 11/2012; 109(18):187402. DOI:10.1103/PhysRevLett.109.187402
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##### Article: Fast electron transfer through a single molecule natively structured redox protein
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ABSTRACT: The electron transfer properties of proteins are normally measured as molecularly averaged ensembles. Through these and related measurements, proteins are widely regarded as macroscopically insulating materials. Using scanning tunnelling microscopy (STM), we present new measurements of the conductance through single-molecules of the electron transfer protein cytochrome b(562) in its native conformation, under pseudo-physiological conditions. This is achieved by thiol (SH) linker pairs at opposite ends of the molecule through protein engineering, resulting in defined covalent contact between a gold surface and a platinum-iridium STM tip. Two different orientations of the linkers were examined: a long-axis configuration (SH-LA) and a short-axis configuration (SH-SA). In each case, the molecular conductance could be 'gated' through electrochemical control of the heme redox state. Reproducible and remarkably high conductance was observed in this relatively complex electron transfer system, with single-molecule conductance values peaking around 18 nS and 12 nS for the SH-SA and SH-LA cytochrome b(562) molecules near zero electrochemical overpotential. This strongly points to the important role of the heme co-factor bound to the natively structured protein. We suggest that the two-step model of protein electron transfer in the STM geometry requires a multi-electron transfer to explain such a high conductance. The model also yields a low value for the reorganisation energy, implying that solvent reorganisation is largely absent.
Nanoscale 10/2012; 4:7106-7113. DOI:10.1039/c2nr32131a
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##### Article: Is Black-Hole Ringdown a Memory of Its Progenitor?
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ABSTRACT: We perform an extensive numerical study of coalescing black-hole binaries to understand the gravitational-wave spectrum of quasinormal modes excited in the merged black hole. Remarkably, we find that the masses and spins of the progenitor are clearly encoded in the mode spectrum of the ringdown signal. Some of the mode amplitudes carry the signature of the binary's mass ratio, while others depend critically on the spins. Simulations of precessing binaries suggest that our results carry over to generic systems. Using Bayesian inference, we demonstrate that it is possible to accurately measure the mass ratio and a proper combination of spins even when the binary is itself invisible to a detector. Using a mapping of the binary masses and spins to the final black-hole spin allows us to further extract the spin components of the progenitor. Our results could have tremendous implications for gravitational astronomy by facilitating novel tests of general relativity using merging black holes.
Physical Review Letters 10/2012; 109(14):141102. DOI:10.1103/PhysRevLett.109.141102
• ##### Article: Direct Binding of a Redox Protein for Single-Molecule Electron Transfer Measurements
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ABSTRACT: An electron transfer protein is engineered with two thiol groups introduced at different positions in the molecular structure to allow robust binding to two gold electrodes. Atomic force microscopy and scanning tunneling microscopy single-molecule studies show that the engineered proteins: (1) bind to a gold electrode in defined orientation dictated by the thiol-pair utilised, and (2) have a higher conductance than the wild-type proteins indicating a more efficient electron transmission due to the strong gold-thiol contacts.
Small 08/2012; 8(15):2341-4. DOI:10.1002/smll.201102416
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##### Article: Engineering the Spin-Flip Limited Exciton Dephasing in Colloidal CdSe/CdS Quantum Dots
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ABSTRACT: We have measured the intrinsic exciton dephasing in high-quality zinc blende CdSe/CdS colloidal quantum dots in the temperature range from 5 to 170 K using a sensitive three-beam photon echo technique in heterodyne detection, which is not affected by spectral diffusion. Pure dephasing via acoustic phonons dominates the initial dynamics, followed by an exponential zero-phonon line dephasing. From the temperature dependence of the zero-phonon line dephasing, the exciton lifetime, and the exciton thermalization within its fine structure, we show that the zero-phonon line dephasing of the lowest bright state originates from the phonon-assisted spin-flip to dark exciton states. Importantly, we can control the dephasing by tailoring the exciton fine structure through its dependence on the dot core size and shell thickness, as expected from the spin-flip mechanism. By reducing the electron-hole exchange interaction with increasing core size and delocalization of the electron wave function in the quasi-type-II core/shell band alignment, we find the longest zero-phonon line dephasing time of ∼110 ps at 5 K in dots with the largest core diameter (5.7 nm) and the thickest CdSe shell (9 monolayers) in the series studied.
ACS Nano 05/2012; 6(6):5227-33. DOI:10.1021/nn300992a
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##### Article: A Demonstration of Spectral and Spatial Interferometry at THz Frequencies
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ABSTRACT: A laboratory prototype spectral-spatial interferometer has been constructed to demonstrate the feasibility of the double-Fourier technique at far infrared (FIR) wavelengths (0.15-1 THz). It is planned to use this demonstrator to investigate and validate important design features and data-processing methods for future astronomical FIR interferometer instruments. In building this prototype, we have had to address several key technologies to provide an end-end system demonstration of this double-Fourier interferometer. We report on the first results taken when viewing single-slit and double-slit sources at the focus of a large collimator used to simulate real sources at infinity. The performance of the prototype instrument for these specific field geometries is analyzed to compare with the observed interferometric fringes and to demonstrate image reconstruction capabilities.
Applied Optics 04/2012; 51(12):2202-11. DOI:10.1364/AO.51.002202
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##### Article: A high signal-to-noise ratio map of the Sunyaev–Zel’dovich increment at 1.1-mm wavelength in Abell 1835
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ABSTRACT: We present an analysis of an 8-arcmin diameter map of the area around the galaxy cluster Abell 1835 from jiggle-map observations at a wavelength of 1.1 mm using the Bolometric Camera (Bolocam) mounted on the Caltech Submillimeter Observatory (CSO). The data are well described by a model including a extended Sunyaev–Zel’dovich (SZ) emission from the cluster gas plus emission from the cluster central galaxy and two bright background submm galaxies magnified by the gravitational lensing of the cluster. We measure flux densities of the two bright point sources in the field: SMM J14011+0252 and SMM J14009+0252 to be 6.5 ± 2.0 ± 0.8 and 11.3 ± 1.9 ± 1.3 mJy, respectively. Fitting the map to a sky model consisting of the point sources and the SZ emission from the cluster gas with a beta model density profile with parameters, θc= 33.6 arcsec and β= 0.69, we find the peak surface brightness of the SZ emission to be Ic= 3.73 ± 0.45 ± 0.60 × 10−21 W m−2sr−1 Hz−1, where the first error is the statistical uncertainty in the fit and the second error represents the calibration uncertainty and additional systematics. Assuming zero cluster peculiar velocity and an X-ray temperature of Te= 9 keV, this surface brightness corresponds to a central Comptonization of y0= (4.41 ± 0.53 ± 0.70) × 10−4. The cluster image represents one of the highest significance SZ detections of a cluster in the positive region of the thermal SZ spectrum to date. We compare the measured central intensity at 1.1 mm to other SZ measurements of Abell 1835 at different wavelengths to obtain values for y0= (3.58 ± 0.28) × 10−4 and the cluster peculiar velocity vz=−538 ± 414 km s−1.
Monthly Notices of the Royal Astronomical Society 03/2012; 421(1). DOI:10.1111/j.1365-2966.2011.20295.x
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##### Article: Measuring a Cosmological Distance-Redshift Relationship Using Only Gravitational Wave Observations of Binary Neutron Star Coalescences
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ABSTRACT: Detection of gravitational waves from the inspiral phase of binary neutron star coalescence will allow us to measure the effects of the tidal coupling in such systems. Tidal effects provide additional contributions to the phase evolution of the gravitational wave signal that break a degeneracy between the system's mass parameters and redshift and thereby allow the simultaneous measurement of both the effective distance and the redshift for individual sources. Using the population of O(10(3)-10(7)) detectable binary neutron star systems predicted for 3rd generation gravitational wave detectors, the luminosity distance-redshift relation can be probed independently of the cosmological distance ladder and independently of electromagnetic observations. We conclude that for a range of representative neutron star equations of state the redshift of such systems can be determined to an accuracy of 8%-40% for z<1 and 9%-65% for 1<z<4.
Physical Review Letters 03/2012; 108(9):091101. DOI:10.1103/PhysRevLett.108.091101
• ##### Article: Orientation-Dependent Electron Transport in a Single Redox Protein
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ABSTRACT: The redox-active protein cytochrome b(562) has been engineered to introduce pairs of thiol groups in the form of cysteine residues at specified sites. Successful STM imaging of the molecules adsorbed on a gold surface indicated that one thiol group controls the orientation of the molecule and that the protein maintains its native form under the experimental conditions. Stable protein-gold STM tip electrical contact was directly observed to form via the second free thiol group in current-voltage and current-distance measurements. Proteins with thiol contacts positioned across the protein's short axis displayed a conductance of (3.48 ± 0.05) × 10(-5)G(0). However proteins with thiol groups placed along the long axis reproducibly yielded two distinct values of (1.95 ± 0.03) × 10(-5)G(0) and (3.57 ± 0.11) × 10(-5)G(0), suggesting that the placement of the asymmetrically located haem within the protein influences electron transfer. In contrast, the unengineered wild-type cytochrome b(562) had conductance values at least 1 order of magnitude less. Here we show that an electron transfer protein engineered to bind gold surfaces can be controllably oriented and electrically contacted to metallic electrodes, a prerequisite for potential integration into electronic circuits.
ACS Nano 11/2011; 6(1):355-61. DOI:10.1021/nn2036818
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##### Article: Effects of Thermal Annealing Upon the Nanomorphology of Poly(3-hexylselenophene)-PCBM Blends
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ABSTRACT: Grazing incidence X-ray diffraction (GI-XRD) is used to characterize the crystallographic dynamics of low molecular weight (LMW) and high molecular weight (HMW) poly(3-hexylselenophene) (P3HS) films and blend films of P3HS with [6-6-]-phenyl-C(61) -butyric acid methyl ester (PCBM) as a function of 'step-by-step' thermal annealing, from room temperature to 250 °C. The temperature-dependent GIXRD data show how the melting point of P3HS crystallites is decreased by the presence of PCBM. P3HS crystallite domain sizes dramatically increase upon annealing to the P3HS melting temperature. The formation of well-oriented HMW P3HS crystallites with the (100) plane parallel to the substrate (edge-on orientation), when cooled from melt, are observed. We compare the behaviour of P3HS pure and blend films with that of poly(3-hexyl)thiophene (P3HT) pure and PCBM blended films and suggest that the similar temperature dependent behaviour we observe may be a common to polythiophene and related polymers and their blends.
Macromolecular Rapid Communications 09/2011; 32(18):1454-60. DOI:10.1002/marc.201100330
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##### Article: Using Computation to Decode the First Known Computer
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ABSTRACT: Researchers have used many different kinds of software to analyze the structure and astronomical functions of the Antikythera mechanism's surviving fragments. This ancient Greek calculator contains 30 gear wheels and has an extraordinarily sophisticated mechanical design.
Computer 08/2011; DOI:10.1109/MC.2011.134
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##### Article: Suppression of the parasitic buffer layer conductance in InSb/AlxIn1-xSb heterostructures using a wide-band-gap barrier layer
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ABSTRACT: InSb/AlxIn1-xSb heterostructures display intrinsic parallel conduction in the buffer layer at room temperature that limits exploitation of the high-mobility two-dimensional electron gas (2DEG), particularly for nanostructured devices where deep isolation etch processing is impractical. Here, we demonstrate a strategy to reduce the parasitic conduction by the insertion of a pseudomorphic barrier layer of wide-band-gap alloy below the QW. We have studied the high-field magnetotransport in two types of InSb/AlxIn1-xSb modulation doped quantum well heterostructures with and without the barrier layer in the temperature range 2-290 K and magnetic fields to 7.5 T. The conduction in the doping layer, the 2DEG, and the buffer layer are analyzed using a multi-carrier model that successfully captures the field dependence of the Hall resistance over the experimental field range. Samples with the barrier layer show significantly reduced buffer layer conduction compared to samples without. Our results are expected to be of importance for ambient temperature nano-electronic operation.
Physical review. B, Condensed matter 08/2011; 84(7). DOI:10.1103/PhysRevB.84.075474
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##### Article: Polypropylene embedded metal mesh broadband achromatic half-wave plate for millimeter wavelengths
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ABSTRACT: We describe a novel multilayered metal-mesh achromatic half-wave plate (HWP) for use in astronomical polarimetric instruments. The HWP is designed to operate across the frequency range from 125 to 250 GHz. The wave plate is manufactured from 12 layers of thin film metallic inductive and capacitive grids patterned onto polypropylene sheets, which are then bonded together using a hot-pressing technique. Transmission line modeling and three-dimensional electromagnetic simulations are used to optimize the parameters of the metal-mesh patterns and to evaluate their optical properties. A prototype HWP has been fabricated, and its performance is characterized in a polarizing Fourier transform spectrometer. The device performance is consistent with the modeling, although the measured differential phase shift for two orthogonal polarizations is lower than expected. This difference is likely to result from imperfect patterning of individual layers and misalignment of the grids during manufacture.
Applied Optics 07/2011; 50(21):3750-7. DOI:10.1364/AO.50.003750
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##### Article: A New Artificial Dielectric Metamaterial and its Application as a THz Anti-Reflection Coating
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ABSTRACT: We describe a novel artificial dielectric material which has applications at millimetre and submillimetre wavelengths. The material is manufactured from layers of metal mesh patterned onto thin polypropylene sheets which are then bonded together using a hot pressing process to provide planar rugged discs which can be reliably cycled to cryogenic temperatures. The refractive index of this material can be tuned by adjusting the geometry and spacing of the metal-mesh layers. We demonstrate its usage by designing and characterising a broadband anti-reflection coating for a Z-cut crystalline Quartz plate. The coating was fabricated and applied to the quartz using the hot press technique and characterized using a Fourier Transform Spectrometer. The performance is shown to be in good agreement with HFSS and transmission line modelling results.
Applied Optics 07/2011; 48(35). DOI:10.1364/AO.48.006635
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##### Article: Time-dependent embedding: surface electron emission
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ABSTRACT: The Dirac-Frenkel variational principle is used to derive the embedding method for solving the time-dependent Schrödinger equation. Embedding allows the time evolution of the wavefunction to be calculated explicitly in a limited region of space, the region of physical interest, the embedding potential ensuring that the wavefunction satisfies the correct boundary conditions for matching on to the rest of the system. This is applied to a study of the excitation of electrons at a metal surface, represented by a one-dimensional model potential for Cu(111). Time-dependent embedding potentials are derived for replacing the bulk substrate, and the image potential and vacuum region outside the surface, so that the calculation of electron excitation by a surface perturbation can be restricted to the surface itself. The excitation of the Shockley surface state and a continuum bulk state is studied, and the time structure of the resulting currents analysed. There is a distinction between emission from the localized surface state, where the charge is steadily depleted, and the extended continuum state, where the current emitted into the vacuum is compensated by current approaching the surface from the bulk. The time taken for the current to arrive outside the surface is studied.
Journal of Physics Condensed Matter 07/2011; 23(30):305004. DOI:10.1088/0953-8984/23/30/305004
• ##### Article: Ultrasensitive TES Bolometers for Space-Based FIR Astronomy
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ABSTRACT: Results of study optical performance of single pixel transition edge sensor (TES) bolometer presented at wavelength band 30-60 μm are presented. FIR radiation is coupled into a multimode horn with entrance aperture of 450 μm, length 4.5 mm and exit aperture of 45 μm , which feeds a metal integrating cavity containing the detector. The radiation band is defined by a pair of lowpass and highpass mesh filters in front of the horn. Here we present measurements of optical noise equivalent power (NEP), optical efficiency, dynamic range and time constant. The results show that measured TES detectors are close to meeting the requirement of the “Band S” of SAFARI FTS imaging instrument on the SPICA mission.
IEEE Transactions on Applied Superconductivity 07/2011; 21(3-21):188 - 191. DOI:10.1109/TASC.2010.2089585
• ##### Article: The effect of a scanning flat fold mirror on a cosmic microwave background B-mode experiment
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ABSTRACT: We investigate the possibility of using a flat-fold beam steering mirror for a cosmic microwave background B-mode experiment. An aluminium flat-fold mirror is found to add ∼0.075% polarization, which varies in a scan synchronous way. Time-domain simulations of a realistic scanning pattern are performed, and the effect on the power-spectrum illustrated, and a possible method of correction applied.
The Review of scientific instruments 06/2011; 82(6):064502. DOI:10.1063/1.3598342
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##### Article: Optical gain in GaInNAs and GaInNAsSb quantum wells
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ABSTRACT: We have measured the absorption, gain and spontaneous emission spectra of GalnNAsSb (3.3%N), GalnNAs (0.5%N) and GalnAs quantum well structures to compare their merits as laser gain media. The parameters describing the relations between peak gain and current provide only limited insight. From the analysis of absorption spectra we have determined the intrinsic properties of the structures, represented by the product [reduced density of states × matrix element × overlap integral], taking account of differences in operating wavelength, well width and confinement. We find only a small variation in this product across the samples. The GalnNAsSb structure has a low radiative recombination current due in part to its low photon energy and also to differences in conduction and valence band densities of states and less inhomogeneous broadening relative to GalnNAs. We speculate that Sb brings benefits as a surfactant producing more homogeneous wells so Sb may also be beneficial in structures at shorter wavelength. However, there is a large non radiative current in GalnNAsSb and achieving further reductions in the non-radiative current is the major challenge in taking advantage of the good gain potential of this system.
IEEE Journal of Quantum Electronics 06/2011; 47(6):870 - 877. DOI:10.1109/JQE.2011.2129492
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##### Article: Models for the clustering of far‐infrared and submillimetre selected galaxies
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ABSTRACT: We discuss and compare two alternative models for the two-point angular correlation function of galaxies detected through the submillimetre emission using the Herschel Space Observatory. The first, now-standard Halo Model, which represents the angular correlations as arising from one-halo and two-halo contributions, is flexible but complex and rather unwieldy. The second model is based on a much simpler approach: we incorporate a fitting function method to estimate the matter correlation function with approximate model of the bias inferred from the estimated redshift distribution to find the galaxy angular correlation function. We find that both models give a good account of the shape of the correlation functions obtained from published preliminary studies of the Herschel Multi-tiered Extragalactic Survey (HerMES) and the Herschel-Astrophysical Terahertz Large Area Survey (H-ATLAS) performed using Herschel, and yield consistent estimates of the minimum halo mass within which the submillimetre galaxies must reside. We note also that both models predict an inflection in the correlation function at intermediate angular scales, so the presence of the feature in the measured correlation function does not unambiguously indicate the presence of intra-halo correlations. The primary barrier to more detailed interpretation of these clustering measurements lies in the substantial uncertainty surrounding the redshift distribution of the sources.
Monthly Notices of the Royal Astronomical Society Letters 03/2011; 412(1):L93 - L97. DOI:10.1111/j.1745-3933.2011.01014.x
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##### Article: Single-Molecule Mapping of Long-range Electron Transport for a Cytochrome b(562) Variant
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ABSTRACT: Cytochrome b(562) was engineered to introduce a cysteine residue at a surface-exposed position to facilitate direct self-assembly on a Au(111) surface. The confined protein exhibited reversible and fast electron exchange with a gold substrate over a distance of 20 Å between the heme redox center and the gold surface, a clear indication that a long-range electron-transfer pathway is established. Electrochemical scanning tunneling microscopy was used to map electron transport features of the protein at the single-molecule level. Tunneling resonance was directly imaged and apparent molecular conductance was measured, which both show strong redox-gated effects. This study has addressed the first case of heme proteins and offered new perspectives in single-molecule bioelectronics.
Nano Letters 01/2011; 11(1):176-82. DOI:10.1021/nl103334q
• ##### Thesis: Single molecule conduction of engineered cytochrome b562 bonded to metallic electrodes

School of Physics and Astronomy, Cardiff University, 01/2011, Degree: PhD
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##### Article: Simulations of black-hole binaries with unequal masses or nonprecessing spins: Accuracy, physical properties, and comparison with post-Newtonian results
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ABSTRACT: We present gravitational waveforms for the last orbits and merger of black-hole-binary systems along two branches of the black-hole-binary parameter space: equal-mass binaries with equal nonprecessing spins, and nonspinning unequal-mass binaries. The waveforms are calculated from numerical solutions of Einstein’s equations for black-hole binaries that complete between six and ten orbits before merger. Along the equal-mass spinning branch, the spin parameter of each black hole is χi=Si/Mi2∈[-0.85,0.85], and along the unequal-mass branch the mass ratio is q=M2/M1∈[1,4]. We discuss the construction of low-eccentricity puncture initial data for these cases, the properties of the final merged black hole, and compare the last 8-10 gravitational-wave cycles up to Mω=0.1 with the phase and amplitude predicted by standard post-Newtonian (PN) approximants. As in previous studies, we find that the phase from the 3.5PN TaylorT4 approximant is most accurate for nonspinning binaries. For equal-mass spinning binaries the 3.5PN TaylorT1 approximant (including spin terms up to only 2.5PN order) gives the most robust performance, but it is possible to treat TaylorT4 in such a way that it gives the best accuracy for spins χi>-0.75. When high-order amplitude corrections are included, the PN amplitude of the (ℓ=2,m=±2) modes is larger than the numerical relativity amplitude by between 2-4%.
Physical review D: Particles and fields 11/2010; 82(12):124008. DOI:10.1103/PhysRevD.82.124008
• ##### Article: Transition Voltage Spectroscopy of Porphyrin Molecular Wires
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ABSTRACT: Measurements are presented of the current-voltage (I-V) characteristics of individual thiol-tethered porphyrin molecules (isolated in an alkanethiol matrix) and of self-assembled monolayers. In both cases, it is found that I/V(2) displays a minimum at a characteristic "transition voltage" V(m). Repeated measurements of the transition voltage enable both its time development and statistical behavior to be determined. For isolated molecules, the transition voltage shows a multipeaked distribution of values, indicating the presence of a small number of distinct molecular/contact configurations, each having different transport characteristics. For self-assembled monolayers, in contrast, a single-peaked distribution was observed, which is consistent with parallel conduction through many molecules.
Small 11/2010; 6(22):2604-11. DOI:10.1002/smll.201001046
• ##### Article: Evaluating design modifications in classical guitars.
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ABSTRACT: The structure and musical usage of the classical guitar has changed considerably over its long history and the instrument still continues to develop. Major the changes in construction of guitars in the latter part of the nineteenth century and changes in their playing style helped establish the classical guitar as a serious solo instrument, which still holds considerable prominence today. Many makers and players strive to "improve" the instrument, but their objectives are rarely clearly defined or the overall consequences of specific changes are misunderstood. This paper will try to establish an objective framework for establishing what constitutes the sound of a guitar and how that sound may be modified by changes in the design or construction of the instrument or through the use of different materials. It will bring together objective experimental and theoretical work on the structural vibrations of guitars and acoustical radiation with subjective psychoacoustical evaluation of real and synthetic sound signals with the aim of helping to answer the many questions posed by guitar makers.
The Journal of the Acoustical Society of America 10/2010; 128(4):2447. DOI:10.1121/1.3508755
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##### Article: Antenna-coupled TES bolometer arrays for BICEP2/Keck and SPIDER
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ABSTRACT: BICEP2/Keck and SPIDER are cosmic microwave background (CMB) polarimeters targeting the B-mode polar-ization induced by primordial gravitational waves from inflation. They will be using planar arrays of polarization sensitive antenna-coupled TES bolometers, operating at frequencies between 90 GHz and 220 GHz. At 150 GHz each array consists of 64 polarimeters and four of these arrays are assembled together to make a focal plane, for a total of 256 dual-polarization elements (512 TES sensors). The detector arrays are integrated with a time-domain SQUID multiplexer developed at NIST and read out using the multi-channel electronics (MCE) developed at the University of British Columbia. Following our progress in improving detector parameters uniformity across the arrays and fabrication yield, our main effort has focused on improving detector arrays optical and noise performances, in order to produce science grade focal planes achieving target sensitivities. We report on changes in detector design implemented to optimize such performances and following focal plane arrays characterization. BICEP2 has deployed a first 150 GHz science grade focal plane to the South Pole in December 2009.
Proc SPIE 09/2010; DOI:10.1117/12.857914
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##### Article: FTIR spectroscopy of synthesized racemic nonacosan-10-ol: A model compound for plant epicuticular waxes
[Hide abstract]
ABSTRACT: As there are no published graphically presented, detailed IR spectra of nonacosan-10-ol (occurring naturally and widely in plant epicuticular waxes of nanotube form), near IR FTIR spectroscopy (fundamentals, overtones and combinations) has been performed on laboratory synthesized racemic nonacosan-10-ol, as a crystalline solid on Mylar and polypropylene substrates. Room temperature, in vacuo data are presented graphically, in full, and show evidence of extensive hydrogen bonding, an orthorhombic perpendicular subcell, a methylene wagging progression, diagnostic of all-trans conformational order, and Fermi resonance. Moderate or stronger anharmonicity is confirmed. Detailed discussion, quantitative in parts, is given of the observed spectral features, especially as to how they inform crystal structure and molecular conformation, and assignments given for some of the features. The results will serve as a reference for future IR studies of the natural epicuticular wax nanotube form of (S)-nonacosan-10-ol.
Journal of Biological Physics 09/2010; 36(4):405-25. DOI:10.1007/s10867-010-9192-6
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##### Article: The Atacama Cosmology Telescope: Extragalactic Sources at 148 GHz in the 2008 Survey
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ABSTRACT: We report on extragalactic sources detected in a 455 square-degree map of the southern sky made with data at a frequency of 148 GHz from the Atacama Cosmology Telescope 2008 observing season. We provide a catalog of 157 sources with flux densities spanning two orders of magnitude: from 15 to 1500 mJy. Comparison to other catalogs shows that 98% of the ACT detections correspond to sources detected at lower radio frequencies. Three of the sources appear to be associated with the brightest cluster galaxies of low redshift X-ray selected galaxy clusters. Estimates of the radio to mm-wave spectral indices and differential counts of the sources further bolster the hypothesis that they are nearly all radio sources, and that their emission is not dominated by re-emission from warm dust. In a bright (>50 mJy) 148 GHz-selected sample with complete cross-identifications from the Australia Telescope 20 GHz survey, we observe an average steepening of the spectra between 5, 20, and 148 GHz with median spectral indices of $\alpha_{\rm 5-20} = -0.07 \pm 0.06$, $\alpha_{\rm 20-148} = -0.39 \pm0.04$, and $\alpha_{\rm 5-148} = -0.20 \pm 0.03$. When the measured spectral indices are taken into account, the 148 GHz differential source counts are consistent with previous measurements at 30 GHz in the context of a source count model dominated by radio sources. Extrapolating with an appropriately rescaled model for the radio source counts, the Poisson contribution to the spatial power spectrum from synchrotron-dominated sources with flux density less than 20 mJy is $C^{\rm Sync} = (2.8 \pm 0.3) \times 10^{-6} \micro\kelvin^2$.
The Astrophysical Journal 07/2010; 731(100). DOI:10.1088/0004-637X/731/2/100
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##### Article: A new method to measure evolution of the galaxy luminosity function
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ABSTRACT: We present a new efficient technique for measuring evolution of the galaxy luminosity function. The method reconstructs the evolution over the luminosity-redshift plane using any combination of three input dataset types: 1) number counts, 2) galaxy redshifts, 3) integrated background flux measurements. The evolution is reconstructed in adaptively sized regions of the plane according to the input data as determined by a Bayesian formalism. We demonstrate the performance of the method using a range of different synthetic input datasets. We also make predictions of the accuracy with which forthcoming surveys conducted with SCUBA2 and the Herschel Space Satellite will be able to measure evolution of the sub-millimetre luminosity function using the method.
Monthly Notices of the Royal Astronomical Society 07/2010; 405(4):2406 - 2418. DOI:10.1111/j.1365-2966.2010.16623.x
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##### Article: Modeling and characterization of the SPIDER half-wave plate
[Hide abstract]
ABSTRACT: Spider is a balloon-borne array of six telescopes that will observe the Cosmic Microwave Background. The 2624 antenna-coupled bolometers in the instrument will make a polarization map of the CMB with approximately one-half degree resolution at 145 GHz. Polarization modulation is achieved via a cryogenic sapphire half-wave plate (HWP) skyward of the primary optic. We have measured millimeter-wave transmission spectra of the sapphire at room and cryogenic temperatures. The spectra are consistent with our physical optics model, and the data gives excellent measurements of the indices of A-cut sapphire. We have also taken preliminary spectra of the integrated HWP, optical system, and detectors in the prototype Spider receiver. We calculate the variation in response of the HWP between observing the CMB and foreground spectra, and estimate that it should not limit the Spider constraints on inflation.
Proceedings of SPIE - The International Society for Optical Engineering 06/2010; 7741. DOI:10.1117/12.857837
• ##### Article: Random Population of Quantum Dots in InAs–GaAs Laser Structures
[Hide abstract]
ABSTRACT: We have measured the spontaneous emission rates, absorption, and gain spectra of quantum dots between 20 K and 350 K using the segmented contact method, exploiting self assembled dots with a bimodal size distribution. At 20 K, we find a linear relationship between the radiative rates of the ground states of small and large sets of dots, and using absorption data we show the slope corresponds to random population of states at different energy with equal probability. The emission spectra indicate relaxation of carriers from excited to ground states in the same set of dots at 20 K. We have developed a rate equation model to describe the transition from thermal to random population, including relaxation between excited and ground states in the same dot. This produces a Fermi-Dirac occupation distribution of electrons in all states at 350 K with a global quasi Fermi level. At 20 K all ground states have the same occupation probability, irrespective of their energy. Using values for spontaneous lifetime and gain cross section from absorption data, these rate equations give a very good description of the experimental results for radiative threshold current density as a function of temperature, reproducing the minimum at 200 K, and thereby modeling the transition from the thermal regime at high temperature, to the random regime at low temperature.
IEEE Journal of Quantum Electronics 05/2010; 46(4-46):525 - 532. DOI:10.1109/JQE.2009.2039198
• ##### Article: BLAST-pol: Balloon-borne Large Aperture Submillimeter Telescope for Polarimetery
[Hide abstract]
ABSTRACT: BLAST-pol (Balloon-borne Large Aperture Submillimeter Telescope for Polarimetery) combines a 1.9 meter Cassegrain telescope and a submillimeter polarimeter that operates in three bands (250, 350, and 500 microns), each with 30% bandwidth. The detection system is comprised of 270 silicon-nitride micromesh bolometers distributed on three focal plane arrays with 30", 42", and 60" FWHM beam sizes, respectively. The detectors are a prototype for the SPIRE instrument used on Herschel. Polarization sensitivity is achieved by placing polarized grids on the bolometer arrays and with a rotatable Achromatic Half Wave Plate (AHWP) located in front of the arrays. The instrument operates on a balloon at an altitude of 40km due to the high opacity of the earth's sea level atmosphere at submillimeter wavelengths. The main scientific objective of BLAST-pol is to determine what role magnetic fields play in star formation. This is achieved by making large, high resolution, linear polarization maps of giant molecular clouds (GMCs), and their substructures. These maps can then be compared to maps created using numerical turbulence simulations to ascertain how magnetic fields affect the morphology and lifetime of these structures. The non-polarized version of BLAST has had two scientific flights and is scheduled to fly again as BLAST-pol in December 2010 from McMurdo, Antarctica.
• ##### Article: Measurements of Superattenuator seismic isolation by Virgo interferometer
[Hide abstract]
ABSTRACT: Each mirror of the interferometric gravitational wave antenna Virgo is attached to a Superattenuator, a chain of mechanical filters designed to suppress seismic vibrations, starting from a few Hz. The filter chain attenuation has been measured by exciting its suspension point with sinuisodal forces and using the interferometer as sensor. The attenuation, measured at different frequencies, is compliant with the requirements of the next generation antenna Advanced Virgo. In the third generation detector Einstein Telescope, the attenuation is sufficient above 3 Hz, independently of the underground site choice.
Astroparticle Physics 04/2010; 33(3-33):182-189. DOI:10.1016/j.astropartphys.2010.01.006
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##### Article: Masses and Luminosities of O and B - type stars and red super giants
[Hide abstract]
ABSTRACT: Massive stars are of interest as progenitors of supernovae, i.e. neutron stars and black holes, which can be sources of gravitational waves. Recent population synthesis models can predict neutron star and gravitational wave observations but deal with a fixed supernova rate or an assumed initial mass function for the population of massive stars. Here we investigate those massive stars, which are supernova progenitors, i.e. with O- and early B-type stars, and also all supergiants within 3 kpc. We restrict our sample to those massive stars detected both in 2MASS and observed by Hipparcos, i.e. only those stars with parallax and precise photometry. To determine the luminosities we calculated the extinctions from published multi-colour photometry, spectral types, luminosity class, all corrected for multiplicity and recently revised Hipparcos distances. We use luminosities and temperatures to estimate the masses and ages of these stars using different models from different authors. Having estimated the luminosities of all our stars within 3 kpc, in particular for all O- and early B-type stars, we have determined the median and mean luminosities for all spectral types for luminosity classes I, III, and V. Our luminosity values for supergiants deviate from earlier results: Previous work generally overestimates distances and luminosities compared to our data, this is likely due to Hipparcos parallaxes (generally more accurate and larger than previous ground-based data) and the fact that many massive stars have recently been resolved into multiples of lower masses and luminosities. From luminosities and effective temperatures we derived masses and ages using mass tracks and isochrones from different authors. From masses and ages we estimated lifetimes and derived a lower limit for the supernova rate of ≈20 events/Myr averaged over the next 10 Myr within 600 pc from the sun. These data are then used to search for areas in the sky with higher likelihood for a supernova or gravitational wave event (like OB associations) (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Astronomische Nachrichten 04/2010; 331(4):349 - 360. DOI:10.1002/asna.200911355
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##### Article: A 350-GHz high-resolution high-sensitivity passive video imaging system
[Hide abstract]
ABSTRACT: We are developing a 350 GHz cryogenic passive video imaging system. This demonstration system uses 800 photon-noise-limited superconducting transition edge sensor bolometers. It will image a 1 m x 1 m area at a standoff distance of 16 m to a resolution of approximately 1 cm at video frame rates (20 frames per second). High spatial resolution is achieved by the use of an f/2.0 Cassegrain optical system with 1.3 m primary mirror. Preliminary testing of prototype detectors indicates that we can achieve a noise equivalent temperature difference (NETD) of 70 mK for the fully sampled 1 m x 1 m image at 20 frames per second.
Proceedings of SPIE - The International Society for Optical Engineering 04/2010; 7670. DOI:10.1117/12.852932
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##### Article: Up on the Jaynes-Cummings ladder of a quantum-dot/microcavity system
[Hide abstract]
ABSTRACT: In spite of their different natures, light and matter can be unified under the strong-coupling regime, yielding superpositions of the two, referred to as dressed states or polaritons. After initially being demonstrated in bulk semiconductors and atomic systems, strong-coupling phenomena have been recently realized in solid-state optical microcavities. Strong coupling is an essential ingredient in the physics spanning from many-body quantum coherence phenomena, such as Bose-Einstein condensation and superfluidity, to cavity quantum electrodynamics. Within cavity quantum electrodynamics, the Jaynes-Cummings model describes the interaction of a single fermionic two-level system with a single bosonic photon mode. For a photon number larger than one, known as quantum strong coupling, a significant anharmonicity is predicted for the ladder-like spectrum of dressed states. For optical transitions in semiconductor nanostructures, first signatures of the quantum strong coupling were recently reported. Here we use advanced coherent nonlinear spectroscopy to explore a strongly coupled exciton-cavity system. We measure and simulate its four-wave mixing response, granting direct access to the coherent dynamics of the first and second rungs of the Jaynes-Cummings ladder. The agreement of the rich experimental evidence with the predictions of the Jaynes-Cummings model is proof of the quantum strong-coupling regime in the investigated solid-state system.
Nature Material 03/2010; 9(4):304-8. DOI:10.1038/nmat2717
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##### Article: STM Study of Molecule Double-Rows in Mixed Self-Assembled Monolayers of Alkanethiols
[Hide abstract]
ABSTRACT: Using scanning tunnelling microscopy (STM), we have studied mixed self-assembled monolayers of linear alkanethiol molecules. Nonanedithiol (C9S2), nonanethiol (C9S), decanethiol (C10S), and dodecanethiol (C12S) were inserted into a self-assembled octanethiol (C8S) host matrix monolayer on an Au(111) surface using a two-step method. Quasi-one-dimensional double-row structures were found in the ordered, close-packed domains of the C8S matrix for each mixed monolayer system. These close-packed domains coexist with ordered striped phase domains (for C9S and C10S) or with a disordered phase (for C9S2 and C12S). Results from high-resolution images suggest that the double-rows are composed of inserted non-nearest-neighbor substitutional molecules, the ordering of which may be a result of locally induced surface stress.
Langmuir 03/2010; 26(11):8174-9. DOI:10.1021/la9044754
• ##### Article: Effect of Growth Temperature on InP QD Lasers
[Hide abstract]
ABSTRACT: We describe the effect of growth temperature on the optical absorption, gain, and threshold current density of 730-nm emitting, metal-organic vapor phase epitaxy (MOVPE) grown, InP-AlGaInP quantum-dot lasers. Decreasing the growth temperature from 750??C to 690??C leads to an increase in ground state absorption, while sufficient optical gain and low 300 K threshold current density is obtained in the growth temperature window between 710??C and 730??C . Wider (16 nm compared to 8 nm) interlayer barriers lead to lower threshold current density with 300 K values as low as 165 Acm<sup>-2</sup> for 2-mm-long lasers with uncoated facets.
IEEE Photonics Technology Letters 02/2010; 22(2-22):88 - 90. DOI:10.1109/LPT.2009.2036245
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##### Article: Exciton-Polariton Bose-Einstein Condensation: Advances and Issues
[Hide abstract]
ABSTRACT: In this review, we present a comprehensive set of experimental results on microcavity-polariton Bose-Einstein Condensation (BEC), obtained in a close collaboration between Institut Néel, Grenoble and EPFL, Lausanne. First, the main observations are presented, i.e. massive occupation of ground state and onset of long range order, which led to the conclusion that polariton BEC indeed occurs. Then, the highly disordered environment in which the condensation takes place is analysed: we show how interactions are a necessary ingredient for polariton BEC. Finally we discuss quantized vortices evidenced for the first time in polariton condensates. Their unusual features are shown to be inherited from the disordered environment and the dissipative-driven character of the polariton BEC. 2001. After his PhD degree in 2004, he stayed at the Institute of Photonics and Quantum Electronics at EPFL, Lausanne, Switzerland, for 2.5 years as a postdoctoral fellow. Since the end of 2007, he is a research associate (CNRS) at Institut Néel, Grenoble. His current research is mainly focused on spontaneous formation and behavior of coherence, in semiconductor nanostructures (Bose-Einstein condensation, superfluidity, superradiance). Exciton-Polariton Bose-Einstein Condensation 2 Le Si Dang was born in Viet Nam. After his doctorate in Grenoble and his post-doctoral stay at Lehigh University (USA), he joined the Laboratoire de Spectrométrie Physique (Université Joseph Fourier) in 1980 and the Institut Néel (CNRS, Grenoble) in 2007. He is a CNRS Directeur de recherche. Presently he is coordinating the Associated International Laboratory "Center for Photonics and Nanostructures" of CNRS and KIST (Korea Institute of Science and Technlogy). His current research interests include macroscopic quantum coherence in low dimensional semiconductor heterostructures (Bose-Einstein condensation, superradiance) and optics and transport in single nanowires.
International Journal of Nanotechnology 02/2010; 7(4). DOI:10.1504/IJNT.2010.031738
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##### Article: Technology Development for Third Generation Ground-based Gravitational Wave Detectors
[Hide abstract]
ABSTRACT: Gravitational wave astrophysics is on the threshold of a first direct detection and in the next few years will begin contributing to astronomy. The groundwork for this advance was laid over the past three decades through a vigorous program of R&D. Significant further advances in sensitivity and frequency coverage for third-generation ground-based interferometers are possible and will address questions of major scientific importance. To accomplish this, a forward-looking program of instrumental research should be supported.The next decade will see the beginning of gravitational wave astrophysics, as a world-wide network of advanced gravitational wave interferometers already under construction or in the advanced planning stage begin to operate, with a substantial increase in sensitivity and bandwidth over the first generation detectors. These advanced detectors were made possible by a program of R&D that began even before the initial detectors began operation. These advanced detectors do not fully exploit the potential for sensitivity and bandwidth— a third generation of ground-based interferometer detectors seems possible with a factor of 10 increase in sensitivity as well as another decade of lower frequency coverage. We urge support for a broad-based effort to explore and master the techniques required to make this leap. The challenges of gravitational wave detection are formidable, but the reward is an entirely new window with which to explore the universe.
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##### Article: Performance of a Microstrip-coupled TES Imaging Module for CMB Polarimetry
[Hide abstract]
ABSTRACT: We have developed a 16-element, 97GHz, low-noise Transition Edge Sensor (TES) module that can be packed easily into large-format polarimetric imaging arrays. The technology was developed originally for the low-frequency instrument of the CLOVER experiment, which was aimed at searching for the signature of primordial gravitational waves in the polarisation state of the Cosmic Microwave Background Radiation. Each module contains 16 slotline and microstrip-coupled TES bolometers, allowing eight polarimetric pixels with external waveguide OMTs, along with time-domain multiplexed SQUID readout. Here we describe the design of the modules, and explain how they were realised in practice, detailing some of the technology developed along the way. Performance was measured by following two parallel paths: (i) detailed characterisation of individual detectors using an exceedingly well understood, fully modelled, analogue SQUID readout system, and (ii) simultaneous measurements of the entire module using CLOVER's multichannel readout electronics. We describe the results of measurements on one of the modules, with an emphasis on uniformity of performance, and we assess the effect of non-uniformity on the operation of a complete array. Although the technology was developed in the context of CLOVER, the work has consequences for many instruments including future space telescopes such as BPol.
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##### Article: A TES focal plane for SPICA-SAFARI
[Hide abstract]
ABSTRACT: We describe the design of a focal plane array assembly incorporating transition edge superconducting bolometers (TES) for the SAFARI instrument aboard the SPICA satellite. SPICA consists of a 3.5 metre telescope actively cooled to a temperature of < 7 K, designed to be diffraction limited at a wavelength of 3 µm. The active cooling of the telescope reduces its emission to a level below the astrophysical background power level over the entire wavelength range of operation (~4 – 210 µm). This represents a reduction in loading at far-infrared (FIR) wavelengths of more than a factor of 10,000 over the 50 K HERSCHEL telescope. SAFARI is a proposed FIR instrument for SPICA consisting of an imaging Fourier Transform Spectrometer (FTS) covering the wavelengths from 34 – 210 µm with science goals of furthering our understanding of planetary system formation, the life cycle of dust and gas in the galaxy and the formation of galaxies. The SAFARI wavelength coverage is divided into 3 bands in the focal plane covering the wavelength ranges from 35 – 60 mm, 60 – 110 µm, and 110 – 210 µm. Each band requires a 2D focal plane array of 0.5 f λ spaced pixels covering a 2' x 2' field of view. The total number of pixels in all three bands is approximately 6000. In order to take advantage of the reduction in background power provided by the cooled telescope, SAFARI requires detectors with a combination of low optical NEP (~ 2  10 -19 W/Hz) and high optical efficiency. There are several detector technologies that have been under development to meet these requirements including Kinetic Inductance Detectors (KIDs), Transition Edge Superconducting bolometers (TES), Silicon bolometers, Ge:Ga photoconductors and Quantum Well detectors. We describe the conceptual design of a TES focal plane array including the detector array, coupling optics, cold electronics and interconnections and mechanical and thermal design. We also describe the results of measurements of performance of single pixel prototypes as well as small prototype arrays and compare these results with the required performance.
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##### Article: Performances of the Virgo interferometer longitudinal control system
[Hide abstract]
ABSTRACT: The performances of the longitudinal sensing and control system of the Virgo gravitational wave detector are described. This system is able to stably maintain the RMS residual fluctuation of the interferometer longitudinal degrees of freedom around or below , compatible with the original Virgo requirements. Moreover the detector sensitivity is not limited by longitudinal control noise at any frequency. Indeed the noise re-introduced by the longitudinal control system does not affect the Virgo design sensitivity.
• ##### Article: The astrophysical consequences of the bimodal hydrodynamic solution of the super star cluster winds
[Hide abstract]
ABSTRACT: The mass reinserted by young stars in an emerging massive compact cluster shows a bimodal hydrodynamic behaviour. In the inner parts of the cluster, it is thermally unstable, while in its outer parts it forms an outflowing wind. The chemical homogeneity/inhomogeneity of low/high-mass clusters demonstrates the relevance of this solution to the presence of single/multiple stellar populations. We show the consequences which the thermal instability of the reinserted mass has on galactic superwinds and discuss the open issues raised by the bimodal solution of stellar winds of massive clusters. KeywordsStars: winds-Stars: clusters-ISM: Hii regions
Astrophysics and Space Science 12/2009; 324(2):195-198. DOI:10.1007/s10509-009-0127-2
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##### Article: BLAST: The Mass Function, Lifetimes, and Properties of Intermediate Mass Cores from a 50 Square Degree Submillimeter Galactic Survey in Vela (l = ~265)
[Hide abstract]
ABSTRACT: We present first results from an unbiased 50 deg2 submillimeter Galactic survey at 250, 350, and 500 μm from the 2006 flight of the Balloon-borne Large Aperture Submillimeter Telescope. The map has resolution ranging from 36'' to 60'' in the three submillimeter bands spanning the thermal emission peak of cold starless cores. We determine the temperature, luminosity, and mass of more than 1000 compact sources in a range of evolutionary stages and an unbiased statistical characterization of the population. From comparison with C18O data, we find the dust opacity per gas mass, κr= 0.16 cm2 g-1 at 250 μm, for cold clumps. We find that 2% of the mass of the molecular gas over this diverse region is in cores colder than 14 K, and that the mass function for these cold cores is consistent with a power law with index α = -3.22 ± 0.14 over the mass range 14 M sun < M < 80 M sun. Additionally, we infer a mass-dependent cold core lifetime of tc (M) = 4 × 106(M/20 M sun)-0.9 yr—longer than what has been found in previous surveys of either low or high-mass cores, and significantly longer than free fall or likely turbulent decay times. This implies some form of non-thermal support for cold cores during this early stage of star formation.
The Astrophysical Journal 11/2009; 707(2):1824-1835. DOI:10.1088/0004-637X/707/2/1824
• ##### Article: Measuring clustering in 2dv space
[Hide abstract]
ABSTRACT: The statistical descriptor is a robust and useful tool for distinguishing and quantifying the degree of radial or multiscale clustering in objects such as open clusters. is calculated as m/s, where is the mean edge length of the minimum spanning tree and is the mean distance between cluster members, or correlation length. is obtained using only two-dimensional position data. Here, we investigate the performance of in three dimensions, both when true three-dimensional data are available and when the radial velocity of cluster components is used as a proxy for position: this is known as 2dv space. True three-dimensional data offer an improvement in the resolution of and as diagnostic indicators of clustering, a scatter plot of versus proving to be a particularly clear method of interpreting the information. Results are not satisfactory when 2dv information is used, as the data from cluster types which are clearly distinguishable using 2d information alone become overlapping and confused when 2dv information is used. We therefore recommend that the 2d method is used, unless true 3d positions of cluster members are available. The use of the versus plot is particularly recommended, as adding extra discrimination between cluster types, compared with that achieved using alone.
Monthly Notices of the Royal Astronomical Society 10/2009; 400(3):1427 - 1430. DOI:10.1111/j.1365-2966.2009.15540.x
• ##### Article: Probing primordial non Gaussianity in the BOOMERanG CMB maps: an analysis based on analytical Minkowski functionals
[Hide abstract]
ABSTRACT: Minkowski functionals are a powerful tool to constrain the Gaussianity of the Cosmic Microwave Background (CMB). In the limit of a weakly non Gaussian field, a perturbative approach can be derived [Hikage C., Komatsu E., & Matsubara T., 2006, ApJ, 653, 11] that is completely based on analytical formulae without requiring computationally intensive, dedicated Monte Carlo non Gaussian simulations of the CMB anisotropy. We apply this machinery to an intensity map derived from the 1998 and 2003 flights of BOOMERanG, analyzed here together for the first time. We set limits on the non-linear coupling parameter fNL as −1020<fNL<390 at 95% CL, markedly improving the previous constraints set by [De Troia G. et al., 2007, ApJ, 670, L73] whose analysis was limited to the BOOMERanG 2003 dataset. These limits are the most stringent ever set among suborbital experiments.
Nuclear Physics B - Proceedings Supplements 10/2009; 194(194):278-286. DOI:10.1016/j.nuclphysbps.2009.07.092
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##### Article: Thermal design and performance of the SCUBA-2 instrument 1-K and mK systems
[Hide abstract]
ABSTRACT: Various research fields require large and complex instruments containing detectors operating at millikelvin temperatures. The materials and techniques traditionally used in cryogenics are often unsuitable for the demanding requirements of such instruments. We describe the thermal design and performance of the 1-K and millikelvin systems of the SCUBA-2 instrument. This is an astronomical “camera” operating at wavelengths of 450 and 850 μm. It is the largest and most complex instrument ever built for sub-mm astronomy, and the first to use a cryogen-free dilution refrigerator. The design consists of a mix of traditional techniques (but used in demanding situations) as well as novel elements. The thermal performance has been stable and very successful, and we hope that the details described here will be useful to the designers of future large instruments.
Cryogenics 09/2009; 49(9-49):504-513. DOI:10.1016/j.cryogenics.2009.07.004
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##### Article: Single source coherent anti-Stokes Raman microspectroscopy using spectral focusing
[Hide abstract]
ABSTRACT: We demonstrate coherent anti-Stokes Raman scattering (CARS) microspectroscopy using a single Ti:sapphire laser oscillator and simple passive optical elements. Spectral selection by dichroic mirrors and linear chirping by glass elements creates a vibrational excitation tuneable over a large spectral range (∼800–2200 cm <sup>-1</sup>) with adjustable spectral resolution (∼10–100 cm <sup>-1</sup>) . We furthermore demonstrate the applicability of differential CARS, enhancing the chemical selectivity, with the proposed single-laser configuration.
Applied Physics Letters 09/2009; 95(8-95):081109 - 081109-3. DOI:10.1063/1.3216073
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##### Article: Detecting relics of a thermal gravitational wave background in the early Universe
[Hide abstract]
ABSTRACT: A thermal gravitational wave background can be produced in the early Universe if a radiation dominated epoch precedes the usual inflationary stage. This background provides a unique way to study the initial state of the Universe. We discuss the imprint of this thermal spectra of gravitons on the cosmic microwave background (CMB) power spectra, and its possible detection by CMB observations. Assuming the inflationary stage is a pure de Sitter expansion we find that, if the number of e-folds of inflation is smaller than 65, the signal of this thermal spectrum can be detected by the observations of Planck and PolarBear experiments, or the planned EPIC experiments. This bound can be even looser if inflation-like stage is the sub-exponential.
Physics Letters B 07/2009; 680(5-680):411-416. DOI:10.1016/j.physletb.2009.09.018
• ##### Conference Paper: Four-wave mixing of gold nanoparticles for three-dimensional cell microscopy
[Hide abstract]
ABSTRACT: Optical microscopy is an indispensable tool for obtaining spatial and temporal resolution within living cells and tissues. In this paper, a novel multiphoton microscopy technique which exploits the third-order nonlinearity called four-wave mixing and applied to gold nanoparticles in resonance with their surface plasmon is developed. The coherent, transient and resonant nature of this signal allows the interferometric time-resolved detection of four-wave mixing specifically originating from GNPs, with particle diameters down to 5nm. This paper demonstrate high-contrast, background-free imaging of gold-labelled Golgi structures in HepG2 cells with a sub-diffraction-limit lateral (axial) resolution of 140nm (470nm) at excitation powers corresponding to less than 3K photothermal heating, compatible to live cell imaging. These results pave the way towards photostable non-toxic and highly sensitive in-vivo optical imaging with sub-micron three-dimensional resolution.
Lasers and Electro-Optics 2009 and the European Quantum Electronics Conference. CLEO Europe - EQEC 2009. European Conference on; 07/2009
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##### Conference Paper: CARS Microscopy using linearly-chirped ultrafast laser pulses
[Hide abstract]
ABSTRACT: We have developed a home-built CARS microscope which exploits linearly-chirped ultrafast laser pulses to achieve spectral focussing of the vibrational excitation. CARS Stokes pulses are generated by a Ti:Sapphire laser, which pumps an optical parametric oscillator (OPO). The intra-cavity frequency-doubled signal of the OPO provides the CARS Pump. By varying the delay time between pump and Stokes their instantaneous frequency difference (IFD) changes so that CARS spectra can be measured. In fact, since CARS is generated modulating the pump by the induced coherent vibration, a pump pulse arriving after the Stokes pulse optimizes the CARS signal and reduces the non-resonant CARS background. Our microscope also features two-photon fluorescence, second harmonic generation, and differential interference contrast imaging modalities. We investigated polystyrene beads in 2% agar solution by detecting the forward-collected CARS at the C-H aromatic band.
Lasers and Electro-Optics 2009 and the European Quantum Electronics Conference. CLEO Europe - EQEC 2009. European Conference on; 07/2009
• ##### Conference Paper: Coherent Dynamics of One- and Two-Photon States in a Strongly Coupled Single Quantum Dot-Cavity System
[Hide abstract]
ABSTRACT: Heterodyne spectral interferometry is employed to perform four-wave mixing spectroscopy on a strongly-coupled system of an exciton confined in a single quantum dot and a photon mode of a pillar microcavity. The coherent dynamics of one and two photon states are directly observed and the validity of the Jaynes-Cummings model is tested.
Lasers and Electro-Optics, 2009 and 2009 Conference on Quantum electronics and Laser Science Conference. CLEO/QELS 2009. Conference on; 07/2009
• ##### Article: Gain and Recombination in Quantum Dot Lasers
[Hide abstract]
ABSTRACT: This paper provides a tutorial review of the physics of recombination and optical gain in quantum dot laser structures using illustrative model computer calculations for an inhomogeneous InAs dot system. Comparisons are made with gain and recombination rate of an InAs quantum well calculated using the same material properties. Attention is drawn to the equivalent role of k -selection in extended state quantum well systems and ldquolocation selectionrdquo in quantum dot ensembles. Comparisons are also made with representative experimental results for gain, current, and temperature sensitivity of threshold current.
IEEE Journal of Selected Topics in Quantum Electronics 07/2009; 15(3-15):808 - 818. DOI:10.1109/JSTQE.2008.2011998
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##### Article: Refractive Index Dynamics and Linewidth Enhancement Factor in -Doped InAs–GaAs Quantum-Dot Amplifiers
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ABSTRACT: Using a pump-probe differential transmission experiment in heterodyne detection, we measured the refractive index dynamics at the ground-state excitonic transition in electrically pumped InAs/GaAs quantum-dot amplifiers emitting near 1.3 mum at room temperature. We compare three samples differing only in the level of p -doping, and interpret the measured index changes taking into account the gain dynamics in these devices. We find that in absorption, the excess hole density due to p -doping accelerates the recovery and reduces the refractive index change, since filling of the hole states by p -doping shifts the induced changes in the hole population toward high energy states. Conversely, in gain, the reduced electron reservoir in the excited states in p -doped devices results in slower gain recovery dynamics and in larger refractive index changes compared to undoped devices operating at the same modal gain. The linewidth enhancement factor inferred from these measurements shows that p -doping is effective in reducing this parameter mainly due to the larger differential gain in p -doped devices in the gain regime.
IEEE Journal of Quantum Electronics 07/2009; 45(6-45):579 - 585. DOI:10.1109/JQE.2009.2013110
• ##### Article: Coherent anti‐Stokes Raman micro‐spectroscopy using spectral focusing: theory and experiment
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ABSTRACT: Coherent anti-Stokes Raman scattering (CARS) microscopy using linearly chirped femtosecond laser pulses and spectrally and time-integrated detection is discussed both theoretically and experimentally. By matching the linear chirp of Stokes and pump pulses, the spectral resolution is reduced to the Fourier limit of the chirped pulse duration (spectral focusing). We discuss the effect of the time-ordering of Pump and Stokes pulses on the signal strength and selectivity, and give analytical expressions for the relevant quantities. We consider linear chirp obtained by glass elements of known group-velocity dispersion, and analyze the limit of the spectral resolution by higher order glass dispersion. Experiments corroborating the theory are presented, using 100 fs laser pulses, SF57 glass elements, and polystyrene beads as test samples. Copyright © 2009 John Wiley & Sons, Ltd.
Journal of Raman Spectroscopy 07/2009; 40(7):800 - 808. DOI:10.1002/jrs.2264
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##### Article: Detecting relic gravitational waves in the CMB: Comparison of Planck and ground-based experiments
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ABSTRACT: We compare the detection abilities for the relic gravitational waves by two kinds of forthcoming cosmic microwave background radiation (CMB) experiments, space-based Planck satellite and the various ground-based experiments. Comparing with the ground-based experiments, Planck satellite can observe all the CMB power spectra in all the multipole range, but having much larger instrumental noises. We find that, for the uncertainty of the tensor-to-scalar ratio Δr, PolarBear (II) as a typical ground-based experiment can give much smaller value than Planck satellite. However, for the uncertainty of the spectral index Δnt, Planck can give the similar result with PolarBear (II). If combining these two experiments, the value of Δnt can be reduced by a factor 2. For the model with r=0.1, the constraint Δnt=0.10 is expected to be achieved, which provides an excellent opportunity to study the physics in the inflationary universe. We also find the observation in the largest scale (ℓ<20) is very important for constraining the spectral index nt. So it is necessary to combine the observations of the future space-based and ground-based CMB experiments to determine the relic gravitational waves.
Physics Letters B 06/2009; 677(1-2-677):16-23. DOI:10.1016/j.physletb.2009.05.015
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##### Article: New method to constrain the relativistic free-streaming gas in the Universe
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ABSTRACT: We discuss a method to constrain the fraction density f of the relativistic gas in the radiation-dominant stage, by their impacts on a relic gravitational waves and the cosmic microwave background (CMB) B-polarization power spectrum. We find that the uncertainty of f strongly depends on the noise power spectra of the CMB experiments and the amplitude of the gravitational waves. Taking into account of the CMBPol instrumental noises, an uncertainty Δf=0.046 is obtained for the model with tensor-to-scalar ratio r=0.1. For an ideal experiment with only the reduced cosmic lensing as the contamination of B-polarization, Δf=0.008 is obtained for the model with r=0.1. So the precise observation of the CMB B-polarization provides a great opportunity to study the relativistic components in the early Universe.
Physics Letters B 05/2009; 677(5-677):235-238. DOI:10.1016/j.physletb.2009.05.046
• ##### Article: Low-Temperature Nonthermal Population of InAs–GaAs Quantum Dots
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ABSTRACT: Measurements of the unamplified spontaneous emission spectra from 80 K to 350 K of a dot ensemble show clear evidence for increased population of higher lying states in the inhomogeneous distribution as the temperature is reduced from 200 K to 80 K, indicating a nonthermal population at low temperature and confirming that the recombination processes are localized in individual dots. These conclusions are supported by modeling an inhomogeneous ensemble of 2 times 10<sup>6</sup> dots. From simultaneous measurements of optical gain, our data show that the increase in threshold current density with decreasing temperature below about 200 K (which is commonly observed) is due to increased population of higher lying states associated with the transition to nonthermal behavior.
IEEE Journal of Quantum Electronics 05/2009; 45(4-45):380 - 387. DOI:10.1109/JQE.2009.2013869
• ##### Chapter: Our Planetary System

Planets and Planetary Systems, 04/2009: pages 1 - 21; , ISBN: 9780470721780
• ##### Chapter: Front Matter
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Planets and Planetary Systems, 04/2009: pages i - x; , ISBN: 9780470721780
• ##### Chapter: Index

Planets and Planetary Systems, 04/2009: pages 183 - 185; , ISBN: 9780470721780
• ##### Article: The effect of Poisson noise on SPH calculations
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ABSTRACT: ABSTRACTA simple algorithm is presented which generates a lattice-like, regular spacing of smoothed particle hydrodynamics (SPH) particles in discs, with any prescribed density gradient. Using this for comparison we demonstrate the effect of Poisson noise on SPH estimates of density, pressure and viscous forces when calculated using randomly distributed particles. The standard deviation of density and pressure is typically increased by greater than an order of magnitude. In a disc with a Keplerian velocity profile, the effectiveness of the Balsara switch in reducing the shear component of SPH artificial viscosity is greatly enhanced when the particles are properly spaced, reducing the magnitude of viscosity by two orders of magnitude. Noise problems are exacerbated, not removed, by increasing the numbers of SPH particles, if the number of neighbours used is kept constant. However, comparison of the evolution of a disc created using perfectly spaced particles and a disc with identical parameters but randomly placed particles, reveals very similar results. Although there are subtle differences in the evolution, and the smooth disc takes longer to begin developing structure, in both cases the identical number of objects is created by gravitational collapse. SPH disc simulations do not depend on initial density disturbances to evolve objects by gravitational collapse, which gives added credence to the validity of the results. It also appears that complicated disc settling procedures are unnecessary.
Monthly Notices of the Royal Astronomical Society 04/2009; 395(4):2373 - 2380. DOI:10.1111/j.1365-2966.2009.14720.x
• ##### Article: Four‐wave mixing from individual excitons: Intensity dependence and imaging
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ABSTRACT: We use heterodyne spectral interferometry (HSI) to investigate the transient four-wave mixing field (FWM) from an individual localized excitonic transition. FWM intensity and spectrum as well as the spatial resolution in FWM hyperspectral imaging are measured as function of excitation intensity. A third-order non-linear response of the FWM followed by saturation and the onset of a Rabi-flop are observed with increasing intensity. The intensity dependence of the spectral response depends on the excitation pulse duration. For short pulses (∼0.2 ps), having a broad spectrum, a weak broadening of the emission line, similar to excitation induced dephasing (EID), is found. For long pulses (∼0.5 ps) instead, no signatures of EID are found, but individual multi-excitonic transitions are observed. When using the FWM intensity as image contrast by spatially scanning a region of the sample surface, a spatial resolution beyond the diffraction limit is measured in the third-order regime, while for higher intensities the FWM saturation deteriorates the spatial resolution. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (b) 04/2009; 246(4):820 - 823. DOI:10.1002/pssb.200880583
• ##### Article: The E and B EXperiment (EBEX); Progress and Status
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ABSTRACT: We report on the status of EBEX, a NASA-funded balloon-borne polarimeter designed to measure the B-mode polarization of the cosmic microwave background radiation. The EBEX receiver is designed to set a 2σ upper limit on an inflationary tensor-to-scalar ratio of 0.02. This limit assumes a 14-day flight, a scan pattern covering 420 square degrees of the sky, and foreground subtraction to levels below detector noise. The instrument employs a 1.5 meter Gregorian-type telescope and 1440 bolometric transition edge sensor detectors distributed over two focal planes. Polarization is measured using a rotating achromatic half wave plate (AHWP) and a fixed polarizing grid. The AHWP is continuously rotated using a superconducting magnetic bearing. Sky signals will be observed in three bands centered at 150, 250 and 410 GHz, providing strong leverage against the polarized dust foreground. Integration of the gondola and the receiver will occur in fall 2008. A short-duration test flight employing 384 detectors in one focal plane is planned for 2009.
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##### Article: Coherent anti-Stokes Raman microspectroscopy using spectral focusing with glass dispersion
[Hide abstract]
ABSTRACT: We demonstrate experimentally that coherent anti-Stokes Raman microspectroscopy with high spectral resolution is achieved using femtosecond laser pulses chirped up to a few picoseconds by glass elements of known group-velocity dispersion without significant intensity losses. By simply choosing the length of the glass, the chirp of Stokes and pump pulses is tailored to obtain a spectral resolution given by the Fourier limit of the chirped pulse duration. We show that for chirped pulse durations shorter than or comparable to the Raman coherence time, maximum signal occurs for a pump arriving after the Stokes pulse, a time-ordering effect confirmed by numerical simulations.
Applied Physics Letters 12/2008; 93(20-93):201103 - 201103-3. DOI:10.1063/1.3028346
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##### Article: All-optical diode action with Thue-Morse quasiperiodic photonic crystals
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ABSTRACT: We theoretically investigate the possibility of realizing a nonlinear all-optical diode by using the unique features of quasiperiodic one-dimensional photonic crystals. The interplay between the intrinsic spatial asymmetry in odd-order Thue–Morse lattices and Kerr nonlinearity, combined with the unconventional field localization properties of this class of quasiperiodic sequences, gives rise to sharp resonances that can be used to give a polarization-insensitive nonreciprocal propagation with a contrast close to unity for low optical intensities.
Journal of Applied Physics 12/2008; 104(9-104):093113 - 093113-5. DOI:10.1063/1.3010299
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##### Article: Analysing the structure of elongated star clusters
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ABSTRACT: We have previously reported a measure which both quantifies and distinguishes between a (relatively smooth) large-scale radial density gradient and multiscale (fractal) subclustering. Here, we extend the applicability of to clusters which deviate significantly from an overall circular shape. varies systematically as clusters assume a more elongated shape, and it is therefore possible to correct for the effect, if the elongation of the cluster is also quantified. therefore remains a useful and robust analytical technique for classifying and quantifying the internal structure of star clusters, even when their overall shape is far from circular. The corrections required are small for individual clusters which are not extremely elongated (not more than three times longer than they are wide) of the same order as the uncertainty in the value of for a particular cluster type. We therefore recommend that no correction be applied to the calculation of for individual clusters, unless they are more than three times longer than their width, but that correction for elongation be applied when is used for statistical analyses of large numbers of observed or simulated clusters.
Monthly Notices of the Royal Astronomical Society 11/2008; 392(1):341 - 345. DOI:10.1111/j.1365-2966.2008.14055.x
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##### Article: The initial conditions of star formation – VIII. An observational study of the Ophiuchus cloud L1688 and implications for the pre‐stellar core mass function
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ABSTRACT: We reanalyse all of the archived observations of the Ophiuchus dark cloud L1688 that were carried out with the submillimetre common-user bolometer array on the James Clerk Maxwell Telescope. For the first time, we put together all the data that were taken of this cloud at different times to make a deeper map at 850 μm than has ever been published. Using this new, deeper map, we extract the pre-stellar cores from the data. We use updated values for the distance to the cloud complex, and also for the internal temperatures of the pre-stellar cores to generate an updated core mass function (CMF). This updated CMF is consistent with previous results in so far as they went, but our deeper map gives an improved completeness limit of 0.1 M⊙ (0.16 Jy), which enables us to show that a turnover exists in the low-mass regime of the CMF. The L1688 CMF shows the same form as the stellar initial mass function (IMF), and can be mapped on to the stellar IMF, showing that the IMF is determined at the pre-stellar core stage. We compare L1688 with the Orion star-forming region and find that the turnover in the L1688 CMF occurs at a mass roughly a factor of 2 lower than the CMF turnover in Orion. This suggests that the position of the CMF turnover may be a function of environment.
Monthly Notices of the Royal Astronomical Society 11/2008; 391(1):205 - 214. DOI:10.1111/j.1365-2966.2008.13750.x
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##### Article: Galaxies appear simpler than expected
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ABSTRACT: Galaxies are complex systems the evolution of which apparently results from the interplay of dynamics, star formation, chemical enrichment and feedback from supernova explosions and supermassive black holes. The hierarchical theory of galaxy formation holds that galaxies are assembled from smaller pieces, through numerous mergers of cold dark matter. The properties of an individual galaxy should be controlled by six independent parameters including mass, angular momentum, baryon fraction, age and size, as well as by the accidents of its recent haphazard merger history. Here we report that a sample of galaxies that were first detected through their neutral hydrogen radio-frequency emission, and are thus free from optical selection effects, shows five independent correlations among six independent observables, despite having a wide range of properties. This implies that the structure of these galaxies must be controlled by a single parameter, although we cannot identify this parameter from our data set. Such a degree of organization appears to be at odds with hierarchical galaxy formation, a central tenet of the cold dark matter model in cosmology.
Nature 11/2008; 455(7216):1082-4. DOI:10.1038/nature07366
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##### Conference Paper: Clover polarimetric detector - a novel design of an ortho-mode transducer at 150 and 225 GHz
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ABSTRACT: This paper presents the design of an ortho-mode transducer (OMT) to separate two orthogonal linearly polarized signals traveling in a circular waveguide with frequency channels at 150/255 GHz band. Simulations (in HFSS) are presented to determine the optimal probe geometry and feed impedance to achieve good coupling to the optimized probes over a wide bandwidth with corresponding feed impedances for telescopes operating at 150 and 225 GHz. The results show that at 150 GHz band, the return loss can reach below -20 dB from 126 GHz to 180 GHz with the cross polarization reach below -40 dB. At 225 GHz band, the return loss can reach below -20 dB from 190 GHz to 260 GHz with the polarization isolation more than 32 dB.
Infrared, Millimeter and Terahertz Waves, 2008. IRMMW-THz 2008. 33rd International Conference on; 10/2008
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##### Article: Instabilities and solitons in systems with spatiotemporal dispersion
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ABSTRACT: A simple model based on the 1D nonlinear Schrödinger equation is studied, which contains both spatially and temporally dispersive terms. Parametric instabilities for plane waves are analyzed in detail, and solitary waves (both bright and dark) are found. The model presented here is able to describe the non-trivial unstable dynamics of intense, nonlinear light propagation near a material resonance in presence of negative spatial dispersion. We provide as a practical example the light propagation near the tail of an exciton-polariton resonance in a specially designed semiconductor superlattice.
Optics Express 10/2008; 16(19):14882-93. DOI:10.1364/OE.16.014882
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##### Article: The origin of the MB and Kormendy relations in dwarf elliptical galaxies
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ABSTRACT: Aims: The present work is aimed at studying the distribution of galaxies of different types and luminosities along different structural scaling relations to see whether massive and dwarf ellipticals have been shaped by the same formation process. Methods: This exercise is done by comparing the distribution of Virgo cluster massive and dwarf ellipticals and star forming galaxies along the B band effective surface brightness and effective radius vs. absolute magnitude relations and the Kormendy relation to the predictions of models tracing the effects of ram-pressure stripping on disc galaxies entering the cluster environment and galaxy harassment. Results: Dwarf ellipticals might have been formed from low luminosity, late-type spirals that recently entered into the cluster and lost their gas because of a ram-pressure stripping event, stopping their star formation. The perturbations induced by the abrupt decrease of the star formation activity are sufficient to modify the structural properties of disc galaxies into those of dwarf ellipticals. Galaxy harassment induces a truncation of the disc and generally an increase of the effective surface brightness of the perturbed galaxies. The lack of dynamical simulations of perturbed galaxies spanning a wide range in luminosity prevents us from deriving any firm conclusion on a possible harassment-induced origin of the low surface brightness dwarf elliptical galaxy population inhabiting the Virgo cluster. Conclusions: Although the observed scaling relations are consistent with the idea that the distribution of elliptical galaxies along the mentioned scaling relation is due to a gradual variation with luminosity of the Sersic index n, the comparison with models indicates that dwarf ellipticals might have been formed by a totally different process to giant ellipticals.
Astronomy and Astrophysics 10/2008; 489(3):1015-1022. DOI:10.1051/0004-6361:200809546
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##### Article: Star formation histories from multiband photometry: A new approach
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ABSTRACT: ABSTRACTA new method of determining galaxy star formation histories (SFHs) is presented. Using the method, the feasibility of recovering SFHs with multiband photometry is investigated. The method divides a galaxy's history into discrete time intervals and reconstructs the average rate of star formation in each interval. This directly gives the total stellar mass. A simple linear inversion solves the problem of finding the most likely discretized SFH for a given set of galaxy parameters. It is shown how formulating the method within a Bayesian framework lets the data simultaneously select the optimal regularization strength and the most appropriate number of discrete time intervals for the reconstructed SFH. The method is demonstrated by applying it to monometallic synthetic photometric catalogues created with different input SFHs, assessing how the accuracy of the recovered SFHs and stellar masses depend on the photometric passband set, signal-to-noise ratio and redshift. The results show that reconstruction of SFHs using multiband photometry is possible, being able to distinguish an early burst of star formation from a late one, provided an appropriate passband set is used. Although the resolution of the recovered SFHs is on average inferior compared to what can be achieved with spectroscopic data, the multiband approach can process a significantly larger number of galaxies per unit exposure time.
Monthly Notices of the Royal Astronomical Society 09/2008; 389(3):1293 - 1305. DOI:10.1111/j.1365-2966.2008.13639.x
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##### Article: The directional analysis of star clusters
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ABSTRACT: We introduce the Fry Plot and demonstrate its use in quantifying the directionality in star clusters. Taurus and IC2391 are both found to demonstrate a bias in the Probability Density Function (PDF) for neighbours in the same direction for both small and large separations. The Chamaeleon I cluster and Orion B North star-forming region, by contrast, show a symmetrical PDF at small scales, but a directional bias at large scale. Rho Ophiucus, NGC 1333, IC348 and Serpens Main show broadly symmetrical distribution of neighbours. The Pipe Nebula shows strong directional bias at all scales due to the existence of a dominating linear feature. This technique will be useful for quantifying features in observed and simulated objects, and for comparisons with other directional information, such as polarization, velocity and magnetic field maps.
Monthly Notices of the Royal Astronomical Society 09/2008; 390(2):807 - 813. DOI:10.1111/j.1365-2966.2008.13791.x
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##### Conference Paper: Opto-mechanical design and performance of a compact three-frequency camera for the Millimeter Bolometer Array Camera on the Atacama Cosmology Telescope
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ABSTRACT: The 6-meter Atacama Cosmology Telescope will map the cosmic microwave background at millimeter wave-lengths. The commissioning instrument for the telescope, the Millimeter Bolometer Array Camera, is based on a refractive optical system which simultaneously images three separate fields of view at three different frequencies: 145, 220, and 280 GHz. Each frequency band contains around twelve individual optical elements at five different temperature stages ranging from 300 K to 300 mK and a 32 x 32 array of Transition Edge Sensor bolometers at 300 mK. We discuss the design of the close-packed on-axis optical design of the three frequencies. The thermal design and performance of the system are presented in the context of the scientific requirements and observing schedule. A major part of the design was the incorporation of multiple layers of magnetic shielding. We discuss Send correspondence to rthornton@wcupa.edu Copyright 2008 Society of Photo-Optical Instrumentation Engineers. This paper was published in the Proceedings of the Astronomical Instrumentation Conference (7020-7) and is made available as an electronic reprint with permission of SPIE. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited. the performance of the 145 GHz optical system in 2007 and the implementation of the additional two frequency channels in 2008.
Millimeter and Submillimeter Detectors and Instrumentation for Astronomy IV, Marseille, France; 08/2008
• ##### Article: Origin of Temperature-Dependent Threshold Current in p-Doped and Undoped In(Ga)As Quantum Dot Lasers
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ABSTRACT: We examine the temperature dependence of threshold current in self-assembled quantum dot lasers focusing on the factors that produce an increase of threshold current at high temperatures. Nonradiative recombination makes up the majority of threshold current and the temperature dependence of threshold current at higher temperatures. We measure radiative efficiencies at low current density for an undoped structure of 18% and 9% at 300 and 360 K, respectively. These values decrease at higher currents and are even lower for p-doped structures. In undoped structures, the incomplete population of the ground state due to the thermal distribution of holes limits the gain, and this is exacerbated at higher temperatures. In p-doped structures, the gain is increased, but the degree of improvement over an undoped sample is reduced at elevated temperatures. The increasing nonradiative current density at high temperatures is a result of both driving the device harder to maintain the threshold gain requirement (~50% of the increase for an uncoated, 2-mm long laser) and due to an increase in nonradiative recombination at fixed injection. The latter is similar in both p-doped and undoped structures with the most likely origin being the population of higher lying dot and wetting layer states.
IEEE Journal of Selected Topics in Quantum Electronics 08/2008; 14(4-14):1162 - 1170. DOI:10.1109/JSTQE.2008.920040
• ##### Article: Dry Etching of Anisotropic Microstructures for Distributed Bragg Reflectors in AlGaInP/GaAs Laser Structures
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ABSTRACT: A process for anisotropically etching AlGalnP/GaAs laser structures in a high-temperature (180degC), single-step BCl<sub>3</sub>/Cl<sub>2</sub>/Ar plasma process is demonstrated. The etch rate is shown to be stable over the duration of the etch due to its in- sensitivity to temperature. However, this etch process is unsuitable for etching high-aspect-ratio features due to the strong aspect ratio dependence of the GaAs etch rate at 180degC. A two-step etch where the GaAs is etched at 25degC and the AlGalnP layers are etched at 180degC is demonstrated that etches both materials anisotropically. The relative amount of Ar in the plasma has a strong effect on both the sidewall quality and the aspect-ratio-dependent etch rate. Finally, a process to etch anisotropic microstructures with a weak etch rate dependence on aspect ratio is reported.
IEEE Journal of Selected Topics in Quantum Electronics 08/2008; 14(4-14):1098 - 1103. DOI:10.1109/JSTQE.2008.918260
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##### Article: The data processing pipelines for the Herschel/SPIRE Imaging Fourier Transform Spectrometer
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ABSTRACT: We present the data processing pipeline to generate calibrated data products from the Spectral and Photometric Imaging Receiver (SPIRE) imaging Fourier Transform Spectrometer. The pipeline processes telemetry from SPIRE point source, jiggle-and raster-map observations, producing calibrated spectra in low-, medium-, high-, and mixed low-and high-resolution modes. The spectrometer pipeline shares some elements with the SPIRE photometer pipeline, including the conversion of telemetry packets into data timelines and the calculation of bolometer voltages from the raw telemetry. We present the following fundamental processing steps unique to the spectrometer: temporal and spatial interpolation of the stage mechanism and detector data to create interferograms; apodization; Fourier transform, and creation of a hyperspectral data cube. We also describe the corrections for various instrumental effects including first-and second-level glitch identification and removal, correction of the effects due to the Herschel primary mirror and the spectrometer calibrator, interferogram baseline correction, channel fringe correction, temporal and spatial phase correction, non-linear response of the bolometers, variation of instrument performance across the focal plane arrays, and variation of spectral efficiency. Astronomical calibration is based on combinations of observations of standard astronomical sources and regions of space known to contain minimal emission.
Proceedings of SPIE - The International Society for Optical Engineering 08/2008; 7731. DOI:10.1117/12.789850
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##### Article: Understanding the Herschel-SPIRE bolometers
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ABSTRACT: Bolometers are very simple devices. In principle, the behaviour of a bolometer can be described by a simple model along with a small number of parameters. The SPIRE instrument for the Herschel Space Observatory contains five arrays of NTD germanium spiderweb bolometers containing up to 139 pixels. We show from characterisation measurements on the ground using the flight read-out system that the bolometers follow the ideal model extremely well, are very stable, and that the read-out system is sufficiently well behaved to take advantage of this. Calibration should be greatly simplified by being able to take advantage of this behaviour.
Proceedings of SPIE - The International Society for Optical Engineering 08/2008; 7020. DOI:10.1117/12.788789
• ##### Article: The Cℓover experiment
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ABSTRACT: C(l)OVER is a multi-frequency experiment optimised to measure the Cosmic Microwave Background (CMB) polarization, in particular the B-mode component. C(l)OVER comprises two instruments observing respectively at 97 GHz and 150/225 GHz. The focal plane of both instruments consists of an array of corrugated feed-horns coupled to TES detectors cooled at 100 mK. The primary science goal of C(l)OVER is to be sensitive to gravitational waves down to r similar to 0.03 (at 3 sigma) in two years of operations.
Proceedings of SPIE - The International Society for Optical Engineering 07/2008; 7020:35. DOI:10.1117/12.788927
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##### Article: Microstrip-coupled TES bolometers for CLOVER
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ABSTRACT: C ℓ over aims to detect the signature of gravitational waves from inflation by measuring the B-mode polarization of the cosmic microwave background. We have produced microstrip-coupled TES detectors for C ℓ over. The dark NEP of these detectors is dominated by the fundamental phonon-noise limit and we have measured high optical detection efficiencies in these devices with two completely different RF architectures: a finline transition and a four-probe OMT. C ℓ over consists of two telescopes: one operating at 97 GHz, and one with a combined 150/220-GHz focal plane. The 220-and 150-GHz detectors use waveguide probes while the 97-GHz detectors use finline transitions to couple waveguide modes into the microstrip. Each detector is fabricated as a single chip to ensure a 100% operational focal plane. The detectors are mounted in eight-pixel modules and the focal planes are populated using 12 detector modules per detection frequency. Each detector module contains a time-division SQUID multiplexer to read out the detectors. Further amplification of the multiplexed signals is provided by SQUID series arrays. We describe the design of the C ℓ over detectors and present measurements of the prototype detectors' performance showing that they satisfy the requirement of photon-noise limited operation on C ℓ over.
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##### Article: The quasi-optical design of the QUaD telescope
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ABSTRACT: We describe the optical design and performance of ’QUEST and DASI’ or ’QUaD’, a ground-based high-resolution experiment designed to measure the polarisation properties of the cosmic microwave background radiation. QUaD uses bolometric detectors at 100 and 150 GHz on a 2.6 m Cassegrain telescope. The QUaD optics are designed to minimise systematic effects as well as to maximise sensitivity, and we report here on the comprehensive quasi-optical analysis used to achieve this design. We also present initial optical performance measurements achieved in operation, and discuss changes made to the optics to overcome some errors in the mechanical construction of the primary mirror. The QUaD experiment is now fully operational and taking world-leading data at the South Pole.
Infrared Physics & Technology 03/2008; 51(4):277-286. DOI:10.1016/j.infrared.2007.10.001
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##### Article: Simple method for antireflection coating ZnSe in the 20 μm wavelength range
[Hide abstract]
ABSTRACT: We present a simple, inexpensive, and effective method of applying antireflection coatings to zinc selenide windows designed to operate in the thermal infrared wavelength region.
Applied Optics 03/2008; 47(7):870-873. DOI:10.1364/AO.47.000870
• ##### Article: Localized Auger Recombination in Quantum-Dot Lasers
[Hide abstract]
ABSTRACT: We have calculated radiative and Auger recombination rates due to localized recombination in individual dots, for an ensemble of 10<sup>6</sup> dots with carriers occupying the inhomogeneous distribution of energy states according to global Fermi-Dirac statistics. The recombination rates cannot be represented by simple power laws, though the Auger rate has a stronger dependence on the ensemble electron population than radiative recombination. Using single-dot recombination probabilities which are independent of temperature, the ensemble recombination rates and modal gain decrease with increasing temperature at fixed population. The net effect is that the threshold current density increases with increasing temperature due to the increase in threshold carrier density. The most significant consequence of these effects is that the temperature dependence of the Auger recombination rate at threshold is much weaker than in quantum wells, being characterized by a T<sub>0</sub> value of about 325 K. Observations of a strong temperature dependence of threshold in quantum dot lasers may have explanations other than Auger recombination, such as recombination from higher lying states, or carrier leakage.
IEEE Journal of Quantum Electronics 01/2008; 43(12-43):1140 - 1146. DOI:10.1109/JQE.2007.907541
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##### Article: Ultrafast carrier dynamics in p-doped InAs/GaAs Quantum-dot amplifiers
[Hide abstract]
ABSTRACT: Using a differential transmission pump-probe experiment in heterodyne detection, the ultrafast gain and refractive-index dynamics of the ground-state transition in InAs/GaAs quantum- dot amplifiers emitting near 1.3 mum at working condition, that is at room temperature and under electrical injection were measured. An ultrafast gain recovery on a subpicosecond time scale is observed at high electrical injection indicating fast carrier relaxation into the dot ground state, which is appealing for high-speed applications with these devices. Comparing p-doped and undoped devices of otherwise identical structure and operating at the same gain, a faster absorption recovery but a slower gain dynamics in p-doped amplifiers was observed. This finding should help in elucidating the role of p-doping in the design of QD-based devices with high-speed performances.
IET Optoelectronics 01/2008; 1(6-1):298 - 302. DOI:10.1049/iet-opt:20070040
• ##### Chapter: Encyclopedia of Acoustics, Volume Four
[Hide abstract]
ABSTRACT: IntroductionPlucked Stringed InstrumentsPlucked String Vibrations and the PlayerBody VibrationsSound Radiation of GuitarsConstruction and ToneReferences
Encyclopedia of Acoustics, Volume Four, 12/2007: pages 1627 - 1634; , ISBN: 9780470172544
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##### Article: An integrated superconducting phase switch for cosmology instruments
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ABSTRACT: We present the design of a low loss planar phase switch operating at millimetre and submillimetre wavelengths. The system comprises a superconducting nanostrip that switches the RF signal between the two branches of a microstrip phase circuit. The employment of superconducting components reduces conduction losses to a negligible level. The required cooling below the transition temperature does not add extra effort since all high performance detectors employ superconducting circuits, and are in fact cooled well below the transition temperature of the phase circuit material. The proposed fully planar design allows the switch to be easily integrated into the detector circuit and eventually the realisation of a fully planar receiver. This avoids the use of bulky, expensive and lossy waveguide components and allows the fabrication of reliable and cheaply mass producible polarimeters that are now routinely used in large format CMB arrays.
Physica C Superconductivity 11/2007; 466(1-2-466):115-123. DOI:10.1016/j.physc.2007.06.014
• ##### Conference Paper: A Terahertz Modulating Stokes Spectropolarimeter.
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ABSTRACT: We describe a spectropolarimeter in the terahertz range (0.25-1.7 THz). The possiblility of simultaneously measuring spectra for unpolarised, linearly and circularly polarised radiation has numerous applications in various fields. A compact modulating system and it's efficiency are described together with it's extension to an imaging instrument.
Infrared and Millimeter Waves, 2007 and the 2007 15th International Conference on Terahertz Electronics. IRMMW-THz. Joint 32nd International Conference on; 10/2007
• ##### Conference Paper: Metal mesh filters for THz applications
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ABSTRACT: The Astronomy Instrumentation Group at Cardiff University has been developing metal mesh optical filters for more than 30 years. They are currently in use in ground-, balloon- and space-based astronomical instruments and in many laboratory-based THz applications. Here we briefly illustrate the current state of the art with respect to these quasi-optical components (low-pass, high-pass and band-pass filters, dichroics and beam-dividers).
Infrared and Millimeter Waves, 2007 and the 2007 15th International Conference on Terahertz Electronics. IRMMW-THz. Joint 32nd International Conference on; 10/2007
• ##### Article: Temperature-Dependent Gain and Threshold in P-Doped Quantum Dot Lasers
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ABSTRACT: The role of changes in gain and nonradiative recombination as a function of temperature in p-doped quantum dot samples that exhibit a minimum in the threshold current versus temperature characteristics are examined using a detailed analysis based on the multisection measurement method. An injection level is defined as the difference between the transparency energy and the transition energy of the dot states so that the results at different temperatures may be compared. The temperature dependence of the injection level necessary to achieve the required gain produces the initial decrease in nonradiative recombination current and threshold current between 250-280 K. At higher temperatures, the nonradiative recombination increases due to both an increase in the injection level required to achieve a fixed value of gain and because of an increase in the nonradiative recombination at fixed injection level. Using measurements of the population inversion function it is shown that the reduction in the injection level required to achieve a fixed value of gain between 250-280 K is caused by the thermalization of the carrier distribution over this temperature range. Subsequent increases in the injection level required to achieve a fixed value of gain are due to the increasing thermal distribution of carriers over the available states at higher temperatures.
IEEE Journal of Selected Topics in Quantum Electronics 10/2007; 13(5-13):1261 - 1266. DOI:10.1109/JSTQE.2007.903375
• ##### Article: Nonradiative Recombination in Multiple Layer In(Ga)As Quantum-Dot Lasers
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ABSTRACT: The segmented contact technique has been used to study the effects on nonradiative recombination of stacking multiple quantum-dot layers. As the number of stacked layers is increased a shift in the balance of dots in the bimodal dot distribution is observed due to a reduction in the number in the smaller dot size subset. This is attributed to an increase in the density of defect islands, as the number of layers is increased, that preferentially take material from the smaller dots, and lead to an increased level of nonradiative recombination per layer at low injection level. A second nonradiative process is apparent at higher injection level, which is related to the population of the small dot size subset. Spontaneous radiative efficiency was improved in a five-layer sample where the large to small dot size energy separation and relative density of the large dot size subset were increased.
IEEE Journal of Quantum Electronics 09/2007; 43(8-43):698 - 703. DOI:10.1109/JQE.2007.901583
• Source
##### Article: The dust temperatures of the pre-stellar cores in the ρ Oph main cloud and in other star-forming regions: Consequences for the core mass function
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ABSTRACT: We estimate the dust temperatures of the clumps in the ρ Oph main cloud taking into account the 3D geometry of the region, and external heating from the interstellar radiation field and from HD 147879, a nearby luminous B2V star, which is believed to dominate the radiation field in the region. We find that the regions where pre-stellar cores are observed (i.e. at optical visual extinctions >7 mag) are colder than ∼10 –11 K. These dust temperatures are smaller than those which previous studies of the same region have assumed. We use the new dust temperatures to estimate the masses of the pre-stellar cores in the ρ Oph main cloud from millimetre observations, and we find core masses that are larger than previous estimates by a factor of ∼2 –3. This affects the core mass function (CMF) of the region; we find that the mass at which the core mass spectrum steepens from a slope α∼ 1.5 to a slope α∼ 2.5 has moved from ∼0.5 to ∼1 M⊙. In contrast with the CMF in other star-forming regions (e.g. Orion), there is no indication for a turnover down to the completeness limit (∼0.2 M⊙), but the CMF may flatten at around ∼0.4 M⊙. We generalize our results to the pre-stellar cores in Taurus and in Orion. In Taurus, the ambient radiation field heating the pre-stellar cores is believed to be weaker than that in ρ Oph. Hence, the dust temperatures of the cores in Taurus are expected to be below ∼10 –11 K. In Orion, the radiation field is believed to be 103 times stronger than the standard interstellar radiation field. Based on this assumption, we estimate that the dust temperatures of the pre-stellar cores in Orion are around ∼20 –30 K.
Monthly Notices of the Royal Astronomical Society 08/2007; 379(4):1390 - 1400. DOI:10.1111/j.1365-2966.2007.11999.x
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##### Article: Millikelvin thermal conductance measurements of compact rigid thermal isolation joints using sapphire-sapphire contacts, and of copper and beryllium-copper …
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ABSTRACT: We present thermal conductance measurements of different types of bolted joint at sub-Kelvin temperatures. Joints containing sapphire surfaces provided good thermal isolation; the mechanism appears to be the existence of a very small area over which the surfaces are actually in contact. Various configurations were measured at temperatures between 100 mK and 4 K. The best joint contained sapphire discs separated by diamond powder and had a conductance of 0.26 µWK −1 (T/1 K) 2.9 , where T is temperature. A mechanical support structure constructed from similar joints, but using alumina powder, had a measured heat leak of 2.57 µW between 80 mK and 1.1 K and was capable of supporting a mass of over 10 kg. Joints between metal surfaces provided good thermal conduction; a bolted joint between copper and a beryllium-copper alloy (C17510 TF00) had a measured conductance of 46 mW K −1 at 100 mK, increasing linearly with temperature. Measurements were also made on a copper-copper compression joint using differential thermal contraction to provide the clamping force. The performance was approximately an order of magnitude worse than for the bolted joint. These measurements were all made as part of the development programme for the SCUBA-2 astronomical instrument; the design requirements were achieved for both insulating and conducting joints. Cryogenics 47(5-6):333-342, 2007 http://dx.doi.
Cryogenics 08/2007; 47(5). DOI:10.1016/j.cryogenics.2007.04.004
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