bluuebunny/crossref_metadata_2025 huggingface dataset:
Nanometer-scale GaAs clusters from organometallic precursors
Authors: Sercel Peter C. , Saunders Winston A. , Atwater Harry A. , Vahala Kerry J. , Flagan Richard C.
Published: July 2002

We report the synthesis of crystalline nanometer-scale GaAs clusters by homogeneous vapor-phase nucleation from organometallic precursors. Cluster synthesis is performed in a hot wall organometallic vapor-phase epitaxy reactor at atmospheric pressure. High resolution transmission electron microscopy studies reveal that the aerosol produced is composed of highly faceted single crystal GaAs particles in the 10–20 nm range. The influence of growth temperature and reactant concentration on cluster morphology is discussed.
Near-field magneto-optics and high density data storage
Authors: Betzig E. , Trautman J. K. , Wolfe R. , Gyorgy E. M. , Finn P. L. , Kryder M. H. , Chang C.-H.
Published: July 2002

Near-field scanning optical microscopy (NSOM) has been used to image and record domains in thin-film magneto-optic (MO) materials. In the imaging mode, resolution of 30–50 nm has been consistently obtained, whereas in the recording mode, domains down to ∼60 nm have been written reproducibly. Data densities of ∼45 Gbits/in.2 have been achieved, well in excess of current magnetic or MO technologies. A brief analysis of speed and other issues indicates that the technique may represent a viable alternative to these and other methods for anticipated high density data storage needs.
Uniform and flexible 24-meter superconducting tape of silver-sheathed Tl0.5Pb0.5Ba0.4Sr1.6Ca2Cu3O8.2
Authors: Ren Z. F. , Wang J. H.
Published: July 2002

Simple geometric considerations show that to maximize the transport Jc for a given type of bulk superconductor, it is only necessary to have the individual grains in the sample oriented with maximal area of close contact with their immediate neighbors. With this guideline, a 24-meter silver-sheathed thallium-containing superconductor of the 1223-type without long-range granular alignment has been made. It is flexible, uniform, and exhibits a transport Jc of 104 A/cm2 at 77 K in the absence of external magnetic field.
Self-stabilization of fundamental in-phase mode in resonant antiguided laser arrays
Authors: Nabiev R. F. , Yeh P. , Botez D.
Published: July 2002

The effect of gain spatial hole burning on antiguided arrays is analyzed for the first time. Nonresonant devices, due to the nonuniformity of the in-phase-mode intensity profile, experience self-focusing and multimode operation with increasing drive level, similar to the behavior of evanescent-wave-coupled devices. Resonant and near-resonant devices [i.e., resonant-optical-waveguide (ROW) arrays], due the uniformity of the in-phase mode, display some mild defocusing with increasing drive level, while the nearest high-order mode cannot reach threshold to drive levels in excess of 4× threshold. These results explain the fundamental single-mode stability of ROW arrays, in excellent agreement with experimental data.
The bidiffractive grating coupler
Authors: Fattinger Ch.
Published: July 2002

A novel optical device for coupling a light beam to a planar waveguide structure and for decoupling the guided wave after a certain distance of propagation is demonstrated. The beam coupler comprises two superimposed uniform dielectric diffraction gratings with different periodicities and/or different orientations incorporated into a planar surface-wave supporting structure. The bidiffractive coupler has translational invariant coupling efficiency and is ideally suited for optical surface probing using electromagnetic surface mode propagation over short distances only.
(Ni80Co20/Cu) multilayers: Giant magnetoresistance with low saturation field
Authors: Bian X. , Ström-Olsen J. O. , Altounian Z. , Huai Y. , Cochrane R. W.
Published: July 2002

We have observed giant magnetoresistance (MR) in Ni80Co20/Cu multilayers. Saturation magnetoresistance exceeding 16% and very well-defined oscillations in MR as a function of Cu spacer thickness are observed at room temperature with an oscillation period and phase quite similar to that found in other Cu-based multilayered systems. Of particular interest are the small saturation fields (HS<120 Oe) for Cu thickness near the second peak (∼20 Å) which result in large MR field sensitivities up to 0.12%/Oe at room temperature.
Time dependent ballistic electron emission microscopy studies of a Au/(100)GaAs interface with a native oxide diffusion barrier
Authors: Talin A. Alec , Ohlberg Douglas A. A. , Williams R. Stanley , Sullivan Patrick , Koutselas Ioannis , Williams Beth , Kavanagh Karen L.
Published: July 2002

Ballistic electron emission microscopy (BEEM) has been used to investigate the effects of a thin interfacial native oxide layer on the electronic properties and stability of a Au/(100) n-GaAs contact as a function of time. The oxide had no effect on the electronic band structure at the interface measured with BEEM, as compared to similar contacts without a diffusion barrier. In addition, the oxide greatly enhanced the electrical homogeneity of the interface and prolonged the ability of the diode to transmit ballistic electrons to more than 35 days.
Submicron electrically conducting wires produced in polyimide by ultraviolet laser irradiation
Authors: Phillips H. M. , Wahl S. , Sauerbrey R.
Published: July 2002

An array of permanently electrically conducting lines with a width of 0.5 μm and a period of 0.9 μm has been produced in polyimide with a KrF laser using a holographic technique. The electrical resistivity of these submicron structures shows an ohmic characteristic and has a value of less than 0.5 Ω cm, while the resistivity perpendicular to the structures exceeds this value by more than a factor of 107. The temperature dependence of the conductivity in these wires is found to be similar to that found for macroscopic regions of polyimide which show laser induced conductivity.
Terahertz four-wave mixing spectroscopy for study of ultrafast dynamics in a semiconductor optical amplifier
Authors: Zhou Jianhui , Park Namkyoo , Dawson Jay W. , Vahala Kerry J. , Newkirk Michael A. , Miller Barry I.
Published: July 2002

Ultrafast dynamics in a 1.5-μm tensile-strained quantum-well optical amplifier has been studied by highly nondegenerate four-wave mixing at detuning frequencies up to 1.7 THz. Frequency response data indicate the presence of two ultrafast physical processes with characteristic relaxation lifetimes of 650 fs and <100 fs. The longer time constant is believed to be associated with the dynamic carrier heating effect. This is in agreement with previous time-domain pump-probe measurements using ultrashort optical pulses.
Pulsed laser deposition of epitaxial silicon/h-Pr2O3/silicon heterostructures
Authors: Tarsa E. J. , Speck J. S. , Robinson McD.
Published: July 2002

Epitaxial Si/praseodymium oxide/Si heterostructures were grown in situ on (111) oriented Si substrates using pulsed laser deposition. Growth of the oxide layer under oxygen deficient conditions resulted in hexagonal Pr2O3 (h-Pr2O3) films which displayed a (001)Pr2O3∥(111)Si, [110]Pr2O3∥[11̄0]Si orientation and x-ray rocking curve full width at half-maximum values of ∼0.8°. The top Si layer grew epitaxially on the oxide film with a twinned (111)Si∥(001)Pr2O3 orientation. The surface structure of both oxide and semiconductor layers was investigated in situ using reflection high energy electron diffraction, and the resulting films were characterized using x-ray diffraction and transmission electron microscopy.
Complete bleaching of the intersubband absorption in GaAs/AlGaAs quantum wells using a far-infrared free-electron laser
Authors: Helm M. , Fromherz T. , Murdin B. N. , Pidgeon C. R. , Geerinck K. K. , Hovenyer N. J. , Wenckebach W. Th. , van der Meer A. F. G. , van Amersfoort P. W.
Published: July 2002

The intensity-dependent intersubband absorption in GaAs/AlGaAs quantum wells with a subband separation smaller than the optical phonon energy has been measured with a pulsed far-infrared free-electron laser. Complete bleaching of the absorption is observed at I=200 kW/cm2. Fitting the data with a two-level system yields a characteristic time constant of 1–2 ps. Possible interpretations, considering the finite pulse width of the laser, are discussed.
Second harmonic generation from SimGen superlattices
Authors: Bottomley D. J. , Lüpke G. , Ledgerwood M. L. , Zhou X. Q. , van Driel H. M.
Published: July 2002

We report the observation of optical second harmonic generation (SHG) from SimGen short-period superlattices grown on different substrates. For a 775 nm fundamental beam, the SHG signals from odd-odd, even-even, and even-odd specimens are comparable, apart from substrate-dependent strain enhancement. It is concluded that the lack of a large bulk dipole-allowed signal in odd-odd superlattices predicted by Ghahramani, Moss, and Sipe [Phys. Rev. Lett. 64, 2815 (1990)] is due to the inevitable presence of monatomic steps on the substrate which lead to antiphase domains; a possible circumvention of this limitation is outlined based on the use of a single domain vicinal Si(001) substrate.
First operation of a photocathode radio frequency gun injector at high duty factor
Authors: Dowell D. H. , Davis K. J. , Friddell K. D. , Tyson E. L. , Lancaster C. A. , Milliman L. , Rodenburg R. E. , Aas T. , Bemes M. , Bethel S. Z. , Johnson P. E. , Murphy K. , Whelen C. , Busch G. E. , Remelius D. K.
Published: July 2002

Tests of the electron beam injector for the Boeing/Los Alamos Average Power Laser Experiment have demonstrated first time operation of a photocathode radio frequency gun accelerator at 25% duty factor, exceeding previous photocathode operating parameters by three orders of magnitude. The macropulse format was 30 Hz and 8.3 ms with a micropulse frequency of 27 MHz. Average beam currents of up to 32 mA have been accelerated to 5 MeV for an average beam power of 160 kW. The macropulse peak current was 128 mA. The 32 mA average beam current exceeds previous cathode performance by a factor of 1000. Emittance measurements demonstrate excellent electron beam quality.
Linear magnetoresistive change at zero field in Cu/Co/Cu/NiFe multilayers
Authors: Miyauchi D. , Araki S.
Published: July 2002

Cu/Co/Cu/NiFe multilayers were prepared by molecular beam epitaxy and their magnetoresistive (MR) properties in dc and ac magnetic field were studied. With a view to application such as MR head, minor loops of MR transfer curves with applied fields of ±50 Oe were particularly investigated. By optimizing the thickness of each layer, a linear and nonhysteretic MR response was obtained. The output voltage was roughly estimated as two and a half times that of a conventional NiFe element in an ac magnetic field at 50 Hz.
Enhanced conductivity of zinc oxide thin films by ion implantation of hydrogen atoms
Authors: Kohiki Shigemi , Nishitani Mikihiko , Wada Takahiro , Hirao Takashi
Published: July 2002

Enhancement of the conductivity of zinc oxide through doping with hydrogen atoms was examined by using ion implantation of highly resistive thin films deposited by rf magnetron sputtering at room temperature. With a doping of 1×1017 atoms cm−2, the conductivity after annealing at 200 °C in an N2 atmosphere at 1 atm rose from the initial 1×10−7 Ω−1 cm−1 to 5.5×102 Ω−1 cm−1.
Electromagnetically excited acoustic resonance for evaluating attenuation coefficient and grain size in polycrystalline metals
Authors: Hirao M. , Ogi H.
Published: July 2002

An experiment on an acoustic resonance spectrometer for evaluating attenuation characteristics and grain size of polycrystalline metals is described. A noncontacting electromagnetic acoustic transducer is excited to establish a shear-wave ringing in a plate sample and the amplitude spectrum is obtained by sweeping the driving rf frequency through the resonances. At each resonance frequency, the sample rings down exponentially with time. The wave form is acquired to determine the time constant, or the attenuation coefficient, whose frequency dependence is finally related to the average grain size through the Rayleigh scattering theory. This nondestructive evaluation agreed with the photomicrographic examination to an accuracy of 6 μm for a relatively narrow distribution of grain size in low carbon steels.
Empty state and filled state image of ZnGa acceptor in GaAs studied by scanning tunneling microscopy
Authors: Zheng Z. F. , Salmeron M. B. , Weber E. R.
Published: July 2002

ZnGa acceptor atoms in the first to sixth layer below the GaAs (110) cleavage plane have been identified. For the first time, we find that the empty state scanning tunneling microscopy image of a ZnGa acceptor is a characteristic equal latitude triangle-shaped feature of ∼4 nm width with a (110) mirror plane. The filled state image, however, is a spherical feature of similar size. These unique features can be used as the signature for the identification of ZnGa in GaAs.
Low energy kinetic threshold in the growth of cubic boron nitride films
Authors: Kidner S. , Taylor C. A. , Clarke Roy
Published: July 2002

We report the growth of cubic boron nitride (cBN) films by magnetron sputtering on Si (100) substrates. The films are grown in the presence of negative substrate bias voltages and a nitrogen plasma produced by an electron cyclotron resonance source. We find evidence for a sharp low-voltage threshold in the substrate bias (−105 V) beyond which the samples are predominantly cBN. The structural quality of the cBN films is optimized in a narrow range of voltages near this threshold. We discuss the important role of energetic ions in the formation of cBN in light of recent theoretical findings.
Reversible control of the nonlinear optical activity of one-dimensional metal complexes
Authors: Kamata T. , Fukaya T. , Matsuda H. , Mizukami F.
Published: July 2002

Reversible controllability of the third-order nonlinear optical activity has been demonstrated on a one-dimensional metal complex. Iodine penetration to a film sample causes disappearance of the d-p transition absorption band of the metal complex and remarkable decline of two-odd orders in the third-order nonlinear susceptibility. The deactivated optical nonlinearity was completely restored by removing iodine from the film with heating. The observed alterations in optical nonlinearities are mainly caused by the fluctuation of metal-metal interaction in the linear metal chain.
Low temperature synthesis of Ge nanocrystals in SiO2
Authors: Craciun Valentin , Boyd Ian W. , Reader Alec H. , Vandenhoudt Dirk E. W.
Published: July 2002

A novel and simple technique for the synthesis of Ge nanocrystals embedded in SiO2 is reported. The method is fully compatible with silicon microelectronic technology and relies solely upon low temperature (only 550 °C) ultraviolet oxidation of Si0.8Ge0.2 strained layers. This temperature is significantly lower than that usually used for the formation of Ge nanocrystals from SiGe oxides by H2 reduction.
Dependence of optical gain on crystal orientation in surface-emitting lasers with strained quantum wells
Authors: Ohtoshi T. , Kuroda T. , Niwa A. , Tsuji S.
Published: July 2002

We analyze theoretically optical gains in vertical-cavity surface-emitting lasers (VCSELs) for various crystal orientations. The calculation based on the multiband effective-mass theory takes into account the effects of anisotropy and nonparabolicity on the valence subband dispersion. It is found that in VCSELs employing InGaAs/InP strained quantum wells (QWs) with non-(001) orientations except (111), the polarization in the QW plane can be controlled and high gains are obtained. In particular, the gains in VCSELs with (NN1)-oriented (N≥2) strained QWs are markedly higher than those in the equivalent (001) lasers.
Size evolution of low energy electron generated Ca colloids in CaF2
Authors: Bennewitz R. , Günther C. , Reichling M. , Matthias E. , Vijayalakshmi S. , Barnes A. V. , Tolk N. H.
Published: July 2002

Metal colloids in CaF2 created by low energy electron irradiation are identified by their optical absorption bands. A mean colloid radius of 18 nm is deduced from a narrow colloid band found in crystals irradiated at room temperature. Absorption data reveal a strong influence of crystal temperature during irradiation on metallization efficiency, mean colloid size, and colloid size distribution. A reversible absorption band shift is observed when turning the electron beam on and off, which may indicate a decrease of colloid size during electron irradiation.
Using x-ray arrays to measure impurity transport in the Alcator-C tokamak
Authors: Petrasso R. D. , Terry J. , Hopf J. , Loter N. , Seguin F. H. , Kerr Del Grande N.
Published: July 2002

Using three different x-ray arrays simultaneously, we have made measurements of the transport of aluminum (Z=13) or silicon (Z=14) injected into MIT’s Alcator tokamak plasma. Each array has ∼2 cm spatial and 15 μs temporal resolution. Since Alcator core temperatures are typically between 1 and 2 keV, fully stripped, H- and He-like ions are the only ionization states of the injected impurity with significant populations for r/a≲0.7. The spectral responses of the arrays have been tailored so that each array has a markedly different response to the x radiations from these three ionization states. Through the use of two of these arrays, time-resolved density profiles of fully stripped and total injected impurities, and studies of their transport, have been previously reported. Current work and description of the analysis method will be presented.
Velocity distribution of supersonic nozzle beams
Authors: Haberland H. , Buck U. , Tolle M.
Published: July 2002

A problem concerning the velocity distribution standardly assumed in molecular beams is investigated. It is shown, theoretically and experimentally, that the sum of all energy contributions based on the standard assumed velocity distribution function is different from that based on pure thermodynamic energy considerations.
High-energy electron impact spectrometer for absolute triple differential cross sections
Authors: Lahmam-Bennani A. , Wellenstein H. F. , Duguet A. , Lecas M.
Published: July 2002

A new high-energy spectrometer for the coincident detection of both outgoing electrons in an electron impact ionization, or (e, 2e), experiment is described. The absolute triple differential cross section can be measured with this instrument to a precision of 10%. Selected results on rare gases are presented, demonstrating the ability of the spectrometer to either probe the target structure in impulsive or binary (e,2e) experiments, or to test the scattering mechanism in nonimpulsive conditions, both for valence or inner shells.
New experimental technique for simultaneously measuring thermal conductivity and heat capacity
Authors: Huang C. C. , Viner J. M. , Novack J. C.
Published: July 2002

A new technique for simultaneously measuring the thermal conductivity and heat capacity of any thin sample is described. Extensive measurements on microscope cover glass slides and on one liquid-crystal compound have been carried out. The results are in good agreement with reported values.
Simple, low-cost planar flow casting machine for rapid solidification processing
Authors: Smith M. T. , Saletore M.
Published: July 2002

The design, fabrication, and operation of a relatively simple low-cost planar flow casting (PFC) machine optimized for small-batch processing were investigated by the Bureau of Mines. Several design features found beneficial to PFC process operation include: a ground nozzle stopper to retain the alloy charge during melting; a remote, large-volume pressure vessel connected to the crucible gas system to reduce temperature-induced pressure fluctuations; and the use of a nested induction coil that allows both the melt charge and the crucible reservoir to be located close to the cooling wheel. The results of several PFC process runs are provided showing typical values of the cooling wheel surface velocity, crucible ejection pressure, and crucible nozzle clearance gap. Examination of the rapidly solidified, Fe-based ribbons for thickness, dimensional uniformity, and atomic structure indicated that good quality glassy ribbon could be produced with proper selection of the controllable process variables. In addition, single-variable linear regression analysis was used to determined the effect of each process variable on the resulting ribbon thickness.
Wafer bonding induced degradation of thermal silicon dioxide layers on silicon
Authors: Afanas’ev V. V. , Ericsson P. , Bengtsson S. , Andersson M. O.
Published: July 2002

Structural and electrical properties of thermal SiO2 layers on Si subjected to wafer bonding were studied. Regions with enhanced etching rate in HF-solutions were found to appear after the bonding and a subsequent etchback of one of the wafers. The bonding procedure also leads to enhancement of interface state generation and accumulation of oxide trapped charge during injection of electrons or holes into the SiO2 layer. The annealing behavior and capture cross section of these defects are close to those of centers generated when hydrogen is incorporated in the oxide network. Enrichment of the oxide by hydrogen during the bonding and postbonding anneal is suggested to be responsible for the degradation of the thermal oxide.
Optimization of the Herzog correction in the hemispherical deflector analyzer
Authors: Nishigaki S. , Kanai S.
Published: July 2002

The theoretical performance of the hemispherical deflector analyzer has been discussed on the basis of a numerical calculation of electron trajectories by taking the fringing field correctly into account. We optimized both the ‘‘Herzog region’’ and the electrode sector angle and got foci of electrons which are as good as those for the ideal-field 180 ° analyzer. In the optimized analyzer, even electrons with their energies different from the pass energy are focused well at the exit plate. Moreover, the arriving position of electrons at the exit plate depends linearly upon their incidence energy. The possibility of using a position-sensitive detector is discussed.
JET x-ray pulse-height analysis system
Authors: Pasini D. , Gill R. D. , Holm J. , van der Goot E. , Weller A.
Published: July 2002

The pulse-height-analysis (PHA) system installed on the Joint European Torus (JET) measures the plasma soft x-ray emission (2–30 keV) with an energy resolution of 450 eV and a time resolution of 200 ms. This diagnostic includes three Si(Li) detectors, equipped with sets of remotely controlled apertures and filters, which view the plasma in the midhorizontal plane of the torus along a single tangential line of sight. Automatic analysis of the spectra yields the central electron temperature, the central concentrations of chlorine, chromium, and nickel, and Zeff. Simulations of the measured spectra using a radiation code provides the basis to construct a consistent picture of the soft x-ray emission in the central region of JET plasmas.
Optical interferometry for measuring instantaneous thickness of transparent solid and liquid films
Authors: Ohyama Tetsuya , Endoh Ken-ichi , Mikami Atsushi , Mori Yasuhiko H.
Published: July 2002

An optical-interference method is proposed which enables measurements of instantaneous, uniform thickness of the order of 10 μm–1 mm of transparent (or weakly absorbing) solid materials or liquids. It uses a converging lens set by its focal length separated from the subject film. The lens turns parallel, monochromatic, coherent rays in an expanded laser beam into converging rays propagating as spherical waves toward the subject film. The rays reflected at the front and rear surfaces of the film pass again through the lens to be retransformed from the spherical waves into the plane waves, and form interference fringes on a screen. Knowing the number of fringes provided by the rays which are incident on the subject film at angles between two specified angles, we can readily calculate the thickness of the film. A prototype apparatus based on this method has been constructed and used successfully in measuring the thicknesses of both solid and liquid films.
Double-pulse Nd:glass laser system for Thomson scattering diagnosis of high-temperature plasmas
Authors: Dooling J. C. , York T. M.
Published: July 2002

Beam properties of a twice Q-switched Nd:glass laser operated at 1053 and 527 nm have been examined with respect to energy, pulse width, divergence, and conversion efficiency at a pulse separation of 75 μs. Q switching occurred twice within a flashlamp burst (FWHM: 160–175 μs). Maximum IR energies of 24 joules per pulse (Jpp) were measured with pulse widths of 25 and 40 ns and divergence values of 0.6 and 0.7 mrad for the first and second pulses, respectively. Maximum second-harmonic energies of 14 J (single pulse) and 4 Jpp were measured with peak power-density conversion efficiencies of 45% and 30%. Use of this laser at 1053 nm, in conjunction with generation III GaAs detectors, offers improvement over current source/detection systems (ruby/S-20) for diagnosis of high-temperature plasmas (Te >10 keV).
Surface acoustic-wave piezoelectric crystal aerosol mass microbalance
Authors: Bowers W. D. , Chuan R. L.
Published: July 2002

The feasibility of applying surface acoustic-wave (SAW) piezoelectric crystal technology to the measurement of subnanogram levels of particles and parts-per-million levels of sulfur dioxide has been demonstrated. Improvements necessary for implementing the development of a practical instrument for the real-time measurement of very low aerosol mass and ambient gases have been identified. Mass sensitivity comparisons of a 158-MHz SAW piezoelectric microbalance and a conventional 10-MHz quartz-crystal microbalance (QCM) showed that the SAW crystal was 266 times more sensitive. This is in good agreement with the theoretical value of 250. Frequency stability of a single SAW resonator was 6 parts in 108 over 1 min. Response to temperature change is found to be very linear over the range +30 to −30 °C. Response to pressure change is found to exhibit an aging effect whereby after one complete cycle of pressure variation from 1 atm to 70 mm Hg and back to 1 atm, the SAW response to pressure change decreases by an order of magnitude. A strong response to 15 ppm SO2 has been demonstrated on a chemically coated SAW crystal. With improvements, SO2 measurements at the ppb level should be achievable.
Enhancement of phosphorus incorporation and growth rate of epitaxial diamond films by the addition of nitrogen
Authors: Cao G. Z. , Enckevort W. J. P. van , Giling L. J. , de Kruif R. C. M.
Published: July 2002

The influence of the addition of nitrogen on the phosphorus incorporation into and the growth rate of epitaxial diamond films grown by hot filament chemical vapor deposition, using a solid compound (NH4H2PO4) as a doping source, was investigated. Experiments show that the introduction of nitrogen gas into the system increases both the segregation of phosphorus in the epitaxial diamond and the film growth rate. The enhancement of phosphorus incorporation was attributed to the local lattice dilatation caused by a significant level of nitrogen dopant, while the increase of growth rate was explained by a model for defect-induced stabilization of diamond. From the secondary ion mass spectrometry analysis, it was found that the effective incorporation efficiency of nitrogen in diamond is about 9×10−4. For phosphorus it can reach a value as high as approximately 1×10−3 when a large amount of nitrogen is introduced simultaneously to the system.
Noise level effects in radiometers
Authors: Harris A.I.
Published: July 2002

Noise in radiometers can often be partitioned between distinct parts of the system. In most cases noise from the front end of the system is difficult to reduce, but should nevertheless dominate the total system noise. We present a simple theory for determining the effects of back-end noise and analog-to-digital conversion on sensitivity and integration time. As general guidelines, we find that the back-end noise should be a quarter of the front-end noise or less, and that the A/D converter should sample the noise so that the rms noise level is twice the converter’s resolution. Although originally derived for a radioastronomical radiometer, these results apply to similar systems which operate close to noise limits and where integration time is at a premium.
Charge-coupled device image detector for Schmidt telescopes
Authors: Natali Giuliano , Pedichini Fernando
Published: July 2002

The application of a charge-coupled device sensor at the focal plane of a Schmidt telescope is described. The device installed on the 60/90/180 cm Schmidt telescope at Campo Imperatore Astrophysical Observatory has a field of view of 17×25 arcmin. A special LN2 cooling system was installed inside the telescope without any modification to the plate holder itself. Tests of the instrument show it is a powerful tool in studying large angular objects like nearby galaxies, globular clusters, and clusters of galaxies. Sensitivity is high enough to reach the 21.8th magnitude V with a signal-to-noise ratio of 1.5 in 40 min of exposure.
A display controller for infrared charge coupled device arrays
Authors: Newbery S. M. , Ollier E.
Published: July 2002

A television compatible display controller is described which forms the image acquisition and display section of a portable infrared camera. The system is capable of accepting a CCD output signal, digitizing it, performing a digital subtraction of a stored reference frame, and increasing each pixel in size so as to ‘‘fill’’ the display area of a TV monitor.
Fiber-optic device acting as a high-accuracy wide-range variable optical attenuator
Authors: Yamashita Mikio
Published: July 2002

A high-accuracy optical attenuator with transmission efficiencies variable in pseudobinary steps over a wide range is described that uses multilegged optical fiber lightguides and a set of optical shutters. It features excellent stability and reproducibility, and thus has proved particularly useful in some specific applications. Procedures for calibrating the relative light transmission efficiencies by the use of scintillations as a stable pulsed light source are also described in some detail.
Simultaneous multivelocity component laser Doppler velocimetry using one digital frequency processor
Authors: Johnson D. A.
Published: July 2002

Digital frequency processing used in conjunction with optical frquency shifting is an ideal way to achieve simultaneous multivelocity component measurements in laser Doppler velocimetry. This becomes obvious once realized that multivelocity component information at different frequency offsets can be obtained directly from the composite frequency spectrum of a single-channel digital frequency processor. The approach has numerous advantages over the very popular approach of using a different laser wavelength, photodetector, and signal processor for each velocity component.
A surface with variable reflectivity
Authors: Horvath H. , Gorraiz J. , Henrich W. , Dellago C.
Published: July 2002

The variable reflectance surface consists of a movable frame and a base plate. The frame carries a series of equidistant black (or white) aluminum stripes; the base plate has a pattern of black and white stripes with the same spacing as the stripes. The reflectivity of the device can be varied by a factor of 10, depending on the location of the movable stripes with respect to the fixed stripes. The position of the frame is controlled by a micrometer screw, giving excellent reproducibility of the reflectivity.
Double frame UV/x-ray Picoframe framing camera systems
Authors: Sibbett W. , Walker D. R. , Sleat W. E. , Eagles R. T. , Freeman N. J.
Published: July 2002

The performance of subnanosecond-resolution, Picoframe I and II electron-optical framing cameras are described for the conditions of UV and soft x-ray illumination. Experimental data are presented to show that an x-ray sensitive Picoframe I camera can produce doublet frames having exposure times of ∼250 ps FWHM, interframe times of ∼1.5 ns with a limiting dynamic spatial resolution of 5 ℓp/mm. The dual-aperture, Picoframe II camera variant was evaluated with UV and x-ray illumination and frame periods of ∼120 ps FWHM and interframe times of 400 ps were obtained in both cases. The limiting dynamic spatial resolution was 8 ℓ;p/mm for the UV-sensitive Picoframe II camera but this was somewhat lower at 4.5 ℓp/mm for the x-ray sensitive counterpart.
Time-of-flight scattering and recoiling spectrometer (TOF-SARS) for surface analysis
Authors: Grizzi O. , Shi M. , Bu H. , Rabalais J. W.
Published: July 2002

A UHV spectrometer system has been designed and constructed for time-of-flight scattering and recoiling spectrometry (TOF-SARS). The technique uses a pulsed primary ion beam and TOF methods for analysis of both scattered and recoiled neutrals (N) and ions (I) simultaneously with continuous scattering angle variation over a flight path of ≊1 m. The pulsed ion beam line uses an electron impact ionization source with acceleration up to 5 keV; pulse widths down to 20 ns with average current densities of 0.05–5.0 nA/mm2 have been obtained. Typical current densities used herein are ≊0.1 nA/mm2 and TOF spectra can be collected with a total ion dose of <10−3 ions/surface atom. A channel electron multiplier detector, which is sensitive to both ions and fast neutrals, is mounted on a long tube connected to a precision rotary motion feedthru, allowing continuous rotation over a scattering angular range 0°<θ<165°. The sample is mounted on a precision manipulator, allowing azimuthal δ and incident α angle rotation, as well as translation along three orthogonal axes. The system also accommodates standard surface analysis instrumentation for LEED, AES, XPS, and UPS. The capabilities of the system are demonstrated by the following examples: (A) TOF spectra versus scattering angle θ; (B) comparison to LEED and AES; (C) surface and adsorbate structure determinations; (D) monitoring surface roughness; (E) surface semichanneling measurements; (F) measurements of scattered ion fractions; and (G) ion induced Auger electron emission.
Electron paramagnetic resonance Q-band bridge with GaAs field-effect transistor signal amplifier and low-noise Gunn diode oscillator
Authors: Hyde James S. , Newton M. E. , Strangeway Robert A. , Camenisch Theodore G. , Froncisz W.
Published: July 2002

A Varian Q-band E-110 microwave bridge for electron paramagnetic resonance (EPR) spectroscopy has been modified by addition of a low-phase noise Gunn diode oscillator of our own design, a low-noise GaAs field-effect transistor microwave signal amplifier, and a balanced mixer requiring high input power (10 mW) at the local oscillator port. The oscillator has previously been found to have −129 dBc/Hz phase noise, 22 dB lower than for the original klystron. Noise measurements indicate that the microwave amplifier and mixer reduce the overall receiver noise figure by 24.6 dB, a very significant improvement. It is shown that reduction of both phase noise and receiver noise are required in order to achieve full improvement in signal-to-noise ratio over the full range of available microwave power. Spectra of 1.6×10−6 M 15N-perdeutero TEMPONE (1-oxyl-2,2,6,6-tetramethyl-4-piperidone) and of 10−6 M Mn2+ are shown in order to demonstrate sensitivity.
High quality AIN and GaN epilayers grown on (00⋅1) sapphire, (100), and (111) silicon substrates
Authors: Kung P. , Saxler A. , Zhang X. , Walker D. , Wang T. C. , Ferguson I. , Razeghi M.
Published: July 2002

The growth of high quality AlN and GaN thin films on basal plane sapphire, (100), and (111) silicon substrates is reported using low pressure metalorganic chemical vapor deposition. X-ray rocking curve linewidths of about 100 and 30 arcsec were obtained for AlN and GaN on sapphire, respectively. Room-temperature optical transmission and photoluminescence (of GaN) measurements confirmed the high quality of the films. The luminescence at 300 and 77 K of the GaN films grown on basal plane sapphire, (100), and (111) silicon was compared.
A very sensitive technique for measuring the temperature derivative of electrical resistance between 4 and 300 K
Authors: Chaussy J. , Gandit P. , Bret J. L. , Terki F.
Published: July 2002

A sinusoïdal temperature modulation method has been developed to provide direct access to the derivative of the resistance with respect to temperature dR/dT. This quantity is of a high physical interest since it gives information on the critical behavior of the specific heat of magnetic materials near the critical points. The original aspect of such an experimental device is the high sensitivity of its detection. Indeed it provides a resolution ΔR/R of 10−9 with R=3 mΩ. Finally, we demonstrate the high accuracy of this technique with experimental measurements of dR/dT of a rare-earth single crystal.
A simple pneumatic pulling mechanism for the production of micropipettes
Authors: Valaskovic G. A. , Morrison G. H.
Published: July 2002

An easily implemented scheme for the fabrication of glass micropipettes is described using a high-velocity pneumatic piston for the production of a strong pulling force. The piston is incorporated into a standard two-stage pipette puller and is capable of producing both patch- and ultramicro-type pipettes. The system has the versatility of a solenoid puller with a greater simplicity and lower cost.
Measuring the homogeneity of ceramic superconductors by eddy current probing
Authors: Jenks W. G. , Bouma P. , Rey C. M. , Testardi L. R. , Vinals J.
Published: July 2002

The homogeneity of a bulk YBa2Cu3Ox sample prepared by standard methods has been determined. Measurements of the conductivity, Tc (by induction), and x-ray analysis showed no deviation from bulk material previously characterized as ‘‘homogeneous’’ in the literature; the sample was shown to be inhomogeneous by the eddy current method. We use the sample’s response to multifrequency eddy current probing as a measure of homogeneity. This response is calculated from measurements of the complex impedance of a coil, which contains the sample, as a function of frequency. The homogeneity of the sample after being quenched in air from three successively higher temperatures (650, 700, 800 °C) has also been determined. A rough conductivity profile in one dimension is obtained for all four sample conditions.
Monte Carlo integration to optimize geometry in gravitational experiments
Authors: Winkler L. I. , Goldblum C. E.
Published: July 2002

The torsion balance has been the experimental apparatus of choice for centuries, both in precision measurements of the Newtonian gravitational constant and in searches for weak anomalous interactions outside of gravity. If the form of the interaction is modeled, it is often possible to optimize the interacting bodies so that the apparatus has the greatest sensitivity to the interaction under study. Other researchers have applied this strategy in the case of the gravitational interaction between cylinders, and between a cylinder and sphere. Whereas their work focused on developing an analytical expression for the force between the masses, we present here a numerical method−Monte Carlo integration−which is general enough to aid in the design of bodies interacting under arbitrary potentials and with any desired geometric shape (as long as an accurate absolute value of the force is not needed). This numerical method is used to compute the gravitational torsion constant produced between an external hollow cylinder and sphere, and demonstrates the behavior studied previously through analysis. However, the main purpose for which we have used this numerical technique is in the design of interacting bodies used in a torsion-pendulum search for interactions that depend on net intrinsic spin. We demonstrate how the method may be used to determine the optimum aspect ratio (l/r) of the polarized test masses, as well as the most sensitive orientation of the masses. Two different interactions are considered: the dipole–dipole interaction between two polarized bodies, and the monopole–dipole interaction between a polarized and unpolarized body. In the case of the monopole–dipole interaction, we also show how the numerical method can indicate which orientation between test bodies is most susceptible to a false signal caused by gravity.
Modified Kelvin technique for measuring strain-induced contact potentials
Authors: Rossi F. , Opat G. I. , Cimmino A.
Published: July 2002

We discuss the principles required to determine the electric field outside a metallic conductor created by strain gradients in the metal as might be engendered by the Earth’s gravity. The experiment we describe measured the contact potential variation between a mechanically differentially strained metal surface and an unstrained capacitive sensing probe. The results are of considerable importance for the investigation of the free fall of charged particles inside vertical conducting drift tubes. Our aim here is to elucidate the large sources of error associated with the application of strain. These include motions of the sample and temperature variations. The solutions we have developed may find application in similar experiments.
A time-resolved x-ray ring coded-aperture microscope for inertial confinement fusion applications
Authors: Ress D. , Bell P. M. , Bradley D. K.
Published: July 2002

For energetic x rays (hv≥3 keV), ring coded-aperture imaging gives better signal-to-noise ratio than equivalent-resolution pinhole cameras for inertial confinement fusion (ICF) targets. We have created a time-resolved ring coded-aperture microscope by combining a 4×3 array of annular apertures with a gated microchannel-plate x-ray imager. The new instrument can produce 500-ps duration sequences of images with a temporal resolution of 80 ps and a spatial resolution of 5 to 6 μm. In demonstration experiments, coded images of imploded targets directly driven by the Omega laser at the Laboratory for Laser Energetics showed the formation and dissolution of the target cores with improved detail. Contour plots of the images indicate that the laser illumination pattern is imprinted in the imploded core region. The gated ring-aperture microscope will be useful in detailed studies of ICF target hydrodynamics.
Recombination activity of iron-related complexes in silicon studied by temperature dependent carrier lifetime measurements
Authors: Kaniava A. , Rotondaro A. L. P. , Vanhellemont J. , Menczigar U. , Gaubas E.
Published: July 2002

Carrier recombination centers related with iron complexes in p-type silicon are studied by microwave and light-induced absorption techniques. Both thermal- and photoactivation are used to decompose iron–boron pairs and to study the impact on the recombination lifetime. Due to photodissociation of iron–boron pairs the lifetime increases for high level injection. Efficient recombination occurs via an acceptor level at Ec−0.29 eV as derived from the temperature dependence of carrier lifetime.
A method for tomography using arbitrary expansions
Authors: Zoletnik S. , Kálvin S.
Published: July 2002

A method for reconstructing two-dimensional distributions from a limited number of integrals is presented. The method can be applied for arbitrary expansions of the distribution; thus different types of distributions can be processed with different expansions. The method is numerically simple, and is especially useful for reconstructing time evolution of two-dimensional images. A comparison with maximum entropy tomography and an example of reconstructing tokamak radiation profiles in the case of impurity injection are presented.
Thermal conductivity measurements in soil using an instrument based on the cylindrical probe method
Authors: Nicolas J. , André Ph. , Rivez JF. , Debbaut V.
Published: July 2002

The article discusses the underlying hypothesis, the limits, the range of application, and the practical operation of a versatile thermal conductivity analyzer based on a transient cylindrical probe method. The best expected precision for the conductivity determination appears to be ∼±10%. A compact analyzer, integrating all the apparatus’ components in the same unit, has been specifically designed for environmental research in the field. Calibration received special attention regarding thermal conductivity values in the range 0.1–3 W/mK. The analyzer was tested on seven materials, in field conditions, and discrepancies from published conductivity values did not exceed 9%. Several checks were performed in the laboratory to study the influence of experimental conditions, such as the probe insertion technique. The cylindrical probe method appears well suited to powdered materials, but may also be applied to viscous liquids and solid matter. As an application, the cylindrical probe was used to characterize nine soils from the Belgian Lorraine. After soil analysis in the dry state, thermal conductivity of each sample was determined at different moisture contents. The transient cylindrical probe method proved to be quick and easy, whether in situ or on samples in the laboratory. It is a powerful tool which can map a country’s soil thermal properties.
An improved method for relaxation measurements using a Faraday balance
Authors: Mexner W. , Heinemann K.
Published: July 2002

Due to an eddy current-free sample holder of a high mass density, it is possible to investigate the magnetic relaxation of high temperature superconductors at early times with a Faraday magnetometer. The experiments can be performed beginning after to≊1 s with a resolution of 10−5 in the susceptibility.
A computer-controlled microscopy system for following protein crystal face growth rates
Authors: Pusey Marc Lee
Published: July 2002

A computer-controlled microscopy system for following the face growth of multiple small (∼10 to 50 μm) crystals during one experimental run is described. The major system components are a controlling PC with a digital oscilloscope board, PC-controlled translation stages for X, Y, and Z axes of motion, a video microscopy system, and a circuit to trigger digitization of preselected video lines. Crystal locations in the growth chamber are stored in an array, and sequentially accessed during each measurement cycle. Operator-selected horizontal video scan lines are digitized and these data are used to calculate the distance between parallel faces of the crystals. The system was assembled from readily available components, and can be easily modified for other microscopy-based tracking and measuring functions.
Measurement of sound pressure via temperature fluctuations by a cold-wire microprobe at high temperatures
Authors: Dubbe Andreas , Göpel Wolfgang
Published: July 2002

A miniature microphone, suitable for measuring periodical pressure variations in gases at frequencies between 0.1 and 6000 Hz and temperatures up to 1200 K, has been developed in order to characterize the frequency response of gas sensors. The temperature variations in the gas due to the heat of compression are measured by a platinum cold-wire probe, from which the pressure variations are calculated. An optimized probe design makes it possible to calculate the transfer function of the microphone by a consideration of thermal boundary layers of the wire holders and surrounding walls. Both a simple approximative formula for the present setup and a more generalized finite-element calculation are presented. Thus, absolute measurements of sinusoidal pressure variations in phase and amplitude with max. 0.2 Pa resolution are possible. Comparison with a fast chemical gas sensor and internal heating measurements demonstrate the proper operation of this device. Possible disturbing effects and operation limits are discussed with respect to modified design or operating conditions in other applications for frequency response studies in physical chemistry.
Simple method for measuring the thermal conductivity of a thin plate
Authors: Graebner J. E.
Published: July 2002

A simple technique is described for measuring the thermal conductivity of a plate of material, with heat flowing parallel to the plane of the plate. The design is aimed at rapid and accurate measurement. Results on copper and diamond indicate an uncertainty of ∼5%.