bluuebunny/crossref_metadata_2025 huggingface dataset:
Efficient Er3+-doped optical fiber amplifier pumped by a 1.48 μm InGaAsP laser diode
Authors: Nakazawa Masataka , Kimura Yasuo , Suzuki Kazunori
Published: July 2002

Optical gain characteristics of an Er3+ -doped silica fiber have been studied by end pumping with a 1.48 μm InGaAsP high-power laser diode. A gain as high as 12.5 dB was obtained for an absorbed pump power of 16 mW with a 3-m-long fiber. By constructing an Er3+ fiber ring cavity with a 3 dB single-mode fiber coupler, we have obtained continuous wave laser oscillation at a wavelength of 1.553 μm.
Linewidth of free excitons in quantum wells: Contribution by alloy disorder scattering
Authors: Basu P. K.
Published: July 2002

A theory is developed for the luminescence linewidth in a quantum well made of ternaries when the two-dimensionally free excitons undergo alloy disorder scattering. The expression for linewidth shows a L−1 dependence on the well width L for infinite barrier height. For thin wells leakage of wave function into the barrier is considered. A comparison between the experimental data and the present values points out the dominant role of alloy disorder scattering. For wells thinner than 40 Å the calculated values decrease with decreasing well width, indicating the importance of surface roughness scattering.
Visible luminescence from silicon wafers subjected to stain etches
Authors: Fathauer R. W. , George T. , Ksendzov A. , Vasquez R. P.
Published: July 2002

Etching of Si in a variety of solutions is known to cause staining. These stain layers consist of porous material similar to that produced by anodic etching of Si in HF solutions. We have observed photoluminescence peaked in the red from stain-etched Si wafers of different dopant types, concentrations, and orientations produced in solutions of HF:HNO3:H2O. Luminescence is also observed in stain films produced in solutions of NaNO2 in HF, but not in stain films produced in solutions of CrO3 in HF. The luminescence spectra are similar to those reported recently for porous Si films produced by anodic etching in HF solutions. However, stain films are much easier to produce, requiring no special equipment.
Pseudomorphic InGaAs-GaAsP quantum well modulators on GaAs
Authors: Cunningham J. E. , Goossen K. W. , Williams M. , Jan W. Y.
Published: July 2002

Previous strain-relief p-i-n InGaAs-GaAs quantum well (qw) modulators have incurred surface striations upon growth due to defect formation, resulting in an optically rough surface. We present here a qw modulator on a GaAs substrate with InGaAs wells and GaAsP barriers which balance the strain in the wells so that the lattice does not relax, leading to much fewer defects, and an optically smooth surface. We obtain a transmission change from 60% to 80% at 1014 nm for a sample with a 1 μm thick intrinsic region.
Luminescence degradation in porous silicon
Authors: Tischler M. A. , Collins R. T. , Stathis J. H. , Tsang J. C.
Published: July 2002

We have studied the stability of the luminescence from porous Si in the presence of a variety of ambient gases (e.g., N2, H2, forming gas, and O2). Although the optical properties are fairly stable under most conditions, illumination in the presence of O2 causes a substantial decrease in luminescence efficiency. Infrared measurements show that the surfaces of degraded samples are oxidized. The luminescence lifetime of the degraded material is found to be substantially reduced, and the density of Si dangling bonds increases by more than two orders of magnitude, which suggests that oxidation of the surface introduces nonradiative recombination channels. These observations indicate that the electronic properties at the surface of the porous Si play a key role in obtaining efficient luminescence from this material.
On the linewidth of microcavity lasers
Authors: Björk Gunnar , Karlsson Anders , Yamamoto Yoshihisa
Published: July 2002

In a recent letter by G. P. Agrawal and G. R. Gray [Appl. Phys. Lett. 59, 399 (1991)] the frequency and phase noise of vertical-cavity surface-emitting microlasers is calculated using rate equations and Langevin noise sources. In this letter we re-examine their calculations and get different results. Specifically, we predict that an increase of the spontaneous emission coupling coefficient will not necessarily lead to an increase in linewidth. It is also found that the reported ‘‘anomalous linewidth enhancement’’ is present in all lasers with a nonzero linewidth enhancement factor.
Dielectric properties of SrTiO3 thin films used in high T
c superconducting field-effect devices
Authors: Walkenhorst A. , Doughty C. , Xi X. X. , Mao S. N. , Li Q. , Venkatesan T. , Ramesh R.
Published: July 2002

The dielectric constant ■ of thin epitaxial SrTiO3 films used as dielectric layers in high Tc superconducting field-effect devices has been studied. ■ in these thin films was found to have much weaker dependence on temperature and applied electric field than the bulk material. The breakdown characteristics of gate structures containing SrTiO3 dielectric layers are strongly influenced by the metal used as the gate electrode.
Stress and its effect on the interdiffusion in Si1−xGex/Si superlattices
Authors: Prokes S. M. , Glembocki O. J. , Godbey D. J.
Published: July 2002

Effects of stress on the interdiffusion behavior in long-period Si0.7Ge0.3/Si is examined using x-ray diffraction and Raman spectroscopy. Symmetrically and asymmetrically strained superlattices have been examined, and an activation energy for interdiffusion of 3.9 and 4.6 eV has been obtained, respectively. Furthermore, an enhanced interdiffusion has been measured for the asymmetrically strained superlattice when subjected to an external tensile stress during annealing. Thus, enhanced intermixing has been measured whenever the Si barrier layers experience tensile stress during annealing. In addition, Raman results confirm the enhancement of Ge diffusion into the Si barriers when annealed under tensile stress.
Rapid-thermal-oxidized porous Si−The superior photoluminescent Si
Authors: Petrova-Koch V. , Muschik T. , Kux A. , Meyer B. K. , Koch F. , Lehmann V.
Published: July 2002

To improve the stability of porous Si (PS) prepared by electrochemical etching, we make use of rapid-thermal oxidation (RTO). During RTO processing, the hydride coverage of the internal surfaces of the pores is replaced by a high-quality oxide while retaining nm-sized Si grains. With increasing process temperature Tox the luminescence is first quenched. It is recovered with comparable intensity for Tox≥700 °C.
Spin-dependent effects in porous silicon
Authors: Brandt M. S. , Stutzmann M.
Published: July 2002

Luminescent anodically etched porous silicon is studied with electron spin resonance, optically detected magnetic resonance, and spin-dependent photoconductivity. The Pb center, the silicon dangling bond at the crystalline Si/SiO2 interface, is found to be the dominant paramagnetic defect, influencing both photoconductivity and photoluminescence. The assignment is supported by the observation of the corresponding 29Si hyperfine lines. A second hyperfine split pair is attributed to Si-F complexes formed during the etching process and remaining in the porous material.
Nonequilibrium effects in quantum well lasers
Authors: Tessler N. , Nagar R. , Eisenstein G. , Chandrasekhar S. , Joyner C. H. , Dentai A. G. , Koren U. , Raybon G.
Published: July 2002

We present a simple technique of measuring the effect of the finite capture time of carriers in quantum well lasers on the homogeneity of the gain. The effect is measured using an extended cavity laser configuration in which we control the feedback level and compare the two extreme cases of a laser and a nonlasing amplifying gain medium. Broadband measurements of the spontaneous emission at energies near the top of the well and above the barriers show an inhomogeneous gain saturation which depends on the photon density inside the cavity. The results agree with a simple model for carrier injection in quantum well lasers.
Low temperature Si3N4 direct bonding
Authors: Bower Robert W. , Ismail Mohd S. , Roberds Brian E.
Published: July 2002

Extremely strong bonds can be formed between smooth, clean layers of Si3N4 at temperatures ranging between 90 and 300 °C. These bonds have been formed between Si3N4 layers deposited on various substrates with deposition temperatures as low as 300 °C. The bond is initially formed at room temperature and subsequently annealed at temperatures ranging between 90 and 300 °C. Thus, the materials bonded in this manner are never exposed to temperatures higher than 300 °C. This low temperature bond greatly expands the range of applications of direct bonding which had heretofore been restricted by the temperatures of 700 to 1000 °C required by conventional wafer bonding.
Surface-enhanced Raman scattering of amorphous silicon-carbon films
Authors: Tzolov M. B. , Tzenov N. V. , Dimova-Malinovska D. I. , Yankov D. Y.
Published: July 2002

Surface-enhanced Raman scattering has been used to study the surface of magnetron sputtered amorphous silicon-carbon alloys applying the silver overlayer method. The presence of clusters from the sputtered material and different types of carbon–carbon bond configurations has been detected on the film surface. It has been shown that structural transformations which are not related to the hydrogen in the plasma take place on the surface.
In-plane effective mass in narrow quantum wells of nonparabolic semiconductors
Authors: Nag B. R. , Mukhopadhyay Sanghamitra
Published: July 2002

A formula is derived for the in-plane effective mass in narrow quantum wells, taking into account the effects of energy band nonparabolicity. The variation of the mass with the width of the well is studied by using the formula for four systems of wells. The mass is nearly the same as the velocity effective mass of the bulk material of the well in GaAs/Ga0.7Al0.3As wells. It is about 8% larger in InAs/InP wells, but is significantly larger in very narrow wells of Ga0.47In0.53As/InP systems. In the case of InAs/Ga0.58Al0.42Sb wells, the in-plane mass differs from the well mass by large amounts for all well widths of interest.
Phonon dispersion and density of states of solid C60
Authors: Yu Jin , Kalia Rajiv K. , Vashishta Priya
Published: July 2002

Dispersion and density of states (DOS) of inter- and intramolecular phonons are calculated for solid C60 using a tight-binding molecular dynamics method. Intermolecular phonon DOS extends up to 7.6 meV and shows libron peaks at 2.3 and 3.7 meV. Orientational disorder softens libron modes. Intramolecular phonons below 70 meV also show significant dispersion. The calculated phonon spectra are in good agreement with inelastic neutron scattering experiments.
Processing of Bi2Sr2CaCu2Ox thick films from nitrate precursors
Authors: Li C. , Patel S. , Ye J. , Narumi E. , Shaw D. T. , Sato T.
Published: July 2002

High-quality Bi2Sr2CaCu2Ox/Ag superconducting tapes were fabricated directly from the nitrate salts of Bi, Sr, Ca, and Cu by the partial-melt-growth technique. Nitrate particles were deposited electrostatically on the Ag substrate and heat treated. After process optimization of the superconducting tapes, the best samples showed typical Tc of 89 K with zero applied field critical current density of 2×104 A/cm2 (Ic=11.2 A) at 77 K and 2.6×105 A/cm2 (Ic=190 A) at 4.2 K. Unlike films grown using solid state reacted precursor powders, these films lacked the typical Bi-free, needlelike defects and possessed a smooth morphology. X-ray diffraction patterns showed the film to be highly c-axis oriented. The processing of these tapes significantly reduces the overall fabrication time from precursor materials preparation to final heat treatment.
Imaging of magnetic vortices in superconducting networks and clusters by scanning SQUID microscopy
Authors: Vu L. N. , Wistrom M. S. , Van Harlingen D. J.
Published: July 2002

We have imaged the configuration of trapped magnetic vortices in two-dimensional superconductor arrays and clusters using a scanning SQUID microscope (SSM). The SSM scans a dc SQUID detector over the array surface, achieving better than 10 μm spatial resolution over a scan range up to 1 cm×1 cm and a magnetic flux sensitivity of about 10−4Φ0. We have obtained images of large square arrays cooled at different values of applied magnetic flux per cell Φ. For low rational values (1/2,1/3,1/4,...) of the frustration parameter f=Φ/Φ0, we find regions of periodically arranged vortices separated by domain walls; at other values of the field, the vortex pattern is disordered. We also present observations of vortex patterns in ensembles of 3×3 clusters.
Selective excitation of the photoluminescence and the energy levels of ultrasmall InGaAs/GaAs quantum dots
Authors: Fafard S. , Leonard D. , Merz J. L. , Petroff P. M.
Published: July 2002

The energy levels of nanometer size InGaAs quantum dots epitaxially grown on GaAs by the coherent islanding effect are probed using selectively excited photoluminescence (PL), and PL excitation. A lateral-confinement-induced interlevel spacing of ∼30 meV between the first two states can be deduced from the spectra.
Thickness dependence of La2−xSrxCuO4 films
Authors: Cieplak Marta Z. , Berkowski M. , Guha S. , Cheng E. , Vagelos A. S. , Rabinowitz D. J. , Wu B. , Trofimov I. E. , Lindenfeld P.
Published: July 2002

Pulsed laser deposition was used to grow c-axis aligned films of La2−xSrxCuO4−δ, with Sr content, x, in the range of 0.03 to 0.2, on substrates prepared from single-crystalline SrLaAlO4, with a lattice mismatch of 0.5%. The thickness dependence of Tc0, the temperature at which the resistance falls to zero, is much reduced compared to that of films deposited on SrTiO3, where the lattice mismatch is 3.4%. The maximum Tc0 is achieved at x=0.15 for both types of substrates. While the thickness dependence is related to the strain resulting from the lattice mismatch, the value of the maximum Tc0 is determined by the oxygen concentration.
Giant photoexpansion in As2S3 glass
Authors: Hisakuni H. , Tanaka K.
Published: July 2002

When illuminated with a focused beam from He-Ne lasers, As2S3 films with thickness of ∼50 μm exhibit the thickness expansion reaching to 3 μm, which is approximately 10 times as great as that expected from the conventional photoexpansion phenomenon. The enhancement seems to be caused by photorelaxation of strain generated by photoexpansion.
Quasi-phase-matched second-harmonic generation in AlGaAs waveguides with periodic domain inversion achieved by wafer-bonding
Authors: Yoo S. J. B. , Bhat R. , Caneau C. , Koza M. A.
Published: July 2002

Quasi-phase-matched second-harmonic generation is observed in an AlGaAs waveguide. The AlGaAs waveguide is epitaxially grown on a template substrate where a periodic crystal domain inversion is achieved using wafer bonding and organometallic chemical vapor deposition. A scanning electron micrograph of the waveguide cross section reveals a distinct propagation of the crystal domain boundaries in the epitaxial growth direction. Second-harmonic generation measurements on a fabricated rib-loaded waveguide show a clear quadratic dependence of the second-harmonic power to the input fundamental power. The peak conversion efficiency is 4.9%/W whereas the theoretical value is 124%/W for an ideal waveguide with no loss and with equal domain dimensions. A significant increase in the conversion efficiency is expected with reduced scattering losses realized by improved epitaxial growth and fabrication processes.
Characterization of a point-contact on silicon using force microscopy-supported resistance measurements
Authors: De Wolf P. , Snauwaert J. , Clarysse T. , Vandervorst W. , Hellemans L.
Published: July 2002

A conductive atomic force microscope (AFM) tip based on B-implanted diamond has been developed for the determination of the spatial distribution of charge carriers in semiconducting structures. The characteristics of this tip have been determined by studying the current–voltage behavior as a function of substrate resistivity and tip load. From this work a model of the electrical properties of the microcontact is emerging. It includes an Ohmic contribution to the overall resistance, which is related to the plastically deformed area, and contributions from a barrier. The tip imprints have been imaged with AFM and their physical dimensions are seen to match the requirements of the model. From resistance measurements on uniformly doped silicon a calibration curve has been established which can be used as a standard to convert measured resistance into resistivity.
Tandem-Mirror Experiment-Upgrade neutral pressure measurement diagnostic systems
Authors: Pickles W. L. , Allen S. L. , Hill D. N. , Hunt A. L. , Simonen T. C.
Published: July 2002

The Tandem-Mirror Experiment-Upgrade (TMX-U) has a large and complex system of Bayard–Alpert, magnetron, and Penning gauges, in addition to mass spectrometers (RGA), all of which measure neutral pressures in the many internal regions of TMX-U. These pressure measurements are used as part of the confinement physics data base as well as for management of the TMX-U vacuum system. Dynamic pressures are modeled by a coupled-volumes simulation code, which includes wall reflux, getter pumping, and plasma pumping.
Time-resolved x-ray line intensities for diagnostics of laser-produced plasmas (abstract)
Authors: Kauffman R. L. , Medecki H. , Lee R. W. , Whitten B. L. , Estabrook K. G. , Drake R. P. , Turner R. E. , Letts S.
Published: July 2002

We have developed a streaked crystal spectrograph for time-resolving x-ray lines from laser-produced plasmas. The spectrograph combines an ellipsoidally curved crystal and x-ray streak camera to optimize spectral coverage and resolution, even though the instrument is located a meter from the source. The instrument has been used at Novette to measure S lines doped in concentrations of 4% in the center of CH foils. Time histories of the temperature and density of the CH plasma have been derived from relative intensities of the S lines. Details of the experiment and analysis will be presented along with the instrument design.
Time-resolving multispatial grazing incidence spectrograph for plasma fusion diagnostics (abstract)
Authors: Nudelfuden A. , Moos H. W. , Lippmann S.
Published: July 2002

A grazing incidence spectrograph which operates in the EUV (40 to 350 Å) that has multispectral, temporal, and spatial resolving capabilities is being constructed for plasma fusion diagnostics. The spectrograph achieves a simultaneous spectral coverage of 20 and 60 Å when centered on 40 and 350 Å, respectively, with ∼1-Å resolution. The detector consists of an image intensifier fiber optically coupled to 3 area array detectors (CCDS), which can be read out in 5 ms, thereby determining the time resolution of the instrument. The spatial resolution is accomplished by using the astigmatism inherent to a concave grating in grazing incidence, coupled with the pinhole camera effect produced by an entrance slit of limited height. The spectrograph can view ∼54 cm of plasma which is 2 m away from the entrance slit with 4- and 8-cm resolution at 350 and 40 Å, respectively. We will present the results of a feasibility study, the spectrograph design, and the results of the data reduction and interpretation codes which are under development.
Multichordal charge exchange recombination spectroscopy on Doublet III (abstract)
Authors: Seraydarian R. P. , Burrell K. H. , Kahn C.
Published: July 2002

Single shot, multipoint ion temperature and plasma rotation profiles have been routinely obtained on the Doublet III tokamak for 32 consecutive time slices with 20-ms resolution. A six-chord tangentially viewing spectroscopic diagnostic has been built to look at radiation emitted by fully stripped low-Z impurity ions (He, C, O) that have undergone charge exchange recombination with hydrogen atoms from a 3-MW heating beam. The main components of the instrument are a single monochromator for wavelength dispersion, a single image intensifier tube for photon gain, and a pair of 1024-element linear photodiode arrays for detection. A special arrangement of fiber optics allows simultaneous data acquisition from all chords without the use of scanning mirrors or other moving parts. Ion temperature profiles taken under a variety of plasma conditions will be presented.
Movable Faraday cup for high-intensity electron beam pulses
Authors: Luches A. , Nassisi V. , Perrone M. R.
Published: July 2002

A simple inexpensive movable Faraday cup is described. It is used for current measurements in pulsed electron beam accelerators. Current pulses of tens of killoamps, of 1-ns rise time and up to a few mega-electron-volts of electron energy, can be measured. This cup can be moved along the e-beam drift tube maintaining its response characteristics to study the e-beam evolution.
Time-resolved ion temperature measurements on the HBTX1A reversed field pinch
Authors: Bunting C. A. , Carolan P. G. , Field A. R.
Published: July 2002

The energy distribution of the neutral particle emission (200 eV–2 keV) from the HBTX1A reversed field pinch has been observed, using a five-channel analyzer, at 20-μs intervals throughout the duration of the discharge. The relative number of high-energy neutrals emitted increases with currents up to 250 kA above which there is little change. Preliminary analyses show that the ion temperature of the bulk of the plasma is in the range 70–120 eV, rather than the few electron volts expected from electron–ion collisions. Equally, the rapid evolution to this temperature (<0.5 ms) is not accountable by equipartition (τeq∼3 ms). Long thermalization times (∼1 ms), compared with short time-scale excursions (∼100 μs) in the high-energy neutrals indicate that there are nonthermal ions at ∼10 times the Maxwellian temperature.
Convenient mounting method for electrical measurements of thin samples
Authors: Matus L. G. , Summers R. L.
Published: July 2002

A novel method of mounting thin samples for electrical measurements is described. A vacuum chuck holds a ceramic mounting plate, which, in turn, holds the sample. Contacts on the mounting plate establish electrical connection to the sample. The attachment of wires directly to the sample is unnecessary.
Thermodynamics in the design of long-life, high-temperature devices
Authors: Zollweg R. J.
Published: July 2002

The application of chemical thermodynamics to the design of long-life, high-temperature devices is advocated and illustrated by its use in selecting the material components for a high-temperature lead vapor Raman cell. It is shown that the reduction of alumina and of a calcia–alumina sealing frit by tantalum lead to equilibrium aluminum and calcium partial pressures orders of magnitude greater than those over pure alumina at the desired cell operating temperature.
Computer-controlled system for measuring bulk resistivity of insulating solids as a function of temperature
Authors: Amin Ahmed
Published: July 2002

A computer-controlled system for measuring bulk resistivity of insulating solids as a function of temperature is described. The measuring circuit is a modification of that given in the ASTM standard D257-66, to allow for a number of operations during the data-acquisition cycle. The bulk resistivity of an acceptor-doped morphotropic lead zirconate–titanate piezoelectric composition has been measured over the temperature range +40 to +200 °C. The activation energy derived from the experimental data is compared to the published values of similar morphotropic compositions.
Low-distortion detection system for frequency-swept ion cyclotron resonance spectrometry
Authors: Wise M. B. , Freiser B. S.
Published: July 2002

A high-performance frequency-swept capacitance bridge detector for ion cyclotron resonance (ICR) spectrometry has been constructed in our laboratory. Although the basic design of the system is similar to that of previously reported bridge circuits, careful design, layout, construction, and component selection have resulted in excellent frequency-swept performance over a bandwidth of 15 kHz to 1 MHz. At a magnetic field strength of 1.0 T, this corresponds to a mass range of 15–1000 Daltons. Problems with base-line drift and frequency-dependent signal distortion common to many other designs have been significantly reduced. Circuit diagrams are included for all parts of the detector and frequency response curves have been included where appropriate. In addition, several simple circuit diagrams for support devices have also been included.
Removal of phototubes from Pb-glass detectors II
Authors: Alburger D. E. , Akiba Y. , Olness J. W. , Tannenbaum M. J.
Published: July 2002

A previously described technique for removing faulty phototubes from Pb-glass detectors with heated chloroform has been modified to allow the removal of good phototubes.
Octave bandwidth constant beamwidth acoustical end-fire line array
Authors: Lardies Joseph , Guilhot Jean-Pierre
Published: July 2002

A simple and an inexpensive technique for maintaining the beamwidth of a transducer constant over frequency bands up to an octave in width is described. An acoustical end-fire line array of five microphones is used to produce the desired radiation pattern at frequency F1. A half-scale model is implemented to give the same beam pattern at frequency 2F1. Two compensation filters, whose frequency responses are derived by imposing the constant beamwidth constraints, are used in the respective array outputs to produce a constant beamwidth over the octave.
Conductivity salinometer for continuous monitoring of small density changes over an extended region
Authors: Sinha Sumon Kumar
Published: July 2002

Experiments involving mixing of solute-induced stratified fluids often require continuous monitoring of small changes in local fluid densities over an extended region. An instrument has been constructed for this purpose using multiple miniature bipolar conductivity probes controlled by a sequential electronic switching circuit. The accuracy of the system is within 0.04% in the range of specific gravities from 0.9967 to 1.0200. It has been used to study destratification due to molecular diffusion, as well as surface shear-induced destratification, in a two-layer fluid.
Large-area low-noise seven-channel dc SQUID magnetometer for brain research
Authors: Knuutila Jukka , Ahlfors Seppo , Ahonen Antti , Hällström Jari , Kajola Matti , Lounasmaa Olli V. , Vilkman Visa , Tesche Claudia
Published: July 2002

The design, construction, and performance of a new high-sensitivity dc SQUID magnetometer, covering a circular area of 93-mm diameter, is described. The device, now used routinely in our brain research, comprises seven asymmetric first-order gradiometers, located on a spherical surface of 125-mm radius and with the symmetry axis tilted 30° with respect to the vertical. The pickup coil diameter is 20 mm, and the channels are separated by 36.5 mm from each other in a hexagonal array. The overall field sensitivity of the system, measured inside our magnetically shielded room, is 5 fT/(Hz)1/2, mainly limited by the thermal noise in the radiation shields of the Dewar. The optimization of the coil configuration and the measurement system is discussed in detail, and a system to determine automatically the position and orientation of the Dewar with respect to certain fixed points on the subject’s head is described. Finally, some examples of measurements carried out with the new device are given.
Dye strip dosimeter
Authors: Saisomboon S. , Siri-Upathum C.
Published: July 2002

This paper describes a new method for measuring radiation dose by using natural pigments. The pigments were extracted from Hibiscus rosa sinensis L. and Canna indica L. and were irradiated with gamma ray. Doses of 30 rad and above are indicated by color changes.
Three-dimensional computer simulation of ion beam scattering by an electrostatic reflector
Authors: Nakamura M. , Tsuruda K. , Hayakawa H.
Published: July 2002

A three-dimensional computer model was used to study the scattering of a reflected lithium ion beam by an electrostatic reflector. Results show that for ion gun systems for active space plasma experiment applications, beam scattering of less than 1° can be attained with electrostatic reflectors.
A new method for measuring absolute total electron-impact cross sections with forward scattering corrections
Authors: Ma Ce , Liescheski Phillip B. , Bonham R. A.
Published: July 2002

In this article we describe an experimental technique to measure the total electron-impact cross section by measurement of the attenuation of an electron beam passing through a gas at constant pressure with the unwanted forward scattering contribution removed. The technique is based on the different spatial propagation properties of scattered and unscattered electrons. The correction is accomplished by measuring the electron beam attenuation dependence on both the target gas pressure (number density) and transmission length. Two extended forms of the Beer–Lambert law which approximately include the contributions for forward scattering and for forward scattering plus multiple scattering from the gas outside the electron beam were developed. It is argued that the dependence of the forward scattering on the path length through the gas is approximately independent of the model used to describe it. The proposed methods were used to determine the total cross section and forward scattering contribution from argon (Ar) with 300-eV electrons. Our results are compared with those in the literature and the predictions of theory and experiment for the forward scattering and multiple scattering contributions. In addition, Monte Carlo simulations were performed as a further test of the method.
Very strong negative differential resistance real-space transfer transistor using a multiple δ-doping GaAs/InGaAs pseudomorphic heterostructure
Authors: Wu C. L. , Hsu W. C. , Tsai M. S. , Shieh H. M.
Published: July 2002

We present the observation of an effective real-space transfer process of hot electrons resulting in a very strong negative differential resistance in GaAs/In0.25Ga0.75As/GaAs pseudomorphic heterostructure by growing symmetrically double δ-doping layers on both sides of the InGaAs channel. By Hall measurements, the proposed structure shows carrier mobility as high as 4500 (14 100) cm2/V s at 300 (77) K which is suitable for high-frequency operations. Meanwhile, this structure with a 5×100 μm2 emitter channel reveals extremely sharp charge injection, broad current valley range (≳3 V), high transconductance (over 23.5 S/mm), high current driving capability, and high peak-to-valley current ratio (up to 156 000). We also carried out secondary-ion mass spectrometry profiles to confirm the quality of the proposed structure.
A simple system for an accurate calibration of grating spectrometers
Authors: Pelletier-Allard N. , Pelletier R.
Published: July 2002

Various calibrating systems for grating spectrometers are proposed. The most simple system leading to a stable and accurate calibration consists in the generation of a channeled spectrum by means of a calcite plate put between two crossed polarizers.
Trigger unit for systems that use diode lasers mounted on a closed-cycle refrigerator
Authors: Rodriguez Miguel , Alvarez Ramon A.
Published: July 2002

Impact shocks produced by the closed-cycle refrigerator produce jitter in the output frequency of a diode laser. An electronic device is described which allows triggering of the data acquisition system when the influence of the vibration-induced noise is at a minimum.
A detachable semiconductor bolometer for heat pulses
Authors: Shen Y. , Boragno C. , Buatier de Mongeot F. , Valbusa U.
Published: July 2002

A semiconductor detachable bolometer using a superconducting thin film as coupling medium is described. The film acts as a low pass band filter for the phonons having a frequency below the superconductive gap. A comparison between different coupling media is reported.
Large range coherence tuned fiber optic interferometric system for application in accelerometers
Authors: Ribeiro A. B. L. , Santos J. L. , Jackson D. A.
Published: July 2002

A low coherence interferometric system with large tracking range and self-initialization to be used for remote signal processing of fiber optic accelerometers is investigated. The tracking range of the low coherence system is 1.1 mm at low frequency. Phase resolution better than 1 mrad/√Hz at frequencies below 600 Hz is obtained, with a dynamic range of ≊90 dB. Data are presented which show that if the system were used to process the output from a compliant cylinder-type accelerometer, resolutions of ∼10−7 g/√Hz could be achieved.
Resonance-enhanced low-pressure optoacoustic cell
Authors: Hartemink M. , Godfried H. P.
Published: July 2002

A low-pressure optoacoustic cell is described that can be used to lock the emission frequency of a laser. A model is developed which describes the low-pressure behavior of the optoacoustic cell as a function of cell dimensions, gas properties, and operating pressure. Resonant optoacoustic cells are predicted to improve the acoustic signal levels significantly. Experiments were carried out with a cell filled with CF4. The model was found to accurately predict resonator quality, resonance frequency, and acoustic response for pressures ranging from 0.1 to 3.0 kPa. At these low pressures acoustic attenuation processes, slow vibration to translation (VT) relaxation and diffusion to the cell wall strongly influence the acoustic behavior of the cell. Using the relaxation time of the ν4 vibrational mode of CF4 as a fitting parameter its value was determined to be three times slower than VT relaxation from the ν2 level. The experimental values for the response were predicted by the model with an error of less than 10% in the whole pressure range. Predictions for the optoacoustic signal for different resonator dimensions were also confirmed. Model predictions for the optoacoustic signal for mixtures of gases and the influence of the temperature are also given. Especially the option of cooling the gas seems to be attractive for the case of CF4.
Robust bi-directional turbulence probe
Authors: Cortese R. , Perlee H.
Published: July 2002

A robust bidirectional probe useful in hostile environments (such as found in gas and dust explosions) has been modified for measuring gas velocity in turbulent flow. The results of the study show that the probe can measure mean velocities in turbulent flow up to Reynolds numbers of 50 000. Results of this research also suggest that the probe can also measure turbulent intensity over the same range of Reynolds numbers.
Environmental isolation platform for microrobot system development
Authors: Brenan Colin J. H. , Charette Paul G. , Hunter Ian W.
Published: July 2002

Environmental isolation of a microrobot is important to achieve the demanding operating specifications intrinsic to such a system. Our experience with the current generation microrobot, MR1, indicates the next generation microrobot, MR2, will require a high degree of isolation from the mechanical, thermal, electromagnetic, and optical environment of a typical laboratory. Accordingly, this paper presents the design, construction, and isolation test results of an environmental isolation platform built to house MR2 and its many subsystems under development in our laboratory. Our design approach used qualitative estimates of the magnitudes of external environmental noise acceptable to microrobot operation to determine the required attenuation of each noise input. The resulting platform consisted of a custom environmental enclosure mounted on a modified commercial vibration isolation optical table. The performance measurements quantified the attenuation of each unwanted disturbance within the frequency bands of interest and a final test was made with a laser heterodyne interferometer installed on the platform in a typical laboratory environment. The results of this experiment show that the isolation specifications required for microrobot operation were met thereby verifying our platform design. We expect that our design can be used in environmental isolation of not only microrobots but also other sensitive electro-optomechanical instruments.
Parametric analysis of the reliability of igniter systems−PARIS
Authors: Leeuw M. W. , Bal E. A. , Prinse W. C.
Published: July 2002

A fully automated improved thermal transient test set is used to measure the thermoelectrical response of a number of different fuze heads. The fitted wire model is used to describe the heat dynamics in the fuze head and to calculate a number of intrinsic thermal properties. These properties are used as input for a parametric analysis of the reliability of igniter systems (PARIS). Comparison of PARIS with firing levels obtained with the classical Robbins–Monro method has been used to validate this new way to estimate the sensitivity of fuze heads.
Metal wire cutting by repeated application of low-power laser pulses
Authors: Bernstein Joseph B. , Cohen Simon S. , Wyatt Peter W.
Published: July 2002

Thermomechanical fatigue, induced by the repeated application of a sufficiently powerful laser pulse, will eventually cause a metallic film to fail. This principle is proposed to cut metal lines for deletive redundancy in microelectronics. Aluminum alloy wires were cut with a series of 5 μs pulses from an argon ion source. The power required to cut 4-μm-wide, 0.75-μm-thick lines was reduced by as much as 87% from that of a single 5 μs laser pulse. By comparison to a single pulse of equal total time, a 70% reduction in power has been realized.
A magnetic suspension system with a large angular range
Authors: Britcher Colin P. , Ghofrani Mehran
Published: July 2002

In order to explore and develop technology required for the magnetic suspension of objects over large ranges of orientation, a small-scale laboratory system, the large-angle magnetic suspension test fixture (LAMSTF) has been constructed at NASA Langley Research Center. This apparatus falls into the category of large-gap, actively stabilized magnetic levitation systems. The hardware comprises five conventional electromagnets in a circular arrangement, each driven from a separate bipolar power amplifier. Electromagnet currents are commanded by a digital control system, implemented on a microcomputer, which in turn derives the position and attitude of the suspended element from an infrared optical system. The suspended element is a cylindrical, axially magnetized, permanent magnet core, within an aluminum tube. The element is ‘‘levitated’’ by repulsive forces, with its axis horizontal, 0.1 m above the top plane of the electromagnet conductor. The element is stabilized in five degrees-of-freedom, with rotation about the cylinder axis not controlled. By mechanical rotation of the sensor assembly, the suspended element can be made to undergo a full 360° rotation about the vertical axis. The controller accommodates the changes in magnetic coupling between the electromagnets and the suspended element by real-time adaptation of a decoupling matrix. This report presents a review of the background to the problem of magnetic suspension over large ranges of orientation. Next, the design and operation procedures adopted for LAMSTF, and the system hardware are described. Finally, some performance measurements are shown, together with illustration that the major design objective—the 360° rotation, has been accomplished.
An automated 55 GHz cryogenic Josephson sampling oscilloscope
Authors: Bodin P. , Jacobsen M. L. , Kuehle A. , Hansen J. Bindslev , Davidson A. , Brady M. , Olsen L. , Qualmann W.
Published: July 2002

A computer-automated superconductive 55 GHz sampling oscilloscope based on 4 kA/cm2, Nb/Nb2O5/Pb edge Josephson junctions is presented. The Josephson sampler chip was flip-chip bonded to a carrier chip with a coplanar transmission line by use of a novel flip-chip bonding machine. A 5.6 ps step pulse was successfully coupled in to the transmission line and 18.5 GHz multiple reflections plus a parasitic oscillation at 43 GHz were observed.
Localization of current dipoles with multichannel SQUID systems
Authors: Dössel O. , David B. , Fuchs M. , Krüger J. , Lüdeke K.-M. , Wischmann H.-A.
Published: July 2002

Current sources in the human body can be localized by measuring the biomagnetic fields with multichannel SQUID systems. Important system aspects are the noise level, the ambient field suppression, the dynamic range, the reliability, the number of channels, and the arrangement of gradiometers. From the user’s point of view the most important quality factor is the accuracy with which a current dipole can be localized. A test procedure is proposed to determine the localization power of the system. A 31-channel-SQUID system is presented together with the results of the test. The crucial parts of the system determining the accuracy are pointed out.
High-flux beamline for small-angle x-ray scattering at ELETTRA
Authors: Amenitsch H. , Bernstorff S. , Laggner P.
Published: July 2002

The optical layout and the expected performance of the new high-flux SAXS beamline at ELETTRA is presented. From the high-power wiggler spectrum the three discrete energies 5.4, 8, and 16 keV will be selected with a double-crystal monochromator which contains three pairs of separated asymmetrically cut plane Si(111) crystals. Downstream, the beam will be focused by a torodial mirror. The optical axis of the beamline will be horizontally 1.25 mrad off wiggler axis and the beamline will accept about 1 mrad horizontally and 0.3 mrad vertically. The beamline will operate with a SAXS resolution between 10 and at least 1000 Å in d spacing at 8 keV and has been optimized with respect to extreme flux. A flux at the sample in the order of 1013 ph/s is expected for 8 keV photons (2 GeV, 400 mA). It will be possible to perform wide angle scattering measurements in the range of 3.5 and 7 Å d spacing at 8 keV simultaneously.
Polarized electroluminescence from oriented p-sexiphenyl vacuum-deposited film
Authors: Era Masanao , Tsutsui Tetsuo , Saito Shogo
Published: July 2002

Oriented sexiphenyl (6P) film was prepared through epitaxial growth on rubbed substrates by vacuum deposition. Polarized absorption spectra demonstrated that the long axis of 6P molecule was highly oriented along the rubbing direction. Using the epitaxial growth film of 6P, we fabricated electroluminescent (EL) devices consisting of indium-tin-oxide anode, emissive layer of 6P, electron-transporting layer of an oxadiazole derivative, and MgAg alloy cathode. Owing to the orientation of 6P molecules, EL polarized in the rubbing direction was observed in the devices.
Advanced experimental application of a digital signal processor in high resolution x-ray spectroscopy
Authors: Sampietro M. , Geraci A. , Fazzi A. , Lechner P.
Published: July 2002

‘‘Optimum’’ digital filtering of the digitized pulses obtained from a radiation detector had been shown to improve the energy resolution with respect to suboptimum analog processing [Rev. Sci. Instrum. 66, 975 (1995)]. This note shows that the improvement does not vanish in very high resolution systems, as it depends on the kind and relative amount of the noises present and not on their absolute value. Using one of the best available silicon detectors, the electronic noise has again been lowered by 18% from 9.6 to 7.9 electrons rms.
Pulse dynamics in stretched-pulse fiber lasers
Authors: Tamura K. , Ippen E. P. , Haus H. A.
Published: July 2002

Stretched-pulse additive pulse mode-locked fiber lasers use fibers with opposite signs of group velocity dispersion to introduce large changes of pulse width per pass and avoid excessive nonlinearity. We have investigated these intracavity pulse dynamics. Optimal points for output coupling and for the placement of the artificial absorber element in the cavity are determined for a fiber ring laser. The dispersion balance is important for the generation of clean sub-100 fs pulses, but the net cavity length is found to be relatively uncritical.
The new 1.2 T–14.5 GHz Caprice source of multicharged ions: Results with metallic elements
Authors: Hitz D. , Bourg F. , Delaunay M. , Ludwig P. , Melin G. , Pontonnier M. , NGuyen T. K.
Published: July 2002

The 1.2 T–14.5 GHz ECR Caprice source is an upgrade version of previous Caprice sources where the three main ingredients of an ECR ion source of multicharged ions have been optimized: (i) the magnetic configuration has higher axial fields (1.4–1.45 T at the mirror throats) and radial field (1.05 T at the wall inside the plasma chamber) as well as higher mirror ratios, (ii) the 14.5 GHz rf frequency is convenient to higher magnetic fields, and (iii) more efficient electron sources allow the electron density to reach higher values, and thus high extracted ion currents (both first stage and wall coating). Emphasis is given to the metallic elements, the source is able to produce 10 eμA of Ca14+, 3 eμA of Fe17+ and Ni20+, 1 eμA of U37+, while being able to deliver 1130 eμA of O6+, 190 eμA of O7+, and 100 eμA of Ar12+.
A novel micromovement actuator manufactured using plastic electromechanical film
Authors: Hämäläinen Matti K. , Parviainen Juha K. , Jaaskelainen Timo
Published: July 2002

A novel multilayer actuator from a fairly new polymer dielectric material, electromechanical film (EMF), has been fabricated. The characterization of the actuator performance, piezoelectric constant (d33=29.7×10−12 m/V), displacement frequency response and resonances (f=1–40 kHz), and displacement behavior of dc and ac electric fields has been determined. An interferometric displacement measurement system was used for these measurements. The mechanical displacement hysteresis was found to be less than 7% (input voltage Vp−p=485 V) in quasistatic low-frequency (f=1 Hz) measurement.
A simple model to describe ion source extraction optics—Simulations and experimental test
Authors: Biri S. , Báder A. , Pálinkás J.
Published: July 2002

The main goal of this work was to experimentally check the value of the ‘‘cos+lin’’ method previously developed by the authors. The design and optimization of the beam extraction optics for a hollow-cathode ion source have been carried out. Starting from an existing electrode arrangement the optical system (e.g., the shape and potential of the electrodes) was gradually fitted to the requirements of the model. The final optimization of the optics was carried out by the SIMION computer code. We tested the extraction and focusing optics with H+ and Ar+ ions at different energies up to 30 keV. The electrode potentials, optimized for the best beam parameters (the largest intensity and smallest diameter), are in good agreement with the calculated values.
A small, insertable oven for boronization
Authors: Brouchous D. A. , Diebold D. A. , Doczy M. L.
Published: July 2002

A small insertable oven for benchmarking the boronizing characteristics of solid compounds, such as decaborane and carborane, has been developed for the Phaedrus-T tokamak. Assembly and installation of the oven are relatively easy as the oven design utilizes a Langmuir probe drive assembly, which is standard equipment on most tokamaks and allows the oven to be inserted into the tokamak without requiring a vent. Films deposited by heating carborane into the vapor state with the oven are found to be spatially nonuniform in both thickness and in the ratio of boron to carbon as compared to films deposited with trimethylboron, a gaseous compound. Overall plasma performance is not found to be greatly affected by whether decaborane, carborane or trimethylboron is used for boronization in Phaedrus-T.
Thermal stability and annealing of columnar defects in Bi2Sr2Ca1Cu2O8/Ag superconductor
Authors: Thompson J. R. , Paul David , Wang Z. L. , Kroeger D. M. , Christen D. K.
Published: July 2002

This work establishes the stability at elevated temperatures of columnar defects, artificially formed in the Bi-based cupric oxide superconductor Bi2Sr2Ca1Cu2O8 for enhanced vortex pinning. Isochronal anneals, conducted in air, led to losses of critical current density in two stages. The defects were relatively stable up to ∼550 °C, where second stage annealing began; above this, the pinning diminished rapidly. The recrystallization and loss of columnar defects were corroborated by transmission electron microscopy.