bluuebunny/crossref_metadata_2025 huggingface dataset:
Sensitive optical detection of the shock front and fast moving surface for shock study in condensed media in the 1 GPa stress region
Authors: Mori Y. , Tamura T. , Nagayama K.
Published: July 2002

Based on the light extinction of total internal reflection by triangular prisms, very sensitive detection of the shock front and fast moving free surface has been demonstrated for 1 GPa shock waves in polymers. This is the first experimental verification of the feasibility of the inclined-prism technique for the relatively low shock stress region, which was originally proposed and used by Eden and Wright for obtaining the high-explosive generated shock Hugoniot of several condensed materials. The present method has the advantage that it only requires closing the gap between the free surface of the target and the reflected surface of the prisms. The feasibility and the limitation of the method have been discussed both theoretically and experimentally. A typical streak photograph for a PMMA specimen is shown to compare with the published data. It is seen that the present method is very sensitive for shock arrival detection even in the 1 GPa shock stress region.
An interferometric device to measure the mechanical transfer function of the VIRGO mirrors suspensions
Authors: Calloni E. , Di Fiore L. , Grado A. , Milano L.
Published: July 2002

The design of servoloops for automating alignment and locking of long baseline gravitational wave detectors requires an accurate knowledge of the mechanical transfer function (TF) of the mirror suspensions. We present a suspended interferometric device devoted to the measurement of the TF of the multipendular seismic isolators of the VIRGO antenna.
A new method used in constructing a noise-immune trigger generator
Authors: Wang Xin Xin , Zhang Zhuo
Published: July 2002

A new method used in constructing a noise-immune multichannel trigger generator of high voltage was presented. The attention was paid to controlling the charge/discharge process of the energy storage capacitor in every channel of the trigger generator. This is much different from the old methods in which the attention was paid to decreasing the electromagnetic noise invading the trigger generator.
Spatially resolved Stokes vector measurements
Authors: Cady Fredrick M. , Krings Duane
Published: July 2002

A procedure for making spatially resolved, Stokes vector image maps of polarized light is described. The imaging system consists of a circular polarizer and a charge coupled device (CCD) camera. A series of four images are captured and the polarization maps are computed and displayed using MATLAB software.
Precision sample rotator with active angular position readout for a superconducting quantum interference device susceptometer
Authors: Bhattacharya Anand , Tuominen M. T. , Goldman A. M.
Published: July 2002

A technique has been devised for precise in situ sample angular positioning and readout in a commercial superconducting quantum interference device (SQUID) susceptometer. The axis of rotation is perpendicular to the applied magnetic field and the temperature range is from 2 K up to room temperature. The technique has an accuracy in angular position of about 0.3%, approximately one degree over one complete rotation. A small coil with an ac current excitation is fixed on the sample stage, and the ac SQUID response is used to determine the angular position of the stage.
Experimental investigation for the time-dependent effect in electrorheological fluids under time-regulated high pulse electric field
Authors: Wen Weijia , Zheng D. W. , Tu K. N.
Published: July 2002

A new time-regulated high-voltage power supply is designed to investigate the dynamic process of the particle–particle interaction in electrorheological (ER) fluids. Its shortest acting time on the particle is less than 200 ns and the field strength can be adjusted linearly from 0 to 15 kV. At a field strength of 1.2 kV/mm, it is found that the initial response time of the glass microsphere is 18 ms and the respective chain and column formation times are measured to be about 125 and 900 ms. During particle aggregation, a sensitive shear stress device is applied to test the time dependence of ER effect and the results indicate that there exist three different stages of increasing effect. In addition, it is found that the aggregation of the microspheres becomes faster and the interaction among particles is much stronger under higher field strength.
Use of a nanoscale Kelvin probe for detecting wear precursors
Authors: DeVecchio Derik , Bhushan Bharat
Published: July 2002

A version of a Kelvin probe that has been developed for an atomic force microscope (AFM), here referred to as a nano-Kelvin probe, is used to create maps of surface potential for study of samples that have been abraded by an AFM tip. The focus is on the wear at very low loads that involve the absence of wear debris and/or wear scars. Wear scars at higher loads, where significant damage to surface has occurred, have also been studied for reference purposes. Samples studied include single crystal aluminum, alumina, gold, and silicon. It is shown that even in cases where there is little or no damage to the surface, as observed by topography scans of an AFM, there is often a large change in the potential at the surface of the sample. The change in surface potential is believed to be the result of chemical and structural changes in the first few nanometers of the sample. We have shown that, even in the case of “zero wear” (that is, no visible deformation of the surface), there can be a significant change in the surface potential inside the wear scar. This allows for the study of the onset of wear in the ultralow wear regime that is not possible with other techniques. The study of wear precursors is particularly important for the hard disk drive industry, the microelectromechanical systems industry, and other industries where the presence of any microscopic wear or debris can represent total failure.
Saturation in “nonmagnetic” stainless steel
Authors: Weber C. , Fajans J.
Published: July 2002

Scientific equipment often uses “nonmagnetic” stainless steel, relying on the steel’s nonmagnetic behavior to leave external magnetic fields unaltered. However, stainless steel’s permeability can rise significantly when it is welded or machined, possibly perturbing an external field. Such perturbations will diminish well above the stainless steel’s saturation point. The authors measured the permeability of both welded and machined 304 stainless steel as a function of an external magnetic field, and found that both saturate at fields of approximately 0.25 T.
A demountable cryogenic feedthrough for plastic optical fibers
Authors: Butterworth J. S. , Brome C. R. , Huffman P. R. , Mattoni C. E. H. , McKinsey D. N. , Doyle J. M.
Published: July 2002

A superfluid-helium-tight optical fiber feedthrough has been developed. Heat shrinkable Kynar provides a removable seal around a plastic optical fiber. The seal preserves the continuity of the fiber and is reliable after repeated thermal cycles.
Development of a two-dimensional space-resolved high speed sampling camera
Authors: Kodama R. , Okada K. , Kato Y.
Published: July 2002

We developed a two-dimensional spatial resolved high-speed UV sampling camera (HISAC) to study energy transport in laser-produced plasmas. The HISAC is composed of an optical bundle of fibers coupled to a streak camera to obtain a two-dimensional spatial resolution with a temporal resolution of less than a few 10 ps. This HISAC system was demonstrated in the experiments on inferred uniformity measurements of laser-ablation pressures as well as on energy transport in ultraintense laser-plasma interactions. Two-dimensional shock heating was temporally resolved in a hemispherical target, resulting in the distribution of the ablation pressure as a function of laser incidence angle.
Characterization of a novel micromachined optical vibrating rate gyroscope
Authors: Seter D. J. , Bochobza-Degani O. , Socher E. , Kaldor S. , Scher E. , Nemirovsky Y.
Published: July 2002

The present article presents experimental results measured on a new class of micromachined rate-gyroscope, incorporating integrated optical sensing. A hybrid cantilever type device was fabricated by bulk micromachining, bonded to a complementary metal-oxide-semiconductor chip and encapsulated in a vacuum package. The proof mass is electrostatically actuated and the mechanical output motion is sensed by modulated integrative differential optical sensing method. Characterization of the device, including measurements of the natural frequencies and angular rate, is reported. Noise measurements indicate noise equivalent rate of 10−2–10−3 deg/s/√Hz. Currently the measured minimum detectable rate is 1 deg/s due to experimental setup limitations.
Structure and magnetocrystalline anisotropy of R2Fe17−xGax compounds with higher Ga concentration
Authors: Shen Bao-gen , Cheng Zhao-hua , Liang Bing , Guo Hui-qun , Zhang Jun-xian , Gong Hua-yang , Wang Fang-wei , Yan Qi-wei , Zhan Wen-shan
Published: July 2002

The effect of Ga substitution for Fe in R2Fe17 (R=Y, Sm, Gd, Tb, Dy, Ho, Er, and Tm) compounds on the structure and magnetocrystalline anisotropy has been studied by means of x-ray diffraction and magnetization measurements. Both iron sublattice anisotropy and rare earth sublattice anisotropy are found to be modified by the introduction of the gallium atoms. A uniaxial anisotropy is shown in R2Fe17−xGax (for R=Y, Gd, Tb, Dy, Ho, Er, and Tm) compounds with high Ga concentration, whereas a reversal change in the easy magnetization direction is observed in the samples for R=Sm. The contributions to the uniaxial orientation of the magnetization in these compounds result from not only the rare earth sublattice, but also the iron sublattice.
Optoelectronic standardization of laser Doppler perfusion monitors
Authors: Liebert A. , Lukasiewicz P. , Boggett D. , Maniewski R.
Published: July 2002

An optoelectronic method for standardization of laser Doppler (LD) instruments is proposed and tested. The optoelectronic standardization is based on detection of modulated near-infrared light from a light emitting diode (LED) emitted into the optic probe (detecting fiber) of the LD instrument. Changes of the frequency and amplitude of the light enables evaluation of the sensitivity and linearity of the instrument response. Measurements of the LD instrument response show linear increase of perfusion signal with the modulation frequency of light. Analysis of the LD signal components (flux, speed, concentration) show that for fixed amplitude and varying frequency of the modulation, the flux and speed are linearly related to the frequency and the concentration is constant. The linear increase of flux and concentration signals was observed for increasing amplitude of modulation with fixed frequency. The proposed technique allows for automatic evaluation of the instrument response.
Characterizing the emittance contribution due to rotated quadrupoles and canonical angular momentum using the quadrupole scan technique in electron accelerators
Authors: Carlsten Bruce E.
Published: July 2002

Standard emittance measurement techniques for bright electron beams are either insensitive to the emittance due to correlated effects like misaligned quadrupoles or a nonzero axial magnetic field at the cathode, or are unable to differentiate that emittance from the beam’s intrinsic emittance corresponding to the beam’s actual entropy. We describe a technique based on the standard quadrupole scan, in which the beam correlations leading to emittance growth can be directly measured, and the emittance from these correlations can be found for special cases.
Angle chord integral inverter and a rotating type optical probe
Authors: Yoon N. S. , Na H. K. , Hwang S. M.
Published: July 2002

The inversion problem of angle dependent chord integrals is theoretically studied and a rotating type optical probe is developed for its application. The proposed solution is computationally efficient and robust to measurement additive noise. The probe system constructed is tested in an inductively coupled plasma source by measuring a spectral line intensity of argon plasma.
Vibrating reed apparatus with optical detection and digital signal processing: Application to measurements on thin films
Authors: Harms U. , Kempen L. , Neuhäuser H.
Published: July 2002

We present an apparatus that permits quasisimultaneous measurements of several vibrating modes in a vibrating reed experiment. Position detection by laser beam deflection offers a simple setup and good sensitivity for higher modes of flexural as well as torsional vibrations. Frequency and damping from free decay data are determined by software which permits high accuracy, especially at low damping. This is particularly interesting for the measurement of the mechanical properties of thin films deposited on a low damping vibrating reed. As an example results on an Al film on a microstructurized silicon resonator are shown.
Differential reflectance spectroscopy using sample rotation
Authors: Zha F. X. , Shen S. C. , Huang X. L. , Shi G. L.
Published: July 2002

Differential reflectance (DR) is one of the modulation spectroscopies, and its modulation signal arises from the differential reflectance due to the spatial inhomogeneity of the sample. There are several methods to achieve the difference reflectance and we report here an experimental configuration using sample rotation. This configuration looks similar to that adopted previously by reflectance difference spectroscopy (RDS), however in the DR experiments we trace the signal synchronizing with the fundamental rotation frequency (f ) of the sample. In RDS one used the second harmonic frequency of the rotation (2f ) as the reference frequency. We check this method with an InGaAs/GaAs quantum well and an AlGaAs/GaAs heterojunction bipolar transistor (HBT). Both samples show well resolved spectral features which are in good agreement with photoreflectance measurements.
Modification of a high vacuum, crossed molecular beam scattering system to perform angle-resolved, gas-surface scattering studies under ultrahigh vacuum conditions
Authors: Yoshikawa Hiroshi , Ohashi Haruhiko , Tabayashi Kiyohiko , Suzui Mitsukazu , Horigome Toshio , Hayakawa Kazuo , Kato Shinji , Shobatake Kosuke , Ito Koji , Fujimoto Tetsuo
Published: July 2002

A molecular beam apparatus which was formerly used for crossed molecular beam chemiluminescence experiments under high vacuum conditions has been modified into an ultrahigh vacuum apparatus with a rotatable mass spectrometer detector equipped with an electron bombardment ionizer to study (1) the molecule-surface scattering process, (2) molecular beam-surface reaction dynamics, and (3) light-excited molecular desorption. Every flange surface originally sealed using a Viton o ring is sealed with a combination of an inner spring-loaded Teflon or aluminum seal with an outer Viton o ring, the intermediate region being pumped with a diffusion pump. Due to the limited space available for a rotatable detector chamber and to make the neutral flight length in time-of-flight (TOF) measurements as long as possible, the electron bombardment ionizer is installed at the farthest end of a triply differentially pumped detector chamber; thus, the ions formed are extracted toward the reverse direction of the scattered neutral molecular beam, mass selected with a quadrupole mass filter, and detected by an electron multiplier. Using this apparatus, angular and TOF distributions have been measured for Xe scattered from crystalline graphite surface and for the desorbed product, SiCl2, from the etching reaction of Si(111) with a beam of hyperthermal Cl2 molecules.
Microindentation device for in situ study of pressure-induced phase transformations
Authors: Gogotsi Yury , Miletich Thomas , Gardner Michael , Rosenberg Michael
Published: July 2002

In situ microscopic and spectroscopic studies of samples allow us to understand the mechanisms and measure kinetics of phase transformations in materials. We use a light microscope and a Raman microspectrometer to study phase transformations induced by contact loading. Many interesting phenomena occur in materials during indentation that can only be analyzed during indentation, in situ. By analyzing what occurs to ceramics and semiconductors in situ we can gain valuable insight into the mechanisms and kinetics of phase transformation. A microindentation device has been designed and fabricated to achieve these objectives. The microindentation device can provide the means to study pressure-induced phase transformations in real time. The basic design of the device is adaptable to several configurations, so that the device may be used in a wide variety of applications. The device consists of a piezoelectric actuator (piezoelectric translator), load cell, linear microscrew stage, translation stage containing the specimen mount and specimen holder, and diamond-tip indenter. For the first time, an indentation tester has been coupled with a Raman microspectrometer to conduct in situ studies of pressure-induced phase transformations. This article describes the design, operation, and experimentation of a microindentation device for the in situ analysis of pressure-induced phase transformations in materials.
Computer-controlled Teflon flow control valve
Authors: Gao R. S. , McLaughlin R. J. , Schein M. E. , Neuman J. A. , Ciciora S. J. , Holecek J. C. , Fahey D. W.
Published: July 2002

A butterfly vane valve is described for the servo control of a gas or liquid flow. The valve is suitable for reactive species since nearly all wetted parts are Teflon. The valve is operated by a servo motor that is controlled by a dedicated microprocessor programmed in BASIC. The valve offers many other desirable features, including constant and relatively small surface area, low dead volume, high conductance in the full-open position, vacuum compatibility, firmware control, and scalability.
Reply to “Comment on ‘Noise averaging and measurement resolution’ ” [Rev. Sci. Instrum. 70, 2038–2040 (1999)]
Authors: Potzick James
Published: July 2002

The Comment expands the scope of the original article. A subsequent expanded version of that article includes references to the work mentioned in the Comment.
Properties of a differential pressure pseudospark device
Authors: Karkari S. K. , Mukherjee S. , John P. I.
Published: July 2002

A differential pressure pseudospark device is developed to produce a discharge at a pressure of 10−4 mbar near the anode. This pressure range is two orders in magnitude lower than conventional pseudospark devices. In this device a pressure gradient is maintained between the cathode and the anode by providing a gas flow through the discharge column. The pressure gradient helps in shifting the Paschen curve more towards the left in comparison to the conventional case. The empirical relationship V∝(p2dD)−2, valid without a gas flow, is not applicable when the pressure difference between the cathode and the anode is over two orders in magnitude. Self-biasing collector technique reveals the presence of energetic electrons (0.4–1.2 keV) present in the plasma downstream of the anode. The nature of this plasma at two distinct pressure ranges of operation of the device shows marked difference in properties. A qualitative discussion is presented that explains the possible discharge mechanism in this device.
Measurements of poloidal rotation velocity using charge exchange spectroscopy in a large helical device
Authors: Ida K. , Kado S. , Liang Y.
Published: July 2002

Absolute measurements of poloidal rotation velocity with the accuracy up to 1 km/s (2 pm in wavelength) were done using charge exchange spectroscopy in a large helical device. Radial profiles of the absolute Doppler shift of charge exchange emission with a beam are obtained from spectra measured with four sets of optical fiber arrays that view downward and upward at the poloidal cross section with and without neutral beam injection. By arranging the optical fiber from four arrays close to each other at the entrance slit, the apparent Doppler shift due to aberrations of the spectrometer and due to interference of the cold component (the charge exchange between He-like oxygen and thermal neutrals 8 pm from the charge exchange emission with a beam) can be eliminated from the measurements. The measured poloidal rotation velocity is 1–3 km/s in the electron diamagnetic direction at half of the plasma minor radius.
Analysis of field emission characteristics of hydrogen-adsorbed silicon surface
Authors: Jo Sung Ho , Park Byung Gook , Lee Jong Duk
Published: July 2002

It has been reported elsewhere that the intercept and the absolute value of slope of the Fowler–Nordheim plot for a hydrogen-adsorbed silicon field emitter are smaller than those for a clean silicon field emitter. In this letter it is shown that this experimental result can be explained by taking account of the change of surface potential barrier due to the polarization of adsorbed hydrogen atom in addition to the change of work function. The amounts of decrease of the intercept and the absolute value of slope calculated for the hydrogen-adsorbed silicon surface are in good agreement with the experimental data.
Pressure-enhanced interdiffusion in amorphous Si/Ge multilayers
Authors: Theiss Steven D. , Spaepen Frans , Aziz Michael J.
Published: July 2002

We have investigated the effect of hydrostatic pressure on interdiffusion in multilayers composed of alternating layers of amorphous Si (2.7 nm) and Ge (3.1 nm). Samples were annealed at 420 °C at pressures between 0 and 2.8 GPa in an externally heated diamond anvil cell. Interdiffusion was measured by monitoring the decay with annealing time of the intensity of the first-order x-ray reflection resulting from the effects of composition modulation. The decay curves for all pressures could be made to coincide by scaling the annealing times. This made it possible to separate the effects of pressure on the interdiffusivity from those of composition and structural relaxation. The interdiffusivity increased with applied pressure, with an activation volume of −5.0 cm3/mole, or −0.42 times the atomic volume of crystalline Si.
Plasma beam deposited amorphous hydrogenated carbon: Improved film quality at higher growth rate
Authors: Gielen J. W. A. M. , van de Sanden M. C. M. , Schram D. C.
Published: July 2002

High quality diamondlike a-C:H has been deposited, at low ion bombardment energies, from an expanding thermal argon/acetylene plasma at high growth rate. It is observed that quality improvement, in terms of hardness, is equivalent to maximization of the refractive index. The highest refractive indices are obtained when the admixed acetylene flow and the argon ion flux emanating from the plasma source are comparable in magnitude, which suggests critical loading. This also indicates that the acetylene has to be dissociated only once. Combination with the observed quality behavior at higher deposition rates suggests that there is one preferred hydrocarbon radical for deposition, probably C2H.
Transverse-electric/transverse-magnetic polarization converter using twisted optic–axis waveguides in poled polymers
Authors: Oh Min-Cheol , Hwang Wol-Yon , Kim Kwangjoon
Published: July 2002

Polymeric passive TE/TM polarization converters are fabricated based on an adiabatic following in twisted optic–axis waveguides. Waveguides are formed by a reactive ion etching, and the optic axis of the waveguide is twisted by 90° with specially designed poling electrodes. Both TE and TM modes confined in the waveguides are converted to their counterparts by following the twisted ordinary and extraordinary axes, respectively. Conversion efficiency better than 20 dB and polarization-dependent loss less than 0.5 dB are obtained. The device provides wavelength insensitive operations and has large fabrication tolerances.
Noncontact nanosecond-time-resolution temperature measurement in excimer laser heating of Ni–P disk substrates
Authors: Chen Shaochen , Grigoropoulos Costas P.
Published: July 2002

The thermal emission from a Ni–P disk substrate heated by a pulsed excimer laser is measured with nanosecond time resolution. A fast InGaAs photodetector is employed to capture the thermal emission signal. The spectral surface reflectivity is simultaneously measured in situ. The transient surface temperature is derived from the spectral thermal emission signal on the basis of Planck’s blackbody radiation intensity distribution. The experimental results and analytical solutions are compared and an important parameter involving the thermal diffusivity and conductivity in the transient temperature response of the material is evaluated.
Photolithographic patterning of vacuum-deposited organic light emitting devices
Authors: Tian P. F. , Burrows P. E. , Forrest S. R.
Published: July 2002

We demonstrate a photolithographic technique to fabricate vacuum-deposited organic light emitting devices. Photoresist liftoff combined with vertical deposition of the emissive organic materials and the metal cathode, followed by oblique deposition of a metal cap, avoids the use of high processing temperatures and the exposure of the organic materials to chemical degradation. The unpackaged devices show no sign of deterioration in room ambient when compared with conventional devices fabricated using low-resolution, shadow mask patterning. Furthermore, the devices are resistant to rapid degradation when operated in air for extended periods. This work illustrates a potential foundation for the volume production of very high-resolution, full color, flat panel displays based on small molecular weight organic light emitting devices.
Generation of ultrashort, discrete spectrum microwave pulses using the dc to ac radiation converter
Authors: Muggli P. , Liou R. , Hoffman J. , Katsouleas T. , Joshi C.
Published: July 2002

The output radiation of a dc to ac radiation converter is characterized. A relativistic ionization front passing through a capacitor array of period d=1 cm produces short pulses of tunable radiation between 39 and 84 GHz with a gas pressure between 0 and 30 mT. The frequency spectra of the produced pulses are discrete and exhibit full widths at half maximum between 12% and 28%, consistent with the expected width for six cycles’ pulses. An upper bound of 750 ps (detection bandwidth limited) is placed on the pulse widths. These are the shortest pulses produced by a source of coherent radiation in this frequency range.
Birefringence phase-matched blue light second-harmonic generation in a KNbO3 ridge waveguide
Authors: Pliska Tomas , Fluck Daniel , Günter Peter , Gini Emilio , Melchior Hans , Beckers Lutz , Buchal Christoph
Published: July 2002

Ridged channel waveguides in KNbO3 were fabricated by a technique using He+ ion implantation, photolithographic masking, and subsequent Ar+ ion sputtering. A continuous-wave second-harmonic output power of 14 mW at 438 nm was obtained with an in-coupled fundamental power of 340 mW in a 0.73 cm long waveguide. Phase matching was provided by material birefringence without need of periodic poling.
Characteristics of InAs/AlGaAs self-organized quantum dot modulation doped field effect transistors
Authors: Phillips J. , Kamath K. , Brock T. , Bhattacharya P.
Published: July 2002

We have investigated the dc characteristics of InGaAs/AlGaAs modulation doped field effect transistors in which a layer of self-organized InAs quantum dots is inserted adjacent to the pseudomorphic quantum well channel. Distinct steps and a negative differential resistance are observed in the current–voltage characteristics at room temperature and lower temperatures. These are attributed to conduction through the bound states in the quantum dots.
Domain broadening in quasi-phase-matched nonlinear optical devices
Authors: Rosenman G. , Garb Kh. , Skliar A. , Oron M. , Eger D. , Katz M.
Published: July 2002

Dominant factors influencing domain broadening in periodically poled ferroelectric domain structures for quasi-phase-matched nonlinear optical converters are studied. It is shown that fabricated patterned switching electrode gives rise to a high tangential field causing polarization inversion behind an isolated layer of the patterned electrode. The domain broadening depends on basic electron parameters of a ferroelectric crystal such as surface electron concentration and electron mobility.
Resonant Raman scattering in polycrystalline silicon thin films
Authors: Paillard V. , Puech P. , Laguna M. A. , Temple-Boyer P. , Caussat B. , Couderc J. P. , de Mauduit B.
Published: July 2002

In this letter, we report the results obtained on polycrystalline silicon thin films using Raman spectrometry in resonance with the silicon direct band gap. First, we show that accurate information about crystallites can be obtained in these experimental conditions, without any deconvolution of Raman spectra. Second, we apply the technique to estimate the mechanical stress of polycrystalline silicon thin films.
Red-light-emitting injection laser based on InP/GaInP self-assembled quantum dots
Authors: Zundel M. K. , Jin-Phillipp N. Y. , Phillipp F. , Eberl K. , Riedl T. , Fehrenbacher E. , Hangleiter A.
Published: July 2002

Red-light-emitting quantum dot injection lasers have been prepared by solid-source molecular beam epitaxy. The separate confinement heterostructure contains densely stacked layers of self-assembled InP quantum dots embedded in Ga0.51In0.49P waveguide and Si/Be-doped Al0.53In0.47P cladding layers. Edge-emitting laser diodes are processed, which show quantum dot lasing at 90 K. Thereby, the threshold current density is 172 A/cm2. The energy of the laser line is at 1.757 eV, which is very close to the peak energy of subthreshold electroluminescence.
Submicron stacked-junction fabrication from Bi2Sr2CaCu2O8+δ whiskers by focused-ion-beam etching
Authors: Kim S.-J. , Latyshev Yu. I. , Yamashita T.
Published: July 2002

We fabricated submicron-sized intrinsic Josephson junctions by the focused-ion-beam (FIB) etching method. The principal result was a reduction of the in-plane junction area to 0.3 μm2 by direct FIB etching with no degradation in the critical transition temperature (Tc). In the current (I)–voltage (V) characteristics of these stacks, the gap structure and the normal state resistance are clearly observed together with a reduction of the Joule heating and disappearance of the branch structure. The Coulomb staircase structure was found in the I–V curves of submicron junctions as a result of their small effective capacitance of fF order.
Low-temperature crystallization of amorphous silicon using atomic hydrogen generated by catalytic reaction on heated tungsten
Authors: Heya Akira , Masuda Atsushi , Matsumura Hideki
Published: July 2002

A method for crystallizing amorphous silicon (a-Si) films at low temperatures is proposed. In the method, a-Si films are crystallized at temperatures lower than 400 °C by annealing in the presence of atomic hydrogen. The hydrogen atoms are generated by catalytic cracking reaction of H2 gas on a heated tungsten catalyzer in the catalytic chemical vapor deposition apparatus. It is found that the crystalline fraction of such an a-Si film is increased from 0% to several tens %, and at the same time the a-Si film itself is etched with the rate of several tens nm/min by annealing in atomic hydrogen. This increment of crystalline fraction appears dependent on the quality of initial a-Si films. It is implied that there are several types of a-Si even if the difference among a-Si films cannot be detected by Raman scattering spectroscopy and other means for measurements.
Room-temperature persistent spectral hole burning in SrFCl:Sm2+ films: Temporal and spatial response
Authors: Schnieper M. , Trotta F. , Bersier S. , Bill H.
Published: July 2002

Persistent spectral hole burning was performed on the 7F0–5D1 transition of Sm2+ in thin films of SrFCl. Depending on the substrate and the growth conditions, a total hole depth between 47% and 70% was reached. The holes were Lorentzians of width 4(±0.3) cm−1. The time evolution of the hole depth was studied. It is described by two exponentials: a short time decay (t1=0.37 days) and a long time decay (t2=20.4 days) with a 20% infinite time limit. One- and two-photon burning mechanisms act.
Nonlinear optical effects in chalcogenide photoresists
Authors: Rosenblum G. , Sfez B. G. , Kotler Z. , Lyubin V. , Klebanov M.
Published: July 2002

Both the “after-pulse effect” and the dynamic characteristics of photostructural transformations induced in glassy As0.5Se0.5 films by pulsed 532 nm excitation have been studied. The after-pulse effect investigation demonstrated more than a 103 times increase of the photosensitivity in case of pulsed excitation. Dynamic characteristics showed a dual time scale behavior and different intensity dependence of transient and long time scale signals. The obtained data indicate that the strong increase of photosensitivity following short intense pulsed light excitation is due to a two-photon effect that aids the process of structural rearrangement.
Mechanism of efficient ultraviolet lasing in GaN/AlGaN separate-confinement heterostructures
Authors: Bidnyk S. , Lam J. B. , Little B. D. , Kwon Y. H. , Song J. J. , Bulman G. E. , Kong H. S. , Schmidt T. J.
Published: July 2002

We report the results of an experimental study on efficient laser action in an optically pumped GaN/AlGaN separate-confinement heterostructure (SCH) in the temperature range of 10–300 K. The lasing threshold was measured to be as low as 15 kW/cm2 at 10 K and 105 kW/cm2 at room temperature. Strongly polarized (TE:TM⩾300:1) lasing peaks were observed over the wavelength range of 358–367 nm. We found high-finesse lasing modes that originate from self-formed microcavities in the AlGaN and GaN layers. Through analysis of the relative shift between spontaneous emission and lasing peaks, combined with the temperature dependence of the lasing threshold, we conclude that exciton–exciton scattering is the dominant gain mechanism leading to low-threshold ultraviolet lasing in the GaN/AlGaN SCH over the entire temperature range studied. Based on our results, we discuss possibilities for the development of ultraviolet laser diodes with a GaN active medium.
Feasibility of transmission x-ray microscopy at 4 keV with spatial resolutions below 150 nm
Authors: Kaulich B. , Oestreich S. , Salome M. , Barrett R. , Susini J. , Wilhein T. , Di Fabrizio E. , Gentili M. , Charalambous P.
Published: July 2002

The feasibility of a transmission x-ray microscope operating in the 3-7 keV photon energy range using phase zone plates (ZPs) has been tested. It has been demonstrated that, at a photon energy of 4 keV, structures smaller than 150 nm can be resolved with good contrast using exposure times in the second to minute range. A large diameter gold ZP was used as condenser and a tungsten ZP with an outermost zone width of 128 nm was used as a high spatial resolution imaging objective lens. Images with a field size of 10×10 μm2 were acquired using a charge coupled device camera optically coupled to a phosphor screen. The corresponding pixel size in the object plane was 70 nm.
Towards ordered arrays of magnetic semiconductor quantum wires
Authors: Chen Limei , Klar Peter J. , Heimbrodt Wolfram , Brieler Felix , Fröba Michael
Published: July 2002

The diluted magnetic semiconductor (Cd, Mn)S has been incorporated into ordered wire-like pores of hexagonal mesoporous silica. X-ray and Raman spectra reveal the wurtzite structure of the incorporated material. Photoluminescence and photoluminescence excitation spectra of the (Cd, Mn)S-wire samples show clearly the optical transitions within the half-filled Mn 3d shell, typical for Mn incorporated in a II–VI host material. The blueshift of the absorption edge of (Cd, Mn)S-wire samples compared to reference crystalline and powder samples of the same Mn content is due to quantum confinement in the nanowires.
Theoretical investigation of nitrogen-doping effect on vacancy aggregation processes in Si
Authors: Kageshima Hiroyuki , Taguchi Akihito , Wada Kazumi
Published: July 2002

The nitrogen-doping effect on vacancy aggregation in Si is studied by comparing total energies of various complexes of nitrogen atoms and Si vacancies in terms of first-principles calculations. Two nitrogen atoms are found to form a stable complex with two Si vacancies, strongly suggesting that a supersaturation of “isolated” Si vacancies in growth cooling can be suppressed. The delayed void formation observed in N-doped Czochralski Si indeed supports this suggestion.
Effect of charge carriers on the barrier height for vacancy formation on InP(110) surfaces
Authors: Semmler U. , Ebert Ph. , Urban K.
Published: July 2002

We determine the energy barrier height for the formation of positively charged phosphorus vacancies in InP(110) surfaces using the rate of formation of vacancies measured directly from scanning tunneling microscope images. We found a barrier height in the range of 1.15–1.21 eV. The barrier height decreases with increasing carrier concentration. These results are explained by a charge separation during the vacancy formation process.
Electronic properties of small neutral and charged beryllium clusters
Authors: Kolchin Andrew M. , Hall Randall W.
Published: July 2002

We determine the atomic and electronic structures for neutral and singly positively charged beryllium clusters containing from two to six atoms using density functional theory in the local spin density approximation. Ions are moved with a steepest descent method and the electronic wave functions optimized using a fictitious dynamics with simulated annealing, as conceived by Car and Parrinello [Phys. Rev. Lett. 55, 2471 (1985)]. Shell-like orbitals, filling angular momentum states in the order: 1s 1p 2s 1d are obtained. We employ a Mulliken population analysis using an atomic basis to examine how the shell orbitals arise from atomic orbitals. This analysis also allows us to associate the electron density distribution and, in the case of a charged cluster, the distribution of the hole with atomic sites and with regions of overlap between atom pairs. We show quantitatively that the contribution to the bonding density from delocalization of the 1s state is hampered by the appearance of the antibonding 2s state. In the case of charged clusters we observe the tendency of the hole to distribute itself near the most exterior atomic sites in geometries of high symmetry.
Efficient multidimensional free energy calculations for ab initio molecular dynamics using classical bias potentials
Authors: VandeVondele Joost , Rothlisberger Ursula
Published: July 2002

We present a method for calculating multidimensional free energy surfaces within the limited time scale of a first-principles molecular dynamics scheme. The sampling efficiency is enhanced using selected terms of a classical force field as a bias potential. This simple procedure yields a very substantial increase in sampling accuracy while retaining the high quality of the underlying ab initio potential surface and can thus be used for a parameter free calculation of free energy surfaces. The success of the method is demonstrated by the applications to two gas phase molecules, ethane and peroxynitrous acid, as test case systems. A statistical analysis of the results shows that the entire free energy landscape is well converged within a 40 ps simulation at 500 K, even for a system with barriers as high as 15 kcal/mol.
Evolution of structure in thin microcrystalline silicon films grown by electron-cyclotron resonance chemical vapor deposition
Authors: Birkholz M. , Selle B. , Conrad E. , Lips K. , Fuhs W.
Published: July 2002

The growth of microcrystalline silicon, μc-Si, films has been studied by infrared spectroscopy and x-ray diffraction. Thin films of various thickness have been prepared from SiH4–H2 mixtures by electron-cyclotron resonance chemical vapor deposition. Two structural transitions were observed during film growth. The first transition at a critical thickness of dac=9 nm manifested itself by a change from an initially amorphous growth to polycrystalline growth. The second structural transition was related to an increasing amount of silicon grains of preferred orientation with (110) lattice planes parallel to the substrate. The population of such (110)-oriented grains N110 was found to become dominant at about d110=310 nm, which may be considered as a second critical thickness above which the film exhibits a (110) fiber texture. The increase of N110 with increasing thickness follows a d1/6 dependence. The effect is understood in terms of an interplay between etching and deposition during growth.
Electroluminescence with high and stable quantum efficiency and low threshold voltage from anodically oxidized thin porous silicon diode
Authors: Gelloz Bernard , Koshida Nobuyoshi
Published: July 2002

Highly efficient electroluminescence (EL) is obtained at low operating voltage (<5 V) from n+-type silicon-electrochemically oxidized thin porous silicon–indium–tin–oxide junctions. Continuous wave external quantum efficiency greater than 1% and power efficiency of 0.37% have been achieved. Considerable reduction of leakage current accounts for the enhancement of EL efficiency upon oxidation. The EL time response (≈30 μs) is slower than the photoluminescence one, due to slow electrical charging of porous silicon. No degradation of quantum efficiency is observed during operation and upon aging. This is attributed to the electrochemically grown oxide, which should provide a better surface passivation than the initial hydrogen coverage.
Time-resolved photoluminescence lifetime measurements of the Γ5 and Γ6 free excitons in ZnO
Authors: Reynolds D. C. , Look D. C. , Jogai B. , Hoelscher J. E. , Sherriff R. E. , Harris M. T. , Callahan M. J.
Published: July 2002

Time-resolved photoluminescence spectroscopy at 2 K was used to measure the radiative recombination lifetime of the allowed (Γ5) and forbidden (Γ6) free excitons in ZnO. The measurements were made on a sample containing internal strain, which altered the sample symmetry, and resulted in relaxed selection rules, allowing the Γ6 exciton to be observed. A radiative recombination lifetime of 259 ps was measured for the Γ5 exciton and 245 ps for the Γ6 exciton. The decay of the free excitons was of single-exponential form, and the decay times were obtained using a least-squares fit of the data.
Temperature variation of near-infrared emission from Cr4+ in aluminate glass for broadband telecommunication
Authors: Tanabe Setsuhisa , Feng Xian
Published: July 2002

Temperature dependence of fluorescence spectra of Cr4+ in an aluminate glass in the range of 1.2–1.6 μm was investigated from 15 to 300 K. The emission spectra were broadband centered at about 1.3 μm, wider than 200 nm width. The emission intensity drastically increased with decreasing temperature and that of 15 K was 20 times larger than that of 300 K. The bandwidth and mean energy of the emission band changed very slightly with temperature. The emission of Cr4+ in the glass fiber is expected as a tunable laser source and a broadband amplifier for the wavelength-division-multiplexed telecommunication.
Self-healing effects in the fabrication process of photonic crystals
Authors: Kawashima Takayuki , Miura Kenta , Sato Takashi , Kawakami Shojiro
Published: July 2002

We demonstrate a self-healing effect of unwanted defects in the autocloning process: autocloning is a previously proposed technique to fabricate multidimensional periodic nanostructures by stacking up corrugated multilayers. With the self-healing effect, aperiodic perturbations in the initial periodic shape of the structure immediately disappear and the surface shape becomes periodic in a few cycles of stacking. If the perturbations exist in photonic crystal, they cause light-scattering loss and also make the boundaries between photonic bands and band gaps unclear. In other words, they make the attenuation of light at the frequency in photonic band gaps weak. Consequently, to attain uniformity of the shape automatically with this effect is very important in the fabrication process for practical photonic crystal devices. In this letter, we verify this phenomenon experimentally, and discuss the mechanism by comparison with process simulation.
Interference effects in the predissociation of the Cs2 C  1Πu and (2)3Πu states through the dissociative (2)3Σu+ state
Authors: Kimura Yasuyuki , Lefebvre-Brion Hélène , Kasahara Shunji , Katô Hajime , Baba Masaaki , Lefebvre Roland
Published: July 2002

The excitation spectrum of the Cs2 molecule monitoring the emission from the dissociated Cs(6p2P3/2) atoms was measured by employing a pulsed supersonic molecular beam crossed with a laser beam. Strong predissociation was found in the 16 600–17 200 cm−1 region. Vibrational progressions with broad asymmetric profiles, which were found via the ionization of dissociated Cs(6p2P3/2) atoms by Kim and Yoshihara, were observed. Transitions from the ground state X  1Σg+ to the dissociative state (2)3Σu+ are allowed by the spin-orbit interaction between the (2)3Σu+ and B  1Πu states. The (2)3Σu+, (2)3Πu, and C  1Πu states are mixed by the spin-orbit interaction. By solving the coupled differential equations using the three-channel model, the transition probability to the mixed levels, (2)3Πu(v) and C  1Πu(v′) was calculated. The coupling between the (2)3Σu+ and (2)3Πu states is larger than that between the (2)3Σu+ and C  1Πu states. Accordingly, the linewidth of the (2)3Πu←X  1Σg+ transition is observed to be wider than that of the C  1Πu←X  1Σg+ transition. The sign of the Fano shape index q was shown to coincide with the sign of the vibrational overlap integral between the discrete level and the continuum.
Efficient particle-mesh Ewald based approach to fixed and induced dipolar interactions
Authors: Toukmaji Abdulnour , Sagui Celeste , Board John , Darden Tom
Published: July 2002

We have implemented classical Ewald and particle-mesh Ewald (PME) based treatments of fixed and induced point dipoles into the sander molecular dynamics (MD) module of AMBER 6. During MD the induced dipoles can be propagated along with the atomic positions either by iteration to self-consistency at each time step, or by a Car–Parrinello (CP) technique using an extended Lagrangian formalism. In this paper we present the derivation of the new algorithms and compare the various options with respect to accuracy, efficiency, and effect on calculated properties of a polarizable water model. The use of PME for electrostatics of fixed charges and induced dipoles together with a CP treatment of dipole propagation in MD simulations leads to a cost overhead of only 33% above that of MD simulations using standard PME with fixed charges, allowing the study of polarizability in large macromolecular systems.
Oxidation kinetics of nickel nanoparticles
Authors: Karmhag Richard , Niklasson Gunnar A. , Nygren Mats
Published: July 2002

The oxidation kinetics of nickel particles with an approximate median size of 15 nm was determined by thermogravimetry. The particles had a thin initial oxide shell amounting to 36 wt % of the sample. The kinetics was studied in pure oxygen in the temperature range 135–235 °C, giving power law behavior of each isotherm with an exponent between 0.48 and 0.58. By use of a master plot technique an apparent activation energy of 1.34 eV was determined. The power law exponent is considerably lower than the one expected for diffusion limited oxidation of a spherical particle with an initial oxide shell. The deviations from parabolic kinetics are discussed in terms of nonlinear diffusion, grain growth, and particle agglomeration.