bluuebunny/crossref_metadata_2025 huggingface dataset:
Associations between tree growth, soil fertility and water availability at local and regional scales in Ghanaian tropical rain forest
Authors: Baker Timothy R. , Burslem David F. R. P. , Swaine Michael D.
Published: February 2003

Relating patterns of species abundance to the processes that determine them at local and regional spatial scales is one of the central aims of ecological research. To examine the relationship between distribution and growth in tropical rain forest, seasonal and annual changes in tree diameter of two tree species with contrasting distribution patterns, Celtis mildbraedii and Strombosia glaucescens, were monitored across topographic gradients in moist semi-deciduous and moist evergreen forest in Ghana over 2 years, 1997-9. Concurrent measurements were made of soil water availability, and during 1997/8, of rainfall, nutrient availability and photosynthetically active radiation (PAR). The two sites received similar, low, total rainfall during 1997/8, but soil water availability in the dry season at the evergreen site exceeded that at the semi-deciduous forest site. Soil water availability was higher during 1998/9 at both sites. During 1997/8, PAR was similar at the two sites. The evergreen site had soils of lower pH, available P, Mg, Ca and K concentrations than the semi-deciduous forest site. Water availability and the sand content of the soil increased and concentrations of total N and C decreased, from summit to valley positions at both sites. Celtis mildbraedii had significantly faster growth rates than S. glaucescens, and growth of both species was greater at the semi-deciduous than the evergreen forest site during the wet year (1998/9) but not during the dry year (1997/8). Celtis mildbraedii in semi-deciduous forest grew less in valley than summit and slope positions. We conclude that in the absence of a severe dry season, growth is higher in semi-deciduous than evergreen forest, and is related to the higher soil fertility in more seasonal forest. The patterns are consistent with the hypothesis that concentrations of available P and/or cations in the soil limit growth in evergreen forest, while concentrations of N in valley soils limit growth of C. mildbraedii in semi-deciduous forest. There was no evidence that variation in PAR influenced growth rates in these forests. A reduction in growth rate on less fertile soils may be a factor determining the distribution limit of dry forest species, such as C. mildbraedii, in evergreen forest.
Effect of litter on the incidence of leaf-fungal pathogens and herbivory in seedlings of the tropical tree Nectandra ambigens
Authors: García-Guzmán Graciela , Benítez-Malvido Julieta
Published: February 2003

Effects of litter cover on insect herbivory, leaf-fungal damage, and seedling performance (survival, growth in height and leaf number), were assessed on established Nectandra ambigens seedlings in three sites of a Mexican tropical rain forest. Naturally occurring seedlings were measured for height, leaf number and standing levels of leaf damage by herbivores and pathogens. Thereafter, seedlings were exposed for 1 y to three litter treatments: (1) litter addition, (2) control and (3) continuous litter removal. There was a significant effect of site on the proportion of surviving seedlings and on the relative growth rates in terms of leaf number. Overall, seedlings with higher initial leaf number showed higher survival and produced more leaves after 12 mo than seedlings with lower initial leaf number. Litter addition resulted in seedlings with lower relative growth rates in height and higher levels of damage by herbivores and pathogens. On average, leaf area damaged by pathogens was < 6% and herbivory was < 27%. Seedlings with higher levels of herbivory showed higher levels of pathogen attack. Herbivory and pathogen attack had no effect on seedling performance. This study reveals the importance of litter-cover on the interactions between seedlings, herbivores and pathogens, as well as on the performance of tropical seedlings.
Risk perception, emotion and policy: the case of nuclear technology
Authors: SJÖBERG LENNART
Published: February 2003

This paper addresses the general question of the role of emotions in risk management, with an application to the siting of a repository for spent nuclear fuel. Although it is true that such siting has been very difficult to carry out without meeting with strong local opposition, there are some exceptions in recent Swedish experience. This recent experience constitutes an occasion for reconsidering the received message from risk perception research since the 1970s. This research has purportedly demonstrated a very strong impact of emotional processes on risk attitudes, which in turn is supposed to imply that these attitudes are rigid and unchangeable. However, when this assertion is looked at more closely, it is found that a number of questionable assumptions and overly strong generalizations have been made. The central role played by the concept of ‘dread’ is particularly important. Dread, or a ‘gut reaction’, is by no means demonstrated in the traditional selection of hazard attributes, which contain such dimensions as concern for future generations or voluntariness. Such dimensions may have a relationship to emotional processes but they are not emotional per se and their emotional significance remains to be documented. The items that do measure emotional reactions directly, by asking about fear and anxiety, are found in data on nuclear waste repository siting to have only a weak influence on policy attitudes. The most important weight is instead carried by items measuring the ‘expected severity of consequences’. The argument is, in a concluding section, carried to the level of attitudes to technologies, which may or may not be related to perceived risk. It is found that an important role is played by substitutability of a technology. If people see no good substitute for a technology that has important benefits, they are ready to accept the risks. The current situation in Sweden with regard to nuclear power can be understood in this light, since no realistic alternatives are available.
The effect of anthropogenic habitat usage on the social behaviour of a vulnerable species, Cyclura nubila
Authors: Lacy Kathryn E. , Martins Emília P.
Published: February 2003

With the increasing human population world‐wide, animals and humans are sharing more habitat. In this study, we consider the consequences of this habitat usage overlap on the behaviour patterns and social structure of a threatened species. Specifically, we used focal animal samples to collect data on the Cuban rock iguana, Cyclura nubila, in six field sites. Three of these sites are subject to considerable anthropogenic habitat usage, whereas three are in relatively low anthropogenic usage areas. Individuals in high anthropogenic usage sites were more closely assembled, with more males and females sharing a smaller amount of space. These animals exhibited even more aggressive behaviour and social interactions than expected when taking into account the larger number of possible interactants. High anthropogenic usage sites also had more male–male interactions and fewer males interacting with females. We suggest that social and mating system changes have occurred. Ramifications of these recent changes in the behavioural repertoires on the long‐term survival of the species are discussed.
Long‐distance wolf recolonization of France and Switzerland inferred from non‐invasive genetic sampling over a period of 10 years
Authors: Valière Nathaniel , Fumagalli Luca , Gielly Ludovic , Miquel Christian , Lequette Benoît , Poulle Marie‐Lazarine , Weber Jean‐Marc , Arlettaz Raphaël , Taberlet Pierre
Published: February 2003

In the early 1900s, the wolf (Canis lupus) was extirpated from France and Switzerland. There is growing evidence that the species is presently recolonizing these countries in the western Alps. By sequencing the mitochondrial DNA (mtDNA) control region of various samples mainly collected in the field (scats, hairs, regurgitates, blood or tissue; n= 292), we could (1) develop a non‐invasive method enabling the unambiguous attribution of these samples to wolf, fox (Vulpes vulpes) or dog (Canis familiaris), among others; (2) demonstrate that Italian, French and Swiss wolves share the same mtDNA haplotype, a haplotype that has never been found in any other wolf population world‐wide. Combined together, field and genetic data collected over 10 years corroborate the scenario of a natural expansion of wolves from the Italian source population. Furthermore, such a genetic approach is of conservation significance, since it has important consequences for management decisions. This first long‐term report using non‐invasive sampling demonstrates that long‐distance dispersers are common, supporting the hypothesis that individuals may often attempt to colonize far from their native pack, even in the absence of suitable corridors across habitats characterized by intense human activities.
Electron−beam−excited DFB laser in CdS
Authors: Aoyagi Yoshinobu , Namba Susumu
Published: February 2003

Distributed feedback (DFB) laser oscillation under electron−beam excitation was observed in CdS. The linewidth of the laser spectrum was found to be less than 0.8 Å. Compared to a conventional laser, the spectrum was less sensitive to temperature. The DFB lasing was observed to occur only at the start of the excitation pulse and the pulsewidth was much shorter than that of a conventional laser.
Optical waveguide of LiNbO3 thin film grown by liquid phase epitaxy
Authors: Miyazawa Shintaro , Fushimi Shoichi , Kondo Susumu
Published: February 2003

We report optical waveguiding of a LiNbO3 single−crystal film grown onto a c−plate LiTaO3 substrate by liquid phase epitaxy from a new Li2O−V2O5 flux. A film of about 3−μm thickness supported seven TE and two TM modes. Measurements of the mode indices indicated that the film is a ’’uniform’’ waveguide having constant refractive indices no=2.2884 and ne=2.1914 throughout the film. Attenuation was measured to be 5 and 11 dB/cm for TM and TE, respectively.
Very rapid tuning of cw dye laser
Authors: Telle John M. , Tang C. L.
Published: February 2003

A method is described which allows the cw dye laser to be tuned over hundreds of angstroms on a nanosecond time scale in spite of the 100-nsec cavity decay and buildup time. Data are provided to verify the proposed method. This tuning rate is fast enough to do, for example, chemical kinetic studies on a nanosecond time scale.
Diffused junction photovoltaic infrared detectors using Pb1−xGexTe with 0.05⩽x⩽0.11
Authors: Antcliffe G. A. , Chapman R. A.
Published: February 2003

Photovoltaic infrared detectors were fabricated in p-type Pb1−xGexTe with 0.05?x?0.11 using antimony impurity diffusion to form n-p junctions. Measurements were performed between 77 and 195 K and the long-wavelength response cutoffs varied from 3.5 to 4.5 μm. The temperature coefficient of the band gap changed from positive below the ferroelectric Curie temperature to negative above this temperature.
Conversion of cw light into a train of subnanosecond pulses using frequency modulation and the dispersion of a near-resonant atomic vapor
Authors: Bjorkholm J. E. , Turner E. H. , Pearson D. B.
Published: February 2003

Sinusoidally frequency-modulated cw laser light is converted into a train of repetitive short pulses by passing it through a nearly resonant atomic vapor. Using a 200-MHz modulation frequency and sodium vapor, pulses as short as 240 psec, having peak powers of 6.3 times the input power, have been obtained at 5890 Å. Calculation and experiment are in good agreement.
Table-top Ar-N2 laser
Authors: Ault E. R.
Published: February 2003

A compact Ar−N2 excitation transfer laser pumped by a coaxial electron gun is demonstrated. This laser emits a 40-ns, 300-kW pulse at a nominal repetition rate of 1 HZ. With 45 J of energy delivered to the electron gun, 12 mJ of laser output at 3577 Å is obtained. A 2.8% laser efficiency is estimated from simple electron deposition calculations, and the over-all electrical efficiency is estimated to be 0.03%. Because the stored energy needed in previously reported Ar−N2 lasers has been reduced by more than an order of magnitude while maintaining a comparable laser energy output, this device represents a significant step forward in the study of high-pressure electron-beam-pumped lasers.
Effect of reconstruction of a semiconductor surface on the crystal growth
Authors: Van Vechten J. A.
Published: February 2003

It is well established that most low-index semiconductor surfaces reconstruct in contact with vacuum or vapor. Covalent bonding considerations imply that they also reconstruct in contact with the liquid. Conventional precipitation theory shows that these first-order transitions should have important consequences on the growth of the crystal. The kinetics of the Si (111) 2×1 to 7×7 reconstruction are explained and the consequent effect upon the growth is found in the observation by de Kock et al. of a 6-orders-of-magnitude drop in vacancy cluster concentration at a pull rate of 0.5 cm/min.
Avalanche buildup time of silicon avalanche photodiodes
Authors: Kaneda T. , Takanashi H.
Published: February 2003

The avalanche buildup time in silicon n+-p avalanche photodiodes is studied by a shot noise investigation. The avalanche buildup time t is given by t∼5×10−13M sec, where M is the multiplication factor.
Optical waveguides in LiNbO3 by ion exchange technique
Authors: Shah Manhar L.
Published: February 2003

Optical waveguides produced in LiNbO3 by immersing the x-cut plates into molten AgNO3 are reported. An increase in the extraordinary refractive index of LiNbO3 at the surface of the plate due to silver exchange is estimated to be 0.12. In such a waveguide, optical losses of 6 dB/cm were measured for the highest-order mode; losses were higher for the lower-order modes.
Plasma electron heating by injection of low-energy electrons
Authors: Hershkowitz Noah , Leung K. N.
Published: February 2003

It is shown that the electron temperature of a multidipole plasma can be significantly increased by injecting electrons with energy less than the ionization potential of the neutral gas. Electron temperatures determined from Langmuir probes and ion-acoustic waves show close agreement.
A method for improving continuous wave laser penetration of metal targets
Authors: Fox Jay A.
Published: February 2003

A method for increasing the penetrating efficiency of a continuous wave (cw) laser beam has been discovered. By combining a Q-switched laser pulse with the cw beam, the irradiation time necessary for penetration of 20-mil steel targets decreased by more than a factor of 2. The hole thus produced was substantially free of resolidified fragments commonly associated with cw penetration.
Applicability of x-ray filter method to thermometry of a noncorona (CH2)n laser-produced plasma
Authors: Rager J. P. , Robouch B. V.
Published: February 2003

It is shown that the temperature as deduced by the filter method is unaffected by departures from the corona ionization equilibrium for a low-temperature carbohydride laser-produced plasma.
Temperature dependence of transverse planar voltages in laser-irradiated Pt and Pd films
Authors: von Gutfeld R. J. , Tynan E. E.
Published: February 2003

The temperature dependence of laser-induced transverse voltages in Pt and Pd films has been investigated in the range ∼25–300 °K. When the data are normalized to a constant average film temperature gradient, a close similarity is found between the normalized values and bulk thermopower data throughout the temperature range. These results suggest a possible model requiring only scalar thermopower coefficients.
Preferential evaporation of In from GaxIn1−xAs
Authors: Goldstein Bernard , Szostak Daniel
Published: February 2003

We have observed a significant change in the surface composition of the ternary GaxIn1−xAs at temperatures ≳500 °C due to the preferential evaporation of In. Both Auger electron analysis of the surface and quadrupole mass analysis of the evaporation products were used to study this evaporation. Controlled sputter etching indicated that the In-depleted region was 5–6 atom layers deep. The reconstruction of both the normal and In-depleted surfaces was the same, as indicated by LEED and there was no change in the basic III-V stoichiometry. Some possible consequences of such a surface composition change are briefly discussed for the case of photoemission from cesiated surfaces.
TeO2 anisotropic Bragg light deflector without midband degeneracy
Authors: Yano T. , Kawabuchi M. , Fukumoto A. , Watanabe A.
Published: February 2003

An advanced acousto-optic light deflector has been designed using the acoustic shear wave traveling slightly off the [110] axis in TeO2. The midband mode degeneracy reported by Warner et al., which decreases the diffracted light intensity, has been shifted off from the active frequency region. A linearly polarized wave can be used as the incident light instead of the elliptically polarized wave used in the conventional deflector designed by Warner et al. The deflector yields a larger number of resolvable spots due to the broadened bandwidth. Experimentally, with 6328-Å light, a 3-dB bandwidth of 50 MHz was observed with a diffraction efficiency of 80% and an electric power of 350 mW. The new deflector has a capacity of 500 resolvable spots with a random access time of 10 μsec.
PbS MIS devices for charge-coupled infrared imaging applications
Authors: Leonberger F. J. , McWhorter A. L. , Harman T. C.
Published: February 2003

Results of an experimental investigation of PbS MIS devices suggest the feasibility of developing a monolithic two-dimensional infrared charge-coupled imaging device (CCID). This CCID would be operable at 77 K and have moderate imaging sensitivity out to about 3.5 μm. Measurements on MIS capacitors fabricated with a pyrolytic SiO2 insulator have shown that the PbS surface potential is variable with bias from accumulation through inversion, the interface state density is ∼1×1012 cm−2 eV−1, and the storage time at 77 K is about 2 sec. Calculations indicate that efficient charge transfer should be achievable with fat-zero operation, and that imaging sensitivity will probably be controlled by the degree of uniformity that can be attained.
Temperature effects in Schottky-barrier silicon solar cells
Authors: Vernon S. M. , Anderson W. A.
Published: February 2003

Experimental results are reported concerning temperature effects from 25 to 125 °C on Schottky-barrier solar cells which were fabricated using a semitransparent Cu/Cr barrier metal layer on p-type silicon. The open-circuit voltage decreased by 2.3 mV/°C and the fill factor by 0.11%/°C, while the short-circuit current increased slightly with increased temperature. These results are consistent with previous work on p-n–junction silicon solar cells. The diode quality factor n was shown to decrease with increased temperature, as predicted by field emission theory. The room-temperature photovoltaic output of cell 96 remained at 0.54 V, 25.4 mA/cm2, and 8.5–10.6% efficiency using 80–100-mW/cm2 sunlight illumination after repeated temperature cycling.
Frequency doubling in KB5O8⋅4H2O and NH4B5O8⋅4H2O to 217.3 nm
Authors: Dewey C. F. , Cook W. R. , Hodgson R. T. , Wynne J. J.
Published: February 2003

We have produced tunable phase-matched second-harmonic radiation in a KB5O8⋅4H2O crystal between 217.3 and 234.5 nm. Phase matching was achieved by crystal rotation. We also studied frequency doubling in NH4B5O8⋅4H2O.
Electroluminescence and photovoltaic detection in Cd-implanted CuInSe2 p-n junction diodes
Authors: Yu Phil Won , Park Y. S. , Faile S. P. , Ehret J. E.
Published: February 2003

p-n junction diodes have been prepared by the ion implantation of Cd into p-type CuInSe2. Electroluminescence is observed near 1.3 μ with internal quantum efficiency of 15% at 77 °K and ∼0.1% at room temperature. The photovoltaic response of typical diodes has a quantum efficiency of 60–70% in the wavelength region 0.7–1.1 μ.
Influence of gyromagnetic ratio on magnetic domain wall dynamics
Authors: Vella-Coleiro G. P. , Blank S. L. , LeCraw R. C.
Published: February 2003

We have investigated the dynamics of magnetic domain walls in three epitaxial garnet films with g values of 1.07, 4.8, and ≳30, and having wall mobilities of 880, 930, and 820 cm/sec Oe, respectively. Bubble collapse measurements in the films with g=1.07 and 4.8 showed velocity saturation with the approximate values 1500 and 8000 cm/sec, respectively. The film with g≳30 showed no saturation, and velocities up to 6×104 cm/sec were observed. These results are consistent with the expected increase in the Slonczewski and Walker critical velocities with increasing g value. Large g values make it possible to operate magnetic bubble circuits at higher frequencies than is otherwise the case.
A hydrogen−sensitive MOS field−effect transistor
Authors: Lundström I. , Shivaraman S. , Svensson C. , Lundkvist L.
Published: February 2003

An MOS transistor in silicon with 10−nm silicon dioxide as gate insulator and 10−nm palladium as gate electrode was fabricated. The threshold voltage of this transistor was found to be a function of the partial pressure of hydrogen in the ambient atmosphere. At a device temperature of 150 °C it was possible to detect 40 ppm hydrogen gas in air with response times less than 2 min.
Phase−matched harmonic generation in a laminar structure with wave propagation in the plane of the layers
Authors: van der Ziel J. P.
Published: February 2003

A technique for obtaining phase−matched second harmonic generation using a finely layered structure with the radiation propagating parallel to the layers is analyzed.
Elastic compression to 30 kbar along 〈111〉 in shocked LiF
Authors: Gupta Y. M.
Published: February 2003

Experimental data are presented for shock propagation along the 〈111〉 crystallographic direction in single−crystal LiF. Within experimenal bounds, an elastic response to 30 kbar is observed. This is in marked contrast with identical 〈100〉 crystals and earlier studies on other single crystals. The elastic behavior along the 〈111〉 direction is in good agreement with low stress and strain rate studies but raises interesting questions regarding crystal strength and dislocation mechansim operative under shock loading.
Temperature dependence of the exchange constant in magnetic−bubble materials
Authors: Enoch R. D. , Jones M. E. , Waters D. G. P.
Published: February 2003

A recent theory has related, through their different Curie temperatures, the exchange constant of an epitaxial garnet film to that of pure YIG at the same temperature. We have experimentally determined the exchange constants A of a SmGa:YIG and of a EuGa:YIG epitaxial garnet film from anisotropy and domain wall energy measurements in the range −30 to 120°C. Despite comparable Curie temperatures, the variation of A with temperature in the two compositions showed a markedly different behavior, which would not be expected from the theoretical model. There is reasonable agreement with the behavior of EuGa:YIG, but not for that of SmGa:YIG or of three other compositions whose exchange constants have been obtained from data in the literature.
First evidence for 1−mm wavelength parametric amplification in a Josephson junction microwave source
Authors: Ulrich B. T. , Levinsen M. T.
Published: February 2003

Our observation of 1−mm wavelength radiation generated in a cavity containing a Josephson junction provides evidence for a negative effective microwave resistance in the Josephson junction, and for parametric amplification in the reflection mode by the Josephson junction. These parametric oscillations were observed at wavelengths corresponding to cavity resonances between 1.1− and 2−mm wavelength. The frequency of the emitted radiation differed from the frequency given by ν=2eV/h, and thus the radiation was not simply from the ac Josephson effect. We develop a theory that predicts a negative microwave resistance at junction bias voltages just below a cavity−induced step for resistively shunted junction, and just above the cavity−induced step for a junction with sufficient shunt capacitance. Our observations are consistent with the theory.
Energy velocity and effective gain in distributed−feedback lasers
Authors: Wang Shyh
Published: February 2003

Here, we establish a relation G = gv/vℰ between the effective gain coefficient G in a periodic medium (for a Bloch wave) and the ordinary gain coefficient g in a corresponding uniform medium (for an ordinary plane wave). In distributed−feedback (DFB) lasers, G is greater (or smaller) than g because the energy velocity vℰ of a Bloch wave is smaller (or greater) than the phase velocity v of a plane wave. The relation between G and g can be regarded as a first principle for DFB lasers. We can use this principle to test the validity of a DFB−laser theory and to guide us to a correct interpretation of the theoretic result.
Quasistatic and transient threshold switching in amorphous semiconductors
Authors: Thornburg D. D.
Published: February 2003

Measurements on thin−film sandwich structure devices using several amorphous chalcogenides have shown that either current−controlled negative differential resistance (CNDR) or threshold switching (TS) phenomena can be displayed under quasistatic bias conditions, depending on whether the amorphous semiconductor is coextensive with or extends beyond the boundary of the top electrode. While the phenomena under these conditions result primarily from bulk thermal processes, transient pulse experiments on CNDR devices have shown that, for sufficiently fast−rise−time pulses, thermal CNDR can be suppressed and sufficient fields can be applied to initiate electronic TS behavior.
A novel technique for measuring small absorption coefficients in semiconductor infrared laser window materials
Authors: Nurmikko A. V.
Published: February 2003

A novel technique for measuring small absorption coefficients in semiconductor infrared laser window materials is presented. The method consists of optically measuring small shifts at the fundamental absorption edge when a thin sample wafer is heated by an incident laser beam. Experimental results are presented for CdTe. The technique is simple and sensitive; it offers several advantages over methods of standard laser calorimetry.
Thermal oxidation of GaAs
Authors: Murarka S. P.
Published: February 2003

Thermal oxidation of GaAs has been carried out at 500 °C in air. Uniform amorphous oxide films as thick as 3.4 μm were grown. This oxide, which adheres well to the substrate and can be etched off in hydrochloric acid, shows promise as a masking oxide for GaAs.
On etching very narrow grooves in silicon
Authors: Kendall Don L.
Published: February 2003

The etch rate on the {110} is shown to be at least 400 times faster than that on the {111} using 44% KOH : H2O at a temperature of 85 °C and below. A model is presented which attributes essentially all the etching on the near {111} planes to the misorientation ledges. Grooves of 0.6−μm width and 44−μm depth have been produced. Such grooves have been used to make high−value SiO2 capacitors and vertical multijunction solar cells.
The effects of degeneracy on doping efficiency for n−type implants in GaAs
Authors: Gibbons J. F. , Tremain R. E.
Published: February 2003

The effect of degeneracy on doping efficiency for n−type implants in GaAs is computed by assuming the donor energy level to be coincident with the bottom of the conduction band. For such a case the Fermi level will always lie in the conduction band, and as a result the sheet carrier concentration can be substantially less than the implanted dose. Computations are given which show that peak carrier concentrations cannot readily exceed ∼5×1018/cm3 and doping efficiency (as conventionally measured) can be as low as 5−10% under common implantation conditions.
Dynamic conversion of magnetic bubble domain walls at megahertz frequencies in epitaxial garnet films
Authors: Hagedorn F. B. , Blank S. L. , Peirce R. J.
Published: February 2003

We have measured the circuit propagation characteristics of magnetic bubble domains in 15 different epitaxial films of Y1.9Sm0.1Ca1Ge1Fe4O12, using a TX propagation circuit with 28.8−μm period. For film thicknesses of 2 μm or less, reliable propagation was observed up to 1.7 MHz. Thicker films ceased to propagate reliably at frequencies as low as 0.5 MHz. Propagation failure is attributed to dynamic conversion of the bubble domain walls, and reasonably good quantitative agreement is shown between our measured critical frequencies and the critical frequencies calculated from a recently published model for dynamic conversion.
High−performance acousto−optic guided−light−beam device using two tilting surface acoustic waves
Authors: Tsai C. S. , Nguyen Le T. , Yao S. K. , Alhaider M. A.
Published: February 2003

An acousto−optic guided−light−beam device utilizing two surface acoustic wave interdigital transducers, which are staggered in their center frequency and tilted in their propagation direction, on an essentially single−mode optical guiding layer of a Y−cut LiNbO3 substrate has been shown to be capable of providing a very wide bandwidth. Deatiled measurements on the diffraction efficiency and the deflection angle of the light beam versus the frequency of the driving signal, usign a 6328−Å He−Ne laser light, are presented. The measured −3 dB bandwidth of the device is close to 200 MHz, which is more than an order of magnitude larger than that obtained in previous devices, with the measured electric driving power of 200 mW at 50% diffraction efficiency. The new device configuration introduced here should be very useful for wide−band applications such as a guided−wave acousto−optic rf spectrum analyzer and high−speed multiport switches for fiber/integrated optics.
Starting times of laser−induced intrinsic damage in NaCl
Authors: Kelly Paul , Bräunlich P. , Schmid A.
Published: February 2003

The starting time for laser−induced intrinsic damage in NaCl is calculated as a function of the rms optical field strength at the peak of the laser pulse. Three different damage mechanisms are considered: (i) avalanche ionization for λ⩾1 μ, (ii) five−photon assisted avalanche ionization at λ=0.694 μ, and (iii) three−photon assisted avalanche ionization at λ=0.347 μ. It is shown that the probability to measure the true damage threshold E*th is zero. (E*th is the optical peak field strength of a laser pulse that heats a small sample volume so that the melting point is reached exactly at the end of the pulse.) For this reason a ’’practical’’ damage threshold E*T is defined which is the peak field strength of a pulse that causes damage by melting at the time the laser flux reaches its peak.
p−InP/n−CdS solar cells and photovoltaic detectors
Authors: Wagner Sigurd , Shay J. L. , Bachmann K. J. , Buehler E.
Published: February 2003

We have prepared p−InP/n−CdS heterodiode photovoltaic detectors with a uniform quantum efficiency of ∼70% for wavelengths between 550 and 910 nm. On a cloudy day in New Jersey, (53 mW/cm2) solar power conversion efficiencies of 12.5% have been measured on cells provided with antireflection coatings.
Optically pumped cw InSb lasers for NO spectroscopy
Authors: Pine A. S. , Menyuk N.
Published: February 2003

Tunable cw laser emission has been obtained near 5.3 μm from InSb samples at ∼10 K optically pumped with GaAs, Nd : YAG, and CO lasers. InSb laser thresholds have been determined as a function of incident pump wavelength and applied magnetic fields from 0 to 9.5 kOe. Temperature and magnetic field tuning have been utilized to obtain NO absorption spectra over a 10−cm−1 range.
FM noise of high−efficiency GaAs IMPATT oscillators and amplifiers
Authors: Mircea A. , Constant E. , Perrichon R.
Published: February 2003

It is found experimentally that the FM noise deviation and noise measure of high−efficiency Read−type GaAs oscillators improve with increasing output power, in contrast with the behavior of conventional (flat doping profile) devices of similar technology. It is suggested that this novel potentially attractive feature may be due to tunnel carrier generation in the Read−type device.
Electron trapping at positively charged centers in SiO2
Authors: Ning T. H. , Osburn C. M. , Yu H. N.
Published: February 2003

Evidence is presented which indicates that positive oxide charge centers in thin films of thermally grown silicon dioxide are electron traps with an average capture cross section of 3±2×10−13 cm2 at room temperature and at an average oxide field of about 7×105 V/cm. Positive charge centers of other origins are also expected to be electron traps with about the same capture cross section.
Effects of diffusion on gap− and strip−coupled acoustic surface wave convolvers
Authors: Otto O. W.
Published: February 2003

Diffusion in gap− and strip−coupled acoustic surface wave convolvers reduces the coupling of the surface wave to the semiconductor. It is shown theoretically, however, that the optimized convolver coupling coefficient, which is proportional to the ratio of the convolver coupling coefficient and the acoustoelectric attenuation, actually increases in the presence of diffusion. An equivalence between the gap− and strip−coupled devices is established so that the same conclusions are applicable to both.
Reduced gain of ion−implanted transistors
Authors: Nicholas K. H. , Ford R. A. , Daniel P. J. , Sullivan C. W. , Sant P. , Bull C. , Booker G. R.
Published: February 2003

Ion implantation is increasingly being used as a deposition technique because of the good uniformity and reproducibility that can be obtained with it. In this letter it is reported that lower gains of bipolar transistors can result from this use of implantation rather than diffusion for depositions. The degradation of gain is shown to be a lifetime effect associated with crystal damage. A technique for avoiding the degradation is described.
Electro−optic intensity modulation in LiTaO3 ridge waveguide
Authors: Noda Juichi , Uchida Naoya , Minakata Makoto , Saku Tadashi , Saito Shoichi , Ohmachi Yoshiro
Published: February 2003

A narrow rectangular ridge for 24 μm (width of the top) ×6 μm (height) ×5.4 mm (length) has been formed on Cu−diffused x−plate LiTaO3 using the sputter−etching technique. The intensity modulation of the optical wave confined in the ridge has been made up to 1 GHz with a half−wave voltage of 25−30 V at 6328 Å.
Electrically based spectral power measurements through use of a tunable cw laser
Authors: Geist Jon , Steiner Bruce , Schaefer Russell , Zalewski Edward , Corrons Antonio
Published: February 2003

A new approach to radiant power measurements is described. A continuously tunable cw dye laser was used to measure the absolute spectral response of a silicon photodiode and narrow band−pass filter by comparison with an electrically calibrated pyroelectric detector. The filtered photodetector was then used to measure the spectral power density from a standard lamp that had been calibrated by the classical technique. The agreement of better than 1% between the two measurements is well within the uncertainties identified with each measurement individually, of the order of 1%. A number of advantages in the new technique are identified.
Single−crystal fiber optical devices: A Nd:YAG fiber laser
Authors: Burrus C. A. , Stone J.
Published: February 2003

Single−crystal fibers of Nd:YAG have been grown in diameters as small as, but not limited to, 50 μm by a pedestal growth (modified zone melting) technique. Room−temperature cw laser operation has been achieved with 0.5−1−cm−long samples. The method is directly applicable to the growth of single−crystal fibers of various materials for devices covering a wide range of applications, especially for optical communications.
Response of thermal filaments in VO2 to laser−produced thermal perturbations
Authors: Jelks E. C. , Walser R. M. , Bené R. W. , Neal II W. H.
Published: February 2003

The response of thermal filaments in thin films of VO2 to small thermal perturbations induced by a He−Ne laser has been measured. With the aid of additional noise spectrum measurements the senstivity of a planar device with a 127−μ gap between electrodes is estimated to be approximately 0.6×10−9 W/(Hz)1/2 at 40 kHz. These devices appear, therefore, to be attractive as fast thermal detectors and may have applications as elements in high−density arrays. Radiance profiles of a thermal filament are also given.
Glass−sealed GaAs−AlGaAs transmission photocathode
Authors: Antypas G. A. , Edgecumbe J.
Published: February 2003

A GaAs/GaAlAs/GaAs/GaAlAs heterostructure has been prepared on a GaAs susbtrate, bonded to 7056 Corning glass, and the substrate and first AlGaAs removed chemically, utilizing the differential etching characteristics of GaAs and AlGaAs in NH4OH−H2O2 and HF solutions. The resulting structure of GaAs/AlGaAs/glass has excellent layer morphology, uniform thickness, and good transmission photocathode performance.
Rotational coupling and laser dynamics in TE CO2 lasers
Authors: Ballik E. A. , Garside B. K. , Reid J.
Published: February 2003

A model of the gain−switched TE CO2 laser is presented which explicitly includes rotational relaxation within the sublevels of the 10.4−μ laser band. Careful comparison with experiment is made employing a Q−switched cavity, and it is shown that rotational coupling dominates the laser dynamics at pressures below 150 Torr. Calculations demonstrate that the effect of rotational coupling is very significant even at atmospheric pressure, and its inclusion in the laser dynamical equations accounts for most of the difficulties obtained with previous models.
GaxIn1−xP−GayAl1−yAs heterojunction close−confinement injection laser
Authors: Schul G. , Mischel P.
Published: February 2003

Heterojunction laser diodes with GaxIn1−xP active layer have been fabricated from GaAs−Ga0.51In0.49P−Ga0.32 Al0.68As−GaAs heterostructures made by liquid phase epitaxy from Ga−rich and In−rich solutions. Stripe−geometry lasers have been operated at 77 K with threshold current densities as low as 2500 A/cm2 with an emission wavelength of λ=638 nm.
Thin−film optical magnetic mode converters
Authors: Henry R. D.
Published: February 2003

A new type of thin−film magneto−optic mode converter is described which has the potential to perform both reciprocal and nonreciprocal mode conversion. As a reciprocal mode converter, the fabrication of the device is straightforward and presents some features not available in previous designs. The same basic structure can be used as a starting point to attack the problem of nonreciprocal devices.
Hole mobility and transport in thin SiO2 films
Authors: Hughes R. C.
Published: February 2003

The first measurement of the hole mobility and its temperature dependence in thermally grown SiO2 on Si is reported and found to follow μ≃20 exp(−0.6eV/kT) cm2/V sec. In agreement with previous studies, the energy required to form an electron−hole pair with ionizing radiation is found to be field dependent and at very high fields is in the range of 18 eV/electron−hole pair, and is nearly temperature independent from 77 to 370 °K.
High−efficiency graded band−gap AlxGa1−xAs−GaAs solar cell
Authors: Hutchby James A.
Published: February 2003

A detailed theoretical analysis of an n−on−p graded band−gap AlxGa1−xAs−GaAs solar cell yields a maximum air mass zero power conversion efficiency of 17% compared to 9% for a similar GaAs cell. The analaysis includes surface and bulk minority carrier recombination, junction recombination current, spectrally varying surface reflection, and series resistance loss. The maximum efficiency is determined for a surface recombination velocity of 1×105 cm/sec and hole and electron diffusion lengths of 2.1 and 7.6 μm, respectively. The improved efficiency is primarily due to a built−in electric field, caused by the band−gap gradation, accelerating photogenerated holes toward the p−n junction. This field reduces the surface and bulk recombination of the holes, and thereby enhances their collection.
Transmission properties of a low−loss near−parabolic−index fiber
Authors: Cohen L. G. , Kaiser P. , Mac Chesney J. B. , O’Connor P. B. , Presby H. M.
Published: February 2003

Pulse dispersion, refractive−index profile, and loss measurements were made on a low−loss graded−index optical fiber fabricated by a chemical vapor deposition technique. The transmission loss spectra had a minimum value of 3.8 dB/km at 1.06−μm wavelength. The core refractive index had a power−law profile with an exponential coefficient of 2.2±0.1. Mode−mixing effects and the near−parabolic−index profile in a 5.6−km extrapolated fiber length (actual length =1.12 km) reduces modal dispersion by a factor of 17 from the result expected for an ungraded step−index fiber.
Polarity effects in high−pressure z pinch
Authors: Preston J. M. , Curzon F. L.
Published: February 2003

Significant differences in the electron density and temperature of the pinching plasma in a z pinch in helium are caused simply by reversing the polarity of the applied voltage. These effects are attributed to differences in the breakdown mechanism.
Transverse self-sustained discharge CO2 laser
Authors: Cohn David B.
Published: February 2003

A scalable transverse self-sustained discharge maintained between an array of flow-stabilized cathodes and a common anode has demonstrated specific input powers of 800 W/g/sec and power densities of 100 W/cm3 per cathode for CO2-N2-He mixtures at pressures of 50–70 Torr. Optical extraction at 15% efficiency has been obtained under nonoptimized conditions in a geometry with the parallel fluid flow and discharge electric field perpendicular to the optical axis.
Polarization effects in four-photon conductivity in quartz
Authors: Venable Demetrius D. , Kay R. B.
Published: February 2003

We report on photoconductivity in SiO2 induced by the absorption of four Nd-glass laser photons in the intensity domain of 108 W/cm2. The relative cross section for circular versus linear polarization of the laser photons is found to be 2.05±0.9.
Application of scanning electron microscopy to determination of surface recombination velocity: GaAs
Authors: Jastrzebski L. , Lagowski J. , Gatos H. C.
Published: February 2003

A method is reported for the determination of the surface recombination velocity by scanning electron microscopy; this method is based on an established relationship between the effective diffusion length of the minority carriers, the penetration depth of the electron beam, and the surface recombination velocity. Values of surface recombination velocity, up to about 2.5×106 cm/sec were determined in n-type GaAs with a bulk minority-carrier lifetime of the order of 10−8–10−10 sec; in GaAs, with carrier concentrations exceeding 1018 cm−3, recombination velocity of about 3×106 cm/sec represents a saturation value.
Efficient strip-waveguide modulator
Authors: Kaminow I. P. , Stulz L. W. , Turner E. H.
Published: February 2003

A strip-waveguide electro-optic phase modulator has been fabricated in Ti-diffused LiNbO3. For a modulation index of 1 rad at 0.63 μm, the modulating voltage is 0.3 V and the power is 1.7 μW/MHz of bandwidth.
Hopping conduction in Langmuir films
Authors: Sugi M. , Fukui T. , Iizima S.
Published: February 2003

The dc conductance of Langmuir films of the Cd salts of fatty acids CH3- (CH2)n-2-COOH, with various n, has been measured at liquid-nitrogen temperature. The conductivity decreases exponentially with increasing chain length of the fatty acid molecules. This agrees well with the theoretical prediction for hopping conduction in a multilayer system associated with interface states.
Tunable cw difference-frequency generation in tellurium at ∼11 μm
Authors: Bridges T. J. , Nguyen V. T. , Burkhardt E. G. , Patel C. K. N.
Published: February 2003

The narrow spectral width and the nearly continuous tunability of the cw spin flip Raman (SFR) laser operating near 5.3 μm can be transferred to other wavelengths from 5 to 30 μm by difference-frequency mixing in tellurium. In a demonstration experiment, the SFR laser output is mixed with a fixed-frequency CO2 laser line to give cw tunable power output near 11 μm. An example of high-resolution spectroscopy possible with this new source is shown by the spectrum of NH3 which indicates a linewidth of ⩽0.01 cm−1.
Improved composite-processed V3Ga through partial substitution of aluminum for gallium in the matrix
Authors: Yoshida Y. , Tachikawa K. , Iwasa Y.
Published: February 2003

The effects of the partial substitution of aluminum for gallium in the matrix on the superconducting properties of composite-processed V3Ga have been investigated. At an aluminum concentration of 2–3% there is considerable increase in Jc and a slight increase in Hc2.
Efficient continuum sources for x-ray lithography
Authors: Greeneich James S.
Published: February 2003

Efficient x-ray sources whose dominant output is from the x-ray continuum are examined experimentally for use in x-ray lithography. At 10 kV with a 6-μm Mylar mask and 50-μm Be window, a Au continuum source is more than 3 times as efficient as the Al Kα source. Submicrometer resolution can be obtained with x-ray continuum sources.
D2-CO2 and D2-N2O electric discharge gas-dynamic lasers
Authors: Stregack J. A. , Wexler B. L. , Hart G. A.
Published: February 2003

cw CO2 and N2O lasing has been obtained in a new electric discharge gas-dynamic laser device. The device consists of a subsonic plenum discharge region where vibrational excitation occurs, a supersonic nozzle array where the laser species is injected, and a supersonic cavity where mixing, energy transfer, and lasing occur. In the present experiments both CO2 and N2O are pumped by energy transfer from D2 excited in the discharge. Comparisons are made for CO2 pumped by both N2 and D2, and N2O and CO2 pumped by D2.
Statistical model for coalescence of islands in discontinuous films
Authors: Granqvist C. G. , Buhrman R. A.
Published: February 2003

A simple statistical model for coalescence of islands in discontinuous films is formulated. It predicts log-normal size distributions, which appears to be well corroborated by experiments.
Determination of energy band and surface-state locations in GaAs using the separated-medium surface-acoustoelectric effect
Authors: Gilboa H. , Motamedi M. E. , Das P.
Published: February 2003

The wavelength dependence of the peak transverse acoustoelectric voltage and the surface acoustic wave attenuation determine the energy band and the locations of the surface states in the energy gap of GaAs. The dependence is explained by direct optical transition between the bands and optical transition from surface states. This method needs no contacts to the semiconductor surface, is simple, very sensitive, and accurate.
Luminescence due to iron centers in lithium niobate
Authors: Parsons R. , Cornish W. D. , Young L.
Published: February 2003

A luminescence band at 770 nm due to iron centers has been observed in lithium niobate crystals treated at 520 °C in lithium carbonate. Without this treatment only a broad background luminescence was observed. The main effect of the treatment is to reduce iron centers to the Fe2+ state but some evidence was found that destruction of shallow traps is also involved in increasing the luminescence.
Low-energy photoionization cross sections from proton-induced x-ray spectroscopy
Authors: Lurio A. , Reuter W.
Published: February 2003

It is shown that photoionization cross sections in the soft-x-ray region can be determined accurately with proton-induced x-ray spectroscopy. A film of the absorber is evaporated onto a substrate. The characteristic substrate radiation, generated with a proton beam, is measured with and without the absorber. The ionization cross section can then be calculated after a small and often negligible correction is made for the intensity change of the substrate radiation due to the energy loss of the proton beam passing through the absorber.