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  • 1995-1999  (138)
  • 1935-1939  (9)
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  • 11
    Electronic Resource
    Electronic Resource
    Polarization bistability in strained ridge-waveguide InGaAsP/InP lasers: Experiment and theory (1995)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 77 (1995), S. 6135-6144 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: New experimental and theoretical results on TE/TM bistability in 1.3 μm ridge-waveguide InGaAsP/InP bulk lasers at room temperature are presented. Measured polarization resolved light power–current (P–I) characteristics as well as lateral near- and far-field patterns are compared with results from a theoretical model based on the paraxial wave equations for TE- and TM-polarized modes and the diffusion equation for the carrier distribution. The model was numerically evaluated by use of the beam propagation method. The observed TE/TM bistability is explained by the interplay of three different effects: (i) Tensile stress of about 109 dyn/cm2 promotes the TM gain strongly enough to compete with the TE mode. (ii) Improved TM waveguiding due to an enhancement of the effective refractive index near the beam axis caused by carrier depletion with increasing current leads to the onset of TM lasing and TE/TM switching. (iii) The TE/TM transition is accompanied by an abrupt increase of spatial hole burning in the lateral carrier distribution. Because of this nonlinear effect, a lower current is needed to switch the laser back to TE, giving rise to a hysteresis loop in the P–I characteristics and to TE/TM polarization bistability. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.359139
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  • 12
    Electronic Resource
    Electronic Resource
    Control of light polarization in InGaAsP/InP lasers by injection of light pulses (1997)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 81 (1997), S. 2064-2069 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Light emission from a ridge waveguide 1.3 μm InGaAsP/InP semiconductor laser pumped both electrically and optically was analyzed by polarization- and time-resolved measurements. The electric and the optical excitation was realized with a dc-bias current and with 150 ps pulses from a Q-switched Nd-YAG laser at 1.064 μm wavelength, respectively. The pump light was introduced into the InGaAsP/InP laser through a window opened in the substrate gold contact. The steady-state P–I characteristics of the semiconductor laser exhibited a transition from TM- to TE-polarized light emission if the injection current surpasses a certain value that depends on the heatsink temperature. Depending on the dc-bias current and the optical pulse power, a variety of different emission characteristics of the semiconductor laser were observed: pure TE or TM pulsations, in combination with a background cw emission in some cases; simultaneous emission of TE and TM pulses and switching between TM and TE emission states with switching times as short as 30 ps. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.364257
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  • 13
    Electronic Resource
    Electronic Resource
    Time evolution of grating decay during photorefractive fixing processes in LiNbO3 (1995)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 77 (1995), S. 308-312 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Decays of the diffraction efficiency of gratings written in Fe:LiNbO3 have been studied in connection with photorefractive fixing. Measurements have been carried at five different temperatures within the range of interest in photorefractive fixing, 144–168 °C. The decay curves are nonexponential and can be fitted to a sum of three simple exponential components, whose effective activation energies and preexponential factors have been determined. The dependence of the last exponential component on the grating period Λ has been measured and a 1/Λ2 law has been found. The decay constants have also been found to depend on the time the crystal is kept at the setting temperature before writing the grating. The difficulties for establishing a model taking into account the occurrence of several sites for protons in LiNbO3 are discussed. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.359393
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  • 14
    Electronic Resource
    Electronic Resource
    Spectroscopic measurement of packaging-induced strains in quantum-well laser diodes (1999)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 86 (1999), S. 1196-1201 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Packaging-induced strain is studied in high-power semiconductor lasers by a noninvasive optical technique. Fourier-transform photocurrent measurements with intentionally strained laser array devices for 808 nm emission reveal spectral shifts of optical transitions within the active region. These shifts by up to 10 meV serve as a measure for the strain status within the active layer of the devices and are compared with model calculations. For different packaging architectures we quantify the strain portion which is transmitted to the optically active region of the semiconductor device. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.370870
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  • 15
    Electronic Resource
    Electronic Resource
    Adjustment of the critical current in a Nb–InxGa1−xAs/InP Josephson contact by light exposure (1999)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 75 (1999), S. 391-393 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The critical current of a Nb–InGaAs/InP Josephson junction is increased stepwise by light exposure. Shubnikov–de Haas effect measurements under illumination show that the increase of the critical current originates from photogenerated electrons in the quantum well. A further enhancement of the critical current is gained under continuous illumination. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.124385
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  • 16
    Electronic Resource
    Electronic Resource
    Diel and seasonal patterns of food intake and prey selection by Coregonus sp. in re-oligotrophicated Lake Lucerne, Switzerland (1998)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of fish biology 52 (1998), S. 0 
    Details
    ISSN: 1095-8649
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Feeding intensity by whitefish Coregonus sp., in oligotrophic Lake Lucerne in Switzerland was high during dusk when the bulk of potential prey items were in the depth zone occupied by the fish. Diet composition was fairly uniform throughout the day but changed substantially over the seasons. The fish fed opportunistically; differences between seasons reflected changes in prey availability. During the intensive feeding and growing period (May-September), fish were found in the upper 20 m of the lake feeding primarily on cladocerans. Large and non-evasive species, Daphnia spp. and Bythotrephes longimanus, were the most numerous and frequent organisms in the diet during the major part of the growing season. Smaller (Bosmina spp.) as well as evasive species (cyclopoid copepods) were consumed in large numbers when larger, non-evasive species were rare in the lake. The fish showed strong preference for the least abundant crustacean, B. longimanus, while the most abundant crustaceans, calanoid copepods, were rare in the diet. The fish not only selected particular species but, within each species, selected the larger individuals. Diel vertical migration of the prey items in this lake could be, at least in part, attributed to fish predation pressure. The observed selectivity patterns shown by the fish are explained in terms of prey visibility, escape ability, the overlap in distribution of predators and prey in time and space, the profitability of the prey and the present trophic state of the lake.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1095-8649.1998.tb02009.x
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  • 17
    Electronic Resource
    Electronic Resource
    Catechol 2,3-dioxygenase from the thermophilic, phenol-degrading Bacillus thermoleovorans strain A2 has unexpected low thermal stability (1999)
    Springer
    Extremophiles 3 (1999), S. 185-190 
    Details
    ISSN: 1433-4909
    Keywords: Key words Catechol 2 ; 3-dioxygenase ; Bacillus thermoleovorans ; Thermophilic ; Enzyme stability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Catechol 2,3-dioxygenase from the thermophilic Bacillus thermoleovorans A2 was purified and characterized. The catechol 2,3-dioxygenase has a molecular mass of 135 000 Da and consists of four identical subunits of 34 700 Da. One iron per enzyme subunit was detected using atom absorption spectroscopy. Enzyme activity was not inhibited by EDTA, suggesting that the iron is tightly bound. Addition of hydrogen peroxide to the enzyme completely destroyed activity, indicating that the iron was in the divalent state. The isoelectric point of the enzyme was 4.8. The enzyme displayed optimal activity at pH 7.2 and 70°C. The half-life of the catechol 2,3-dioxygenase at the optimum temperature was 1.5 min under aerobic conditions and 10 min in a nitrogen atmosphere. This stability of the enzyme is comparable to the stability of the enzyme from the mesophilic Pseudomonas putida mt-2. The stability of the cloned enzyme in E. coli extracts was identical to the stability in wild-type extracts, suggesting that no stabilizing factors were present in Bacillus thermoleovorans A2 In whole cells the half-life of the enzyme at 70°C was approximately 26 min, when protein synthesis was disrupted by chloramphenicol; however, the activity remained constant when protein synthesis was not inhibited. From these results we concluded that catechol 2,3-dioxygenase from Bacillus thermoleovorans A2 is not particularly thermostable, but that the organism retains the ability to degrade phenol at high temperatures because of continuous production of this enzyme.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s007920050115
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  • 18
    Electronic Resource
    Electronic Resource
    A Microtomographic System for the Nondestructive Evaluation of Bone Architecture (1996)
    Springer
    Calcified tissue international 58 (1996), S. 24-29 
    Details
    ISSN: 1432-0827
    Keywords: Key words: Microtomography — Computed tomography — Bone structure — Bone strength — Osteoporosis.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine , Physics
    Notes: Abstract. Microtomography (micro-computed-tomography, μ-CT) is a method to image and quantify trabecular bone. It has the capability to address the role of trabecular architecture on the mechanical properties of bone and to study trabecular bone remodeling. The system described in this work is based on a compact fan-beam type tomograph that can work in spiral scanning or multislice mode. An X-ray tube with a microfocus is used as a source, a CCD-array as a detector. Samples with diameters from a few millimeters to a maximum of 14 mm can be measured, typically, bone biopsies with a diameter of 8 mm and a length of approximately 10 mm are measured. Spatial resolution is 28 μm. Usually the volume of interest contains 4 × 4 × 4 mm3 and is represented in 14 × 14 × 14 μm3 voxels. 3D stereological indices are extracted according to the standard definitions used in histomorphometry. Triangular surface representation is effected with an extended marching cube algorithm and forms a convenient basis for finite element analysis. Microtomographic measurements may be employed to ``calibrate'' lower-dose, lower-resolution images in vivo as well as to nondestructively assess unprocessed surgical bone biopsy specimens. These specimens remain intact for mechanical or histological testing.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002239900006
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  • 19
    Electronic Resource
    Electronic Resource
    Prevalence of Trabecular Microcallus Formation in the Vertebral Body and the Femoral Neck (1997)
    Springer
    Calcified tissue international 60 (1997), S. 479 -484 
    Details
    ISSN: 1432-0827
    Keywords: Key words: Trabecular microcallus formation — Vertebra — Femur — Histomorphometry — Bone density — Osteoporosis.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine , Physics
    Notes: Abstract. Trabecular microcallus formation (TMF) has been described previously in the human vertebra and femur, but the difference in TMF prevalence at these two sites has not been studied and the role of TMF remains controversial. In this study, the 4th lumbar vertebra (L4) and right proximal femur were removed from 27 male and 23 female cadavers. A 2 cm cube cut from the center of L4 and a 1 cm-thick slice cut from the femoral neck were cleaned, defatted, and dried. The apparent density of the L4 cubes was determined as dry weight/bulk bone volume. Using a dissecting microscope at low magnification (4–60×), TMF were identified and counted in both the vertebral and femoral samples. A 8 mm diameter core was then cut from the center of the L4 cubes in the vertical direction, and selected histomorphometric parameters of the core were evaluated with an X-ray microcomputed tomography system (μ-CT). There was a significantly greater prevalence of TMF in vertebral cubes (82%) than in the femoral slices (11%) (P 〈 0.001). TMF prevalence did not differ significantly between males and females, but the mean number of TMF in the vertebra was significantly (P 〈 0.05) greater in females (15.0/vertebra) than in males (7.7/vertebra). In the vertebra, the majority of the observed TMF were in vertical trabeculae. Subjects over 60 years old had a higher TMF prevalence than those under 60 years old (P 〈 0.01). TMF numbers increased with decreasing apparent density (P 〈 0.05), whereas no significant correlations were found between TMF and bone volume (BV/TV), trabecular number (Tb.N), or trabecular thickness (Tb.Th) as assessed by μ-CT. In two fractured vertebra, very few TMFs (2 and 4, respectively) were observed. These results demonstrated that the occurrence of TMF is strongly related to the anatomical site, probably due to differences in the applied loads and the trabecular structure between sites. The results were consistent with the hypothesis that TMF is a mechanism acting to maintain bone strength, but further studies are needed to clarify this important issue.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002239900266
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  • 20
    Electronic Resource
    Electronic Resource
    Validity and reproducibility of benthic cover estimates made during broadscale surveys of coral reefs by manta tow (1999)
    Springer
    Coral reefs 18 (1999), S. 353-356 
    Details
    ISSN: 1432-0975
    Keywords: Key words Coral Reef ; Manta tow ; Survey ; Benthic video ; Observer ; Comparisons
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Geosciences
    Notes: Abstract  There is limited information on the validity and reproducibility of estimates of benthic cover from manta tow surveys. To address this, benthic cover estimates from the same reef area were compared (i) among observers and (ii) with an independent assessment using under-water video. Benthic cover was classified into 11 categories. There was generally unbiased agreement within one cover category, both among observers (89%) and for comparisons between manta tow and video (86%). While estimates of dead coral cover were reproducible, they were not valid because the concordance between observer estimates and video estimates was not greater than would be expected by chance. Manta tow estimates of the cover of sand and rubble were biased in that they consistently overestimated sand and rubble cover in comparison with estimates from video. The results indicate that manta towing is generally effective for the broadscale estimation of live coral cover, providing observers receive adequate training.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s003380050212
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