摘要:

The thermal rms fluctuation in position of a levitated microparticle in a Paul trap in N2near standard temperature and pressure is calculated numerically from a stochastic equation of motion and is measured experimentally by long term imaging. The calculations reveal loci of minima within the lowest stability region and enhanced fluctuations as the first instability is approached. Pseudopotential results deviate below our calculations progressively with increased drive potential. This disparity grows by more than an order of magnitude near instabilities, although the pseudopotential approach provides a good asymptotic approximation for small drive potentials. Long term imaging experiments are performed in a trap modified to eliminate stray static fields at its ac ‘‘null’’ point. These experiments are well‐described by the stochastic model and suggest the use of such a trap as a stage for long term fluorescence and Raman microphotography experiments on individual microparticles in a gaseous environment.

ISSN:0021-8979    

DOI:10.1063/1.354392

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Periodically segmented waveguides were fabricated by the ion exchange process in KTiOPO4. The waveguides were characterized by measuring the spectra of the transmitted and reflected guided infrared light and of the second‐harmonic‐generated power. The near‐field distribution of the fundamental and second‐harmonic wave was also investigated. Infrared to blue light conversion efficiency up to 250% W−1 cm−2was obtained. Waveguide characteristics were calculated by replacing the segmented structure with an equivalent continuous waveguide. Good agreement between the calculated and measured values was obtained.

ISSN:0021-8979    

DOI:10.1063/1.354393

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A magnetically embedded coaxial z‐pinch configuration is proposed for the resonant photoexcitation laser at 228 A˚ in Mg ixpumped by the 48.338 A˚ line of Al xi. This laser configuration consists of an annular Al z‐pinch plasma imploded (final electron density of about 1020cm−3) onto an inner, Mg plasma (final electron density of about 1018cm−3) on the axis. The Mg plasma is confined and stabilized by an axial magneticBzfield, which is compressed to a final magnitude of about 130 T from an initially applied field of about 1 T by the Al implosion, thus serving to stabilize and physically separate the Al and Mg plasmas. Separate numerical simulations of the Al and Mg plasmas designed to guide exploratory experiments are reported. A 0D dynamic z‐pinch model captures the Al implosion, including trappedBzfield and power losses due to line radiation. Initial conditions required to produce the intense Al xipump line at 48.338 A˚ are derived assuming an 800 kA, 100 ns implosion current pulse. Assuming a pinch length of 2 cm, a pump power of 3.5 GW/eV is predicted for the 48.338 A˚ Al xiline. With the pump so defined, a separate, detailed atomic physics model is used to study the photoexcitation kinetics in the Mg ixlasant plasma. It is found that a gain of 1.0–1.5 cm−1is obtainable on the 2s3d 1D2 ‐ 2s4f 1F3transition at 228 A˚ in Mg ix.

ISSN:0021-8979    

DOI:10.1063/1.354394

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Phase plates have been used in holography and laser fusion to produce a well‐defined far‐field intensity envelope that is relatively independent of the input‐beam profile. However, superposed on this envelope is unwanted highly modulated speckle from the interference between different phase‐plate elements. The speckle can have deleterious effects such as creating intensities in the nonlinear region of a hologram recording medium or by initiating filamentation and hydrodynamic instabilities in laser‐fusion applications. A technique has been developed for choosing the phase‐plate elements (in combination with polarization rotation) such that these intensity fluctuations approach zero in the limit of plane‐wave near‐field irradiation. For applications where the near‐field phase and amplitude have only small variations, the resulting speckle is also small. Conversely, the observation of far‐field intensity fluctuations can be used to estimate the quality of the near‐field beam.

ISSN:0021-8979    

DOI:10.1063/1.354395

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Analytical expressions for the upper bounds of power and efficiency of an internal combustion engine are obtained taking into account finite rate of heat exchange with the environment and nonzero entropy production due to chemical reactions. Recommendations of theoretically possible ways of improving internal combustion engines are presented.

ISSN:0021-8979    

DOI:10.1063/1.354396

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A Langmuir probe and a microwave interferometer have been combined to measure the electron density of argon glow discharges in the Gaseous Electronics Conference reference reactor [Bull. Am. Phys. Soc.36, 207 (1991)]. The two techniques indicate the same charge density at 100 mTorr to within 30%. This 30% difference is easily explained by the experimental peculiarities. While the predicted charge densities obtained from the two techniques track one another as the applied rf voltage is varied at constant pressure, they do not track one another as the pressure is varied at constant rf voltage. In fact, the charge densities predicted from the Langmuir probe using Laframboise’s theory (Tech. Rep. 100, Univ. Toronto Inst. Aerospace Study, 1966) are factors of 2 and 4 times lower than those from the interferometer at 250 and 500 mTorr, respectively. It appears that the probe alters the charge density in its vicinity when the probe radius becomes greater than the ion mean free path. The interferometer has also been used to investigate the macroscopic perturbation of the plasma electron density caused by the Langmuir probe. It was found that presence of the probe in a radio‐frequency discharge does not appreciably alter the electron density measured by the interferometer at a set rf voltage irrespective of the potential placed upon it. This was not the case for dc discharges in the same apparatus where the volume‐averaged electron density could be depleted by as much as 70% when the probe was biased even a few volts above plasma potential.

ISSN:0021-8979    

DOI:10.1063/1.354397

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A high‐temperature x‐ray‐diffractometer stage specifically designed for low‐angle work has been built. With careful alignment, the stage provides resolution sufficient to probe temperature‐induced structural changes in multilayers. A series of studies using the stage was performed on W/Cu multilayers with modulation wavelengths between 65 and 110 A˚ over the temperature range 25–180 °C. Reversible, temperature‐dependent changes in the modulation wavelength, and an irreversible increase in the intensity of the scattered radiation along both specular and nonspecular directions in reciprocal space, have been observed. The observed thermal‐expansion coefficients in the growth direction are marginally greater than would be expected from bulk behavior even when interactions with the Si substrate are taken into account. Analysis of the intensity changes indicates that, upon annealing, the interfaces within these multilayers become more rough, but that this roughness comes from a thermally induced buckling of the film. This results in a mosaic texture and increased reflectivity with a concomitant increase in diffuse scattering.

ISSN:0021-8979    

DOI:10.1063/1.354398

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Undopedn‐GaAs, grown by organometallic vapor phase epitaxy, was irradiated with neutrons from a clinicalp(66)/Be(40) source for a range of fluences. Deep level transient spectroscopy (DLTS), employing Pd Schottky barrier diodes, indicated that four electron traps,En1,En2,En4, andEn5, with energy levels at 0.04, 0.14, 0.36, and 0.66 eV, respectively, below the conduction band were created during neutron radiation. Their introduction rates varied from 1 cm−1for theEn1 to 11 cm−1for theEn5. It was found that theEn1,En2, andEn4 defects have DLTS ‘‘signatures’’ similar to theE1,E2, andE3 point defects introduced during high energy electron irradiation, indicating their point defect nature. TheEn5 has a very large capture cross section, its emission rate exhibits a strong electric field dependence, and there are indications that it has a band‐like energy distribution, that results in a broad DLTS peak. We speculate that this trap is related to the presence of extended defects in the neutron irradiated GaAs.

ISSN:0021-8979    

DOI:10.1063/1.354399

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Damages induced by 69 MeV oxygen ions (fluence: 1.3×1014ions cm−2) in gold–silicon‐nitride–silicon metal‐insulator‐semiconductor (MIS) structures were studied by measuring the pre‐ and postirradiation junction and interface characteristics. Amount of the space‐charge produced at the silicon–silicon‐nitride interface was estimated by measuring capacitance‐voltage characteristics and the energies of the defects were measured with the technique of deep‐level transient spectroscopy. Two new defect levels having energies (Ec−0.34) eV and (Ec−0.63) eV were found in addition to the defects detected in the unirradiated devices. The values of the surface recombination velocities, measured with a variable‐energy electron beam, indicate that the radiation‐induced defect concentration on the surface was negligible; however, about 7% degradation in the lifetime of minority carriers in the bulk was observed. The results indicate that oxygen ions can be used to modify the junction characteristics of MIS structures while keeping the surface characteristics of the structure almost unaltered.

ISSN:0021-8979    

DOI:10.1063/1.354400

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Ion‐beam synthesis of &bgr;‐FeSi2is demonstrated both in (111) Si and (001) Si substrates by 450 keV Fe ion implantation at elevated temperatures using a dose of 6×1017Fe/cm2and subsequent annealing at 900 °C. The structure of the buried layers has been analyzed using Rutherford backscattering spectrometry, x‐ray diffraction, and (cross‐section) transmission electron microscopy. In (111) Si an epitaxial layer is formed consisting of grains with lateral dimensions of approximately 5 &mgr;m. Epitaxy of &bgr;‐FeSi2(110) and/or (101) planes parallel to the (111) Si substrate plane is observed. In (001) Si a layer is formed consisting of grains with lateral dimensions of typically 0.5 &mgr;m. Several grain orientations have been observed in this material, among others &bgr;‐FeSi2{320}, {103}, and {13,7,0} parallel to (001) Si. Selected (111) Si samples were investigated optically using spectroscopic ellipsometry, and near‐infrared transmittance and reflectance spectroscopy. The results confirm that the &bgr;‐FeSi2layer has an optical band gap of 0.87 eV. The ellipsometry results indicate that the layers formed by ion‐beam synthesis are more dense than those formed by surface growth techniques. Hall measurements show that the &bgr;‐FeSi2layers obtained areptype. Mobilities observed are 1–4 cm2/V s at room temperature and approximately 25 cm2/V s at liquid‐nitrogen temperature. These results show that the electrical properties of ion‐beam‐synthesized &bgr;‐FeSi2is comparable with those of surface‐grown material. The results confirm that optoelectronic applications of &bgr;‐FeSi2are limited.

ISSN:0021-8979    

DOI:10.1063/1.354401

出版商:AIP    

年代:1993

数据来源: AIP