摘要:

The capability of diffraction intensity fluctuations to probe dynamic phenomena at equilibrium is studied. The summation of fluctuations over different domains that scatter incoherently, the role of statistically “white-noise” generated fluctuations and the time necessary to collect a sufficiently long signal are investigated. The factors that determine the number of incoherent regions, and maximize the relative fluctuation without reducing the count rate to such low values that impractically long acquisition times are needed are discussed. Monte Carlo simulations modeling thermally generated step fluctuations are used to illustrate the results. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365950

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Two-dimensional dipole (uniform) fields produced by parallel currents are generally considered to have two symmetry planes, with a current direction reversal in reflection across one plane. Similarly, quadrupoles have four symmetry planes, and so on for higher multipoles. Expansion of the fields of filamentary currents shows that there are lower symmetry multipoles with highly uniform field characteristics. These solutions include configurations in which there is no current reversal across reflection planes, producing unipolar-current dipoles, quadrupoles, and other multipoles. In all cases, the solutions can yield systems that have the best uniformity of field characteristics (minimum undesired multipole content over the largest area) that can be achieved with the number of wires used, although the unipolar systems are less efficient and produce a lower field magnitude for a given current. These optimum solutions are not limited to circular configurations, or to systems with all wire current magnitudes identical. The configurations are easily extended by summation or integration to wire bundles or finite radius conductors. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365949

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Optical gains in wurtzite–GaN strained quantum-well (QW) lasers are estimated theoretically for various crystallographic directions. The calculation of the valence subbands is based on thek⋅ptheory, where deformation potentials are determined by a semiempirical tight-binding method. It is found that the gains in GaN strained QW lasers with non-(0001) orientations, particularly around the (101¯5) orientation, are markedly high and anisotropic, unlike those in (0001)-oriented lasers. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365951

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Binary diffraction gratings inAs2S3films were prepared with the aid of an electron beam. The dose of the electrons was varied. The gratings were read with a HeNe laser, and the zero-order and first-order diffraction efficiencies were noted. Rigorous diffraction theory was used to give a homogeneous approximation for the refractive index change. Reactive ion etching was applied to reduce the thickness of the film, and a new value for the refractive index change was evaluated. The refractive index change versus film thickness dependence was found to be linear at low electron doses and Gaussian shaped at higher doses. Through a simple mathematical analysis, the absolute value of the refractive index was determined as a function of the position inside the film. At higher doses, the refractive index change was found to have a maximum value of 3&percent;, approximately 1.2 &mgr;m from the film surface. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365952

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Irradiation of an absorbing material with a short laser pulse generates a thermoelastic stress wave, from which the distribution of absorbed energy can be derived. This method is ideal to measure the light penetration in biological tissue. Especially forin vivoapplications, we developed an optical stress transducer that can be positioned directly in front of the irradiated surface, inside the laser beam, in order to avoid distortion of the stress wave due to acoustic diffraction. The detector is based on stress wave-induced changes of optical reflectance of a glass-water interface, probed with a continuous laser beam that is incident at an angle close to the critical angle of total internal reflection to achieve maximum sensitivity. In this study, we describe the theory for the calibration of the transducer and compare the measured with the theoretically predicted signals. In the experiments, an aqueous dye solution is irradiated with pulses from either aQ-switched, frequency-doubled Nd:yttrium aluminum garnet (YAG) laser or from an optical parametric oscillator with pulse durations of 8 and 6 ns, respectively. Good agreement between the measured and calculated waveforms as well as the possibility to obtain photoacoustic absorption spectra from the shape of the recorded signals is demonstrated. From our experimental and theoretical findings, it follows that the detector is characterized by a high temporal and spatial resolution and by an adjustable sensitivity, depending on the incident angle of the probe beam at the glass-water interface. Apart from the applications proposed in the present work, it seems to be possible to use this kind of transducer for the two-dimensional recording of stress waves. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365953

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

In this article, we concentrate on the optical penetration and laser pulse duration effects on the features of the ultrasonic waves generated in a solid by a laser impact. We consider the simple case of a uniform irradiation of the sample, which we describe with the help of a simple analytical one-dimensional model. In spite of its simplicity, this model clearly highlights the competition that occurs between two temporal convolution sources, related to the optical penetration and the laser pulse duration, respectively, to produce the longitudinal arrivals on the rear side of the sample. The model is also used for an accurate description of the features of the longitudinal precursor. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365954

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The dynamics of a vacuum-arc operated rail-gun plasma were numerically simulated for the undercritically damped regime and confirmed in experiments. Analytical solutions of arc current, its velocity, and position also showed good agreement with the results. The spatial development of the arc is shown here to follow a linear behavior, even though the propulsion velocity followed an exponential-step function due to the sinusoidal nature of the arc current. Peak arc current∼100 kA,15 &mgr;s period was propelled to∼106 cm/sas indicated in time-of-flight measurements and simulations. The voltage signal induced on aB-dot probe by the dynamic sinusoidal arc was simulated, which compared well with the measured probe signal. A Gaussian current distribution inside the measured arc length∼1–2 cm,was established by unfolding theB-dot probe signal. A peak magnetic field of the order 1–5 kG was also interpreted inside the arc. The emerging plasma contained ion current over tens of amperes in the pulse duration encompassing the arc envelope. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366275

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Particle trapping in different areas of a parallel-plate, radio frequency silane discharge, and its effect on plasma optical emission of SiH andH&agr;,has been studied under high gas-flow and low power-density conditions, as used for “device-quality” hydrogenated amorphous silicon(a-Si:H) film deposition. The largest density of particles occurs between the electrodes, near the downstream corners of the rectangular electrodes. Particles are trapped in these positions by sheath fields, until reaching sufficient size to escape with the flow. The region of strong particle trapping has an increased intensity of optical emission, withH&agr;increased nearly fourfold. Slow oscillatory behavior of particle scattering and discharge emission was observed for pressures near 30 Pa. Power deposited in the discharge has also been measured; for a constant rf voltage and gas-flow speed it changes weakly with pressure, with the maximum at ∼40 Pa. Combined with film growth-rate measurements, this yields a discharge energy deposition of ∼17 eV per deposited Si atom. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365955

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Macrostrain variations in engineering components are frequently examined using neutron diffraction, at both reactors and pulsed sources. It is desirable to minimize the sampling volume in order to maximize the spatial resolution, although this increases the required measurement time. At reactors, macrostrain behavior is inferred from a single lattice reflection (deemed to be representative of the bulk response). At a pulsed source, a complete diffraction pattern is recorded and accordingly it is natural to fit the entire diffraction spectra using a Rietveld [J. Appl. Cryst.2, 65 (1969)] refinement. This means that an idealized crystal structure is fit to the measured distorted crystal structure, which includes deviation of the measured lattice reflections from the ideal due to elastoplastic strain anisotropies, which are dependent on the particular lattice reflection (hkl) considered. We show that elastic macrostrains calculated from lattice parameter changes in Rietveld refinements (without accounting for hkl dependent anisotropies) are almost identical to the bulk elastic response and are comparable to the response obtained from a single lattice reflection typically used by practitioners at a steady state source. Moreover good refinements on the complete pattern are obtained with short measurement times compared to what is required for good statistics for single reflections. By incorporating a description of the elastic strain anisotropy expected in cubic materials into the Rietveld code, an empirical prediction of plastic strain history is possible. The validity of these arguments is demonstrated by analysis of a uniaxial tensile load test and a reanalysis of previously reported data taken on a deformed stainless steel ring. The plastic strain predictions compare favorably with a finite element model. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365956

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Plastic deformation in submicron wide Al-1 wt &percent;Cu interconnects was studiedin situusing a straining device in the transmission electron microscope. Dislocation motion occurred readily in unpassivated lines but was nonexistent in passivated lines due to the presence of stiff oxide sidewalls. Instead heterogeneous void nucleation occurred on straining to a critical limit. The void morphology was always near hemispherical and the nucleation always took place at the line edges. Further stretching of the lines led to a rupture of the sidewalls away from the lines, resulting in immediate dislocation motion. Void nucleation, cross slip, and operation of dislocation sources at line edges were all recorded on video. It was noted that dislocations almost parallel to the plane of the lines are rarely observed and furthermore, their movement is sluggish. Based on the dislocation configuration observed in these lines, a generalized geometrical model was arrived at in order to determine the significance of grain orientation on yield stress of passivated lines with columnar, bamboo grains. Frequent occurrence of twinning within the grains indicated that plastic deformation was indeed restricted in confined metal lines. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365957

出版商:AIP    

年代:1997

数据来源: AIP