摘要:

Positron annihilation lifetime spectroscopy is a powerful method for characterizing free volumes in a variety of materials. Correlations between positron annihilation rates and the size of free‐volume regions in whicho‐Ps localizes are well described. Unfortunately, difficulties in the analysis of positron annihilation lifetime data have limited the approach to the determination of average lifetimes and average free volumes. Recent advances in the development of numerical integral transform methods now make it possible to extract continuous distributions of positron annihilation rates. The application of these methods to the determination of free‐volume distributions is described. The variable transformations required to convert positron annihilation rate probability density functions (PDF) to the corresponding lifetime, radius, and free‐volume PDFs are given and the approach is illustrated by application to positron annihilation lifetime data for amorphous polytetrafluoroethylene.

ISSN:0021-8979    

DOI:10.1063/1.349057

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Photon‐gated persistent spectral hole burning was observed for metal‐free tetraphenylporphine with halogenated anthracene derivatives in poly(methylmethacrylate) or poly(ethylene) at liquid‐helium temperatures. The hole formation yield was markedly dependent on the polymer matrices and the sample composition as well as on the gating wavelength. The irreversible broadening of holes in the systems measured by temperature cycling experiment was smaller than that by proton tautomerization of tetraphenylporphine. We concluded that the hole formation mechanism is donor‐acceptor electron transfer on the basis of combined analysis of the matix and acceptor concentration dependence of hole formation yield, the action spectrum of the gating photon and the photoproduct spectrum.

ISSN:0021-8979    

DOI:10.1063/1.349058

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Measurements have been made of the dielectric response of distributions of metallized fibers aligned in epoxy matrices for different loading fractions and fiber lengths. These have been shown to be in reasonable agreement with a simple independent particle theory. Use of the theory allows classification of fiber dielectric behavior by fiber conductivity and a characteristic length. Some aggregation of the fibers has been observed, even at relatively low loading densities. Computer simulation suggests enhanced dielectric constants of the composites for a moderate range of separations where the fibers are offset about 75% of their lengths with respect to each other and are closer than about 10 radii. This effect may be significant for aligned fibers which tend to aggregate in the magnetic field with separations and offsets within this range. The resulting composites are rugged and easily machined, and with less than 5% fiber weight loading have large, highly anisotropic dielectric constants of 60 or more at microwave frequencies in the X band. In conjunction with their (predicted) broadband performance characteristics such media should prove useful for a variety of applications.

ISSN:0021-8979    

DOI:10.1063/1.349059

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The growth, fabrication, and operation of InGaAs/AlGaAs distributed feedback (DFB) grating surface emitting (GSE) lasers are described. These devices contain a continuous, buried second‐order diffraction grating, for which two organometallic vapor phase epitaxial growths are required. Compared to more conventional distributed Bragg reflector surface emitting lasers, the grating and its coupling coefficient are much more uniform, since it is patterned onto a planar surface. Operation of ten‐element linear arrays, and a 10×10 array of DFB‐GSEs is also demonstrated, and the threshold dependence on both the layer and array structure is discussed.

ISSN:0021-8979    

DOI:10.1063/1.349060

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

ArF(B‐X) laser emission has been observed under long pulse electron‐beam excitation at relatively low pump rate. ArF lasing only occurred with Ne buffered gas mixtures. Optimum performance of 1.93 J/lat an intrinsic efficiency of 1.35% occurred with an Ar/F2mixture of 1/0.075% using a Ne buffer to 4.0 amagats. Total energy of 290 mJ in a 2‐cm2beam with a 1.0‐&mgr;s full width at half maximum pulse width was recorded.

ISSN:0021-8979    

DOI:10.1063/1.349061

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The photothermal edge effect was theoretically investigated under the realistic assumption of a Gaussian distributed heating beam, which is not only absorbed at the upper surface of the sample, but because of scattered light also in the vertical plane. The photothermal signal scanned over an edge strongly depends on the detector aperture and in the case of a narrow point‐like detector aperture it depends also on the radiusR. An explanation of elevations in the signal near the edge is given.  

ISSN:0021-8979    

DOI:10.1063/1.349062

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Recording angularly multiplexed holograms in photorefractive media requires an understanding of both the recording and erasure dynamics. In this paper a coupled‐wave analysis is used to describe both the recording and erasure processes. This analysis has been applied to the recording of multiplexed holograms, resulting in a procedure to record maximum diffraction efficiency holograms. Experimental results confirming the theory for both the dynamics of a single exposure and the efficiencies of multiple exposures are presented. Using an uncoupled recording wave analysis, an expression for the dependence of the diffraction efficiency on the number of exposures in the case of equal record/erase time constants is presented. Approximate results are presented for the case of unequal time constants. This serves to set an upper limit on the diffraction efficiencies for a given saturation index of refraction modulation.

ISSN:0021-8979    

DOI:10.1063/1.350340

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

It is shown that the resultant in‐plane acceleration sensitivity of contoured quartz resonators symmetrically supported along rectangular edges always vanishes. This result is a consequence of symmetry and applies to any shape for which the support system is symmetric with respect to the center of the mode shape. When the active plate is supported on one side only, a state of flexure is induced in the resonator which degrades the in‐plane sensitivity. This emphasizes the importance of a symmetric support system to achieve low acceleration sensitivities. The extensional and induced in‐plane flexural biasing deformations are determined by means of our variational approximation procedure using the variational principles in which all conditions appear as natural conditions. The resulting biasing states are employed in an existing perturbation equation along with the mode shapes of the contoured resonators to calculate the nonvanishing acceleration sensitivities when the resonator is supported on one side only.

ISSN:0021-8979    

DOI:10.1063/1.349063

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

A simple new laser diode array model using the boundary element method is presented. This model−which needs few boundary elements and little calculation time−represents a multilayer chip as a single‐layer chip and distributed heat sources as a line heat source in the center of an active region, and neglects the grooves between array elements. For three types of laser diode array chips with element spacing greater than 50 &mgr;m, the simple model estimates thermal interactions to within 2%. The interactive thermal influence of laser diode array packing density and the number of elements is discussed in terms of this simple model: Submounts and stems with high thermal conductivity are shown to be effective for increasing the number of elements packed within an array chip. With a type‐IIa diamond submount, for example, the number of elements packed within a 400‐&mgr;m width on an array chip can be increased from five elements 100 &mgr;m apart to nine elements 50 &mgr;m apart without increasing thermal interaction.

ISSN:0021-8979    

DOI:10.1063/1.349064

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The evolution of stress and strain during the consolidation of a fluid‐saturated porous elastic sphere subjected to a uniform normal surface traction is investigated. The solution of Cryer [Q. J. Mech. Appl. Math.16, 401 (1963)] for the pore fluid pressure is extended by deriving and fully analyzing new analytical solutions for the strain tensor of the elastic skeleton, the total stress tensor, the trace‐free stress deviator tensor, and the effective stress tensor. Asymptotic expansions for small values of time of the stress and strain components, which determine the behavior of these components during the initial stages of the consolidation, are derived. Computer‐generated graphs of the exact analytical solutions are presented, which provide insights regarding the redistribution of stress and strain throughout the consolidation. As fluid is expelled from the outermost layers of the sphere, fluid discharge builds up quickly to a peak rate and then subsides slowly as successively deeper layers are drained. During this initial consolidation, the stresses, strains, deviator stresses, and effective stresses within the outermost layers all change very rapidly. The nature of the Mandel–Cryer effect (pore fluid pressure increase within the sphere during initial fluid expulsion) is illuminated. Two other quantities, the transverse component of the total stress tensor and the total mean normal pressure, also show changes of opposite sign in the near surface and deeper regions, respectively: diminution in the outermost layers and simultaneous increase in the deeper portions. All of these effects are derived from Biot’s theory of poroelasticity, in which the dilatation satisfies the diffusion equation.

ISSN:0021-8979    

DOI:10.1063/1.349065

出版商:AIP    

年代:1991

数据来源: AIP