摘要:

Chains of submicron size rodlike TiOxNyparticles, formed along an electric field by the so‐called electrorheological effect, were softly fixed in a silicone elastomer film interposed between a pair of indium–tin oxide glasses. It has been found that the transmittance of the device was remarkably changed by displacing one substrate relatively to the other. This transmittance variation was linear with the relative displacement between substrates. For a displacement of about 35 &mgr;m, a solar transmittance change of 40.7% was obtained, while it was below 6.9% for a cell containing TiO2particles. The excellent optical properties of the cell using TiOxNyparticles as switchable glazing was imputable to the large absorption coefficient of the particles over the wavelength of the solar spectrum (∼105cm−1) and to a drastic variation in the cross‐sectional area of the particle chain for the light when submitting the substrates to shear stress. This new type of the switchable glazing was referred to as ‘‘shear stress‐type dipole particle chain in elastomer.’’

ISSN:0021-8979    

DOI:10.1063/1.353357

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The electronic structure of coaxial, graphene double‐layer tubules is predicted for various combinations of metallic and insulating constituent inner and outer monolayers, depending on the diameter and chirality of the tubule. For the examples chosen, some of the energy bands of the inner and outer tubules are coupled to each other by commensurate interlayer interactions. Nevertheless, because of symmetry, the energy bands of metallic monolayer tubules remain metallic even after interlayer interactions are considered. The possible implications of these results on molecular metal‐insulator devices are discussed.

ISSN:0021-8979    

DOI:10.1063/1.353358

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The inverse eddy current problem can be described as the task of reconstructing an unknown distribution of electrical conductivity from eddy‐current probe impedance measurements recorded as a function of probe position, excitation frequency, or both. In eddy current nondestructive evaluation, this is widely recognized as a central theoretical problem whose solution is likely to have a significant impact on the characterization of flaws in conducting materials. Because the inverse problem is nonlinear, we propose using an iterative least‐squares algorithm for recovering the conductivity. In this algorithm, the conductivity distribution sought minimizes the mean‐square difference between the predicted and measured impedance values. The gradient of the impedance plays a fundamental role since it tells us how to update the conductivity in such a way as to guarantee a reduction in the mean‐square difference. The impedance gradient is obtained in analytic form using function‐space methods. The resulting expression is independent of the type of discretization ultimately chosen to approximate the flaw, and thus has greater generality than an approach in which discretization is performed first. The gradient is derived from the solution to two forward problems: an ordinary and an ‘‘adjoint’’ problem. In contrast, a finite difference computation of the gradient requires the solution of multiple forward problems, one for each unknown parameter used in modeling the flaw. Two general types of inverse problems are considered: the reconstruction of a conductivity distribution, and the reconstruction of the shape of an inclusion or crack whose conductivity is known or assumed to be zero. A layered conductor with unknown layer conductivities is treated as an example of the first type of inversion problem. An ellipsoidal crack is presented as an example of the second type of inversion problem.    

ISSN:0021-8979    

DOI:10.1063/1.353359

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Propagation of a directed pulse of electromagnetic radiation in a vacuum and in a rarefied, temporally dispersive medium is investigated. The pulse belongs to the class of broadband directed radiation beams termed electromagnetic ‘‘missiles’’ and is superposed on a carrier wave. First the propagation characteristics of the pulse in a vacuum are reviewed. In a vacuum the far‐field time‐integrated Poynting fluxU(z) of the pulse drops off as 1/z1+4&ngr;, with ‖&ngr;‖≤1/4. In the corresponding expression forU(z) in a dispersive medium the leading term also varies as 1/z1+4&ngr;since the high‐frequency end of the spectrum experiences very little dispersion. In a dispersive medium the propagation of the precursors and of the carrier signal is examined in certain limits and expressions for the spatial variation of the vector potential are given.

ISSN:0021-8979    

DOI:10.1063/1.353360

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Quantitative measurements of desorbed ions and neutral molecules by a quadruple mass spectrometer from laser ablation of Tl‐Ca‐Ba‐Cu‐O were obtained. The results indicate that more neutral atoms and molecules desorbed than the corresponding ions. More elementary ions desorbed than the corresponding oxide compounds.

ISSN:0021-8979    

DOI:10.1063/1.353361

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The microbending losses induced by axial strain in double‐coated optical fibers have been analyzed. The microbending loss is dominated by the lateral pressure in the glass fiber, which is proportional to the axial strain and is affected by the material properties of primary coating and secondary coating and their thicknesses. To minimize the microbending losses induced by axial strain, it is found that if the lateral pressure is compressive, the thickness and Poisson’s ratio of primary coating should be increased; but the Young’s modulus of primary coating should be decreased. On the other hand, the thickness, Young’s modulus, and Poisson’s ratio of secondary coating should be decreased.

ISSN:0021-8979    

DOI:10.1063/1.353362

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A semiclassical model is presented for ultrashort‐pulse dye laser amplifiers. The model permits a discussion of important aspects of short‐pulse amplification such as pulse shape, pulse energy, and gain saturation. Coherence effects are found to be important in characterizing the evolution of amplified pulses in the subpicosecond and femtosecond regimes. The results are compared with those obtained using the corresponding rate equation model.

ISSN:0021-8979    

DOI:10.1063/1.353363

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Absorption and scattering, or fanning, have a pronounced effect on the observed dynamics of the photorefractive effect. In this article these effects are included in the coupled wave equations for both recording and erasure, and a comparison is made with experimental observations. It is shown that a single recording and erasure time constant is necessary to fit the experimental measurements, whereas separate time constants would be required if absorption and scattering were neglected.  

ISSN:0021-8979    

DOI:10.1063/1.353364

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

For an active dielectric film waveguide with different passive dielectrics for the cover and the substrate and with periodic corrugations on the upper surface, the characteristics of the second order Bragg interaction are investigated for the transverse electric mode with reference to such applications as frequency‐selective reflector, couplers, and distributed feedback laser. Four canonical equations that are valid for a distributed coupler with two guided waves and two free waves, incident and radiated, one each in the cover and the substrate, are deduced systematically. The coupling coefficients are shown to satisfy all the requirements imposed by symmetry, reciprocity, and power conservation. From the solution of the canonical equations with the appropriate terminal conditions, the characteristics of the devices are obtained. The design parameters leading to the optimum performance of these devices are determined. For the distributed feedback laser, the frequencies of oscillation, the corresponding gain thresholds, and the relative power distributions in the two guided and the two radiated waves are obtained for the first few modes in the neighborhood of the second order Bragg frequency. The modifications of the characteristics introduced by the presence of a phase shifting section on the grating structure are analyzed for symmetrical and asymmetrical grating configurations. The effects of radiation and the phase shifting section on the separation of the gain thresholds of the neighboring lower order laser modes are investigated with a view to increasing the possibility of a single mode oscillation. Selected numerical results are presented to illustrate the important characteristics of the various thin film devices based on the second‐order Bragg interaction.

ISSN:0021-8979    

DOI:10.1063/1.353365

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The influence of surface roughness on the detection of subsurface scatterers using phase‐coherent, ultrasonic immersion transducers is reported. Experiments were conducted with phase‐coherent transducers, immersed in a liquid bath, that insonified samples with roughened surfaces. Measurements were made of: (1) the signal‐to‐noise ratio for a subsurface void when viewed through a rough surface; (2) the frequency‐dependent loss (between 2 and 20 MHz) that was induced in the transmission and reflection coefficients of the coherent beam for flat aluminum plates with root‐mean‐square roughness varying between 5 and 50 &mgr;m; and (3) the effects of surface roughness on the material noise. The following results were found. First, surface roughness greatly degraded the signal‐to‐noise ratio (S/N) for some typical inspection geometries. Second, after appropriate normalization, the transmission and reflection coefficients for the coherent beam were found to be nearly universal functions of the angle of incidence, except near the critical angles. Large reductions (several orders of magnitude) in the phase‐coherent signalSresulted for some typical inspection setups. The noise, on the other hand, remained nearly unchanged for the same setups. The degradation in S/N was attributed to the randomization of the phase of the coherently transmitted and reflected beams. Both the sensitivity of the signal and the relative insensitivity of the backscatter noise are explained in terms of the phase‐screen approximation. This simple approximation is also used to explain the nearly universal form of the normalized transmission and reflection coefficients, and to predict that the backscatter noise may be either slightly enhanced or slightly decreased by surface roughness.

ISSN:0021-8979    

DOI:10.1063/1.353366

出版商:AIP    

年代:1993

数据来源: AIP