摘要:

We present an efficient and fast simulation technique to determine the mean survival time &tgr; of a Brownian particle diffusing among a digitized lattice‐based domain of traps. Following the first‐passage time ideas of Torquato and Kim [Appl. Phys. Lett.55, 1847 (1989)], the algorithm is based on the known solution for the mean first passage time of a Brownian particle in a cube. The mean survival time, the inverse of the trapping rate, is obtained for a variety of configurations involving digitized spheres. Since the survival time is highly sensitive to the surface area and associated resolution, the results provide a means of determining the relation between the survival time of a real material and its digitized representation. In general, lower resolution images give rise to a diminished mean survival time and, thus, a lower bound on the true mean survival time &tgr;. Digitization can affect other transport properties in which the interface plays a major role, e.g., the fluid permeability associated with flow in porous media. We demonstrate both analytically and computationally that the mean survival time for the digitized medium converges to the continuum value in the high‐resolution limit. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358940

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Decay times, or the relaxation times after poling, of nonlinear optical host‐guest systems at room temperature have been evaluated from the behavior at elevated temperatures below glass transition temperatureTgusing Arrhenius plots. The factors were investigated by comparing the behaviors of the nonlinear systems to those of &agr; and &bgr; relaxations in host polymers. The activation energies of the nonlinear systems seem to agree with those of the &bgr; relaxation processes of host polymers; however, the frequency factors were very different. The decay time of nonlinearity seemed to be affected by &agr; relaxation processes belowTgof host polymers. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358941

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The semipermanent charge distribution at saturation in a germanium‐doped silica planar waveguide is modeled for the case of seeding by a low‐intensity fundamental wave and its second harmonic both propagating in the lowest order transverse magnetic mode. Based on the asymmetric photoionization model, the total current in the waveguide is taken to be the sum of the photoinduced current and the current due to the resulting static electric field caused by the charge separation. This current is used in the charge conservation equation to determine the charge distribution at saturation which is then used to compute the resulting static electric field inside and outside the waveguide. To compare with other models, the spatial profile of the resulting static electric field is compared to the direct current (dc) optical field due to a four‐wave mixing process. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.358942

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Single crystals of strontium barium niobate (Sr0.61Ba0.39Nb2O6), normally referred to as SBN:61, have been investigated using optical absorption, micro‐Raman, photoluminescence, and electron paramagnetic resonance (EPR) techniques. Both undoped and cerium‐doped (nominally 0.01 to 0.10% Ce) samples were included in the study. A broad optical absorption band centered near 490 nm and extending to the band edge near 380 nm is observed at room temperature in the Ce‐doped crystals. Additional absorption, which we assign to iron, occurs just below the band edge in some of the samples. Photoluminescence data were taken at room temperature. Sharp emission bands at 545 and 650 nm are assigned to Er3+, an emission band at 755 nm is assigned to Ho3+, and a broader emission band extending from 700 to 850 nm is assigned to Cr3+. The SBN:61 crystals strongly absorb 9.4 GHz microwaves. To minimize the effect of these microwave losses, the EPR data were taken near 7 K. A broad isotropic EPR line due to Ce3+ions is observed atg=0.9 in the Ce‐doped samples. Additional isotropic EPR lines due to Fe3+ions are observed atg=9.35 andg=4.10. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358943

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The third‐harmonic generation of far‐infrared (FIR) laser radiation inn‐doped silicon was measured with full temporal resolution of the power fluctuations during the FIR laser pulse. Thus, the intensity dependence of the nonlinear coefficient &khgr;(3)could be observed. For 2 MW incident power, corresponding to an intensity of 15 MW/cm2, at a wavelength of 676 &mgr;m a power conversion efficiency of 10−3was reached. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.358944

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A radio‐frequency glow discharge between dielectric electrodes is observed by spatiotemporal optical emission spectroscopy. No emission from an excited Ar ion is observed in the dielectric electrode discharge, but it is observed in a discharge with metallic electrodes under the same external conditions. On the other hand, marked excitation of Ar(3p5) on the driven dielectric is observed. The maximum of the spatiotemporal net excitation rate profile is larger and the amplitude of the current is smaller compared with the discharge sustained by metallic electrodes. These differences are attributed to the effect of surface charge on the dielectric. The dielectric immersed in the discharge is exposed to the ion and electron fluxes, and is charged negatively to balance both fluxes. The accumulated charge on the dielectric is estimated by a capacitor in series with the grounded electrode. As a result, the surface charge affects the field just in front of the dielectric and enhances excitation and ionization. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358945

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The unified particle simulation of theentireglow discharge including plasma bulk and electrode sheaths suffers from the bad representation of particles in the cathode fall and/or from expensive computing time. To improve the efficiency and reliability of such simulations we develop asplittingprocedurefor the simulation particles that increases the sampling rate in critical phase space regions. The technique is demonstrated with the help of a simplified model of a dc glow discharge. Simulation particles representing ions and electrons are traced from electrode to electrode by moving the particles in self‐consistent fields. For comparison, the same model of the discharge is analyzed in the framework of transport equations. To improve the spatial density profiles of the macroscopic equations, we accounted for nonlocal effects by adjusting the Townsend ionization coefficient with the help of the particle simulation. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358946

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

In the kinematical approximation, a two‐dimensional model is described for X‐ray scattering from multilayered films that includes discrete thickness fluctuations and interfacial correlated roughness. In this work, the influence of the thickness fluctuations and of the roughness that is introduced as small perturbations at the interfaces is studied on both specular and nonspecular reflectances. For specular direction, the kinematical calculations are compared with those calculated using the full optical Fresnel theory. It is shown that whenever the reflectance is low, both computations are in good agreement. Finally, the model is applied tentatively to fit the experimental specular and nonspecular spectra performed on Fe/Si multilayered samples. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358957

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Yttrium layers on amorphous or crystalline silicon substrates were irradiated with 600 keV Ar++, Kr++, and Xe++ions between liquid‐nitrogen temperatures and 265 °C. Ion‐induced YSi1.7formed in those samples irradiated above 205 °C and fluence ≥1×1015ions/cm2. The growth rates were monitored as a function of fluence and nuclear energy deposition at the Y/Si interface. For each ions species investigated, the growth rate varied linearly with the square root of fluence. The apparent activation energy was determined to be 0.6±0.1 eV. The experimental growth rates also exhibited a linear dependence on the nuclear energy deposition. This finding agrees qualitatively with the premise of nonoverlapping subcascades. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358958

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A wedge‐shaped target was implanted with nitrogen ions using the plasma source ion implantation process, in order to understand the effects of the target edges on the energy and fluence distribution of incident ions. Experimental measurements and analysis of retained dose on silicon samples affixed on the surface of the target, showed results consistent with those predicted by theoretical models. Higher retained dose and greater implantation depths were observed in the vicinity of the edge contained by the normal angle as compared to the edges contained by the acute angles. The target face with smaller area accumulated, on the average, higher dose compared to the face with the larger area. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358959

出版商:AIP    

年代:1995

数据来源: AIP