摘要:

Trajectories of secondary electrons (SEs) as an image signal in scanning electron microscopy are studied analytically in high‐aspect‐ratio submicron wide infinite trenches. It is found that the SE collection efficiency &xgr;, which is defined as the ratio of the number of SEs exiting the trench top surface divided by that emitted from the trench bottom surface, has the following dependencies on magnetic flux densityBzgenerated by an objective lens and electric fieldEz. Here,BzandEzare the components perpendicular to the wafer surface. For a lowBzregion corresponding to a usual operation condition, &xgr; decreases gradually but is almost constant at about 0.15 with increasingBzfor trenches 0.5 &mgr;m wide and 5 &mgr;m deep. For a highBzregion over 3 T, &xgr; increases with increasingBz, since the cyclotron radius is fairly small compared with the trench width. A special superconducting magnet is necessary to reach the highBzregion. An external electric fieldEzbrings about the larger increase of &xgr; for the smaller SE energy &egr;s. For &egr;s=1 eV,Ezas large as 106V/m is necessary to realize a 50% increase of &xgr; compared with the case ofEz=0 V/m, whereas for &egr;s=0.1 eV, only aboutEz=105V/m is sufficient to realize the increase of 50%. On the other hand, experiments show that an electric field as large as 105V/m is effective to achieve bright and clear observations of the bottom image.

ISSN:0021-8979    

DOI:10.1063/1.351552

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A simple theoretical model is developed to explain neutron thermalization properties. Assuming that a fast neutron source is located at the origin, the neutron distribution is described as a function of distance from the source. Based on the continuous slowing‐down model, it is shown that the thermal neutron flux is a simple function of the diffusion constantD, the reciprocal length &kgr;, and the Fermi age &tgr; of a moderator material. Several points are noteworthy from calculation of the thermal neutron flux. First, theoretical results for polyethylene moderator agree remarkably well with simulation data obtained from a Monte Carlo simulation. Second, the thermal neutron flux at the origin is proportional to the square of moderator density. Third, the volume of the large thermal neutron flux increases drastically as the Fermi age increases. Finally, we also note that theoretical results agree reasonably well with experimental data for a broad range of physical parameters. A simple analytical expression of the thermalization factor of a moderator material is derived and it is found that its theoretical values agree rather closely with measured values.

ISSN:0021-8979    

DOI:10.1063/1.351553

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

An analytical model for a longitudinally end‐pumped laser is developed using the circulating photon flux approach, and the threshold pump intensity and slope efficiency are expressed with the laser parameters. The thermal population in the lower laser level is included in the model. The threshold pump intensity decreases as the absorption cross section increases. The model shows that the threshold pump intensity increases as the length of the laser rod increases over the optimum value. The slope efficiency of the system depends on the laser rod length, the absorption cross section, and the reflectance of the output mirror. When the model is applied to a GaAlAs diode laser pumped Nd:YAG laser, the achievable slope efficiency reaches up to 76%, which is the ultimate quantum efficiency, at the output mirror reflectance lower than 50% with a laser crystal of loss coefficient 0.1%/cm. The experimental results of 946 nm Nd:YAG laser by Fan and Byer, Opt. Lett.12, 809 (1987) are compared with the present model, and shows a good agreement with calculations.

ISSN:0021-8979    

DOI:10.1063/1.351554

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The characteristics and properties of proton exchange and annealed proton exchange waveguides in lithium tantalate are investigated. Planar waveguides are first studied using secondary‐ion mass spectrometry (SIMS) to obtain the hydrogen‐ and lithium‐ion concentration profiles. From this data, accurate exchange and anneal diffusion coefficients are calculated. Optical propagation constants are also measured and used in conjunction with the SIMS results to calculate the change in the extraordinary refractive index at &lgr;=0.6328 and 0.829 &mgr;m. Infrared‐absorption measurements are also taken to monitor the exchange process. In addition, the surface damage due to proton exchange of thexfacet of lithium tantalate is qualitatively investigated. Channel waveguides are also characterized. A parameter space study to determine the fabrication conditions necessary to achieve low‐loss, single‐mode waveguides at &lgr;=0.829 &mgr;m is carried out. Mode near‐field sizes and the fiber‐to‐output insertion losses are characterized as a function of anneal time. The polarization extinction ratio and optical damage threshold for typical waveguides are also measured.

ISSN:0021-8979    

DOI:10.1063/1.351555

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Ion implantation of both Li+and Na+inton‐type PbTe0.92Se0.08has been used to convert this material top‐type. Li+‐implanted samples achieved carrier‐type conversion without annealing, while Na+implanted samples required an anneal at 350 °C before carrier conversion was attained. Hall measurements at 77 K on Li+‐implanted,n‐type PbTe0.92Se0.08, annealed at 250 °C, indicated an estimated activation efficiency of 11%, and achievement of a hole concentration of 5×1018/cm3. The implantation of Li+into the top layer of a PbTe0.92Se0.08(n)/PbTe0.82Se0.18Te(p) /PbTe0.92Se0.08(p) double heterostructure laser diode test structure resulted in a factor of 20 increase in series resistance at 200 mV, when compared to the series resistance of a similar unimplanted structure at the same bias voltage. The implanted structure did not exhibit breakdown until voltages exceeding 800 mV had been applied. These results suggest that ion implantation of Li+into the top cladding layer of PbTeSe/PbSnTe laser diodes can be used to significantly reduce current spreading.

ISSN:0021-8979    

DOI:10.1063/1.351556

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A simple analytical model is obtained to describe the effect of carrier heating on the frequency response of a quantum well laser. The principal factors are taken to be injection heating, recombination heating, and hot phonons. The model is applied to the GaAs/GaInAs strained layer system and is shown to qualitatively account for many of the nonideal features observed. The nonlinear effects cannot be described satisfactorily by a single phenomenological ‘‘gain suppression’’ factor. However, at low drives the conventional gain suppression factor can be expressed in terms of the phonon lifetime and the temperature‐relaxation time. The response is mediated by several time constants which, in our example, combine to give an effective time constant of about 10 ps. The modulation frequency response becomes seriously impaired when the differential gain is lowered by a factor of 2 and the time constants describing scattering and phonon lifetime are increased by a factor of 2.

ISSN:0021-8979    

DOI:10.1063/1.351557

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The problem of the generation of guided elastic wave modes in infinite hollow circular cylinders by loading applied to the boundaries of the cylinder, apart from being an interesting problem in itself, has important applications in the field of nondestructive evaluation. Hollow cylinders are commonly used in the nuclear industry in heat exchangers and hence must be inspected for defects regularly to ensure continued safe operation. An exact, closed‐form solution is given for the amplitude with which any propagating waveguide mode is generated due to the application of prescribed (time harmonic) surface tractions. Instead of using classical integral transform techniques to solve the problem, the normal mode expansion technique is used. In this technique, analogous to the expansion of a function in terms of an orthogonal (and complete) set of functions, the fields generated in the cylinder due to the application of the surface loading are expanded in terms of the normal modes of the cylinder. The general results are then specialized to loading configurations of practical importance in nondestructive testing.

ISSN:0021-8979    

DOI:10.1063/1.351558

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The fundamental nature of an interface crack between dissimilar anisotropic media under the uniform heat flux is studied. Based on the Hilbert problem formulation and a special technique of analytical continuation, a simple and compact version of general solutions for the thermal field are given. The temperature gradients or heat fluxes are found to possess the characteristic inverse square‐root singularity in terms of the radial distance from the crack tip. Due to this singular behavior, the heat flux intensity factor is then introduced to quantify the thermal energy cumulated in the neighborhood of the crack tip. Some numerical examples are given for the application of the final results to the intensity factor of heat flux as well as the full field solutions of temperature. It is very interesting to see that the solutions associated with the dissimilar media can be easily obtained from the corresponding problem associated with the homogeneous media by a simple substitution of their own material properties. In general, the system possesses the symmetric properties of the thermal conductivity coefficients which can make the heat flux intensity factor smaller. Consequently, the thermal energy intensification is diminished.  

ISSN:0021-8979    

DOI:10.1063/1.351559

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Cylindrically converging shock waves in solids have been analyzed by the random choice method (RCM). A Riemann solver for fluidlike solids with the Gru¨neisen‐type equation of state is constructed and incorporated into the RCM. It is then applied to the cylindrical shock tube problems for solid copper with driving pressures of 20 and 200 GPa. The numerical results are compared with those of the finite difference method (FDM). The shock speed is smaller and the discontinuity at the shock front is smeared out due to the artificial viscosity in the FDM calculation. Spatial distributions of pressure, density, and particle velocity calculated by the RCM show that the steepness at the shock front is maintained both in the converging and reflecting stages. It is shown that the pressure on the shock front and the total energy contained in the central circular area are much larger in the reflecting stage than in the focusing stage. The dimensional analysis has shown that the similarity solution exists; however the numerical result shows that the flow does not fall within the similarity regime in the region of calculation. It suggests that the self‐similar flow is only limited in extreme proximity to the axis.

ISSN:0021-8979    

DOI:10.1063/1.351560

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Cross‐property relations linking the fluid permeabilitykassociated with viscous flow through a porous medium to effective diffusion properties of the medium have recently been derived. Torquato [Phys. Rev. Lett.64, 2644 (1990)] found thatk≤D&fgr;1&tgr;, where &tgr; is the ‘‘mean survival time’’ associated with steady‐state diffusion of ‘‘reactants’’ in the fluid region of diffusion coefficientDand porosity &fgr;1of a porous medium containing absorbing walls (i.e., trap boundaries). Subsequently, Avellaneda and Torquato [Phys. Fluids A3, 2529 (1991)] relatedkto the electrical formation factorF(inverse of the dimensionless effective electrical conductivity) and the principal (largest) diffusion relaxation timeT1associated with the time‐dependent trapping problem, namely,k≤DT1/F. In this study, we compute the aforementioned bounds, using an efficient first‐passage‐time algorithm, for grain‐consolidation models of porous media and compare them to exact results for these models. We also conjecture a new relation connectingkto &tgr; andFfor a wide class of porous media, namely,k≤D&tgr;/F, and show that it gives the sharpest permeability estimate among the existing bounds. The importance of this relation lies not only in its usefulness as an estimator of the permeability but that it involves the diffusional parameters &tgr; andFwhich can be measuredinsitu.

ISSN:0021-8979    

DOI:10.1063/1.351561

出版商:AIP    

年代:1992

数据来源: AIP