摘要:

We demonstrate the combined effects of optical scattering loss and surface recombination (or carrier diffusion) on the performance and scalability of etched‐post vertical cavity lasers (VCLs). The size dependence of optical losses and threshold gain are determined from pulsed measurements of external quantum efficiency. Deeper etch depths result in a stronger radial dependence of the threshold gain, which quickly increases the threshold current density. With optical loss accounted for, pulsed threshold current density measurements give the extra information needed for evaluating carrier loss. Surface recombination or carrier diffusion also results in threshold current density increases, but scalability is ultimately limited by the ability of the active region to provide enough gain for smaller size, higher optical loss devices. Even with these losses, three‐quantum‐well VCLs with shallow etches have threshold currents as low as 420 &mgr;A. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360588

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We report studies of the optical and electro‐optic properties of guest–host polymeric nonlinear optical materials based on aromatic, fluorinated, fully imidized, organic soluble, thermally, and photochemically crosslinkable, guest–host polyimides. We have introduced temperature stable nonlinear optical chromophores into these polyimides and studied optical losses, electric field poling, electro‐optic properties, and orientational stability. We measured electro‐optic coefficients of 5.5 and 12.0 pm/V for ((2,6‐Bis(2‐(3‐(9‐(ethyl)carbazolyl))ethenyl)4H‐pyran‐4‐ylidene)propanedinitrile (4‐(Dicyanomethylene)‐2‐methyl‐6‐(p ‐dimethylaminostyryl)‐4H‐pyran) DCM‐doped guest–host systems at 800 nm using a poling field of 1.3 MV/cm. Poling induced nonlinearities in single‐layer films were in agreement with the oriented gas model, but were lower in three‐layer films due to voltage division across the layers. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360779

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Dye‐in‐polymer (DIP) films consisting of soluble Pr4VOPc (vanadyl phthalocyanine having four propyl substituents) dye and polymethylmethacrylate (PMMA) were prepared by spin‐coating process. Pr4VOPc precipitates as a glassy phase in pure PMMA matrix during the film formation. Upon exposure of these DIP films to organic solvent vapor, stable crystalline Pr4VOPc, which has stronger absorption in the near‐IR region and larger nonlinear refractive indexn2than the glassy phase, forms. The organic solvent vapor promotes the solubilization and crystallization of the dye, plastication of the polymer chains, and migration of the dye. The nonlinear refractive indexn2and optical spectra of the DIP films were measured, and the change rule has been discussed in terms of electron transition and molecular packing of Pr4VOPc. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360589

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Diffraction of ultrasonic waves by immersed elastic bigratings constructed from the Galois fields GF(23) along one direction and GF(32) along the orthogonal one is studied. Such bigratings permit a significant attenuation of the specular echo in a wide frequency domain. This attenuation is experimentally observed and agrees with the theoretical one predicted by a model which takes into account the elasticity of the structure, the boundary conditions at the fluid–solid interface, and the periodicities of the gratings. Furthermore, numerous sharp minima in the spectrum of the reflection coefficient are experimentally observed and theoretically predicted; they correspond to the excitation of surface waves (generalized Rayleigh and Scholte–Stoneley waves) and they contribute significantly to the attenuation of the specular echo. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.360590

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A mortar is a composite of inert sand grains surrounded by a porous cement paste matrix. We investigate the electrical conductivity of model mortars that include enhanced electrical conduction in the matrix–sand grain interfacial region. The electrical conductivity is evaluated by a combination of finite element, finite difference, and random walk methods for periodic and disordered models of mortar. Since the effective conductivity within the interfacial zone is often much higher than the bulk matrix conductivity, the qualitative features of transport in these systems is often controlled by the connectivity of the interfacial zone. Special attention is thus given to the geometrical percolation of this zone. A family of effective medium approximations give a good qualitative description of the disordered model’s electrical properties. A simple four parameter Pade´ approximant is found to successfully describe the electrical conductivity of the periodic model over the entire range of parameters studied. Finally, we show that our calculations can be used to obtain a reasonable estimate of the permeability to viscous fluid flow. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360591

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A new modeling technique for low‐pressure, low‐temperature discharges is introduced in which electrons, ions, and excited neutrals are equivalently treated using particle‐mesh algorithms. Heavy particles are represented as electrons in bound states having large effective masses and the appropriate charge. An effective density of states is used which has discrete negative energies representing bound states, and a positive continuum representing free electrons. Collisions between continuum electrons and heavy particles are computationally treated as electron–electron collisions. This method has been used as the kinetic portion of a hybrid kinetic‐fluid model for positive column electrical discharges. Densities and electric fields are obtained from the fluid portion of the model. Transport coefficients, source functions, and energy distributions for all species are generated in the kinetic module. The hybrid model has been used to examine electron energy distributions and radial electric fields in positive column discharges. Evidence is presented for diffusion heating of electrons in the sheaths. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360592

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The characteristics of a monopolar space‐charge region under the effect of charge injection with a uniform initial velocity are studied. Poisson’s equation has been solved as a Dirichlet boundary‐value problem, other than by conventional serial integrations, to yield exact analytical expressions for the potential, field, charge density, and current. The structure of the space‐charge region is characterized by a potential well and an effective diode gap, and can be explained in terms of the initial kinetic energy of the charge. The Child–Langmuir limit, when calculated by using the effective diode gap, is a firm limit. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360593

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Oxygen (O) and carbon (C) coprecipitation in Czochralski Si is studied in terms of a diffusion‐limited growth model. The interfacial energy increase upon C incorporation into oxide precipitates as well as the changes of O and C concentrations in the Si matrix with annealing time have been taken into account. A comparison of the model predictions with available experimental data has led to the following conclusions: (i) Regardless of the C content in the crystal, it is necessary to introduce sinks for the precipitation‐induced excess Si self‐interstitials (I) in the matrix for high annealing temperatures. (ii) At annealing temperatures below about 1000 °C, the enhancement effect of C on O precipitation results primarily from an increase in the precipitate density. (iii) The transition in the C precipitation behavior observed in C‐rich Si crystals at annealing temperatures around 800 °C is related to a change in the availability of effectiveIsinks in the Si matrix at these temperatures. (iv) An enhancement of C diffusivity in the presence of excessIplays an important role in increasing the precipitate growth rate, particularly at low temperatures for which no efficientIsinks are available in the Si matrix. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360594

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Electron‐paramagnetic‐resonance linewidths at theM=+1/2↔−1/2 transition of the Fe3+spectrum of Fe/MgO single crystals have been examined at 9 GHz and 300 K before and after ion bombardment. The nominal ion concentrations ranged from 310 to 4300 ppm. Specimens were bombarded at 150 keV with argon ions for up to 12 min. The ion‐bombardment produces reductions in the peak‐to‐peak linewidth of &Dgr;Hpp, which occurred in two stages for all the specimens examined. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360595

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We investigate the effect of Si ion implantation and subsequent rapid thermal annealing (RTA) on the homogeneity of the crystal quality of metalorganic chemical‐vapor‐deposited GaAs (150 A˚)/InGaP (100 A˚)/n+InGaAs:Si (150–200 A˚)/GaAs (800 A˚) epilayers, which is the structure for heterostructure metal–semiconductor field‐effect transistors. It is found that ion implantation to a dose higher than 3×1013cm−2at energies between 30 and 90 keV and RTA causes the appearance of dark regions in and near the InGaP layers and the disappearance of the InGaP ordered structure in the lattice images taken with a transmission electron microscope. In the dark regions, the interfaces between the InGaP layer and the GaAs and InGaAs layer become indistinct. An energy‐dispersive x‐ray analysis shows that compositional atoms intermix through these interfaces. The intermixing seems to be enhanced by implantation damage rather than by the interactions of Si impurities. On the other hand, 1×1013cm−2implantation causes no dark regions. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360596

出版商:AIP    

年代:1995

数据来源: AIP