摘要:

The inability to measure pressure with accuracy at high temperature has been a hindrance to the development of simultaneous high‐temperature, high‐pressure experimental techniques. The results of recent laser‐induced fluorescence studies at high temperature and high pressure indicate that Sm:YAG is a promising pressure calibrant with very low‐temperature sensitivity. The most intense feature in the fluorescence spectrum is a doublet at 16186.5 cm−1. The Sm:YAG doublet exhibits a pressure‐induced peak shift comparable to theR1shift of ruby. However, the temperature‐induced shift of the doublet is almost two orders of magnitude less than that observed for theR1peak. Simultaneous high‐pressure‐temperature experiments indicate that the pressure and temperature effects on the frequency and line shape can be added linearly. An empirical model based on the linear combination of pressure dependent frequency shift and temperature dependent linewidth and intensity ratio successfully predicts the doublet line shape at simultaneous pressure and temperature. Use of the model facilitates measurement of peak position at high temperature resulting in improved accuracy and repeatability of the pressure determination. Pressure measurements at 400 °C and 40 kbar based on the Sm:YAG doublet peak position agree with the temperature‐corrected rubyR1pressure measurement to within 3 kbar. At 15 kbar and 900 °C the uncertainty in the Sm:YAG fluorescence peak wavelength is 5 cm−1due to temperature‐induced line broadening; this corresponds to an uncertainty in the pressure determination of ±2.5 kbar. The high thermal and chemical stability of YAG materials make Sm:YAG an ideal pressure calibrant for high‐temperature applications. In addition the frequency and intensity of the Sm:YAG fluorescence allow simple conversion from experimental setups designed for ruby fluorescence measurement.

ISSN:0021-8979    

DOI:10.1063/1.346593

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A model for current spreading and carrier distribution in the active region of a gain‐guided or weakly index‐guided surface emitting laser is presented. At high currents, current ‘‘crowding’’ takes place which reduces the amount of carrier spreading beyond the contact and hence can help stabilize the fundamental mode oscillation. The fundamental mode is a Gaussian whose width varies approximately as the square root of the contact radius.

ISSN:0021-8979    

DOI:10.1063/1.346594

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Data are presented on step‐graded separate confinement quantum well lasers with Al0.85Ga0.15As outer confining layers, AlxGa1−xAs barriers and a 50‐A˚ GaAs quantum well grown by metalorganic chemical vapor deposition. By varyingxbfrom 0.15 to 0.60, we show that, given an adequate optical waveguide confinement factor and sufficient cavity length, the collection of electrons in thin quantum wells with either direct or indirect barriers can be highly efficient, transfer of electrons from indirect barriers to thin direct wells does not degrade laser performance, and electron confinement in the separate confinement region plays no role in the operation of the laser.

ISSN:0021-8979    

DOI:10.1063/1.347178

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A water‐soluble cyanine dye is adsorbed from the solution to form a salt with an arachidic acid monolayer at the air‐water interface. Complex monolayers thus formed were deposited onto substrates by the vertical dipping method to form Langmuir–Blodgett (LB) films. The optical properties of these films were examined employing a linearly polarized incident light. The films showed a large in‐plane anisotropy of light absorption in both visible (electronic transition) and infrared (vibrational transition) regions. The in‐plane anisotropy of the LB film is explained as due to the alignment of dimers, each showing Davydov splitting, by flow orientation during dipping. The configuration of the dye molecules in the dimer was estimated using Davydov’s theory and the extended dipole model. The layered structure of the film was determined by the x‐ray diffraction technique.

ISSN:0021-8979    

DOI:10.1063/1.346595

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The PVF2film‐based fiber‐optic phase modulator exhibits a resonance around 3.5 kHz. Existing reports of its characterization have considered only the linearity at nonresonance frequencies and the frequency response at a single drive voltage. A large nonlinearity of phase shift with applied voltage at frequencies near the resonance has been measured. The factors that influence the resonance have been identified on the basis of the frequency response for different drive voltages. These measurements were facilitated by the three new methods of phase detection we recently reported.

ISSN:0021-8979    

DOI:10.1063/1.346596

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A new argon‐ion laser based on an electrodeless magnetoplasma with tube diameter of 45 mm has been developed, and optimal operation conditions were experimentally studied. The plasma is thought to be generated by helicon waves excited by an external antenna. Selected excitation of the upper lasing level of Ar iidue to Landau damping of the helicon waves plays an important role in the mechanics of the population inversion in this laser. The plasma is excited externally by radio frequencies, and the lasing active medium does not directly touch the plasma wall, minimizing the problems associated with impurities from both the electrodes and the tube wall. A peak laser output power of 0.5 W with 10−4efficiency has been reached, although the operation condition was not far from that of the lasing threshold.

ISSN:0021-8979    

DOI:10.1063/1.346597

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

An analytical solution to the boundary‐value problem for the existence of a Bleustein–Gulyaev piezoelectric surface acoustic wave in GaAs and other piezoelectric cubic crystals is presented. Numerical solutions for Bleustein–Gulyaev waves in a number of crystals of 4¯3mand 23 symmetry classes are also presented. These include commonly used materials such as GaAs, GaP, InSb, InAs, Bi12GeO20, and Bi4Ge3O12. The results are then used to investigate light diffraction by Bleustein–Gulyaev waves in GaAs. Both plane‐wave acousto‐optic and guided‐optical‐wave interaction cases are considered. Numerical solution for a guided‐wave acousto‐optic collinear interaction in a GaAs‐GaAlAs waveguide is presented and discussed. In addition, a simple method of Bleustein–Gulyaev wave detection using optical techniques is developed and illustrated through numerical calculations.

ISSN:0021-8979    

DOI:10.1063/1.347154

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

In this work, the electrical admittance of this plane‐parallel resonator loaded on one face with a viscoelastic medium is described directly in terms of the physical properties of the system. This description is based on the detailed matrix equations for the piezoelectric quartz and includes the piezoelectric effect of the quartz transducer and the shear modulus and viscosity of the overlayer. This physically based analysis has proven invaluable in relating the admittance behavior directly to the materials’ properties. Initial comparisons of the theory with experimental data are given, including the appearance of even harmonics as described by Benes [E. Benes, J. Appl. Phys.56, 608 (1984)] for the asymmetrically loaded resonator. Additionally, measurements on a series of perfluoropolyether fluids of differing molecular weights indicate that they cannot be described as simple viscous liquids in the MHz range. The assignment of a frequency‐dependent viscosity with a relaxation time proportional to the molecular weight resolves some of the discrepancies. Because this study relates the complete admittance spectrum of the loaded resonator to the overlayer material properties, it opens the possibility of studying more than just the changes in resonant frequency. A detailed analysis of the resonance spectrum using an impedance analyzer would provide data which can be fit to the material’s properties and would be particularly helpful with viscoelastic media.

ISSN:0021-8979    

DOI:10.1063/1.346548

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A two‐dimensional theoretical approach is developed to describe the thermal and elastic fields in the vicinity of a line thermal source located at the junction between two bonded materials. A modified system of thermoelastic equations is obtained and solutions are found. The temperature distributions give evidence for a depth‐dependent transverse thermal flow. From the shape of the mechanical deformations at the interface, the ability of different pairs of materials to be applied as thermoacoustic transducers is qualitatively deduced.

ISSN:0021-8979    

DOI:10.1063/1.346549

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Buoyancy forces in nonuniform dispersions are considered. Using a thermodynamic approach it is shown that Archimedes principle, which holds in cases of pure fluids and uniform dispersions, is incomplete in the case of nonuniform dispersions. For nonuniform dispersions the Archimedes force density must be augmented by two diffusion‐type buoyancy force densities which are dependent on the expectation of particle size and density. The generalized buoyancy force consists of three vectors that may have different directions. It is shown that the effect of buoyancy diffusion is pronounced at diffusion fronts, being enhanced by aggregative motion of particles. Finally, the effect of buoyancy diffusion is expected to prevail over Brownian diffusion in the post‐micron particle size range.

ISSN:0021-8979    

DOI:10.1063/1.346550

出版商:AIP    

年代:1990

数据来源: AIP