摘要:

The dispersion relation for the six waves generated in the suddenly created two‐resonance lossy Lorentz medium has been derived and analyzed by the use of a Brillouin‐like dispersion diagram. In comparison to the single‐resonance Lorentz medium, another two new waves were excited. The amplitude, power transmission, and reflection coefficients for these new modes have been derived and analyzed for several values of particle density. It was shown that less than 4% of the pump wave power is transformed into the reflected modes.

ISSN:0021-8979    

DOI:10.1063/1.355135

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Emission life of small Ba‐impregnated thermionic cathodes suitable for use in TV tubes is investigated systematically. Cathodes are covered with an Os layer to enhance the emissivity. They have an unusual emission life in that the emission current decreases gradually with time and then drops abruptly. The early gradual decrease in emission at high cathode temperatures is caused by the phase change of the Os layer to OsW2. A linear relationship is found to exist between the logarithm of the lifetime and the reciprocal of the cathode temperature. This life end is believed to happen after the end of the reduction reaction between BaO and the porous W body and after ceasing replenishment of the low work function Ba+‐O−monolayer on the metal surface. A formula for the emission lifetime is derived from the assumption that the rate determining step is the Ba diffusion inside the pellet. Emission lifetime &tgr; is shown to be given by &tgr;=Ad2 exp(E/kT), whereAis a constant,dis the pellet thickness, andEis the activation energy of Ba diffusion. This relation is confirmed experimentally.

ISSN:0021-8979    

DOI:10.1063/1.355136

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Charging insulating films in a scanning electron microscope is shown to be a potentially useful thermographic technique which makes it possible to reveal hot regions in microelectronic devices, with a spatial resolution in the submicrometer range. This technique entails depositing an insulating film on the device to serve as thermographic medium. A focused, low‐energy electron beam charges the insulator during the scanning process. Hot regions modify the local charge, which in turn modifies the secondary electron signal and thus generates a thermal contrast. This technique has been applied to investigate mirrors of GaAs/AlGaAs graded index separate confinement single quantum well laser diodes. Thermographic images of these mirrors have been obtained with a spatial resolution of 0.25 &mgr;m. Since the thermal images can be observed using the scanning electron microscope’s TV mode, the course of fast thermal phenomena at laser mirrors can be imaged. As an example, the thermal drift prior to the thermal runaway at laser mirrors has been investigated.

ISSN:0021-8979    

DOI:10.1063/1.355137

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Coupled InGaAs quantum‐well systems which use field‐induced spatial separation of electron and hole states to modulate the magnitude of exciton optical absorption, and hence transmission have been theoretically analyzed and experimentally demonstrated. The samples consisted ofp‐i‐ndiodes with an active region of 20 coupled wells, each coupled well containing a 50 A˚ In0.3Ga0.7As well and a 30 A˚ In0.15Ga0.85As well separated by a 10 A˚ Al0.33Ga0.67As barrier. One structure was grown with the thinner well on then‐type side of each coupled quantum well while in the other sample the thinner well was oriented toward thep‐type side. By applying bias to the structures, either the lowest electron or hole states effectively switch wells, thereby enhancing certain exciton resonances and quenching others. The two devices grown, despite their similar structure, operate through the field‐induced switching of opposite carrier types. Because this method of modulation does not require excitons to Stark shift, the device can produce large absorption/transmission changes with zero refractive index change under bias. These first nonoptimized samples produce changes in absorption per applied bias three times larger than single‐well systems. In addition, optical bistability is realizable in these structures. In addition to their presently displayed use, the coupled quantum‐well structure has numerous applications for waveguide or Fabry–Perot optical modulator systems.

ISSN:0021-8979    

DOI:10.1063/1.355138

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A necessary and sufficient condition for no thermal runaway is given for cw double‐heterostructure semiconductor lasers. No catastrophic damage at the mirror facet should occur when the condition is satisfied. The condition for no thermal runaway contains the dimension, operating conditions, and material properties of the double‐heterostructure laser. As such, it can be used to systematically optimize the laser structure. Use of the condition is illustrated and a typical case analyzed.

ISSN:0021-8979    

DOI:10.1063/1.355139

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

We present a general theoretical formulation for the characteristics of surface acoustic waves (SAW) generated by the phase velocity scanning (PVS) method that employs a scanning single laser beam (SSB) or a scanning interference fringes (SIF). In the SSB approach, a broad band SAW pulse is generated and its amplitude is coherently enhanced when the laser scanning velocityVis equal to the phase velocity &ngr;Rof the SAW. The amplitude of the SAW follows a resonance curve represented by a sinc function of the scanning velocityV, but different spatial frequency components in the SSB significantly suppress the side lobes of the resonance curve. In the SIF approach, the scanning velocity &ngr;fof the fringes is determined by the intersection angle and the frequency difference &ohgr;aof the laser beams. A narrow band tone burst of SAW with frequencies higher than 100 MHz can be excited. The SAW frequency &ohgr; depends upon a characteristic timet*, defined as a propagation time of the SAW across the laser beam spot. The SAW frequency &ohgr; is identical to the frequency difference &ohgr;awhen the laser pulse widthTis longer than the characteristic timet*. But, the SAW frequency &ohgr; is determined as a productkf&ngr;Rof the wave number of the SIF and the SAW velocity when the laser pulse width is shorter than the characteristic time. Precise frequency measurement provided by the amplitude enhancement effect and the narrow frequency bandwidth in the SIF approach make the PVS method particularly promising for the noncontact SAW velocity measurement.

ISSN:0021-8979    

DOI:10.1063/1.355140

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The surface acoustic wave (SAW) properties, i.e., free surface wave velocity, surface coupling coefficient, and static surface permittivity, of a lead titanate composition modified with an additive of samarium and processed by hydrothermal synthesis are determined. The effective permittivity of the piezoelectric ceramic was calculated, and the SAW properties were extracted from the analysis of the curve. In particular, the relations between the square coupling coefficient, the effective permittivity, and the usual definitionk2s=2&Dgr;V/Vare discussed. In parallel, experimental measurements of the SAW properties were carried out by developing a curve‐fitting algorithm on the real and imaginary parts of a unapodized single electrode SAW transducer. A free surface wave velocity of 2595 m/s, a coupling coefficient of 1.8%, and a static permittivity of 196 are found which were predicted by the analysis of the effective permittivity curves. The SAW properties of a Y+128° cut lithium niobate single crystal are also presented and are in agreement with those found in literature. These results are discussed and suggest that the present ceramic has the potential as a substrate for SAW devices.

ISSN:0021-8979    

DOI:10.1063/1.355339

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The effect of gas adjacent to the sample surface on the photo‐acoustic displacement (PAD) measurement was studied by using an extremely sensitive laser interferometric probe with a sensitivity of 0.1 picometers. For silicon, the PAD signal measured at atmospheric pressure increased about 18% as compared to the signal obtained in vacuum, and varied by less than 0.7% for a change in pressure of 5% around 1 atm. It is shown, by a simple theoretical model, that the variation of the PAD is caused by a change in refractive index of the gas and the real PAD can be accurately obtained by correcting this effect.

ISSN:0021-8979    

DOI:10.1063/1.355113

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The heating of a metal surface by ion impact is described by a coupled electron and phonon model that follows from recent research on femtosecond laser heating of metals. The conduction electrons of the metal are directly heated to very high temperature by the impinging ion and then on a longer time scale transfer their energy to the lattice. This results in very efficient heating of the lattice. These processes are described by coupled partial differential equations for the electron and lattice temperatures. The results from three‐dimensional numerical solutions of these equations are presented.

ISSN:0021-8979    

DOI:10.1063/1.355114

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Remote plasma‐activated chemical‐vapor deposition (RPACVD) provides a means to deposit thin dielectric films with low ion bombardment and while having high selectivity in generating precursors. In RPACVD of SiO2, gas mixtures of He/O2or He/N2O are passed through a plasma, producing radicals and excited states that are mixed with silane downstream. Excited states produced in the plasma and precursor species produced by these reactions then flow to the substrate. Although high‐quality SiO2films can be produced by RPACVD, the gas‐phase deposition precursors have not been identified. A two‐dimensional plasma chemistry model is described, and results from that model are used in a discussion of possible gas‐phase precursors for SiO2deposition. In particular, the formation and transport of silanols (SiH2O and SiH3O) are examined as a function of gas mixture, power deposition, and geometry. It is found that the fluxes of SiH2O, SiH3O, and SiH3are sufficient to account for the observed deposition rates; while systematic dependencies of the fluxes of HSiO and SiO discount them as being deposition precursors. He/N2O/SiH4mixtures differ from He/O2/SiH4mixtures by providing larger fluxes of SiH3to the substrate, while the fluxes of SiH2O, SiH3O, and O2(1&Dgr;) are significantly less.

ISSN:0021-8979    

DOI:10.1063/1.355115

出版商:AIP    

年代:1993

数据来源: AIP