Two-step Profile Research Articles

Molecular Beam Epitaxy Growth of AlGaAs/GaAs Vertical Cavity Surface Emitting Lasers and the Performance of PIN Photodetector/Vertical Cavity Surface Emitting Laser Integrated Structures

An all-epitaxial planar top emitting AlGaAs/GaAs multi-quantum well laser is fabricated and characterized. The constructed vertical cavity surface emitting laser (VCSEL) consists of GaAs/Al0.2Ga0.8As (100/80 Å) quantum wells sandwiched between two doped distributed Bragg reflectors characterized by a two-step composition profile. Two Ga and two Al cells are used to facilitate the growth of mirror profile. The gain-guided VCSEL is found to generate continuous wave at a characteristic temperature of 210°K up to 90°C, and can be amplitude modulated at frequencies above 5 GHz. Thresholds as low as 2 mA, and a CW power more than 1.5 mW, are obtained at room temperature. Monolithic integration of a PIN photodetector on top of the VCSEL is demonstrated and discussed. The integrated photodetector shows an effective linear responsivity to the laser emission of 0.25 A/W.

New Non-Axisymmetric Eigenmodes Associated with an Edge Plasma Layer

The effects on Alfven waves of a rarefied plasma layer surrounding a cylindrical main plasma are investigated. The plasma is approximated with a two-step density profile and is assumed to be surrounded by a conducting wall. When the Alfvén resonance exists inside the rarefied plasma layer, two new modes are generated. One has its maximum of the wave intensity at the wall and is thus similar to a surface wave, and the other is a type of coaxial mode. These results are re-examined in a diffuse boundary plasma and the presence of these modes is confirmed.

The interplanetary and solar causes of geomagnetic activity

We present a review of recent work done on the topic of interplanetary and solar causes of geomagnetic activity. During solar maximum (1978–1979), 90% of the major magnetic storms ( D ST ⩽ − 100 nT) are caused by large southward B z events associated with interplanetary shocks. Of these, roughly half of the B z events are located in the sheath and half associated with the driver gas. These two sources of southward IMFs often give magnetic storms a two-step profile. The sheath field events are generated in the interplanetary medium between the outer corona and the Earth from the “shocking” of the slow solar wind upstream of the high speed stream. In contrast, the driver gas events are fields which come from the solar source region. A correlation between the field orientation at the solar source and that at 1 a.u. was sought, but none was found. Thus, quantitative predictions of storm intensities from solar observations appear to be very difficult. Prominence eruptions are shown to be an important cause of the high speed solar wind streams that lead to magnetic storms. The other 10% of the magnetic storms arc not related to interplanetary shocks or high speed streams, but to high density “non-compressional density enhancements”. Following magnetic storms arc “high-intensity long-duration AE activity events” (HILDCAAs) that are series of continuous auroral substorms that last from days to weeks during or after the storm's recovery phase. HILDCAAs can also occur independently of magnetic storms. This continuous auroral activity is caused by the southward component of the magnetic field of interplanetary Alfvén waves, presumably through the process of magnetic reconnection with the Earth's field. These Alfvén wave trains arc often observed in the trailing portions of high speed streams. From an analysis of a year's data during solar maximum, it is found that the interplanetary medium is “Alfvénic” approx. 60% of the time. There appear to be no substantial differences in magnetusphcric response to Alfvénic or non-Alfvénic interplanetary intervals. The magnetopause boundary layer is shown to contain broad-band ELF/VLF plasma waves at least 85% of the time at all daysidc local times. These waves have sufficient amplitude to cause cross-field diffusion of magnetosheath plasma to form the low latitude boundary layer. Pitch angle scattering of the low latitude boundary layer particles is adequate to account for the dayside aurora. The only interplanetary /magnetosheath parameter that appears to affect the wave intensities is the IMF B z . Although the waves arc present at almost all times, they are intensified during southward IMF B z intervals.

Two examples of strong magnetic clouds interacting with the co-rotating stream

Two intense magnetic clouds interacted with the same corotating high-speed solar wind stream in two successive rotations in the fall of 1978. In the August 27 event, the cloud was overtaken by the stream, the front half of the cloud still retained the basic properties of a simple intense cloud while the back half was the interaction region with a two-step velocity profile. In the September 25 event, the cloud was overtaking the same stream, its front half was now the interaction region while the back half had a rather more complex structure than a simple, non-interacting cloud. In the second event, the bulk velocity reached a very high peak value more than 900 km/s in the interaction region, and the Alfvénic speed in the cloud body also reached a high value of more than 600 km/s, which was the local solar wind speed. These two events show further the importance of an enhancement of the Al fvénic speed in accelerating the cloud. The properties of the geomagnetic storm and cosmic ray decrease caused by the clouds are also briefly discussed with reference to the Kp category of geomagnetic storms proposed by the author.

Approximation to the two-step diffusion profile by a series of gaussian functions

The two-step diffusion profile is approximated by a series of gaussian functions. Its accuracy is dependent upon the number of terms used in the series, but independent of the number and the spacing of exact data points.

Approximation to the two-step diffusion profile by a series of gaussian functions

Approximation to the two-step diffusion profile by a series of gaussian functions

Apparatus for HREEL and molecular beam spectroscopy of surfaces: A Gussoni, G Maloberti, L Risso, M Rocca, B Vincenzi and U Valbusa, Dipartimento di Fisica dell'Università di Genova and GNSM, Unità di Genova, Italy

Immiscibility of InGaN hinders epitaxial growth of high-quality AlGaN/InGaN heterojunction, which could have superior performances than AlGaN/GaN in view of high-speed devices. AlGaN/InGaN/GaN double heterostructures have been grown on silicon (111) substrate using plasma assisted molecular beam epitaxy. All growth conditions for each sample have been kept identical except the InGaN channel thickness. Alloy inhomogeneity has been found to occur in the InGaN channel by high resolution (HR) X-ray diffractometer (XRD) and cross-sectional HR- transmission electron microscopy (TEM). This non-uniformity of alloy causes reduced indium incorporation with a decrease of channel thickness along with a thin InN binary alloy. Capacitance-voltage (C-V) profile has revealed non-uniformity of alloy and spatial position of InN in the channel due to variation of band-offset and carrier confinement. Unconventional two-step profile has been obtained for the heterostructure. Higher capacitance at near zero bias corroborates the formation of InN at AlGaN/InGaN due to larger band offset. Conductance-voltage (G-V) profiles further validate mapping of InGaN phase separation in terms of carrier trapping. Lower effect of trapping has been identified due to low bandgap InN formation at the interface. Effect of epilayer relaxation on the phase separation has also been discussed in terms of threading dislocation and V-defects.

Fluid adsorption up to the critical point. Experimental study of a wetting fluid/solid interface

We have measured multilayer adsorption isotherms of propane on graphitized carbon black over a wide temperature range, corresponding to reduced temperatures T/Tc of the fluid from 0.7 to 1.004 and reduced densities ρ/ρc up to 1.4. Experimental isotherms of the surface excess concentration Γgs are analyzed in terms of the Frenkel–Halsey–Hill (FHH) model. The exponent n is somewhat less than 3 (2.55±0.30) and the amplitude parameter Δε/kT becomes nearly independent of temperature, up to T/Tc=0.98, when the simple one-step density profile of the original FHH model is replaced by a two-step profile, to account for the compression of the layer next to the solid substrate. Evidence for a compression of the liquid boundary layer comes from measurements of the surface excess concentration Γls at the liquid/substrate interface. Along the liquid–vapor coexistence curve, Γls0 diverges as T approaches Tc, in qualitative agreement with scaling law theory. The analogy of the present one-component fluid/solid–substrate interface near the critical point of the fluid, with a two-component liquid/vapor interface near the critical solution point of the liquid mixture is discussed.

Isolation, physicochemical properties, and folding of octopine dehydrogenase from Pecten jacobaeus

Two types (isoenzymes) of octopine dehydrogenase (A and B) from Pecten jacobaeus adductor muscle were purified to homogeneity, applying affinity chromatography as an efficient final step of purification. Both forms of the enzyme differ in their electrophoretic mobility. All other physico-chemical and enzymatic properties, as well as the folding behaviour were found to be identical. Interconversion of one form into the other was not detectable. Sedimentation equilibrium, gel permeation chromatography, and NaDodSO4/polyacrylamide gel electrophoresis yield a relative molecular mass of 45000 +/- 1500 for both native and denatured enzyme. The unfolding transition at varying guanidine X HCl concentrations is characterized by a two-step profile: at 0.4-0.8 M, partial unfolding is parallelled by inactivation; at 2.0-2.4 M the residual structure is destroyed in a second unfolding step. Beyond 2.8 M no further changes in fluorescence emission and dichroic absorption are observed. At 0.4-1.8 M guanidine X HCl, partial unfolding is superimposed by aggregation. The emission maximum of the intrinsic protein fluorescence at 327 nm is shifted to 352 nm upon denaturation in 6 M guanidine X HCl. Changes in the far-ultraviolet circular dichroism indicate complete loss of the overall backbone structure in this denaturant, including the native helix content of about 33%. Denaturation in 6 M guanidine X HCl, as monitored by the decrease of protein fluorescence, is fast (less than 8s). Upon reactivation after short denaturation, about 25% of the activity is recovered in a fast initial phase (less than 20s). The product of this phase has a similar stability towards destabilizing additives or proteases as the native enzyme. The slow phase of reactivation, which predominates after long-term denaturation, is determined by a single first-order reaction characterized by tau = 29 +/- 3 min (20 degrees C). This reaction must be a relatively late event on the folding pathway, preceded by the fast formation of a structured intermediate, as indicated by the immediate recovery of the native fluorescence. The structural rearrangements, which are rate-limiting for reactivation after long-term denaturation, are characterized by a high energy of activation (112 +/- 8 kJ/mol). The slow reactivation step is compatible in rate with the first-order folding reactions involved in the reconstitution of several oligomeric dehydrogenases [c.f. R. Jaenicke and R. Rudolph (1983) Colloq. Ges. Biol. Chem. Mosbach 34, 62-90].

Mode birefringence of a three-layer elliptic single-mode fiber wavequide

A vector solution is obtained for the fields of the lowest-order modes in a three-layer elliptic fiber waveguide which is valid in the case of a small ellipticity of the boundaries between the layers, but is not restricted in respect of the differences between the refractive indices of the core and claddings. This solution is used to study mode birefringence as a function of the fiber waveguide parameters. It is shown that for a given core and an outer cladding, a waveguide with the W profile ensuring a high concentration of the energy of the lowest-order modes exhibits a stronger mode birefringence than a waveguide with a two-step profile. Near the limit of single-mode behavior an increase in the thickness of the inner cladding enhances the mode birefringence of a W-type waveguide, but reduces the corresponding birefringence of a two-step waveguide. The sensitivity of a three-layer waveguide to elliptic distortions of the boundaries increases on increase in the difference between the refractive indices of the three media forming the waveguide.

Noise measure and temperature effect in small-signal GaAs IMPATT amplifiers

For two kinds of GaAs IMPATT diodes (flat and two-step doping profile) it is shown that an improved experimental noise measure is obtained by increasing temperature. This can be chiefly explained by a large decrease of GaAs electron saturated drift velocity, as temperature is increased.

The effects of 5-hydroxytryptamine and cyclic AMP on the potential profile across isolated salivary glands.

ABSTRACT The sites of the transepithelial potential changes produced by 5-HT and cyclic AMP in the salivary gland of the blowfly (Calliphora) have been investigated. Microelectrodes recorded a two-step potential profile across this epithelium. The cell was negative to both the bathing medium and the saliva. The basal membrane potential was 44·0 ± 0·2 mV and was affected very little by application of either 5-HT or cyclic AMP. The apical membrane was depolarized by 5-HT and in contrast was hyperpolarized by cyclic AMP. On the basis of these results it is concluded that 5-HT has two actions: (a) to stimulate a potassium pump on the apical membrane using cyclic AMP as an intermediary, (b) to increase chloride movement by a mechanism not directly involving cyclic AMP.

Properties of chromosomal nucleohistone

The deoxyribonucleohistone component may be physically separated from the other components of pea embryo chromatin. Such nucleohistone differs from deproteinized pea embryo DNA in its higher melting temperature, in its histone/DNA mass ratio (1·3: I) and in its inactivity in the support of DNA-dependent RNA synthesis. Pea embryo chromatin contains DNA not only as nucleohistone but also as DNA not so complexed. This is indicated by the two-step melting profile of chromatin as well as by the ability of chromatin to support DNA-dependent RNA synthesis. Approximately 80% of the DNA of pea embryo chromatin is bound in the form of nucleohistone; the remaining 20% appears to be present as DNA not complexed with histone.