Carrier transport effects in quantum well lasers

EditorialSpecial Issue

Carrier transport effects in quantum well lasers     

Analysis of the effects of doping and barrier design on the small-signal modulation characteristics of long-wavelength multiple quantum well lasers

This theoretical work has modelled the small signal response of InGaAsP and InGaAlAs multiple quantum well (MQW) lasers based on an ambipolar carrier transport model. The MQW parameters such as barrier bandgap, barrier width and the number of quantum wells have been optimized for high-speed modulation. The effect of the p-type doping and the strain of the InGaAs well have also been investigated.For the InGaAsP-based system, the optimization for maximum 3 dB bandwitdth shows that the optimum width is about 5 nm for 1.1 m barriers and 7 nm for 1.2 m barriers. The optimum barrier bandgap wavelength is about 1.1 m for the barrier width of 6 nm, about 1.15 m for 8 nm and 10 nm barriers. The p-doped MQW exhibits a higher modulation bandwidth because of its high differential gain and improved carrier distribution among the MQWs. The compressively strained InGaAs quantum well system has the potential for a higher modulation bandwidth. For the InGaAlAs-based system, the optimization for maximum 3 dB bandwidth shows that the optimum width is about 4 nm for a barrier wavelength of 1.10 m, and 6 nm for 1.2 m. The optimum barrier bandgap wavelength is about 1.1 m for a barrier width of 4 nm, and about 1.2 m for 6, 8 and 10 nm.     

Interactive influence and optimization of reaction parameters in the hydrolysis of olive and coconut triacylglycerols byC. cylindracea andR. javanicus lipases in reverse micellar media

The four reaction parameters—pH, reaction temperature, molar ratio (R) of water to surfactant, and Aerosol-OT concentration—have significant linear, quadratic., and interactive effects on the initial rate and the degree of hydrolysis for lipase-mediated hydrolysis of triacylglycerols (TAG) in Aerosol-OT/iso-octane reverse micellar media. Reaction temperature and pH had the most significant influence on the rate and the degree of hydrolysis, whereas Aerosol-OT concentration had the least influence on those parameters. The initial rate was most influenced by the interactive effect of pH with all other variables, whereas the degree of hydrolysis was the most influenced by the interactive effect of reaction temperature with other variables. Lowerlevel variable combinations were favorable over higher-level variable combinations to TAG hydrolysis in reverse micellar media. Regression models, developed for the initial rate and the degree of hydrolysis as a function of reaction variables, accounted for up to 96% of the variation in the two responses. The optimum reaction condition ranges were determined based on maximization of both the rate and the degree of hydrolysis. The differences in the physicochemical characteristics of substrates had no significant effect on the optimum condition ranges. However, the noticeable differences were observed for these ranges between the systems withRhizopus javanicus andCandida cylindracea lipases. Lipase-catalyzed hydrolysis of TAG in Aerosol-OT/iso-octane reverse micellar media was optimum at about 22‡C, 140 mM Aerosol-OT concentration, pH 6.8, andR value of 14.     

Amine-templated linear vanadium sulfates with different chain structures

Amine-templated vanadium sulfates of the formula [HN(CH(2))(6)NH][(V(IV)O)(2)(OH)(2)(SO(4))(2)].H(2)O, I, [H(3)N(CH(2))(2)NH(3)][V(III)(OH)(SO(4))(2)].H(2)O, II, and [H(2)N(CH(2))(4)NH(2)][(V(IV)O)(H(2)O)(SO(4))(2)], III, have been prepared under hydrothermal conditions. These vanadium sulfates add to the new emerging family of organically templated metal sulfates. Compound I has a linear chain structure consisting of V(2)O(8) square-pyramid dimers connected by corner-sharing SO(4) tetrahedra, creating four-membered rings along the chain. Both II and III possess simple linear chain topologies formed by VO(6) octahedra and SO(4) tetrahedra, with II having the tancoite chain structure. Compound I crystallizes in the triclinic space group P1 (No. 2) with a = 7.4852(4) A, b = 9.5373(5) A, c = 11.9177(6) A, alpha = 77.22 degrees, beta = 76.47(2) degrees, gamma = 80.86 degrees, Z = 2. Compound II: monoclinic, space group P2(1)/c (No. 14), a = 6.942(2) A, b = 10.317(3) A, c = 15.102(6) A, beta = 90.64(4) degrees, Z = 4. Compound III: triclinic, space group P1 (No. 2) with a = 6.2558(10) A, b = 7.0663(14) A, c = 15.592(4) A, alpha = 90.46(2) degrees, beta = 90.47(2) degrees, gamma = 115.68(2) degrees, Z = 2. Magnetic susceptibility measurements reveal weak antiferromagnetic interactions in I and III and ferromagnetic interactions in II.     

Anomalous1H NMR spectra of 3,4-dihydroisoquinolines and related compounds

Solutions of 3,4-dihydro-6,7-methylenedioxyisoquinoline (1) and analogues2–5 and9 as well as isoquinolines7 and8 in certain samples of CDCl₃, CCl₄, DMSO-d₆ or acetone-d₆ gave rise to anomalous¹Hnmr spectra with extreme line broadening, signals due to protons at C-l and C-3 most often not being seen. The spectra of1 and3 were most striking in this respect.¹Hnmr spectra of the quaternary salt10 and the model schiff base11 were normal. Several hypotheses for the observed line broadening have been considered and rejected, a slow equilibrium1 ⇆13 being one of them. NaBH₄ reduction of1 and2 followed by mass spectrometric analysis of the crude tetrahydroisoquinolines16 and17 ruled out a slow equilibrium1 ⇆14 and2 ⇆15 as contributory cause for line broadening. The crude reduction products unexpectedly contained N-ethyl species22 and25. Their formation is rationalised     

Molecular dynamics for very large systems on massively parallel computers: The MPSim program

We describe the implementation of the cell multipole method (CMM) in a complete molecular dynamics (MD) simulation program (MPSim) for massively parallel supercomputers. Tests are made of how the program scales with size (linearly) and with number of CPUs (nearly linearly) in applications involving up to 10⁷ particles and up to 500 CPUs. Applications include estimating the surface tension of Ar and calculating the structure of rhinovirus 14 without requiring icosahedral symmetry. © 1997 by John Wiley & Sons, Inc.     

Crystal growth,thermal, optical and microhardness studies of tris (thiourea) zinc sulphate - a semiorganic NLO material

Tris (thiourea) zinc sulphate (ZTS), a semiorganic nonlinear optical (NLO) material has been synthesized at 30 °C. The solubility was determined in different solvents such as water, ethanol and water mixture of ethanol (1:1). Good quality single crystals with size 10 × 8 × 6 mm³ were grown by slow evaporation technique within three weeks with approximate growth rate of 0.5 mm/day. The grown crystals have been subjected to single crystal X-ray diffraction to determine the unit cell dimensions and morphology. The TGA and DTA reveal that the material has good thermal stability. The UV-Vis spectrum shows that the material has wide optical transparency in the entire visible region. The second harmonic generation was confirmed by Kurtz powder method. The birefringence of the crystal was measured in the visible region and it was found to vary with the wavelength. The microhardness test was carried out in (100) plane and the load dependent hardness was measured. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Superconductivity in new quaternary borocarbide system R–Re–B–C (R=Y, Gd, Tb and Lu)

We report here our observation of superconductivity in a new quaternary borocarbide system R–Re–B–C (R=Y, Gd, Tb and Lu). Samples of the nominal compositions RReBC exhibit superconductivity with T_c≈6 K in LuReBC which is high for an intermetallic. TbReBC and GdReBC exhibit superconductivity at ≈4 K. A magnetic transition is also observed in TbReBC around 30 K. From our measurements, we infer that superconductivity originates from a bulk quaternary superconducting phase in these materials.