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Notes: We have fabricated an antiresonant Fabry–Perot saturable absorber (A-FPSA), for potential use in laser passive mode locking, using metalorganic vapor phase epitaxy followed by ion implantation and thermal annealing. We show that the implantation/annealing cycle shortens the free-carrier dwell time without degrading the other optical properties of the A-FPSA. © 1997 American Institute of Physics.

Notes: We report on a study of Al and Ti/Al contacts to n-type GaN. Al contacts on n-GaN (7×1017 cm−3) annealed in forming gas at 600 °C reached a minimum contact resistivity of 8×10−6 Ω cm2 and had much better thermal stability than reported by previous researchers. Ti/Al (35/115 nm) contacts on n-GaN (5×1017 cm−3) had resistivities of 7×10−6 and 5×10−6 Ω cm2 after annealing in Ar at 400 °C for 5 min and 600 °C for 15 s, respectively. Depth profiles of Ti/Al contacts annealed at 400 °C showed that low contact resistance was only achieved after Al diffused to the GaN interface. We propose that the mechanism for Ohmic contact formation in Ti/Al contacts annealed in the 400–600 °C range includes Ti reducing the GaN native oxide and an Al–Ti intermetallic coming into intimate contact with the GaN. © 1997 American Institute of Physics.

Notes: We have measured the nonlinear optical absorption of arsenic and oxygen implanted epitaxial GaAs for a range of ion doses and annealing temperatures. The response time, τA, and a parameter, Mmax, which characterizes the performance of the structures as modulators, are both reduced by implantation, and correspondingly the nonbleachable losses are increased. We show that similar combinations of (τA, Mmax) can be achieved using either ion species and various combinations of dose and annealing temperatures. Furthermore, the data were all located on a well-defined curve in the (τA, Mmax) plane, provided amorphization, which occurs at high implant doses, was avoided. We deduce that there exists a limit to the modulation if a specific response time is required. © 1999 American Institute of Physics.

Notes: Using high resolution transmission electron microscopy, a thin pseudomorphic AlN layer (2–3 nm) has been observed at the metal/GaN interfaces of Ti/Al (35/115 nm) and Pd/Al (25/125 nm) ohmic contacts to n-type GaN annealed in Ar at 600 °C for 15 and 30 s, respectively. The interfacial layer has a c-plane (002) lattice spacing of 2.48±.03 Å and an a-plane (100) spacing matching that of GaN (2.76 Å), and the identification of this layer as AlN is consistent with chemical analysis by energy dispersive x-ray spectroscopy. The formation of this interfacial AlN layer coincides with the onset of ohmic behavior in Ti/Al and Pd/Al contacts annealed at 600 °C and may be the cause. © 1997 American Institute of Physics.

Notes: Quasiclassical trajectory calculations of the energy transfer of highly vibrationally excited benzene and hexafluorobenzene (HFB) molecules colliding with helium, argon and xenon have been performed. Deactivation is found to be more efficient for HFB in accord with experiment. This effect is due to the greater number of low frequency vibrational modes in HFB. A correlation between the energy transfer parameters and the properties of the intramolecular potential is found. For benzene and HFB, average energies transferred per collision in the given energy range increase with energy. Besides weak collisions, more efficient "supercollisions'' are also observed for all substrate–bath gas pairs. The histograms for vibrational energy transfer can be fitted by biexponential transition probabilities. Rotational energy transfer reveals similar trends for benzene and HFB. Cooling of rotationally hot ensembles is very efficient for both molecules. During the deactivation, the initially thermal rotational distribution heats up more strongly for argon or xenon as a collider, than for helium, leading to a quasi-steady-state in rotational energy after only a few collisions. © 1995 American Institute of Physics.

Notes: Quasiclassical trajectory calculations have been applied to study intermolecular potential effects in the collisional deactivation of highly vibrationally excited aromatic molecules by noble gases. For benzene+helium and benzene+argon several potential functions of Lennard-Jones 12-6 and EXP-6 type were used. At low well depths ε (for benzene+helium) energy transfer is dominated by the exact shape of the intermolecular potential, especially of the repulsive part, whereas the dependence on ε itself is less important. At higher well depths (for benzene+argon) the energy transfer parameters 〈ΔE〉 and 〈ΔE2〉 scale with ε and are much less sensitive to details of the interaction. New potential parameters based on experimental scattering data for the benzene+helium system are presented, which provide very good agreement with experimental energy transfer parameters for the deactivation of benzene and azulene by helium. © 1996 American Institute of Physics.

Notes: Objective To assess the influence of hypertensive disorders in pregnancy on the subsequent risk of placental abruption and uterine bleeding of unknown aetiology, and to examine the combined effects of hypertensive disorders and cigarette smoking during pregnancy on the risk of uteroplacental bleeding disorders.Design Retrospective cohort study.Setting Data for this study were derived from the Nova Scotia Atlee Perinatal database, Canada, comprising of women who were delivered in the province between 1980 and 1993.Population 120,666 pregnancies resulting in singleton births, of which 13,360 pregnancies were complicated by pre-eclampsia and/or chronic hypertension.Main outcome measures Risks and relative risks of placental abruption and uterine bleeding of unknown aetiology in pregnancies complicated by chronic hypertension, mild and severe pre-eclampsia, and chronic hypertension with superimposed pre-eclampsia, each compared with normotensive patients. Adjusted relative risks were obtained through the fit of multivariable logistic regression models based on the method of generalised estimating equations.Results Chronically hypertensive women had no increased risk of abruption (RR 1.4; 95% CI 0.5–3.6), while women whose pregnancies were complicated by severe pre-eclampsia (RR 3.8; 95% CI 2.1–6.9), and chronic hypertension with superimposed pre-eclampsia (RR 2.8; 95% CI 1.2–6.3) showed strong associations with placental abruption. However, none of the hypertensive disorders were associated with uterine bleeding of unknown aetiology. The association between placental abruption and hypertensive disorders varied by parity. Parous women with chronic hypertension and superimposed pre-eclampsia were at greater risk of placental abruption (aRR 3.8; 95% CI 1.9–7.8) than nulliparous women with chronic hypertension and superimposed pre-eclampsia (aRR 1.6; 95% CI 0.5–4.9). The joint effects of smoking and hypertension had a greater effect on the risk of placental abruption than would have been expected based on their individual effects.Conclusions The pattern of association between placental abruption and hypertension varied in relation to the specific type of hypertensive disorder. However, uterine bleeding of unknown aetiology was not associated with hypertension. Findings from this study suggest that placental abruption and uterine bleeding of unknown origin are aetiologically distinct obstetric complications with respect to hypertensive disorders during pregnancy.

Notes: Summary: Mouse mammary tumor virus has developed strategies to exploit the immune response. It requires vigorous immune stimulation to achieve efficient infection. The infected antigen-presenting cells present a viral superantigen on the cell surface which stimulates strong CD4-mediated T-cell help but CDS T-cell responses are undetectable. Despite the high frequency of superantigen-reactive T cells, the superantigen-induced immune response is comparable to classical antigen responses in terms of T-cell priming, T-cell—B-cell collaboration as well as follicular and extra-follicular B-cell differentiation. Induction of systemic anergy is observed, similar to classical antigen responses wbere antigen is administered systemically but does not influence the role of the superantigen-reactive T cells in the maintenance of the chronic germinal center reaction. So far we have been unable to detect a cytotoxic T-cell response to mouse mammary tumor virus peptide antigens or to the superantigen. This might yet represent another step in the viral infection strategy.

Notes: The collisional deactivation of highly vibrationally excited 1,3,5-cycloheptatriene (CHT) in compressed gases, supercritical fluids, and in liquids was studied. Different bath gases and solvents (alkanes, alcohols, CO2, CHF3, nitrogen) and pressures up to 200 bars were used. This way, measurements covered the complete gas–liquid transition regime. CHT with a vibrational energy of 41 000 cm−1 was generated by laser excitation into the S1 state and internal conversion to the S0 ground state. From picosecond UV absorption measurements, energy-loss profiles were obtained taking into account the density dependent absorption spectra, which were measured separately. A monoexponential decrease of the internal energy in CHT was observed under all studied conditions. In gases at pressures up to 40 bars, the relaxation rates increase linearly with the density and the energies transferred per collision, 〈ΔE〉, agree with those values obtained in earlier low pressure gas phase experiments. At higher densities, the relaxation rates deviate markedly from predictions based on simple scaling assumptions with appropriate collision frequencies. The deactivation in liquids is slower by a factor of 3–5 than expected from such isolated binary collision (IBC) models. These results are rationalized using a model that takes into account the finite lifetime of collision complexes. © 1997 American Institute of Physics.