Search

The investigation of the gains and threshold currents is carried out for several quantum well lasers of differing potential well shape. The theoretical threshold current density as a function of laser cavity length compares very well with the result obtained from experiments. The gain and photon density as a function of time are reported for several GRIN-SCH potentials of differing shape. The gain of multiple quantum well potentials shows a narrower spectrum than single quantum well potentials. The oscillation frequency of a laser when it is excited by short current pulse is linear with the square root of the power, in agreement with experiments.

Results obtained from recent theoretical investigations of the interaction of intense electromagnetic pulses with gases when the field strength is above the threshold for avalanche breakdown are presented. It is shown how the breakdown affects the shapes of the pulses. In addition, the accuracy of the calculational models is discussed by comparing some calculations with experimental data. It is noted that this is not a comprehensive review of the field of microwave breakdown. >

The state of research on intense electromagnetic pulse propagation in air and in waveguides is reviewed. Some results obtained from recent investigations of the propagation of intense electromagnetic pulses in the atmosphere and in an air-filled waveguide when the field strength is above the threshold for air breakdown are presented. It is shown how the air breakdown affects the shape of the pulse. In addition, the accuracy of the calculational models is discussed, comparing calculations with experimental data. >

Self-consistent equations-Maxwell's equations and the electron fluid transport equations-are used to study the interaction of long, intense microwave pulses with gases. The threshold for breakdown is a factor of seven higher in SF/sub 6/ than in air. This difference is attributed to the fact that SF/sub 6/ is an insulator. When SF/sub 6/ was mixed with air, the breakdown threshold increased with the increase in the percentage of SF/sub 6/. Introducing 5-10% of SF/sub 6/ in air increased the breakdown threshold by a factor of 3-4. The electron density and conductivity and the plasma frequency increased as the pulse intensity increased. The growth rates of the electron density and conductivity were much higher in air than in SF/sub 6/. >

In vitro mutagenic activity and carcinogenic potential of metronidazole in certain animals raised concerns about its possible carcinogenicity in humans,We studied the late incidence of cancer after metronidazole use among persons enrolled in the Group Health Cooperative of Puget Sound, Seattle, a health maintenance organization. Randomly selected nonusers were matched on a one-to-one basis for age, gender, and year of enrollment to persons who used metronidazole on an outpatient basis during the period January 1975 to December 1983; 5,222 metronidazole user/nonuser pairs, for whom the median follow-up was 12.6 years, were analyzed. Forty-nine percent, 39.2%, 9.8%, and 2% of users had 1, 2-4, 5-9, and greater than or equal to 10 prescriptions or refills of metronidazole filled, respectively, The late (after the first 7 years of follow-up) incidence of cancer was nearly identical among users and nonusers (652 and 662 per 100,000 person-years, respectively; relative risk, 0.98; 95% confidence interval, 0.80-1.20). Age-gender stratified analysis did not reveal any association between metronidazole use and cancer. These data support no association between short-term exposure to metronidazole and cancer in humans. Although the results are reassuring, they map not extend to subjects who have used metronidazole for prolonged periods; further epidemiological studies should focus on these individuals.

OBJECTIVE--To estimate the rate and means of suicide among people taking 10 commonly prescribed antidepressant drugs: dothiepin, amitriptyline, clomipramine, imipramine, flupenthixol, lofepramine, mianserin, fluoxetine, doxepin, and trazodone. DESIGN--Open cohort study with a nested case-control analysis. SETTING--General practices in the United Kingdom that used VAMP computers to maintain their patient records from January 1988 to February 1993. SUBJECTS--172,598 people who had at least one prescription for one of the 10 antidepressants during the study period. MAIN OUTCOME MEASURE--Suicide confirmed by general practitioner or on death certificate, or both. RESULTS--143 people committed suicide. The overall rate of suicide was estimated to be 8.5 per 10,000 person years (95% confidence interval 7.2 to 10.0). Rates of suicide were higher in men than women (relative risk 2.8 (95% confidence interval 1.9 to 4.0)), people with a history of feeling suicidal (19.2 (9.5 to 38.7)), and people who had taken several different antidepressants (2.8 (1.8 to 4.3)). People who received high doses of antidepressants and those who had had a prescription in the 30 days before they committed suicide were also at higher risk than those who had received low doses and had had their prescriptions 30 or more days previously (2.3 (1.4 to 3.7) and 2.3 (1.6 to 3.4)) respectively. Rates of suicide were higher in patients who received fluoxetine, but this may be explained by selection biases which were present for those drug users. CONCLUSION--Several factors correlate with the risk of suicide in people taking antidepressants. After controlling for these factors, the risk of suicide was similar among the 10 study antidepressants. Overdose with antidepressants accounted for only 14% of the suicides.

This paper describes our first-principles derivation of the Hamiltonian for the calculation of the bound states for a particle with a spatially varying mass in an arbitrary potential well. Our approach is based on the Feynman path-integral technique and results in a Hermitian form of the Schroedinger equation that conserves current at the boundary. We deduced the correct operator ordering for the Hamiltonian and found a correctional term that can be viewed as an effective potential.

For a number of years, we have numerically investigated intense microwave pulse propagation in the atmosphere. For short pulses (

The ionization coefficient of deep traps in GaAs is determined from a gas breakdown model together with the recent experimental data obtained at LLNL (Lawrence Livermore National Laboratory) and Boeing. Using this coefficient in our nonlinear device transport code, we have investigated theoretically the nonlinear switching phenomena in GaAs devices. The results obtained from our investigations show that if we take into consideration the effect of the field ionization of the deep traps, we can show how the "Lock-On" phenomena could occur in the device.

The gas avalanche switch is a recently conceived, laser-activated, high-voltage, picosecond-speed switch. It basically consists of a set of pulse-charged electrodes immersed in a gas at high-pressure (2--800 atm). A picosecond-scale laser pulse initiates an avalanche discharge in the gas between the electrodes. With the proper configuration, the avalanche, which is fueled by the immense number of electrons available in the gas, causes the applied voltage to collapse in picoseconds and generates electromagnetic waves. A two-dimensional electron fluid computer code solves Maxwell`s curl equations and a set of electron fluid equations for transverse magnetic modes in air between parallel plane conductors. Collision frequencies for ionization and momentum and energy exchange to neutral molecules are taken to scale directly with neutral pressure. Calculations are performed for a Blumlein pulse generator geometry, featuring a charged rectangular center electrode between grounded parallel plates spaced 2 mm apart. In one mode of operation, initial electrons are induced in the lower air gap, the gap between the center electrode and the lower plate. At 292 kV on the center electrode, 27 atm pressure, and uniform ionization under the full width of the center electrode, induced voltage pulses of 300 kV, 2.4 ps rise time, 9.1more » ps full width at half maximum (FWHM), 38 ps duration, and 22 GHz bandwidth at 3 dB occur at the ends of the parallel plates. Reduction of the initial electron number over eleven orders of magnitude has very little gross effect on these pulses. Concentration of the initial electrons into a narrow, centered column generally results in reductions in rise time, FWHM, and pulse duration, as well as increases in peak voltage. Movement of the narrow column to the extreme left of the center electrode causes the voltage pulses at the left and right sides of the parallel plates to differ somewhat in their characteristics. 10 refs.« less

Wideband, high-power microwave pulses are expected to have important applications in the future. One of these applications is ultra-wideband radar. The wide bandwidth should generate increased information for targetcharacterization and identification. The high power should result in increased target detection range for conventional targets and targets with reduced signatures. A way to generate wideband, high-power microwave pulses with relatively conventional technology is to tail erode high-power pulses by passage through a low-pressure air cell. In this process, the tails ofshort(3 — 10 ns), high-amplitude (

Wideband, high-power microwave pulses are expected to have a number of important future applications. One of these is wideband radar. The wide bandwidth should yield increased information for target characterization and identification.1 The high power should yield increased target range for conventional objects. It should also make possible the detection of previously undetectable, reduced-signature objects.

For the case of radio and microwave pulses propagating in the earth’s ionosphere, which motivated this work and is the source of our examples, the dispersion process is nearly timereversible, since collisional losses are very small. As a result, a long transmitted pulse of suitable amplitude and frequency modulation can be received as a compressed pulse of much higher amplitude at a chosen location in the ionosphere. The compression factor will be limited by source bandwidths and modulation control, distortion of broadband signals by antennas, and uncertainties in the propagating medium. In this paper, we present examples of compressed pulses for a variety of source waveforms. Our computer similations were performed by convolving the source signal with the dispersed waveform of a triangular element. We have obained a time-domain expansion, in which terms due to moments of the electron density, an earth’s axial magnetic field, and refraction of oblique trajectories, can be identified.

The gas avalanche switch, a high-voltage, picosecond-speed switch, has been proposed. The basic switch consists ofpulse-charged electrodes, immersed in a high-pressure (7—800 atm) gas. An avalanche discharge is induced in the gasbetween the electrodes by ionization from a picosecond-scale laser pulse. The avalanching electrons move toward the anode,causing the applied voltage to collapse in picoseconds. This voltage collapse, if rapid enough, generates electromagneticwaves. A two-dimensional (2D), finite difference computer code solves Maxwell's equations for transverse magnetic modesfor rectilinear electrodes between parallel plate conductors, along with electron conservation equations for continuity,momentum, and energy. Collision frequencies for ionization and momentum and energy transfer to neutral molecules areassumed to scale linearly with neutral pressure. Electrode charging and laser-driven electron deposition are assumed to beinstantaneous.Code calculations are done for a pulse generator geometry, consisting of an 0.7 mm wide by 0.8 mm high, beveled,rectangular center electrode between grounded parallel plates at 2 mm spacing in air. In one operational mode, a uniformdistribution of initial electrons is induced in the gap between the center electrode and the lower plate. With the centerelectrode at 227 kV positive and 15 atm pressure, voltage pulses of 24 1 kV, 2 Ps rise times, 8 ps full widths at halfmaximum (FWHM), 30 ps durations, and 24 GHz 3 dB bandwidths are induced at the ends of the plates by passingelectromagnetic waves. Similar pulses occur at 292 kV, 27 atm and 3.9 MV, 700 atm. Slower pulses are generated at

Laboratory chamber experiments in low-pressure (0.1–10 Torr) air with long-pulse (150–600 ns), 2.856 GHz crossed microwave beams, created by reflection of an incident beam from a metal plate inclined at 45°, have shown that multiple, luminous electron density layers form toward the source of the incident beam.1 Previous two-dimensional calculations2 of microwave air breakdown in a rectangular waveguide with full resolution of each cycle of the driving electric field required Cray 1 computation times of about 0.5 hr/ns for runs of up to 20 ns of simulation time. To simulate chamber experiments with pulses of 150–600 ns duration, computation times with full-cycle resolution must be expected to be 75–300 hours per run. Because these are prohibitive run times, a code, which follows only the pulse envelope, has been developed to reduce computation time. This envelope approximation is an extension of the one-dimensional technique for atmospheric microwave propagation reported previously.3

To investigate the propagation and reflection of laser light in slab heterostructure configurations, especially quantum wells, we are developing 2-D finite difference computer codes for self-consistent solution of Maxwell’s curl equations in complicated refractive media. The exact Maxwell’s equations are solved without the approximations inherent in the high-frequency envelope methods. Separate codes treat TE and TM modes. These codes will be used in semiconductor laser research and design. The field gradients in the charge free equations are conveniently approximated by central finite differences on a staggered mesh. The three modal components occupy a triangle of grid points within a four-point computational cell. Appropriate boundary conditions are imposed at the mesh extremities. The global set of ordinary differential equations from this discretization are solved with the well known implicit, variable step size solver LSODE. The propagation of low-order eigenmodes, perturbed eigenmodes, and noneigenmodes in step refractive, parabolic, and quantum well GaAs/AlGaAs structures is investigated.