Non-equilibrium dynamics in the dual-wavelength operation of vertical external-cavity surface-emitting lasers.

Microscopic many-body theory coupled with Maxwell's equation is used to study dual-wavelength operation in vertical external-cavity surface-emitting lasers. The intrinsically dynamic nature of coexisting emission wavelengths in semiconductor lasers is associated with characteristic non-equilibrium carrier dynamics, which causes significant deformations of the quasi-equilibrium gain and carrier inversion. Extended numerical simulations are employed to efficiently investigate the parameter space to identify the regime for dual-wavelength operation. Using a frequency selective intracavity etalon, two families of modes are stabilized with dynamical interchange of the strongest emission peaks. For this operation mode, anti-correlated intensity noise is observed in agreement with the experiment. A method using effective frequency selective filtering is suggested for stabilization of genuine dual-wavelength output.

Author Correction: High-temperature operation of electrical injection type-II (GaIn)As/Ga(AsSb)/(GaIn)As "W"-quantum well lasers emitting at 1.3 µm.

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High-temperature operation of electrical injection type-II (GaIn)As/Ga(AsSb)/(GaIn)As "W"-quantum well lasers emitting at 1.3 µm.

Electrical injection lasers emitting in the 1.3 µm wavelength regime based on (GaIn)As/Ga(AsSb)/(GaIn)As type-II double "W"-quantum well heterostructures grown on GaAs substrate are demonstrated. The structure is designed by applying a fully microscopic theory and fabricated using metal organic vapor phase epitaxy. Temperature-dependent electroluminescence measurements as well as broad-area edge-emitting laser studies are carried out in order to characterize the resulting devices. Laser emission based on the fundamental type-II transition is demonstrated for a 975 µm long laser bar in the temperature range between 10 °C and 100 °C. The device exhibits a differential efficiency of 41 % and a threshold current density of 1.0 kA/cm2 at room temperature. Temperature-dependent laser studies reveal characteristic temperatures of T0 = (132 ± 3) K over the whole temperature range and T1 = (159 ± 13) K between 10 °C and 70 °C and T1 = (40 ± 1) K between 80 °C and 100 °C.

Novel type-II material system for laser applications in the near-infrared regime.

The design and experimental realization of a type-II "W"-multiple quantum well heterostructure for emission in the λ > 1.2 µm range is presented. The experimental photoluminescence spectra for different excitation intensities are analyzed using microscopic quantum theory. On the basis of the good theory-experiment agreement, the gain properties of the system are computed using the semiconductor Bloch equations. Gain values comparable to those of type-I systems are obtained.

Influence of many-body interactions during the ionization of gases by short intense optical pulses.

The excitation of atomic gases by short high-intensity optical pulses leads to significant electron ionization. In dilute systems, the generated distribution of ionized electrons is highly anisotropic, reflecting the quantum mechanical properties of the atomic states involved in the many photon transitions. For higher atomic densities, the Coulomb interaction in the electron-ion system leads to the development of an isotropic electron plasma. To study the ionization process in the presence of the many-body interaction, a fully microscopic model is developed that combines a generalized version of the optical Bloch equations describing the optical excitation with a microscopic description of the many-body interactions. The numerical evaluation shows that the Coulomb interaction significantly modifies the distribution anisotropy already during the excitation process. Whereas a reduced anisotropy is still present after the pulse for low ionization degrees and pressures, it is completely absent for elevated gas densities. An ionization degree is predicted that is significantly enhanced by the many-body interactions.

Quantum theory of terahertz emission due to ultrashort pulse ionization of gases.

A microscopic model is developed to analyze terahertz (THz) emission after ultrashort one- and two-color laser-pulse excitations of an atomic gas. Optical Bloch equations are derived to describe the pulse-induced ionization in the many-atom system including the Coulombic scattering of the ionized electrons. The model captures the continuous transition between the tunneling and the multiphoton ionization regimes. Numerical evaluations for a wide range of pulse configurations identify optimized excitation conditions for strong THz emission.

Impact of rural hospital closures in Saskatchewan, Canada.

Canada's health care system has undergone major changes since 1990. In Saskatchewan, 52 small rural hospitals funded for less than eight beds stopped receiving funding for acute care services in 1993. Most were subsequently converted to primary health care centers. Since then, concerns have been raised about the impact of the changes on rural residents' access to care, their health status, and the viability of rural communities. To assess the impact of hospital closures on the affected communities, we conducted a multi-faceted, province-wide study. We looked at hospital use patterns, health status, rural residents' perceptions of the impact of these hospital closures, and how communities responded to the changes. We found the hospital closures did not adversely affect rural residents' health status or their access to inpatient hospital services. Despite widespread fears that health status would decline, residents in these communities reported that hospital closures did not adversely affect their own health. Although some communities continue to struggle with changes to health care delivery, others appear to have adapted as a result of strong community leadership, the development of widely accepted alternative services, and local support for creating innovative solutions. Good rural health care does not depend on the presence of a very small hospital that cannot, in today's environment, provide genuinely acute care. It requires creative approaches to the provision of primary care, good emergency services, and good communication with the public on the intent and outcomes of change.

Hyperbaric oxygen treatment for experimental cyclophosphamide-induced hemorrhagic cystitis.

Acrolein is a toxic metabolite of cyclophosphamide that causes hemorrhagic cystitis in 2 to 40% of treated patients. Hyperbaric oxygen (HBO) is used to treat poorly healing wounds in conditions such as Fournier's gangrene and radiation-induced cystitis. The present study was designed to evaluate the effects of HBO on acute acrolein-induced hemorrhagic cystitis in a rat model. Rats were divided into 4 groups. Group I served as a control and received only HBO prior to sacrifice. Group II received acrolein only, while groups III and IV received acrolein as well as HBO therapy. Hyperbaric oxygen (100% oxygen, 2.8 atmospheres, 90 minutes) was delivered twice a day for 4 days, with group III receiving a fifth HBO treatment just before acrolein and group IV receiving the fifth HBO treatment just after acrolein. After therapy, the amount of urothelial injury was determined morphometrically. Group II untreated rat bladders had only 33% of the urothelium intact after acrolein injury, whereas groups III and IV rat bladders had 93% (p < 0.01) and 55% (p < 0.01) intact urothelium, respectively, after treatment with HBO. The timing of the HBO treatment appeared to be a critical factor, with less injury occurring if the fifth HBO treatment immediately preceded acrolein. These results suggest that HBO may be useful as prophylaxis and treatment of cyclophosphamide-induced hemorrhagic cystitis.