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The far-field emission and spectral characteristics of a photoniccrystal distributed-feedback (PCDFB) laser with an antimonide type-II 'W' active region were studied theoretically and experimentally. A 2nd-order grating was defined on a 2D rectangular lattice tilted by 20° relative to the facet normal, using optical lithography and dry etching. For pulsed optical pumping, the emission line centred on λ = 4.6 4.7µm was up to an order of magnitude narrower (7-10nm) than those of Fabry-Pérot and angled-grating DFB (α-DFB) lasers fabricated from the same wafer. The PCDFB beam quality was also substantially enhanced, for example, by a factor of 5 compared to the α-DFB at a pump-stripe width of 200µm. Theoretical simulations based on a self-consistent time-domain Fourier-transform simulation confirm significant advantages of the PCDFB laser over an α-DFB geometry. [ABSTRACT FROM AUTHOR]

The thermal properties of 5-stage 'W' Interband-Cascade Lasers emitting at 4.1 μm at room temperature (RT) are investigated by measuring the lasing and spontaneous emission properties as a function of temperature and hydrostatic pressure up to 1 GPa. Experiments show that at RT more than 90% of threshold current of these devices is due to non-radiative loss processes. We also find that the threshold current density dependence on temperature can be fitted with a single exponential function over a wide temperature range with a characteristic temperature, T0, of 45 K. The relatively high temperature sensitivity in these devices is attributable to the large non-radiative current contribution coupled with non-pinning of the carrier density above threshold. [ABSTRACT FROM AUTHOR]

The equations for threshold-current density Jth, differential quantum efficiency ηd, and maximum wallplug efficiency ηwp,max for quantum-cascade lasers (QCLs) are modified for electron leakage and backfilling. A thermal-excitation model of “hot” injected electrons from the upper laser state to upper active-region states is used to calculate leakage currents. The calculated characteristic temperature T0 for Jth is found to agree well with experiment for both conventional and deep-well (DW) QCLs. For conventional QCLs ηwp,max is found to be strongly temperature dependent; explaining experimental data. At 300 K for optimized DW-QCLs, front-facet, continuous-wave ηwp,max values >20% are projected. [ABSTRACT FROM AUTHOR]

We report interband cascade lasers operating in a single spectral mode (λ≈3.6 μm) at -5–30 °C. A corrugated pattern etched into both sidewalls of the 6- and 9-μm-wide ridges serves to suppress higher-order lateral modes by increasing their loss, and also provides a fourth-order distributed-feedback grating for longitudinal mode selection. Despite the grating’s weak coupling strength, the 9 μm ridge produced up to 12 mW per facet of single-mode cw output power at 25 °C, with a side-mode suppression ratio of >30 dB. [ABSTRACT FROM AUTHOR]

4.8 μm emitting, quantum cascade (QC) lasers that suppress carrier leakage out of their active regions to the continuum have been realized by using deep (in energy) quantum wells in the active regions, tall barriers in and around the active regions, and tapered conduction-band-edge relaxation regions. The characteristic temperature coefficients T0 and T1 for the threshold current density Jth and slope efficiency, respectively, reach values of 238 K over the 20–60 °C temperature range, which means that Jth and the slope efficiency vary with temperature half as fast as those of conventional QC lasers. In turn, significantly improved continuous wave performance is expected. [ABSTRACT FROM AUTHOR]

Mid-infrared diode lasers with type-II “W” active regions are analyzed using a fully microscopic many-body theory. The Auger carrier losses are found to dominate over radiative losses even at low temperatures. The experimentally observed strong temperature-dependent increase in Auger losses is shown to be a consequence of thermal gain reduction causing increased threshold carrier densities. Good agreement between theory and experiment is demonstrated for temperature-dependent photoluminescence spectra as well as threshold loss currents. [ABSTRACT FROM AUTHOR]

Lifetimes and Auger coefficients for type-II W interband cascade lasers are deduced from correlations of the experimental threshold current densities and slope efficiencies with calculated threshold carrier densities and optical gains. The room-temperature Auger coefficients for a number of low-threshold devices emitting at wavelengths from 2.9 to 4.1 μm fall in the narrow range of (3–5)×10-28 cm6/s, which represents a much stronger suppression of Auger decay than was implied by most earlier experiments and theoretical projections. The Auger coefficient is nearly independent of the thicknesses and compositions of the layers in the W active region. [ABSTRACT FROM AUTHOR]

The magnitude of threshold reduction in a semiconductor laser with electron spin injection is shown to depend on such intrinsic properties of the active region as the dominant recombination mechanism, the ratio of hole-to-electron densities of states, the active-region doping, and the available material gain as well as cavity properties such as the optical loss. The threshold reduction is expected to be greatest when the laser’s active region is undoped, the recombination is strongly dominated by Auger processes, and the threshold gain is low. It can approach a factor of 3.5 for fully spin-polarized electrons in the active region. [ABSTRACT FROM AUTHOR]

We have demonstrated an electrically pumped photonic-crystal distributed-feedback laser with an interband cascade active region emitting at 3.3 μm. At 78 K, the stripe of width 400 μm emits up to 67 mW of cw power into a single spectral mode with side-mode suppression ratio ≈27 dB. The full width at half maximum of the far-field divergence angle is ≈0.5°, which combined with the near-field profile yields an effective M2 of 1.7–2.0. [ABSTRACT FROM AUTHOR]

The authors report the fabrication and characterization of mid-IR HgCdTe negative luminescence (NL) devices grown on silicon substrates. Sensitive optical modulation measurements of the 5 mm square array with ∼5.4 μm cutoff and single-layer antireflection (AR) coating yield an internal NL efficiency of 98% at room temperature. This is the highest ever reported, and represents a factor-of-50 suppression of the blackbody emission. The corresponding external NL efficiency of 86% is consistent with the internal efficiency and the spectrally weighted reflectivity of 15%, which could be improved substantially through the incorporation of a state-of-the-art multilayer AR coating. [ABSTRACT FROM AUTHOR]

The authors report the operating characteristics of ten-stage interband cascade lasers from two wafers with room-temperature wavelengths of 4.1 and 4.3 μm. For 150-μm-wide stripes, the threshold current densities are as low as 4.8 A/cm2 at 78 K (cw) and 1.15 kA/cm2 at room temperature (pulsed). At 78 K, the cw wall-plug efficiency for an 11-μm-wide ridge with 0.5-mm-long cavity and coated facets is 27%, while a 3-mm-long cavity emits a maximum cw power of 200 mW. Devices from the two wafers have maximum cw operating temperatures of 261 K (λ≈4.0 μm) and 243 K (λ≈4.2 μm). [ABSTRACT FROM AUTHOR]

GaAsSbN/InP strained layer superlattice (SL) structures have been grown using low-temperature metal organic chemical vapor deposition with N composition varying from 0.6% to 1.6%. High-resolution x-ray diffraction measurements indicate that GaAsSbN/InP SLs with defect-free layers and abrupt interfaces were achieved. Low-temperature photoluminescence measurements reveal that nitrogen incorporation into the GaAsSbN layers extends the emission wavelength, increases the conduction band offset, and dramatically changes the As/Sb ratio. In parallel with the experimental efforts, simulation studies using a ten-band k·p model are carried out to correlate the emission properties of these SL structures with experiment. Photoluminescence measurements indicate an emission wavelength redshift with respect to the simulated values. [ABSTRACT FROM AUTHOR]

A new W-structured type-II superlattice photodiode design, with graded band gap in the depletion region, is shown to strongly suppress dark currents due to tunneling and generation-recombination processes. The long-wave infrared (LWIR) devices display 19%–29% quantum efficiency and substantially reduced dark currents. The median dynamic impedance-area product of 216 Ω cm2 for 33 devices with 10.5 μm cutoff at 78 K is comparable to that for state-of-the-art HgCdTe-based photodiodes. The sidewall resistivity of ≈70 kΩ cm for untreated mesas is also considerably higher than previous reports for passivated or unpassivated type-II LWIR photodiodes, apparently indicating self-passivation by the graded band gap. [ABSTRACT FROM AUTHOR]

Results are presented for an enhanced type-II W-structured superlattice (WSL) photodiode with an 11.3 μm cutoff and 34% external quantum efficiency (at 8.6 μm) operating at 80 K. The new WSL design employs quaternary Al0.4Ga0.49In0.11Sb barrier layers to improve collection efficiency by increasing minority-carrier mobility. By fitting the quantum efficiencies of a series of p-i-n WSL photodiodes with background-doped i-region thicknesses varying from 1 to 4 μm, the authors determine that the minority-carrier electron diffusion length is 3.5 μm. The structures were grown on semitransparent n-GaSb substrates that contributed a 35%–55% gain in quantum efficiency from multiple internal reflections. [ABSTRACT FROM AUTHOR]

Continuous-wave single-mode emission near λ=3.42 μm is obtained from a distributed-feedback (DFB) interband cascade laser. The low-loss DFB mode in the 15-μm-wide ridge is resonant with the gain peak at 110 K, where the maximum single-mode output power is 3 mW. The instrument-limited linewidth is ≤=0.5 Å, and the wavelength is linearly tunable over a 4 nm range by varying the temperature from 110 to 125 K (0.28 nm/K), or over 0.6 nm by varying the current from 60 to 83 mA (0.031 nm/mA) [ABSTRACT FROM AUTHOR]

We demonstrate a surface-emitting photonic-crystal distributed-feedback laser, emitting at λ≈3.7 μm from an antimonide type-II “W” active region. While single-mode operation was not achieved, the spectral emission linewidth for cw optical pumping was only 1.3 Å. For pulsed operation with a pump-spot diameter of 0.8 mm, 60% of the nearly circular output beam was transmitted through an aperture with an angular diameter approximately five times that of a Gaussian emitted beam, assuming that the emission area and pump area are the same. [ABSTRACT FROM AUTHOR]

A ten-stage interband cascade laser emitting at 3.3–3.6 μm exhibits a voltage drop only 4% above the theoretical minimum, and operates to room temperature in pulsed mode. At 78 K, a five-stage device mounted epitaxial-side-up with coated facets has a maximum cw wallplug efficiency of 21%, and a second device produces up to 1.1 W of cw power. [ABSTRACT FROM AUTHOR]

Low temperature (30 K) long wavelength photoluminescence emission (λ=1400–1600 nm) from metalorganic chemical vapor deposition grown InGaAsN–GaAsSb type II “W” quantum wells (QWs), on GaAs substrates has been demonstrated. Thin layers (2–3 nm) and high antimony-content (30%) GaAsSb were utilized in this study for realizing satisfactory wave function overlap and long wavelength emission. Tensile strained GaAsP barriers effectively improve the material structural and luminescence properties of the compressive strained active region. Room temperature photoluminescence data show that the type-II QW design is a promising candidate for realizing long wavelength GaAs-based diode lasers beyond 1500 nm. [ABSTRACT FROM AUTHOR]

A type-II “W” diode laser with five quantum well periods and emitting at λ≈3.5 μm operated in cw mode to T=218 K. A second device produced more than 500 mW of cw power at 80 K. The threshold current density at T=78 K was 31 A/cm2, and pulsed operation was observed to 317 K. Improvements over previous single-stage devices for this wavelength range may be attributed in part to high growth quality and also to the incorporation of transition regions that smooth discontinuities in the conduction-band profile. [ABSTRACT FROM AUTHOR]

Type-II “W” diode lasers with ten quantum-well periods operated in pulsed mode to 315 K, where the emission wavelength was 4.02 μm. The devices with uncoated facets had a threshold current density of 145 A/cm2 and slope efficiency of 47 mW/A per facet at 78 K, and displayed a characteristic temperature T0≈46 K in the range 78–300 K. [ABSTRACT FROM AUTHOR]

We have investigated the negative luminescence (NL) properties of an array of HgCdTe photodiodes (λ[sub co]=4.6 μm at 295 K) as a function of temperature and wavelength. The internal NL efficiency of ≈93% at λ=4.0 μm is nearly independent of temperature in the 240–300 K range, and at 296 K corresponds to an apparent temperature reduction of 54 K. This is obtained at a reverse-bias saturation current density of only 0.13 A/cm[sup 2] at 296 K, which is attributable in part to an array geometry that reduces the influence of macroscopic defects. These observed NL performance characteristics are compatible with most of the requirements for applications such as cold shielding. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We describe a ternary antimonide superlattice photodiode with a 21 μm cutoff wavelength. The active region consists of 150 periods of 10 monolayers (MLs) of In[SUB0.07]Ga[SUB0.93]Sb and 19 MLs of InAs with InSb-like interfacial bonds. The device has a detectivity of 3 × 10[SUP9] cm√Hz/W, dynamic impedance-area product of 0.18 Ω cm[SUP2], and peak external quantum efficiency of 3% at 40 K. X-ray diffraction and cross-sectional scanning tunneling microscopy show the structure to have a high degree of order with abrupt interfaces. A simulation of the absorption spectrum effectively reproduces the observed spectrum. [ABSTRACT FROM AUTHOR]

The negative luminescence of a midwave-infrared HgCdTe photodiode (cutoff wavelength =5.3 μm at 295 K) was investigated using a modulated, self-referencing, optical technique. Internal efficiencies were measured to be 88% throughout the 3-5-μm spectral region, nearly independent of temperature in the 240-300-K range. This corresponds to an apparent temperature reduction of greater than 50 K at room temperature and greater than 35 K at 240 K when the reverse bias is applied. The reverse saturation current density of 1.3 A/cm[SUP2] is lower than any reported previously for a negative luminescence device with cutoff wavelength beyond 4 mm. [ABSTRACT FROM AUTHOR]

Research has shown that free-space laser communication systems may experience fewer outages due to atmospheric impairments such as haze, fog, clouds, and turbulence by operating at a longer wavelength in the mid-wave or long-wave infrared, if disadvantages such as lower-performance transceiver components may be overcome. Here we report a resonant cavity infrared detector (RCID) with 4.6-µm resonance wavelength that enables 20-dB larger link budget than has been reported previously for ∼ 5 Gb/s operation. The device combines high responsivity, 1.97 A/W, with a low noise equivalent power (NEP) of 0.7 pW/ H z at room temperature, and a high bandwidth of 6.7 GHz at 3-dB. The relatively large surface-normal-incidence device with 30-µm diameter simplifies the coupling relative to intra-subband quantum cascade detectors. Although the RCID NEP is expected to increase with frequency to ∼ 1.5 pW/ H z , we estimate that the total equivalent noise power in a 2.5-GHz bandwidth is less than 200 nW. When combined with a relatively high power (∼100-mW) distributed-feedback quantum cascade laser, the difference of > 50 dB between modulated laser power and RCID noise significantly outpaces that of existing devices.

We demonstrate the sensitive detection of dimethyl methylphosphonate (DMMP, a hydrogen-bond (HB) basic phosphonate ester) using additional optical loss induced in an interband cascade laser with top optical cladding layer replaced by an exposed sensing window coated by a HB acidic sorbent layer. Thin coatings of the sorbents HCSFA2 and oapBPAF were deposited on the sensing window to allow reversible capture and concentration of DMMP for optical interrogation. Analyte levels down to 0.1 mg/m 3 (∼20 ppb) were tested and successfully detected by monitoring the laser's threshold or its output power at a fixed bias as a function of DMMP delivery concentration.

By the end of 2018, 42 years after the landing of the two Viking seismometers on Mars, InSight will deploy onto Mars' surface the SEIS ( S eismic E xperiment for I nternal S tructure) instrument; a six-axes seismometer equipped with both a long-period three-axes Very Broad Band (VBB) instrument and a three-axes short-period (SP) instrument. These six sensors will cover a broad range of the seismic bandwidth, from 0.01 Hz to 50 Hz, with possible extension to longer periods. Data will be transmitted in the form of three continuous VBB components at 2 sample per second (sps), an estimation of the short period energy content from the SP at 1 sps and a continuous compound VBB/SP vertical axis at 10 sps. The continuous streams will be augmented by requested event data with sample rates from 20 to 100 sps. SEIS will improve upon the existing resolution of Viking's Mars seismic monitoring by a factor of ∼ 2500 at 1 Hz and ∼ 200 000 at 0.1 Hz. An additional major improvement is that, contrary to Viking, the seismometers will be deployed via a robotic arm directly onto Mars' surface and will be protected against temperature and wind by highly efficient thermal and wind shielding. Based on existing knowledge of Mars, it is reasonable to infer a moment magnitude detection threshold of M w ∼ 3 at 40 ∘ epicentral distance and a potential to detect several tens of quakes and about five impacts per year. In this paper, we first describe the science goals of the experiment and the rationale used to define its requirements. We then provide a detailed description of the hardware, from the sensors to the deployment system and associated performance, including transfer functions of the seismic sensors and temperature sensors. We conclude by describing the experiment ground segment, including data processing services, outreach and education networks and provide a description of the format to be used for future data distribution., Electronic Supplementary Material: The online version of this article (10.1007/s11214-018-0574-6) contains supplementary material, which is available to authorized users.

We demonstrate a strategy for increasing the operating temperatures of nBn midwave infrared (MWIR) focal plane arrays, based on the use of two-dimensional plasmonic gratings to enhance the quantum efficiency (QE) of structures with very thin absorbers. Reducing the absorber volume correspondingly reduces the dark current in a diffusion-limited photodiode, while light trapping mediated by the plasmonic grating increases the net absorbance to maintain high QE. The plasmonically enhanced nBn MWIR sensors with absorber thicknesses of only 0.5 μm exhibit peak internal QEs as high as 57%, which enables a 5-fold reduction in dark current. Numerical simulations indicate the potential for further improvement.

The application of an interband cascade laser, ICL, to multi-mode absorption spectroscopy, MUMAS, in the mid-infrared region is reported. Measurements of individual mode linewidths of the ICL, derived from the pressure dependence of lineshapes in MUMAS signatures of single, isolated, lines in the spectrum of HCl, were found to be in the range 10-80 MHz. Multi-line spectra of methane were recorded using spectrally limited bandwidths, of approximate width 27 cm -1 , defined by an interference filter, and consist of approximately 80 modes at spectral locations spanning the 100 cm -1 bandwidth of the ICL output. Calibration of the methane pressures derived from MUMAS data using a capacitance manometer provided measurements with an uncertainty of 1.1 %. Multi-species sensing is demonstrated by the simultaneous detection of methane, acetylene and formaldehyde in a gas mixture. Individual partial pressures of the three gases are derived from best fits of model MUMAS signatures to the data with an experimental error of 10 %. Using an ICL, with an inter-mode interval of ~10 GHz, MUMAS spectra were recorded at pressures in the range 1-10 mbar, and, based on the data, a potential minimum detection limit of the order of 100 ppmv is estimated for MUMAS at atmospheric pressure using an inter-mode interval of 80 GHz., (© The Author(s) 2016.)

Narrow-ridge interband cascade lasers were subjected to accelerated aging. The aging curves were statistically evaluated by a log-normal distribution of the failure time, and by the mixed effects of the degradation parameters. Based on 10,000 h of output power trend data for lasers operating at 90°C and the maximum cw power, an unexpectedly long lifetime is predicted. The projected lifetimes range from about 500,000 h (57 years) for the linear degradation model to 183,000 h (21 years) for the exponential one.

We characterize the internal efficiency, internal loss, and optical gain versus current density in 7-stage interband cascade lasers operating at λ=3.1 and 3.45 μm using a cavity-length study of the external differential quantum efficiency (EDQE) and threshold current density at temperatures between 300 and 345 K. We find that the pronounced efficiency droop of the EDQE at high current densities is primarily due to an increase in the internal loss rather than a reduction in the internal efficiency. On the other hand, if the current density J is fixed, the temperature variation of the EDQE at that J is due primarily to a decrease of the internal efficiency. The gain versus current density is fit well by a logarithmic relationship, although the magnitude of the experimental gain is >20% below the theoretical estimate.

An interband cascade laser (ICL) operating at 3.7 μm has been used to perform multimode absorption spectroscopy, MUMAS, at scan rates up to 10 kHz. Line widths of individual modes in the range 10-80 MHz were derived from isolated lines in the MUMAS signatures of HCl. MUMAS data for methane covering a spectral range of 30 nm yielded a detection level of 30 μbar·m for 1 s measurement time at 100 Hz. Simultaneous detection of methane, acetylene, and formaldehyde in a gas mixture containing all three species is reported.

Background and Purpose: Vitamin D deficiency is associated with cognitive impairment in the elderly and with increased white matter T2 hyperintensities in elderly debilitated patients. We investigated the relationship between serum vitamin D and brain MR findings in adult outpatients., Materials and Methods: Brain MR studies of 56 patients ages 30-69 years were selected when vitamin D level had been obtained within 90 days of the MRI. White matter T2 hyperintensities were characterized by size and location by two neuroradiologists. Manual volumetric analysis was assessed in patients more than 50 years of age., Results: The entire cohort showed a significant negative relationship between serum 25-hydroxyvitamin D and the number of confluent juxtacortical white matter T2 hyperintensities (P = .047). The cohort ages 50 years and older showed stronger correlation between confluent white matter T2 hyperintensities and serum 25-hydroxyvitamin D in the juxtacortical region; number (P = .015) and size of white matter T2 hyperintensities (P = .048). Atrophy was not significantly related to serum 25-hydroxyvitamin D by radiologist visual analysis or by the bicaudate ratio., Conclusions: We found a significant relationship between vitamin D and white matter T2 hyperintensities in independent adult outpatients, especially over the age of 50 years., (© 2014 by American Journal of Neuroradiology.)

The interband cascade laser differs from any other class of semiconductor laser, conventional or cascaded, in that most of the carriers producing population inversion are generated internally, at semimetallic interfaces within each stage of the active region. Here we present simulations demonstrating that all previous interband cascade laser performance has suffered from a significant imbalance of electron and hole densities in the active wells. We further confirm experimentally that correcting this imbalance with relatively heavy n-type doping in the electron injectors substantially reduces the threshold current and power densities relative to all earlier devices. At room temperature, the redesigned devices require nearly two orders of magnitude less input power to operate in continuous-wave mode than the quantum cascade laser. The interband cascade laser is consequently the most attractive option for gas sensing and other spectroscopic applications requiring low output power and minimum heat dissipation at wavelengths extending from 3 μm to beyond 6 μm.

Room temperature spasing of surface plasmon polaritons at 1.46 μm wavelength has been demonstrated by sandwiching a gold-film plasmonic waveguide between optically pumped InGaAs quantum-well gain media. The spaser exhibits gain narrowing, the expected transverse-magnetic polarization, and mirror feedback provided by cleaved facets in a 1-mm long cavity fabricated with a flip-chip approach. The 1.06-μm pump-threshold of ~60 kW/cm2 is in good agreement with calculations. The architecture is readily adaptable to all-electrical operation on an integrated microchip.

A compact and versatile source of coherent surface-plasmon polaritions (SPPs) is demonstrated by end-coupling a laser diode operating at 1.46 microm to a plasmonic waveguide integrated on the same microchip. With an optimized overlap between the spatial-modes of the laser and a planar-stripe waveguide, a high coupling efficiency of approximately 36% is achieved, that computations show could approach approximately 60% with smaller, readily achievable gaps between laser and waveguide. This integrated and electrically-activated source, with an available SPP power limited only by the laser diode, appears ideally suited for directly driving plasmonic circuitry or surface-enhanced sensors., ((c) 2010 Optical Society of America.)

Mating disruption theory predicts that high concentrations of female pheromone, and/or large numbers of release sites, should confuse males orienting to "calling" females, reduce the number of successful matings, and decrease the reproductive potential of the population. In this scenario, females are regarded as stationary point sources of pheromone. Past behavioral observations, however, have shown virgin female grape root borers, Vitacea polistiformis Harris, significantly alter their behavior in mating disruption treatments. Treated females call at different heights, move less before call initiation, and move more after call initiation than control females. Pheromone gland dragging and wing fanning also increase significantly during pheromone treatments. These behavioral differences are significant only if they alter the mating success of females. Because long-term field studies are impractical, we used known behavior of male and female GRB to build a Fortran language time step model, adding the effects of female movement to past models of male pheromone plume following. Females were distributed randomly, and then assigned a conditional movement strategy. If females were within the competitive portion of another female's plume, the downwind female moved. Except in the lowest population density tested, females moving upwind and crosswind when in a competing female's pheromone plume mated significantly more often than females remaining stationary. In all population simulations, mating success was significantly reduced when females moved downwind. These field and simulation studies provide strong evidence for female movement as a previously overlooked potential mechanism for resistance to mating disruption treatments, as well as a shaping behavior in the evolution of pheromone communication systems.

We have measured the cosmic ray spectrum above 10(17.2) eV using the two air-fluorescence detectors of the High Resolution Fly's Eye observatory operating in monocular mode. We describe the detector, phototube, and atmospheric calibrations, as well as the analysis techniques for the two detectors. We fit the spectrum to a model consisting of galactic and extragalactic sources.

Aphanomyces euteiches has become recognized as an important root rot pathogen of alfalfa in the north-central United States, and resistant cultivars are now commonly planted. Recent evidence indicates the existence of A. euteiches strains, designated as race 2, that are virulent on resistant cultivars, but there is little information on the prevalence of such strains or their impact on the performance of A. euteiches-resistant cultivars. The purpose of this study was to assess the virulence of A. euteiches isolates obtained from Iowa and Wisconsin soils and to determine the frequency of isolates virulent on race 1-resistant alfalfa populations. In addition, the yield performance of susceptible and resistant alfalfa populations was compared in four Iowa locations and one Wisconsin location. Fourteen isolates of A. euteiches from different Iowa locations were used to challenge two race 1-resistant cultivars (Paramount and Quantum), a susceptible cultivar (Agate or Vernal), and two resistant breeding populations (WAPH-1 and WAPH-2). Fifty-nine isolates of A. euteiches from one location in Wisconsin were used to challenge one susceptible cultivar (Saranac) and WAPH-1 and WAPH-2. Every isolate was virulent to one or more alfalfa cultivars or populations. Emergence of seedlings in growth chamber experiments did not differ significantly among isolates or alfalfa populations. Alfalfa population and A. euteiches isolate had significant effects on disease severity index (DSI, 1-5 scale), but there were significant interactions (P < 0.05) between these two effects. All 14 Iowa isolates of A. euteiches were virulent (DSI ≥ 3.0) on Agate (mean DSI = 4.4, range 3.8 to 4.9), WAPH-1 (mean DSI = 3.9, range 3.0 to 4.4), and the two commercial resistant cultivars (mean DSI = 3.9 and 4.1, range 3.2 to 4.4). On WAPH-2, only three isolates were virulent (mean DSI = 2.5, range 1.8 to 3.2). Of 59 Wisconsin isolates, all were virulent on Saranac (mean DSI = 4.6, range 3.9 to 5.0), 21 were virulent on WAPH-1 (mean DSI = 2.9, range 1.8 to 4.8), and only four were virulent on WAPH-2 (mean DSI = 2.3, range 1.8 to 3.4). In field studies, we compared yield performance of alfalfa cultivars that were resistant or susceptible to A. euteiches or Phytophthora medicaginis at four Iowa locations for one to three harvest years, and one Wisconsin location for two harvest years. Mean yields of cultivars with resistance to one or both pathogens were significantly higher than those of susceptible cultivars in only one of the four Iowa locations. In Wisconsin, WAPH-4, a Race 2-resistant alfalfa population, expressed a significant yield advantage when compared with both WAPH-1, a Race 1-resistant alfalfa population, and Columbia 2000, a cultivar susceptible to both race 1 and 2 of A. euteiches. These results indicate that race 2 of A. euteiches is prevalent in Iowa and Wisconsin soils and may be limiting the yield benefits of currently available race 1-resistant alfalfa cultivars. Incorporation of race 2 resistance is likely to improve the performance of alfalfa cultivars in A. euteiches-infested soils.

We review the recent progress of electrically injected and optically pumped mid-IR lasers based on antimonide quantum wells with the type II W configuration. W quantum-well diodes have achieved cw operation up to 195 K at lambda = 3.25 mum. Optically pumped devices that employ the diamond pressure bond heat sink have reached 290 K at 3 mum and 210 K at 6 mum. Pulsed power conversion efficiencies of up to 7% at 220 K have been attained by use of an optical pumping injection cavity approach, in which an etalon cavity for the pump beam significantly enhances its absorptance. The angled-grating distributed-feedback configuration has been used to obtain near-diffraction-limited output for an optical pumping stripe width of 50 mum.

Background and Purpose: Recent technological advances in MR instrumentation allow acquisition of whole-brain diffusion-weighted MR scans to be obtained with b values greater than 1,000. Our purpose was to determine whether high-b-value diffusion-weighted MR imaging improved contrast and detection of signal changes in acute and chronic brain infarction., Methods: We prospectively evaluated the MR scans of 30 subjects with a history of possible brain infarction on a 1.5-T MR imager with 40 mT/meter gradients (slew rate 150 T/m/s) by use of the following single-shot echo-planar diffusion-weighted MR sequences: 1) 7,999/ 71.4/1 (TR/TE/excitations, b = 1,000; 2) 999/ 88.1/3, b = 2,500; and 3) 7,999/ 92.1/4, b = 3,000. Diffusion-weighted MR imaging was performed in three orthogonal directions during all sequences. All subjects were scanned with fast fluid-attenuated inversion recovery (FLAIR) (10,006/145/2,200/1 [TR/TE/TI/excitations]) and fast spin-echo T2-weighted (3,650/95/3 [TR/TE/excitations], echo train length, 8). The diagnosis of brain infarction was established by clinical criteria., Results: Twenty women and 10 men with a mean age of 67.7 years were enrolled in the study. One subject was excluded owing to poor image quality. Twelve of 29 subjects had a clinical diagnosis of acute infarction. All 12 had lesions that were hyperintense on diffusion-weighted images at all three b values; five were cortical and seven subcortical. There was increased contrast of all lesions on high-b-value scans (b = 2,500 and 3,000). Lesions that were hypointense on diffusion-weighted images were identified and evaluated at the three different b values. At b = 1,000, there were 19 hypointense lesions, whereas at b = 2,500 and 3,000 there were 48 and 55 lesions, respectively. On FLAIR and T2-weighted images, these low-signal lesions were predominantly chronic, subcortical, ischemic lesions and lacunar infarcts, but four chronic cortical infarcts, one porencephalic cyst, and one primary brain tumor were also found. Low-signal lesions were also noted to have increased contrast on high-b-value diffusion-weighted scans., Conclusion: High-b-value diffusion-weighted MR imaging (b = 2,500 or b = 3,000) had no impact on diagnosis of acute infarction. High-b-value diffusion-weighted MR imaging (b = 2,500) combined with diffusion-weighted MR imaging at b = 1,000 improves tissue characterization by increasing the spectrum of observed imaging abnormalities in patients with suspected brain infarction.

The purpose of this essay is to stimulate academic discussion about the ethical justification of using human primordial stem cells for tissue transplantation, cell replacement, and gene therapy. There are intriguing alternatives to using embryos obtained from elective abortions and in vitro fertilisation to reconstitute damaged or dysfunctional human organs. These include the expansion and transplantation of latent adult progenitor cells.

Background and Purpose: A new option in the treatment of recurrent malignant glioma is surgical placement of chemotherapy-laden biodegradable wafers. We describe the CT and MR appearance of chemotherapy wafers in patients after surgery for recurrent malignant glioma, Methods: Eighteen patients had carmustine (BCNU) wafers implanted during reoperation for malignant glioma; three patients had empty, placebo wafers placed. The 21 patients had a total of 22 CT and 57 MR imaging studies. Repeat CT studies were conducted for up to 6 months, the MR studies for up to 1 year. Examinations were evaluated for attenuation on CT scans, signal abnormalities on MR images, and changing appearance during the follow-up period. Enhancement characteristics were also assessed., Results: On CT scans, 13 of 16 acute (<7 days) cases showed linear high-attenuation wafers, with three showing low attenuation. On MR images, all T1 and T2 studies performed in the acute stage showed decreased signal of the wafers. Eight of 15 studies showed a transient increase in T1 only at about 2 months. Wafers decreased in conspicuity on both CT and MR studies after 2 months. The wafers did not enhance. One postoperative tumor showed a transient increase in edema and increased enhancement at 5 weeks. The presence or absence of BCNU within the wafers did not change their appearance., Conclusion: BCNU wafers have a characteristic appearance: in the first 7 days after implantation they are linear, usually of increased attenuation on CT scans, and always show decreased signal on MR images; they do not enhance, and become less conspicuous after 2 months.

Functional MRI was used to examine cerebral activations in 12 subjects while they performed a spatial attention task. This study applied more stringent behavioural and cognitive controls than previously used for similar experiments: (i) subjects were included only if they showed evidence of attentional shifts while performing the task in the magnet; (ii) the experimental task and baseline condition were designed to eliminate the contributions of motor output, visual fixation, inhibition of eye movements, working memory and the conditional (no-go) component of responding. Activations were seen in all three hypothesized cortical epicentres forming a network for spatial attention: the lateral premotor cortex (frontal eye fields), the posterior parietal cortex and the cingulate cortex. Subcortical activations were seen in the basal ganglia and the thalamus. Although the task required attention to be equally shifted to the left and to the right, eight of 10 subjects showed a greater area of activation in the right parietal cortex, consistent with the specialization of the right hemisphere for spatial attention. Other areas of significant activation included the posterior temporo-occipital cortex and the anterior insula. The temporo-occipital activation was within a region broadly defined as MT+ (where MT is the middle temporal area) which contains the human equivalent of area MT in the macaque monkey. This temporo-occipital area appears to constitute a major component of the functional network activated by this spatial attention task. Its activation may reflect the 'inferred' shift of the attentional focus across the visual scene.

The first (to our knowledge) III-V mid-IR vertical-cavity surface-emitting lasers (lambda = 2.9 microm) are demonstrated and show promising characteristics for chemical detection applications. The cw optical-pumping threshold is low (4 mW at 80 K) and efficiency is high (5.6% W/W). Pulsed operation is obtained up to 280 K and cw up to 160 K. Lateral-mode confinement will lead to spectrally pure, single-mode output for chemical identification.

We report three patients with spontaneous intracranial hypotension in whom spinal MR imaging revealed ventral extradural fluid collections that were centered at the cervicothoracic junction in two patients and extended throughout the entire spine in the third patient. These spinal fluid collections most likely resulted from the accumulation of CSF at the site of dural leakage. Knowledge of this association can be helpful in the selection of imaging studies to facilitate diagnosis and treatment.

In two orchard experiments, interplanting wheat (Triticum aestivum cv. Stacy) around either newly planted or 4-year-old well-established peach trees did not suppress (P ≤ 0.05) the population density of the ring nematode, Criconemella xenoplax, after 3 years. Furthermore, inter-planting wheat around newly planted trees reduced tree growth, perhaps the result of competition for water and (or) nutrients. Wheat root exudate was not as attractive to C. xenoplax as peach root exudate, but wheat root exudate did not repel the nematode either. Stacy wheat appeared to be more beneficial as a preplant rather than as a postplant ground cover management tool for suppressing the population density of C. xenoplax.

Intracranial hemorrhage is a common initial symptom of intracranial vascular malformations. Although CT remains important in the acute setting, MR imaging has proved invaluable for diagnosis and characterization of intracranial hemorrhage. MR imaging of hemorrhage and the differential diagnosis of nontraumatic intracranial hemorrhage is presented in this article.