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Single and multi-mode room temperature laser action was observed in GaN pyramids under strong optical pumping. The 5- and 15-micron-wide hexagonal-based pyramids were laterally overgrown on a patterned GaN/AlN seeding layer grown on a (111) silicon substrate by metal-organic chemical vapor deposition. The pyramids were individually pumped, imaged, and spectrally analyzed through a high magnification optical system using a high density pulsed excitation source. We suggest that the cavity formed in a pyramid is of a ring type, formed by total internal reflections of light off the pyramids’ surfaces. The mode spacing of the laser emission was found to be correlated to the size of pyramids. The effects of pyramid geometry and pulse excitation on the nature of laser oscillations inside of the pyramids is discussed. Practical applications of the results for the development of light-emitting pixels and laser arrays are suggested.
We studied and compared the emission properties of optically excited (Al)GaN structures grown by two different techniques: hydride vapor phase epitaxy (HVPE) and metalorganic chemical vapor deposition (MOCVD). We successfully achieved stimulated emission (SE) in an HVPE-grown GaN epilayer and a GaN/AlGaN double heterostructure at 10 K and room temperature. We found that the SE threshold and photoluminescence efficiency of the HVPE-grown samples are similar to those of high-quality MOCVD-grown structures. Photoluminescence measurements from 10 to 300 K show that the HVPE GaN has a high density of non-radiative recombination channels, especially those activated below 100 K. This study represents the first demonstration of SE in HVPE-grown (Al)GaN heterostructures.
Thin films of (RE)-Ba-Cu-O perovkites containing silver were prepared by pulsed Nd:YAG (wavelength 1064 nm) laser ablation. Presence of the 1:2:3 phase was verified by scanning Auger microscopy. Onset of the d.c. resistive superconducting transition of 85 K was observed. Films with and without silver were compared and the potential benefits of silver inclusion are discussed.
Single and multi-mode room temperature laser action was observed in GaN pyramids under strong optical pumping. The 5- and 15-micron-wide hexagonal-based pyramids were laterally overgrown on a patterned GaN/AlN seeding layer grown on a (111) silicon substrate by metal-organic chemical vapor deposition. The pyramids were individually pumped, imaged, and spectrally analyzed through a high magnification optical system using a high density pulsed excitation source. We suggest that the cavity formed in a pyramid is of a ring type, formed by total internal reflections of light off the pyramids' surfaces. The mode spacing of the laser emission was found to be correlated to the size of pyramids. The effects of pyramid geometry and pulse excitation on the nature of laser oscillations inside of the pyramids is discussed.
We present the results of an experimental study on stimulated emission (SE) in optically pumped InGaN/GaN multi-quantum well (MQW) structures in the temperature range of 175 K to 575 K. Samples used in this work consisted of 12 QWs and the GaN barriers were intentionally doped with different Si concentrations. The effect of doping on the SE thresholds of the MQWs were investigated. We observed that the SE spectra were comprised of many narrow peaks of less than I Å full width at half maximum (FWHM). No broadening of the FWHMs of the peaks occurred as the temperature was raised from 175 to 575 K. The SE threshold was measured as a function of temperature and compared with that of a thin GaN film. Low SE thresholds were attributed to high quantum efficiency of the MQWs, possibly associated with large carrier localization. A characteristic temperature of 162 K was derived from the temperature dependence of the SE threshold. The integrated emission intensity versus pumping density was examined for different temperatures. This study shows that InGaN/GaN MQWs are suitable for the development of laser diodes that can operate well above room temperature.
Dr. George Kaplan, the current Vice-President of the Commission was nominated to be the new President. Dr. Catherine Hohenkerk was elected to be the next Vice-President of the Commission. As for the Membership of the Organizing Committee, Dr. Vondrak stepped down and Drs William Folkner of JPL and Steve Bell of HMNAO have been added. In the below, we present summaries of the reports from various institutions presented at the business session.
JPL planetary ephemeris development has been very active assimilating measurements from current planetary missions and supporting future missions. The NASA Mars Science Laboratory (MSL) mission with launch in 2009 requires knowledge of the Earth and Mars ephemerides with 30m accuracy. By comparison, the accuracy of the Mars ephemeris in the widely used DE405 ephemeris was about 3 km. Meeting the MSL needs requires an ongoing program of range and very-long baseline interferometry measurements of Mars orbiting spacecraft. The JPL ephemeris DE421 was released three months before the landing of the Phoenix mission on Mars, and has met the 300m requirement. Continued measurements are planned to support the MSL landing.
The IAU Working Group (WG) on Numerical Standards for Fundamental Astronomy has been tasked with updating the IAU Current Best Estimates (CBEs), conforming with the IAU Resolutions, IERS Conventions and Système International d'Unités whenever possible. As part of its effort to achieve this, the WG is working in close cooperation with IAU Commissions 4 and 52, the IERS, and the BIPM Consultative Committee for Units.
The main goal of the Working Group was to gather astrometric observations made during the triennum as well as old observations not yet published in the data base. The WG encouraged the making of new observations. A Spring School was organized in China in order to teach the observational techniques of natural satellites to students and young astronomers. New theoretical models of the motion of the satellites and fit of the current models to new observations were used in order to make ephemerides of all the planetary satellites with tools useful for observations such as configurations. These ephemerides named MULTISAT are available at <www.imcce.fr/sat> or at <lnfm1.sai.msu.ru/neb/nss/nssephme.htm>.
This business meeting was held from 16:00 to 17:30. Toshio Fukushima and George Kaplan were welcomed as the next president and vice-president, respectively. The following, in no particular order, are the summary reports from the various offices. The full versions will be made available on the Commission 4 website at <http://iau-comm4.jpl.nasa.gov/>.
X-ray photoelectron spectroscopy, scanning Auger, and optical microscopy studies of polycrystalline superconducting pellets of Y–Ba–Cu–O/Ag are presented. Silver-laced samples have a lower porosity and a drastically reduced hydrocarbon contamination. Results indicate no detectable substitution of A g into the Y–Ba–Cu–O but a collection of metallic silver in voids and possibly along grain boundaries Intergranular silver could mitigate adverse grain boundary effects in polycrystalline Y–Ba–Cu–O.
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