Low Threshold Two-Dimensional Annular Bragg Lasers

Lasing at telecommunication wavelengths from annular resonators employing radial Bragg reflectors is demonstrated at room temperature under pulsed optical pumping. Sub milliwatt pump threshold levels are observed for resonators with 0.5-1.5 wavelengths wide defects of radii 7-8 mm. The quality factors of the resonator modal fields are estimated to be on the order of a few thousands. The electromagnetic field is shown to be guided by the defect. Good agreement is found between the measured and calculated spectrum.Comment: 8 pages, 4 figure

Collapsar Jets, Bubbles and Fe Lines

In the collapsar scenario, gamma ray bursts are caused by relativistic jets expelled along the rotation axis of a collapsing stellar core. We discuss how the structure and time-dependence of such jets depends on the stellar envelope and central engine properties, assuming a steady jet injection. It takes a few seconds for the jet to bore its way through the stellar core; most of the energy output during that period goes into a cocoon of relativistic plasma surrounding the jet. This material subsequently forms a bubble of magnetized plasma that takes several hours to expand, subrelativistically, through the envelope of a high-mass supergiant. Jet break-through and a conventional burst would be expected not only in He stars but possibly also in blue supergiants. Shock waves and magnetic dissipation in the escaping bubble can contribute a non thermal UV/X-ray afterglow, and also excite Fe line emission from thermal gas, in addition to the standard jet deceleration power-law afterglow.Comment: Ap.J. Letters, accepted 6/20/01, first subm 4/24/01; aaspp4, 9 pages, no figures; minor revision

Vertically emitting annular Bragg lasers using polymer epitaxial transfer

Fabrication of a planar semiconductor microcavity, composed of cylindrical Bragg reflectors surrounding a radial defect, is demonstrated. A versatile polymer bonding process is used to transfer active InGaAsP resonators to a low-index transfer substrate. Vertical emission of in-plane modes lasing at telecom wavelengths is observed under pulsed optical excitation with a submilliwatt threshold

Band splitting and Modal Dispersion induced by Symmetry braking in Coupled-Resonator Slow-Light Waveguide Structures

We study the dispersion relations in slow-light waveguide structures consisting of coupled microdisk resonators. A group theoretical analysis of the symmetry properties of the propagating modes reveals an interesting phenomenon: The degeneracy of the CW and CCW rotating modes is removed, giving rise to two distinct transmission bands. This effect induces symmetry-based dispersion which may limit usable bandwidth of such structures. The properties of this band splitting and its impact on CROW performance for optical communications are studied in detail

Assessment of lithographic process variation effects in InGaAsP annular Bragg resonator lasers

Optical microresonators based on an annular geometry of radial Bragg reflectors have been designed and fabricated by electron-beam lithography, reactive ion etching, and an epitaxial transfer process. Unlike conventional ring resonators that are based on total internal reflection of light, the annular structure described here is designed to support optical modes with very small azimuthal propagation coefficient and correspondingly large free spectral range. The effect of lithographic process variation upon device performance is studied. Laser emission wavelength and threshold optical pump power are found to vary between similar devices given different electron doses during electron-beam lithography. As the resonance wavelength and quality factor of these resonators are very sensitive to environmental changes, these resonators make ideal active light sources that can be integrated into large arrays for gas and liquid sensing applications and are easily interrogated

Asymmetry of jets, lobe size and spectral index in radio galaxies and quasars

We investigate the correlations between spectral index, jet side and extent of the radio lobes for a sample of nearby FRII radio galaxies. In Dennett-Thorpe et al. (1997) we studied a sample of quasars and found that the high surface brightness regions had flatter spectra on the jet side (explicable as a result of Doppler beaming) whilst the extended regions had spectral asymmetries dependent on lobe length. Unified schemes predict that asymmetries due to beaming will be much smaller in narrow-line radio galaxies than in quasars: we therefore investigate in a similar manner, a sample of radio galaxies with detected jets. We find that spectral asymmetries in these objects are uncorrelated with jet sidedness at all brightness levels, but depend on relative lobe volume. Our results are not in conflict with unified schemes, but suggest that the differences between the two samples are due primarily to power or redshift, rather than to orientation. We also show directly that hotspot spectra steepen as a function of radio power or redshift. Whilst a shift in observed frequency due to the redshift may account for some of the steepening, it cannot account for all of it, and a dependence on radio power is required.Comment: accepted for publication in MNRAS, 10 pages; typos/minor correctio

Radiative Efficiencies of Continuously Powered Blast Waves

We use general arguments to show that a continuously powered radiative blast wave can behave self similarly if the energy injection and radiation mechanisms are self similar. In that case, the power-law indices of the blast wave evolution are set by only one of the two constituent physical mechanisms. If the luminosity of the energy source drops fast enough, the radiation mechanisms set the power-law indices, otherwise, they are set by the behavior of the energy source itself. We obtain self similar solutions for the Newtonian and the ultra-relativistic limits. Both limits behave self similarly if we assume that the central source supplies energy in the form of a hot wind, and that the radiative mechanism is the semi-radiative mechanism of Cohen, Piran & Sari (1998). We calculate the instantaneous radiative efficiencies for both limits and find that a relativistic blast wave has a higher efficiency than a Newtonian one. The instantaneous radiative efficiency depends strongly on the hydrodynamics and cannot be approximated by an estimate of local microscopic radiative efficiencies, since a fraction of the injected energy is deposited in shocked matter. These solutions can be used to calculate Gamma Ray Bursts afterglows, for cases in which the energy is not supplied instantaneously.Comment: 28 LaTeX pages, including 9 figures and 3 table

Evidence of nitric acid uptake in warm cirrus anvil clouds during the NASA TC4 campaign

Uptake of HNO3 onto cirrus ice may play an important role in tropospheric NOx cycling. Discrepancies between modeled and in situ measurements of gas-phase HNO3 in the troposphere suggest that redistribution and removal mechanisms by cirrus ice have been poorly constrained. Limited in situ measurements have provided somewhat differing results and are not fully compatible with theory developed from laboratory studies. We present new airborne measurements of HNO3 in cirrus clouds from anvil outflow made during the Tropical Composition, Cloud, and Climate Coupling Experiment (TC4). Upper tropospheric (\u3e9 km) measurements made during three flights while repeatedly traversing the same cloud region revealed depletions of gas-phase HNO3 in regions characterized by higher ice water content and surface area. We hypothesize that adsorption of HNO3 onto cirrus ice surfaces could explain this. Using measurements of cirrus ice surface area density and some assumptions about background mixing ratios of gas-phase HNO3, we estimate molecular coverages of HNO 3 on cirrus ice surface in the tropical upper troposphere during the TC4 racetracks to be about 1 × 1013 molecules cm-2. This likely reflects an upper limit because potential dilution by recently convected, scavenged air is ignored. Also presented is an observation of considerably enhanced gas-phase HNO3 at the base of a cirrus anvil suggesting vertical redistribution of HNO3 by sedimenting cirrus particles and subsequent particle sublimation and HNO3 evaporation. The impact of released HNO3, however, appears to be restricted to a very thin layer just below the cloud. Copyright 2010 by the American Geophysical Union

Seasonal distributions of fine aerosol sulfate in the North American Arctic basin during TOPSE

We used the mist chamber/ion chromatography technique to quantify fine aerosol SO4=(\u3c2.7 μm) in the Arctic during the Tropospheric Ozone Production about the Spring Equinox Experiment (TOPSE) with about 2.5 min time resolution. Our effective sample area ranged from 50° to 86°N and 53° to 100°W. The seasonal evolution of fine aerosol sulfate in the Arctic troposphere during TOPSE was consistent with the phenomenon of Arctic haze. Arctic haze has been attributed to pollution from sources in the Arctic and pollution transported meridionally along stable isentropes into the Arctic in geographically broad but vertically narrow bands. These layers became more prevalent at higher altitudes as the season progressed toward summer, and the relevant isentropes are not held so close to the surface. Mean fine particle SO4= mixing ratios during TOPSE in February below 1000 m were elevated (112 pptv) and highly variable (between 28 and 290 pptv) but were significantly lower at higher altitudes (about 40 pptv). As the season progressed, elevated mixing ratios and higher variability were observed at higher altitudes, up to 7 km. In May, mixing ratios at the lowest altitudes declined but still remained higher than in February at all altitudes. The high variability in our measurements likely reflects the vertical heterogeneity of the wintertime Arctic atmosphere as the airborne sampling platform passed in and out of these layers. It is presumed that mixing ratios and variability will continue to decline at all altitudes into the summer as wet deposition processes become important in removing aerosol SO4= from the troposphere

Synchrotron Emission from Hot Accretion Flows and the Cosmic Microwave Background Anisotropy

Current estimates of number counts of radio sources in the frequency range where the most sensitive Cosmic Microwave Background (CMB) experiments are carried out significantly under-represent sources with strongly inverted spectra. Hot accretion flows around supermassive black holes in the nuclei of nearby galaxies are expected to produce inverted radio spectra by thermal synchrotron emission. We calculate the temperature fluctuations and power spectra of these sources in the Planck Surveyor 30 GHz energy channel, where their emission is expected to peak. We find that their potential contribution is generally comparable to the instrumental noise, and approaches the CMB anisotropy level at small angular scales. Forthcoming CMB missions, which will provide a large statistical sample of inverted-spectra sources, will be crucial for determining the distribution of hot accretion flows in nearby quiescent galactic nuclei. Detection of these sources in different frequency channels will help constrain their spectral characteristics, hence their physical properties.Comment: 10 pages, 4 figures, accepted for publication in Ap