Using time-resolved photoluminescence measurements, the recombination rate in an In0.18Ga0.82N/GaN quantum well (QW) is shown to be greatly enhanced when spontaneous emission is resonantly coupled to a silver surface plasmon. The rate of enhanced spontaneous emission into the surface plasmon was as much as 92 times faster than normal QW spontaneous emission. A calculation, based on Fermi's golden rule, reveals the enhancement is very sensitive to silver thickness and indicates even greater enhancements are possible for QWs placed closer to the surface metal coating.The spontaneous emission (SE) decay constant τ for radiating dipoles at r e is given by Fermi's golden rulewhere ρ(hω) is the photon density of states (DOS) and f | d · E( r e )|i is the dipole emission matrix element. As pointed out by Purcell, SE may be enhanced by altering the photon DOS 1 . For example, the ratio of enhanced to free space emission (the Purcell factor F ) has been measured as large as 5 in an atomic system by placing the radiating atoms in a high Q, low volume cavity 2,3 . A Purcell factor of up to 6 has been observed from quantum well (QW) and quantum dot emitters in vertical cavity surface emitting laser structures, while an enhancement of 15 has been observed from quantum dots in a microdisk cavity 4,5 . Photonic crystals and distributed Bragg gratings have also been used to enhance the SE rate by as much as a factor of 4.5 6,7,8 . Such enhanced SE rates, achieved by increasing the photonic DOS in a small cavity, permit lower threshold, higher modulation frequency lasers as well as more efficient light emitting diodes.The SE rate can also be modified when semiconductor or dye emitters are coupled to a surface plasmon (SP) of a metallic film 9,10,11,12 . A single QW can experience strong quantum electrodynamic coupling to a SP mode if placed within the SP fringing field penetration depth. An electron-hole pair in the QW recombines and emits a photon into a SP mode instead of into free space. The degree of SE rate modification for a given wavelength depends on the SP DOS at that wavelength. The strongest enhancement occurs near the asymptotic limit of the SP dispersion branch, the SP "resonance" energy E sp , where the SP DOS is very high. Non-resonant, SPmediated SE enhancements as large as 6 have been observed from GaAs QWs near thin Ag films 9 . Even greater enhancements are possible for wide bandgap semiconductors whose emission wavelength is coincident with E sp . In this report, time-resolved photoluminescence (TRPL) measurements of a partially silver-coated InGaN/GaN QW directly demonstrate the SP-mediated resonant enhancement of the SE rate for the first time in a semiconductor QW. An InGaN/GaN QW was used in these experiments, grown by metal-organic chemical vapor deposition (MOCVD) on sapphire substrate 13 . Over a 1.5 µm Si-doped GaN buffer layer was grown a 28 nm In 0.04 Ga 0.96 N reference layer, a 6 nm GaN layer, and the 3 nm In 0.18 Ga 0.82 N QW as shown in Fig. 1. Above the QW was a 12 nm Si-doped GaN cap layer, pl...

Current analysis of exosomes focuses primarily on bulk analysis, where exosome-to-exosome variability cannot be assessed. In this study, we used Raman spectroscopy to study the chemical composition of single exosomes. We measured spectra of individual exosomes from 8 cell lines. Cell-line-averaged spectra varied considerably, reflecting the variation in total exosomal protein, lipid, genetic, and cytosolic content. Unexpectedly, single exosomes isolated from the same cell type also exhibited high spectral variability. Subsequent spectral analysis revealed clustering of single exosomes into 4 distinct groups that were not cell-line specific. Each group contained exosomes from multiple cell lines, and most cell lines had exosomes in multiple groups. The differences between these groups are related to chemical differences primarily due to differing membrane composition. Through a principal components analysis, we identified that the major sources of spectral variation among the exosomes were in cholesterol content, relative expression of phospholipids to cholesterol, and surface protein expression. For example, exosomes derived from cancerous versus non-cancerous cell lines can be largely separated based on their relative expression of cholesterol and phospholipids. We are the first to indicate that exosome subpopulations are shared among cell types, suggesting distributed exosome functionality. The origins of these differences are likely related to the specific role of extracellular vesicle subpopulations in both normal cell function and carcinogenesis, and they may provide diagnostic potential at the single exosome level.

We present Spitzer Space

We present the Ðrst results of a survey of 220 starless cores selected primarily by their optical obscuration and observed in CS (2È1), (1È0), and C18O (1È0) using the Northeast Radio Observatory N 2 HC orporation (NEROC) Haystack 37 m telescope. We have detected 163 out of 196 sources observed in CS, 72 out of 142 in and 30 out of 30 in C18O. In total, 69 sources were detected in both CS and N 2 H`, The isolated component of the (1È0) spectrum usually shows a weak, 2) symmetric proÐle that is optically thin. In contrast, a signiÐcant fraction of the CS spectra show nonGaussian shapes, which we interpret as arising from a combination of self-absorption due to lower excitation gas in the core front and kinematics in the core. The distribution of the normalized velocity di †erence between the CS and peaks appears signiÐcantly skewed to the blue as (dV CS ) N 2 H`(dV CS \ 0), was found in a similar study of dense cores with embedded young stellar objects (YSOs). The incidence of sources with blue asymmetry tends to increase as the total optical depth or the integrated intensity of the line increases. This overabundance of "" blue ÏÏ over "" red ÏÏ sources suggests that inward N 2 Hm otions are a signiÐcant feature of starless cores. We identify seven strong infall candidates and 10 probable infall candidates. Their typical inward speeds are subsonic, approximately 0.04È0.1 km s~1, so they contain "" thermal ÏÏ infall motions, unlike the faster inward speeds associated with most YSOs. We discuss the importance of the choice of a consistent set of line frequencies when using the velocity shift between an optically thick and a thin line as a tracer of infall, and show how the results of the survey depend on that frequency choice.

No abstract

We determine the magnetic field strength in the OMC 1 region of the Orion A filament via a new implementation of the Chandrasekhar-Fermi method using observations performed as part of the James Clerk Maxwell Telescope (JCMT) B-Fields In Star-Forming Region Observations (BISTRO) survey with the POL-2 instrument. We combine BISTRO data with archival SCUBA-2 and HARP observations to find a plane-of-sky magnetic field strength in OMC 1 of B pos = 6.6 ± 4.7 mG, where δB pos = 4.7 mG represents a predominantly systematic uncertainty. We develop a new method for measuring angular dispersion, analogous to unsharp masking. We find a magnetic energy density of ∼ 1.7 × 10 −7 J m −3 in OMC 1, comparable both to the gravitational potential energy density of OMC 1 (∼ 10 −7 J m −3 ), and to the energy density in the Orion BN/KL outflow (∼ 10 −7 J m −3 ). We find that neither the Alfvén velocity in OMC 1 nor the velocity of the super-Alfvénic outflow ejecta is sufficiently large for the BN/KL outflow to have caused large-scale distortion of the local magnetic field in the ∼500-year lifetime of the outflow. Hence, we propose that the hour-glass field morphology in OMC 1 is caused by the distortion of a primordial cylindrically-symmetric magnetic field by the gravitational fragmentation of the filament and/or the gravitational interaction of the BN/KL and S clumps. We find that OMC 1 is currently in or near magnetically-supported equilibrium, and that the current large-scale morphology of the BN/KL outflow is regulated by the geometry of the magnetic field in OMC 1, and not vice versa.

We present a new catalogue of 406 dense cores optically selected by using the STScI Digitized Sky Survey (DSS). In this catalogue 306 cores have neither an Embedded YSO (EYSO) nor a Pre-Main-Sequence (PMS) star, 94 cores have EYSOs (1 core has both an EYSO and a PMS star), and 6 cores have PMS star only. Our sample of dense cores in the catalogue is fairly complete within a category of northern Lynds class 5, 6 clouds, and southern Hartley et al. (1986)'s class A clouds, providing a database useful for the systematic study of dense cores. Most of the cores listed in the catalogue have diameters between 0.05 − 0.36 pc with a mean of ∼ 0.24 pc. The sizes (∼ 0.33 pc in the mean ) of cores with EYSOs are found to be usually larger than the sizes (∼ 0.22 pc in the mean) of starless cores. The typical mean gas density of the cores is ∼ 7 × 10 3 cm −3 . Most of the cores are more likely elongated than spherical (mean aspect ratio: ∼ 2.4). The ratio of the number of cores with EYSOs to the number of starless cores for our sample is about 0.3, suggesting that the typical lifetime of starless cores is 0.3 − 1.6 Myr, about 3 times longer than the duration of the Class 0 and Class I phases. This lifetime is shorter than expected from models of ambipolar diffusion, by factors of 2 − 44.

ABSTRACT. Crucial steps in the formation of stars and planets can be studied only at mid-to far-infrared wavelengths, where the Space Infrared Telescope (SIRTF) provides an unprecedented improvement in sensitivity. We will use all three SIRTF instruments (Infrared Array Camera [IRAC], Multiband Imaging Photometer for SIRTF [MIPS], and Infrared Spectrograph [IRS]) to observe sources that span the evolutionary sequence from molecular cores to protoplanetary disks, encompassing a wide range of cloud masses, stellar masses, and starforming environments. In addition to targeting about 150 known compact cores, we will survey with IRAC and MIPS (3.6-70 mm) the entire areas of five of the nearest large molecular clouds for new candidate protostars and substellar objects as faint as 0.001 solar luminosities. We will also observe with IRAC and MIPS about 190 systems likely to be in the early stages of planetary system formation (ages up to about 10 Myr), probing the evolution of the circumstellar dust, the raw material for planetary cores. Candidate planet-forming disks as small as 0.1 lunar masses will be detectable. Spectroscopy with IRS of new objects found in the surveys and of a select group of known objects will add vital information on the changing chemical and physical conditions in the disks and envelopes. The resulting data products will include catalogs of thousands of previously unknown sources, multiwavelength maps of about 20 deg 2 of molecular clouds, photometry of about 190 known young stars, spectra of at least 170 sources, ancillary data from ground-based telescopes, and new tools for analysis and modeling. These products will constitute the foundations for many follow-up studies with ground-based telescopes, as well as with SIRTF itself and other space missions such as SIM, JWST, Herschel, and TPF/Darwin.