17,009 research outputs found
Photon Distribution Function for Long-Distance Propagation of Partially Coherent Beams through the Turbulent Atmosphere
The photon density operator function is used to calculate light beam
propagation through turbulent atmosphere. A kinetic equation for the photon
distribution function is derived and solved using the method of
characteristics. Optical wave correlations are described in terms of photon
trajectories that depend on fluctuations of the refractive index. It is shown
that both linear and quadratic disturbances produce sizable effects for
long-distance propagation. The quadratic terms are shown to suppress the
correlation of waves with different wave vectors. We examine the intensity
fluctuations of partially coherent beams (beams whose initial spatial coherence
is partially destroyed). Our calculations show that it is possible to
significantly reduce the intensity fluctuations by using a partially coherent
beam. The physical mechanism responsible for this pronounced reduction is
similar to that of the Hanbury-Braun, Twiss effect.Comment: 28 pages, 4 figure
Manipulation of ultracold atoms in dressed adiabatic radio frequency potentials
We explore properties of atoms whose magnetic hyperfine sub-levels are
coupled by an external magnetic radio frequency (rf) field. We perform a
thorough theoretical analysis of this driven system and present a number of
systematic approximations which eventually give rise to dressed adiabatic radio
frequency potentials. The predictions of this analytical investigation are
compared to numerically exact results obtained by a wave packet propagation. We
outline the versatility and flexibility of this new class of potentials and
demonstrate their potential use to build atom optical elements such as
double-wells, interferometers and ringtraps. Moreover, we perform simulations
of interference experiments carried out in rf induced double-well potentials.
We discuss how the nature of the atom-field coupling mechanism gives rise to a
decrease of the interference contrast
Six-wave mixing: secular resonances in a higher-order mechanism for second-harmonic generation
Herschel evidence for disk flattening or gas depletion in transitional disks
Transitional disks are protoplanetary disks characterized by reduced near-
and mid-infrared emission with respect to full disks. This characteristic
spectral energy distribution indicates the presence of an optically thin inner
cavity within the dust disk believed to mark the disappearance of the
primordial massive disk. We present new Herschel Space Observatory PACS spectra
of [OI] 63 micron for 21 transitional disks. Our survey complements the larger
Herschel GASPS program "Gas in Protoplanetary Systems" (Dent et al. 2013) by
quadrupling the number of transitional disks observed with PACS at this
wavelength. [OI] 63 micron traces material in the outer regions of the disk,
beyond the inner cavity of most transitional disks. We find that transitional
disks have [OI] 63 micron line luminosities two times fainter than their full
disk counterparts. We self consistently determine various stellar properties
(e.g. bolometric luminosity, FUV excess, etc.) and disk properties (e.g. disk
dust mass, etc.) that could influence the [OI] 63 micron line luminosity and we
find no correlations that can explain the lower [OI] 63 micron line
luminosities in transitional disks. Using a grid of thermo-chemical
protoplanetary disk models, we conclude that either transitional disks are less
flared than full disks or they possess lower gas-to-dust ratios due to a
depletion of gas mass. This result suggests that transitional disks are more
evolved than their full disk counterparts, possibly even at large radii.Comment: Accepted for publication in ApJ; 52 pages, 16 figures, 8 table
Phosphorothioate Anti-sense Oligonucleotides: The Kinetics and Mechanism of the Generation of the Sulfurising Agent from Phenylacetyl Disulfide (PADS)
The synthesis of phosphorothioate oligonucleotides is often accomplished in the pharmaceutical industry by the sulfurisation of the nucleotide-phosphite using phenylacetyl disulfide (PADS) which has an optimal combination of properties. This is best achieved by an initial ‘ageing’ of PADS for 48 hrs in acetonitrile with 3-picoline to generate polysulfides. The initial base-catalysed degradation of PADS occurs by an E1cB-type elimination to generate a ketene and acyldisulfide anion. Proton abstraction to reversibly generate a carbanion is demonstrated by H/D exchange, the rate of which is greatly increased by electron-withdrawing substituents in the aromatic ring of PADS. The ketene can be trapped intramolecularly by an o-allyl group. The disulfide anion generated subsequently attacks unreacted PADS on sulfur to give polysulfides, the active sulfurising agent. The rate of degradation of PADS is decreased by less basic substituted pyridines and is only first order in PADS indicating that the rate-limiting step is formation of the disulfide anion from the carbanion
Laser-controlled fluorescence in two-level systems
The ability to modify the character of fluorescent emission by a laser-controlled, optically nonlinear process has recently been shown theoretically feasible, and several possible applications have already been identified. In operation, a pulse of off-resonant probe laser beam, of sufficient intensity, is applied to a system exhibiting fluorescence, during the interval of excited- state decay following the initial excitation. The result is a rate of decay that can be controllably modified, the associated changes in fluorescence behavior affording new, chemically specific information. In this paper, a two-level emission model is employed in the further analysis of this all-optical process; the results should prove especially relevant to the analysis and imaging of physical systems employing fluorescent markers, these ranging from quantum dots to green fluorescence protein. Expressions are presented for the laser-controlled fluorescence anisotropy exhibited by samples in which the fluorophores are randomly oriented. It is also shown that, in systems with suitably configured electronic levels and symmetry properties, fluorescence emission can be produced from energy levels that would normally decay nonradiatively. © 2010 American Chemical Society
Quantum carpet interferometry for trapped atomic Bose-Einstein condensates
We propose an ``interferometric'' scheme for Bose-Einstein condensates using
near-field diffraction. The scheme is based on the phenomenon of intermode
traces or quantum carpets; we show how it may be used in the detection of weak
forces.Comment: 4 figures. Submitted to Phys. Rev.
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