2,619 research outputs found
Competition between Electromagnetically Induced Transparency and Raman Processes
We present a theoretical formulation of competition among electromagnetically
induced transparency (EIT) and Raman processes. The latter become important
when the medium can no longer be considered to be dilute. Unlike the standard
formulation of EIT, we consider all fields applied and generated as interacting
with both the transitions of the scheme. We solve Maxwell equations
for the net generated field using a fast-Fourier-transform technique and obtain
predictions for the probe, control and Raman fields. We show how the intensity
of the probe field is depleted at higher atomic number densities due to the
build up of multiple Raman fields.Comment: 3.5 pages, 7 figure
In-situ electrochemical fabrication of natural contacts on single nanowires
We report a template-based in-situ electrochemical method for fabricating
natural electric contacts on single nanowires using a pair of cross-patterned
electrodes. Such electric contacts are highly stable upon thermal cycling
between room temperature and milli-Kelvin temperatures. Direct imaging of the
single-nanowire contacts using scanning electron microscopy is also
demonstrated.Comment: 13 pages, 4 figure
The role of intermolecular coupling in the photophysics of disordered organic semiconductors: Aggregate emission in regioregular polythiophene
We address the role of excitonic coulping on the nature of photoexcitations
in the conjugated polymer regioregular poly(3-hexylthiophene). By means of
temperature-dependent absorption and photoluminescence spectroscopy, we show
that optical emission is overwhelmingly dominated by weakly coupled
H-aggregates. The relative absorbance of the 0-0 and 0-1 vibronic peaks
provides a powerfully simple means to extract the magnitude of the
intermolecular coupling energy, approximately 5 and 30 meV for films spun from
isodurene and chloroform solutions respectively.Comment: 10 pages, 4 figures, published in Phys. Rev. Let
Buried AGNs in Advanced Mergers:Mid-infrared color selection as a dual AGN finder
A direct consequence of hierarchical galaxy formation is the existence of
dual supermassive black holes (SMBHs), which may be preferentially triggered as
active galactic nuclei (AGN) during galaxy mergers. Despite decades of
searching, however, dual AGNs are extremely rare, and most have been discovered
serendipitously. Using the all-sky WISE survey, we identified a population of
over 100 morphologically identified interacting galaxies or mergers that
display red mid-infrared colors often associated in extragalactic sources with
powerful AGNs. The vast majority of these advanced mergers are optically
classified as star-forming galaxies suggesting that they may represent an
obscured population of AGNs that cannot be found through optical studies. In
this work, we present Chandra/ACIS observations and near-infrared spectra with
the Large Binocular Telescope of six advanced mergers with projected pair
separations less than ~ 10 kpc. The combined X-ray, near-infrared, and
mid-infrared properties of these mergers provide confirmation that four out of
the six mergers host at least one AGN, with four of the mergers possibly
hosting dual AGNs with projected separations less than ~10 kpc, despite showing
no firm evidence for AGNs based on optical spectroscopic studies. Our results
demonstrate that 1) optical studies miss a significant fraction of single and
dual AGNs in advanced mergers, and 2) mid-infrared pre-selection is extremely
effective in identifying dual AGN candidates in late-stage mergers. Our
multi-wavelength observations suggest that the buried AGNs in these mergers are
highly absorbed, with intrinsic column densities in excess of N_H >10^24cm^-2,
consistent with hydrodynamic simulations.Comment: 23 pages, 11 figures, accepted for publication to Ap
Evidence for Excimer Photoexcitations in an Ordered {\pi}-Conjugated Polymer Film
We report pressure-dependent transient picosecond and continuous-wave
photomodulation studies of disordered and ordered films of
2-methoxy-5-(2-ethylhexyloxy) poly(para-phenylenevinylene). Photoinduced
absorption (PA) bands in the disordered film exhibit very weak pressure
dependence and are assigned to intrachain excitons and polarons. In contrast,
the ordered film exhibits two additional transient PA bands in the midinfrared
that blueshift dramatically with pressure. Based on high-order configuration
interaction calculations we ascribe the PA bands in the ordered film to
excimers. Our work brings insight to the exciton binding energy in ordered
films versus disordered films and solutions. The reduced exciton binding energy
in ordered films is due to new energy states appearing below the continuum band
threshold of the single strand.Comment: 5.5 pages, 5 figure
Buried Black Hole Growth in IR-selected Mergers: New Results from Chandra
Observations and theoretical simulations suggest that a significant fraction
of merger-triggered accretion onto supermassive black holes is highly obscured,
particularly in late-stage galaxy mergers, when the black hole is expected to
grow most rapidly. Starting with the Wide-Field Infrared Survey Explorer
all-sky survey, we identified a population of galaxies whose morphologies
suggest ongoing interaction and which exhibit red mid-infrared colors often
associated with powerful active galactic nuclei (AGNs). In a follow-up to our
pilot study, we now present Chandra/ACIS and XMM-Newton X-ray observations for
the full sample of the brightest 15 IR-preselected mergers. All mergers reveal
at least one nuclear X-ray source, with 8 out of 15 systems exhibiting dual
nuclear X-ray sources, highly suggestive of single and dual AGNs. Combining
these X-ray results with optical line ratios and with near-IR coronal emission
line diagnostics, obtained with the near-IR spectrographs on the Large
Binocular Telescope, we confirm that 13 out of the 15 mergers host AGNs, two of
which host dual AGNs. Several of these AGNs are not detected in the optical.
All X-ray sources appear X-ray weak relative to their mid-infrared continuum,
and of the nine X-ray sources with sufficient counts for spectral analysis,
eight reveal strong evidence of high absorption with column densities of
~cm. These observations demonstrate that a
significant population of single and dual AGNs are missed by optical studies,
due to high absorption, adding to the growing body of evidence that the epoch
of peak black hole growth in mergers occurs in a highly obscured phase.Comment: 29 pages, 22 figures; (Main text: 17 pages, 4 figures
Obscured AGNS in Bulgeless Hosts Discovered By Wise : The Case Study of Sdss J1224+5555
There is mounting evidence that supermassive black holes form and grow in bulgeless galaxies. However, a robust determination of the fraction of AGNs in bulgeless galaxies, an important constraint to models of supermassive black hole seed formation and merger-free models of AGN fueling, is unknown, since optical studies have been shown to be incomplete for low mass AGNs. In a recent study using the Wide-field Infrared Survey Explorer, we discovered hundreds of bulgeless galaxies that display mid-infrared signatures of extremely hot dust suggestive of powerful accreting massive black holes, despite having no signatures of black hole activity at optical wavelengths. Here we report X-ray follow-up observations of J122434.66+555522.3, a nearby (z=0.052) isolated bulgeless galaxy that contains an unresolved X-ray source detected at the 3 sigma level by XMM-Newton with an observed luminosity uncorrected for intrinsic absorption of L2-10~keV=1.1+/-0.4 1040 ergs s-1. Ground-based near-infrared spectroscopy with the Large Binocular Telescope together with multiwavelength observations from ultraviolet to millimeter wavelengths together suggest that J1224+5555 harbors a highly absorbed AGN with an intrinsic absorption of ~NH \u3e1024 cm-2. The hard X-ray luminosity of the putative AGN corrected for absorption is L2-10~keV~3x1042 ergs s-1, which, depending on the bolometric correction factor, corresponds to a bolometric luminosity of the AGN of 6x1043 ergs s-1 - 3x1044 erg s-1, and a lower mass limit for the black hole of MBH~2x106 Msun, based on the Eddington limit. While enhanced X-ray emission and hot dust can be produced by star formation in extremely low metallicity environments typical in dwarf galaxies, J1224+5555 has a stellar mass of ~2.0 x 1010 Msun and an above solar metallicity (12 + logO/H = 9.11), typical of our WISE-selected bulgeless galaxy sample. While collectively these observations suggest the presence of an AGN, we caution that identifying obscured AGNs in the low-luminosity regime is challenging and often requires multiwavelength observations. These observations suggest that low-luminosity AGNs can be heavily obscured and reside in optically quiescent galaxies, adding to the growing body of evidence that the fraction of bulgeless galaxies with accreting black holes may be significantly underestimated based on optical studies
Partially-Time-Ordered Schwinger-Keldysh Loop Expansion of Coherent Nonlinear Optical Susceptibilities
A compact correlation-function expansion is developed for nth order optical
susceptibilities in the frequency domain using the Keldysh-Schwinger loop. By
not keeping track of the relative time ordering of bra and ket interactions at
the two branches of the loop, the resulting expressions contain only n+1 basic
terms, compared to the 2n terms required for a fully time-ordered density
matrix description. Superoperator Green's function expressions for the nth
order suscpeptibility derived using both expansions reflect different types of
interferences between pathways .These are demonstrated for correlation-induced
resonances in four wave mixing signals.Comment: article: 19 pages (preprint style!; including figures) ``paper.tex''
figures:
Not In Our Backyard: Spectroscopic Support for the CLASH z=11 Candidate MACS0647-JD
We report on our first set of spectroscopic Hubble Space Telescope
observations of the z~11 candidate galaxy strongly lensed by the
MACSJ0647.7+7015 galaxy cluster. The three lensed images are faint and we show
that these early slitless grism observations are of sufficient depth to
investigate whether this high-redshift candidate, identified by its strong
photometric break at ~1.5 micron, could possibly be an emission line galaxy at
a much lower redshift. While such an interloper would imply the existence of a
rather peculiar object, we show here that such strong emission lines would
clearly have been detected. Comparing realistic, two-dimensional simulations to
these new observations we would expect the necessary emission lines to be
detected at >5 sigma while we see no evidence for such lines in the dispersed
data of any of the three lensed images. We therefore exclude that this object
could be a low redshift emission line interloper, which significantly increases
the likelihood of this candidate being a bona fide z~11 galaxy.Comment: 14 Pages. 6 Figures. 2nd revised version. Accepted. To appear in ApJ.
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