18,136 research outputs found
Enhancement of second-order nonlinear-optical signals by optical stimulation
Second-order nonlinear optical interactions such as sum- and
difference-frequency generation are widely used for bioimaging and as selective
probes of interfacial environments. However, inefficient nonlinear optical
conversion often leads to poor signal-to-noise ratio and long signal
acquisition times. Here, we demonstrate the dramatic enhancement of weak
second-order nonlinear optical signals via stimulated sum- and
difference-frequency generation. We present a conceptual framework to
quantitatively describe the interaction and show that the process is highly
sensitive to the relative optical phase of the stimulating field. To emphasize
the utility of the technique, we demonstrate stimulated enhancement of second
harmonic generation (SHG) from bovine collagen-I fibrils. Using a stimulating
pulse fluence of only 3 nJ/cm2, we obtain an SHG enhancement >10^4 relative to
the spontaneous signal. The stimulation enhancement is greatest in situations
where spontaneous signals are the weakest - such as low laser power, small
sample volume, and weak nonlinear susceptibility - emphasizing the potential
for this technique to improve signal-to-noise ratios in biological imaging and
interfacial spectroscopy
Central dark matter content of early-type galaxies: scaling relations and connections with star formation histories
We examine correlations between masses, sizes and star formation histories for a large sample of low-redshift early-type galaxies, using a simple suite of dynamical and stellar population models. We confirm an anticorrelation between the size and stellar age and go on to survey for trends with the central content of dark matter (DM). An average relation between the central DM density and galaxy size of 〈ρDM〉∝R−2eff provides the first clear indication of cuspy DM haloes in these galaxies – akin to standard Λ cold dark matter haloes that have undergone adiabatic contraction. The DM density scales with galaxy mass as expected, deviating from suggestions of a universal halo profile for dwarf and late-type galaxies. We introduce a new fundamental constraint on galaxy formation by finding that the central DM fraction decreases with stellar age. This result is only partially explained by the size–age dependencies, and the residual trend is in the opposite direction to basic DM halo expectations. Therefore, we suggest that there may be a connection between age and halo contraction and that galaxies forming earlier had stronger baryonic feedback, which expanded their haloes, or lumpier baryonic accretion, which avoided halo contraction. An alternative explanation is a lighter initial mass function for older stellar populations
Should One Use the Ray-by-Ray Approximation in Core-Collapse Supernova Simulations?
We perform the first self-consistent, time-dependent, multi-group
calculations in two dimensions (2D) to address the consequences of using the
ray-by-ray+ transport simplification in core-collapse supernova simulations.
Such a dimensional reduction is employed by many researchers to facilitate
their resource-intensive calculations. Our new code (F{\sc{ornax}}) implements
multi-D transport, and can, by zeroing out transverse flux terms, emulate the
ray-by-ray+ scheme. Using the same microphysics, initial models, resolution,
and code, we compare the results of simulating 12-, 15-, 20-, and
25-M progenitor models using these two transport methods. Our
findings call into question the wisdom of the pervasive use of the ray-by-ray+
approach. Employing it leads to maximum post-bounce/pre-explosion shock radii
that are almost universally larger by tens of kilometers than those derived
using the more accurate scheme, typically leaving the post-bounce matter less
bound and artificially more "explodable." In fact, for our 25-M
progenitor, the ray-by-ray+ model explodes, while the corresponding multi-D
transport model does not. Therefore, in two dimensions the combination of
ray-by-ray+ with the axial sloshing hydrodynamics that is a feature of 2D
supernova dynamics can result in quantitatively, and perhaps qualitatively,
incorrect results.Comment: Updated and revised text; 13 pages; 13 figures; Accepted to Ap.
Egg Production in the Boxelder Bug \u3ci\u3eBoisea Trivittata\u3c/i\u3e (Hemiptera: Rhopalidae)
Boxelder bug females emerged from overwintering sites in the spring and rapidly provisioned eggs with yolk materials. Five discrete egg stages were identified based on egg size, protein content, and degree of chorion sclerotization. Females did not accumulate yolk materials into the egg until after melanization was completed, as unmelanized animals rarely possessed even stage 2 eggs. All adult females entering overwintering sites possessed only immature stage eggs (stage 1 and 2). The rate of egg vitellogenesis in the spring was rapid; a major change in numbers of more mature stage eggs (stage 3 and above) in the ovary occurred within approximately 6 days. Most mating pairs recovered in the field (92%, 12/13) possessed ovaries full of eggs in stages 3, 4 or 5. The remaining female contained only immature eggs of stage 1 and 2. This finding indicates that fully provisioned ovaries are not an absolute requirement for mating to occur. The signals that initiate vitellogenesis and control the movement of materials from fat body into eggs are unknown for the boxelder bug
Exciton Trapping Is Responsible for the Long Apparent Lifetime in Acid-Treated MoS2
Here, we show that deep trapped "dark" exciton states are responsible for the
surprisingly long lifetime of band-edge photoluminescence in acid-treated
single-layer MoS2. Temperature-dependent transient photoluminescence
spectroscopy reveals an exponential tail of long-lived states extending
hundreds of meV into the band gap. These sub-band states, which are
characterized by a 4 microsecond radiative lifetime, quickly capture and store
photogenerated excitons before subsequent thermalization up to the band edge
where fast radiative recombination occurs. By intentionally saturating these
trap states, we are able to measure the "true" 150 ps radiative lifetime of the
band-edge exciton at 77 K, which extrapolates to ~600 ps at room temperature.
These experiments reveal the dominant role of dark exciton states in
acid-treated MoS2, and suggest that excitons spend > 95% of their lifetime at
room temperature in trap states below the band edge. We hypothesize that these
states are associated with native structural defects, which are not introduced
by the superacid treatment; rather, the superacid treatment dramatically
reduces non-radiative recombination through these states, extending the exciton
lifetime and increasing the likelihood of eventual radiative recombination
Ultrafast Charge Transfer at a Quantum Dot/2D Materials Interface Probed by Second Harmonic Generation
Hybrid quantum dot (QD) / transition metal dichalcogenide (TMD)
heterostructures are attractive components of next generation optoelectronic
devices, which take advantage of the spectral tunability of QDs and the charge
and exciton transport properties of TMDs. Here, we demonstrate tunable
electronic coupling between CdSe QDs and monolayer WS using variable length
alkanethiol ligands on the QD surface. Using femtosecond time-resolved second
harmonic generation (SHG) microscopy, we show that electron transfer from
photoexcited CdSe QDs to single-layer WS occurs on ultrafast (50 fs - 1 ps)
timescales. Moreover, in the samples exhibiting the fastest charge transfer
rates ( 50 fs) we observed oscillations in the time-domain signal
corresponding to an acoustic phonon mode of the donor QD, which coherently
modulates the SHG response of the underlying WS layer. These results reveal
surprisingly strong electronic coupling at the QD/TMD interface and demonstrate
the usefulness of time-resolved SHG for exploring ultrafast
electronic-vibrational dynamics in TMD heterostructures
A Pneumatically Actuated Manipulandum for Neuromotor Control Research
Functional magnetic resonance imaging (fMRI) techniques have great potential for identifying which neural structures are involved in the control of goal-directed reaching movements. However, fMRI techniques alone are not capable of probing the neural mechanisms involved in acquisition of novel motor behaviors because such studies require that the moving limb be perturbed in a controlled fashion. We outline a plan to design and develop a non-metallic, pneumatically actuated tool that, along with systems identification techniques and functional magnetic resonance imaging (fMRI), will characterize and quantify how the human central nervous system uses sensory information during practice-based motor learning
An Enhanced Spectroscopic Census of the Orion Nebula Cluster
We report new spectral types or spectral classification constraints for over
600 stars in the Orion Nebula Cluster (ONC) based on medium resolution R~
1500-2000 red optical spectra acquired using the Palomar 200" and Kitt Peak
3.5m telescopes. Spectral types were initially estimated for F, G, and early K
stars from atomic line indices while for late K and M stars, constituting the
majority of our sample, indices involving TiO and VO bands were used. To ensure
proper classification, particularly for reddened, veiled, or
nebula-contaminated stars, all spectra were then visually examined for type
verification or refinement. We provide an updated spectral type table that
supersedes Hillenbrand (1997), increasing the percentage of optically visible
ONC stars with spectral type information from 68% to 90%. However, for many
objects, repeated observations have failed to yield spectral types primarily
due to the challenges of adequate sky subtraction against a bright and
spatially variable nebular background. The scatter between our new and our
previously determined spectral types is approximately 2 spectral sub-classes.
We also compare our grating spectroscopy results with classification based on
narrow-band TiO filter photometry from Da Rio et al. (2012, finding similar
scatter. While the challenges of working in the ONC may explain much of the
spread, we highlight several stars showing significant and unexplained bona
fide spectral variations in observations taken several years apart; these and
similar cases could be due to a combination of accretion and extinction
changes. Finally, nearly 20% of ONC stars exhibit obvious Ca II triplet
emission indicative of strong accretion.Comment: Accepted to the Astronomical Journal; 37 pages, including 11 Figures
and 3 Tables (one long table not reproduced here but available upon request
or from the journal
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