4,572 research outputs found
Application of remote sensing in the study of vegetation and soils in Idaho
There are no author-identified significant results in this report
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
Velocity vector control system augmented with direct lift control
A pilot-controlled stability control system that employs direct lift control (spoiler control) with elevator control to control the flight path angle of an aircraft is described. A computer on the aircraft generates an elevator control signal and a spoiler control signal, using a pilot-controlled pitch control signal and pitch rate, vertical velocity, roll angle, groundspeed, engine pressure ratio and vertical acceleration signals which are generated on the aircraft. The direct lift control by the aircraft spoilers improves the response of the aircraft flight path angle and provides short term flight path stabilization against environmental disturbances
A digital scene matching technique for geometric image correction and autonomous navigation
A technique is described for precise registration of two images of the same area, taken under different conditions. The technique, called AUTO-MATCH, involves digital preprocessing of the images to extract edge contours, followed by a correlation of corresponding edge end points. The availability of an array of endpoints makes possible subpixed registration accuracy. The technique is applied to a system for assessment of geometric image quality, to be installed at NASA-Goddard. This system measures the registration vectors over an array of window pairs from LANDSAT images, so as to determine the distortion between them. Similar measurements could be used to infer relative positioning of the spacecraft. The technique is considered for updating the inertial navigation systems of missiles or aircraft. Scene matching between an image obtained aboard the vehicle and a stored reference can eliminate the drift of an inertial platform in order to demonstrate this capability in real time. A laboratory demonstration of AUTO-MATCH is described
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
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
Application of remote sensing in the study of vegetation and soils in Idaho
There are no author-identified significant results in this report
Can disorder enhance incoherent exciton diffusion?
Recent experiments aimed at probing the dynamics of excitons have revealed
that semiconducting films composed of disordered molecular subunits, unlike
expectations for their perfectly ordered counterparts, can exhibit a
time-dependent diffusivity in which the effective early time diffusion constant
is larger than that of the steady state. This observation has led to
speculation about what role, if any, microscopic disorder may play in enhancing
exciton transport properties. In this article, we present the results of a
model study aimed at addressing this point. Specifically, we present a general
model, based upon F\"orster theory, for incoherent exciton diffusion in a
material composed of independent molecular subunits with static energetic
disorder. Energetic disorder leads to heterogeneity in molecule-to-molecule
transition rates which we demonstrate has two important consequences related to
exciton transport. First, the distribution of local site-specific diffusivity
is broadened in a manner that results in a decrease in average exciton
diffusivity relative to that in a perfectly ordered film. Second, since
excitons prefer to make transitions that are downhill in energy, the steady
state distribution of exciton energies is biased towards low energy molecular
subunits, those that exhibit reduced diffusivity relative to a perfectly
ordered film. These effects combine to reduce the net diffusivity in a manner
that is time dependent and grows more pronounced as disorder is increased.
Notably, however, we demonstrate that the presence of energetic disorder can
give rise to a population of molecular subunits with exciton transfer rates
exceeding that of subunits in an energetically uniform material. Such
enhancements may play an important role in processes that are sensitive to
molecular-scale fluctuations in exciton density field.Comment: 15 pages, 3 figure
Nonequilibrium dynamics of localized and delocalized excitons in colloidal quantum dot solids
Self-assembled quantum dot (QD) solids are a highly tunable class of
materials with a wide range of applications in solid-state electronics and
optoelectronic devices. In this perspective, we highlight how the presence of
microscopic disorder in these materials can influence their macroscopic
optoelectronic properties. Specifically, we consider the dynamics of excitons
in energetically disordered QD solids using a theoretical model framework for
both localized and delocalized excitonic regimes. In both cases, we emphasize
the tendency of energetic disorder to promote nonequilibrium relaxation
dynamics and discuss how the signatures of these nonequilibrium effects
manifest in time-dependent spectral measurements. Moreover, we describe the
connection between the microscopic dynamics of excitons within the material and
the measurement of material specific parameters, such as emission linewidth
broadening and energetic dissipation rate.Comment: 4 figure
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Stranger Compass of the Stage: Difference and Desire in Early Modern City Comedy
In periods of social and political upheaval like ours, it is more important than ever to interrogate constructions of identity and difference and to understand the histories of alterity that separate us from one another. Stranger Compass of the Stage: Difference and Desire in Early Modern City Drama reimagines the cultural and social effect of alien, foreign, and stranger characters on the early modern stage and re-envisions how these characters contribute to, alter, and imaginatively build new epistemologies for understanding difference in early modern London. Resisting the field’s current critical inclination toward English identity formation, this project works intersectionally to exhume the delicate cultural and theatrical networks in which difference was negotiated. In doing so, it rescopes the limits of what counts as difference in the period.
Stranger Compass addresses fundamental questions of how early modern theater navigated difference on the stage by looking to four areas of performed difference: geographic and social difference, sexual difference, physical difference/disability, and gender non-conformance. Each chapter focuses on one of these areas, and each chapter is treated with a similar analytical framework that draws on transformation and desire as socially constitutive forces. Rescoping the cultural and theatrical landscape of London allows this project to begin with geographic and social difference and to work ever closer to negotiations of individual difference in the theatrical space.
Ultimately, Stranger Compass brings together methodologies that demonstrate how theatrical performance stimulated audience members to engage, participate, and revise their intimate attitudes toward difference. Looking to Thomas Middleton’s Michaelmas Term (1604) and A Trick to Catch the Old One (1605), the anonymously authored Fair Maid of the Exchange (1607), Thomas Dekker and Thomas Middleton’s The Roaring Girl (1607/10), and Ben Jonson’s Every Man in His Humour (1616 folio), I highlight that figures of difference are also often figures of familiarity and locality, who were integral to London’s most basic social relationships as a growing city with a malleable culture. Vital in their difference and desirable in their tangible divergence, the characters in these works call on us to reconsider difference, identity, and desire
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