547 research outputs found
Tailored Raman-resonant four-wave-mixing processes
Nonlinear optical processes are strongly dominated by phase relationships among electromagnetic fields involved. In this paper, we theoretically and experimentally show that in a Raman-resonant four-wave-mixing process, the first anti-Stokes and Stokes generations can be tailored in a variety of ways by manipulating the phase relationships among the relevant electromagnetic fields
Numerical simulations of chromospheric hard X-ray source sizes in solar flares
X-ray observations are a powerful diagnostic tool for transport,
acceleration, and heating of electrons in solar flares. Height and size
measurements of X-ray footpoints sources can be used to determine the
chromospheric density and constrain the parameters of magnetic field
convergence and electron pitch-angle evolution. We investigate the influence of
the chromospheric density, magnetic mirroring and collisional pitch-angle
scattering on the size of X-ray sources. The time-independent Fokker-Planck
equation for electron transport is solved numerically and analytically to find
the electron distribution as a function of height above the photosphere. From
this distribution, the expected X-ray flux as a function of height, its peak
height and full width at half maximum are calculated and compared with RHESSI
observations. A purely instrumental explanation for the observed source size
was ruled out by using simulated RHESSI images. We find that magnetic mirroring
and collisional pitch-angle scattering tend to change the electron flux such
that electrons are stopped higher in the atmosphere compared with the simple
case with collisional energy loss only. However, the resulting X-ray flux is
dominated by the density structure in the chromosphere and only marginal
increases in source width are found. Very high loop densities (>10^{11}
cm^{-3}) could explain the observed sizes at higher energies, but are
unrealistic and would result in no footpoint emission below about 40 keV,
contrary to observations. We conclude that within a monolithic density model
the vertical sizes are given mostly by the density scale-height and are
predicted smaller than the RHESSI results show.Comment: 19 pages, 9 figures, accepted for publication in Ap
Flare Ribbons Observed with G-band and FeI 6302A Filters of the Solar Optical Telescope on Board Hinode
The Solar Optical Telescope (SOT) on board Hinode satellite observed an X3.4
class flare on 2006 December 13. Typical two-ribbon structure was observed, not
only in the chromospheric CaII H line but also in G-band and FeI 6302A line.
The high-resolution, seeing-free images achieved by SOT revealed, for the first
time, the sub-arcsec fine structures of the "white light" flare. The G-band
flare ribbons on sunspot umbrae showed a sharp leading edge followed by a
diffuse inside, as well as previously known core-halo structure. The underlying
structures such as umbral dots, penumbral filaments and granules were visible
in the flare ribbons. Assuming that the sharp leading edge was directly heated
by particle beam and the diffuse parts were heated by radiative back-warming,
we estimate the depth of the diffuse flare emission using the intensity profile
of the flare ribbon. We found that the depth of the diffuse emission is about
100 km or less from the height of the source of radiative back-warming. The
flare ribbons were also visible in the Stokes-V images of FeI 6302A, as a
transient polarity reversal. This is probably related to "magnetic transient"
reported in the literature. The intensity increase in Stokes-I images indicates
that the FeI 6302A line was significantly deformed by the flare, which may
cause such a magnetic transient.Comment: 14 pages, 7 figures, PASJ in pres
Human Rehearsal Processes and the Frontal Lobes: PET Evidence
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71395/1/j.1749-6632.1995.tb38134.x.pd
New Computer-Aided Diagnosis of Dementia Using Positron Emission Tomography: Brain Regional Sensitivity-Mapping Method
Purpose: We devised a new computer-aided diagnosis method to segregate dementia using one estimated index (Total Z
score) derived from the Brodmann area (BA) sensitivity map on the stereotaxic brain atlas. The purpose of this study is to
investigate its accuracy to differentiate patients with Alzheimer’s disease (AD) or mild cognitive impairment (MCI) from
normal adults (NL).
Methods: We studied 101 adults (NL: 40, AD: 37, MCI: 24) who underwent 18FDG positron emission tomography (PET)
measurement. We divided NL and AD groups into two categories: a training group with (Category A) and a test group
without (Category B) clinical information. In Category A, we estimated sensitivity by comparing the standard uptake value
per BA (SUVR) between NL and AD groups. Then, we calculated a summated index (Total Z score) by utilizing the sensitivitydistribution
maps and each BA z-score to segregate AD patterns. To confirm the validity of this method, we examined the
accuracy in Category B. Finally, we applied this method to MCI patients.
Results: In Category A, we found that the sensitivity and specificity of differentiation between NL and AD were all 100%. In
Category B, those were 100% and 95%, respectively. Furthermore, we found this method attained 88% to differentiate ADconverters
from non-converters in MCI group.
Conclusions: The present automated computer-aided evaluation method based on a single estimated index provided good
accuracy for differential diagnosis of AD and MCI. This good differentiation power suggests its usefulness not only for
dementia diagnosis but also in a longitudinal study.浜松医科大学学位論文 医博第695号(平成27年3月16日
Benefits of Artificially Generated Gravity Gradients for Interferometric Gravitational-Wave Detectors
We present an approach to experimentally evaluate gravity gradient noise, a
potentially limiting noise source in advanced interferometric gravitational
wave (GW) detectors. In addition, the method can be used to provide sub-percent
calibration in phase and amplitude of modern interferometric GW detectors.
Knowledge of calibration to such certainties shall enhance the scientific
output of the instruments in case of an eventual detection of GWs. The method
relies on a rotating symmetrical two-body mass, a Dynamic gravity Field
Generator (DFG). The placement of the DFG in the proximity of one of the
interferometer's suspended test masses generates a change in the local
gravitational field detectable with current interferometric GW detectors.Comment: 16 pages, 4 figure
Experimental demonstration of distance measurement with a femtosecond frequency comb laser
We experimentally demonstrate that a stabilized femtosecond frequency comb can be applied as a tool for distance measurement in an interferometric scheme. A proof of principle of this method, as proposed by Ye [5], is provided by measuring a displacement of about 15 cm in air and comparing it to a reference value from a calibrated laser interferometer. The experiment shows that the new scheme easily achieves an accuracy better than one optical fringe. [DOI: 10.2971/jeos.2008.08003
In vivo mapping of cholinergic terminals in normal aging, Alzheimer's disease, and Parkinson's disease
To map presynaptic cholinergic terminal densities in normal aging (n = 36), Alzheimer's disease (AD) (n = 22), and Parkinson's disease (PD) (n = 15), we performed single-photon emission computed tomography using [ 123 I]iodoben-zovesamicol (IBVM), an in vivo marker of the vesicular acetylcholine transporter. We used coregistered positron emission tomography with [ 18 F]fluorodexyglucose for metabolic assessment and coregistered magnetic resonance imaging for atrophy assessment. In controls (age, 22–91 years), cortical IBVM binding declined only 3.7% per decade. In AD, cortical binding correlated inversely with dementia severity. In mild dementia, binding differed according to age of onset, but metabolism did not. With an onset age of less than 65 years, binding was reduced severely throughout the entire cerebral cortex and hippocapus (about 30%), but with an onset age of 65 years or more, binding reductions were restricted to temporal cortex and hippocampus. In PD without dementia, binding was reduced only in parietal and occipital cortex, but demented PD subjects had extensive cortical binding decreases similar to early-onset AD. We conclude that cholinergic neuron integrity can be monitored in living AD and PD patients, and that it is not so devastated in vivo as suggested by postmortem choline acetylransferase activity (50–80%).Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/50361/1/410400309_ftp.pd
Statistics of low-energy levels of a one-dimensional weakly localized Frenkel exciton: A numerical study
Numerical study of the one-dimensional Frenkel Hamiltonian with on-site
randomness is carried out. We focus on the statistics of the energy levels near
the lower exciton band edge, i. e. those determining optical response. We found
that the distribution of the energy spacing between the states that are well
localized at the same segment is characterized by non-zero mean, i.e. these
states undergo repulsion. This repulsion results in a local discrete energy
structure of a localized Frenkel exciton. On the contrary, the energy spacing
distribution for weakly overlapping local ground states (the states with no
nodes within their localization segments) that are localized at different
segments has zero mean and shows almost no repulsion. The typical width of the
latter distribution is of the same order as the typical spacing in the local
discrete energy structure, so that this local structure is hidden; it does not
reveal itself neither in the density of states nor in the linear absorption
spectra. However, this structure affects the two-exciton transitions involving
the states of the same segment and can be observed by the pump-probe
spectroscopy. We analyze also the disorder degree scaling of the first and
second momenta of the distributions.Comment: 10 pages, 6 figure
Optical technology for arbitrarily manipulating amplitudes and phases of coaxially propagating highly discrete spectra
The development of new optical technology plays a key role in the essential evolution of optical science. In this article, we experimentally demonstrate an optical technology that enables the arbitrary manipulation of amplitudes and phases of highly discrete spectra. This optical technology is simple and practically useful: we may only place a few fundamental optical elements on an optical axis and control their thicknesses precisely. As a typical application, we show generation of a train of ultrashort pulses in the time domain, with a pulse duration close to 1 fs and a repetition rate exceeding 100 THz
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