3,258 research outputs found
Multimode Spontaneous Parametric Down-Conversion in the Lossy Medium
We study the process of multimode Spontaneous Parametric Down--Conversion
(SPDC) in the lossy, one dimensional waveguide. We propose a description using
first order Correlation Functions (CF) in the fluorescence fields, as a very
fruitful and easy approach providing us with a complete information about the
final multimode state. We formulate the equation of the evolution of the
multimode CF along the crystal using four characteristic length scales. We
solve it analytically in the one mode case and numerically in the multimode
case. We capture simultaneous effects of three wave mixing with ultrashort
pump, linear propagation and attenuation, and we are able to divide the
evolution into three stages and predict it qualitatively. We find that losses
do not destroy the quantum properties of SPDC but stabilize the final state
Efficient infrared upconversion via a ladder-type atomic configuration
We have demonstrated experimentally that infrared light at 1529.4nm can be
converted into the visible at 780nm with 54% efficiency through a ladder-type
atomic configuration in 85Rb. Specifically we theoretically analyze that high
efficiency is due to the large nonlinear dispersion of the index of refraction
from the off-resonant enhancement in a four-wave mixing (FWM) process. By using
two perpendicular polarized pump fields, the coherence of two FWM processes in
this configuration is verified.Comment: The new version is published in Journal of Modern Optic
Nondegenerate parametric down conversion in coherently prepared two-level atomic gas
We describe parametric down conversion process in a two-level atomic gas,
where the atoms are in a superposition state of relevant energy levels. This
superposition results in splitting of the phase matching condition into three
different conditions. Another, more important, peculiarity of the system under
discussion is the nonsaturability of amplification coefficients with increasing
pump wave intensity, under "sideband" generation conditions
Squeezing spectra from s-ordered quasiprobability distributions. Application to dispersive optical bistability
It is well known that the squeezing spectrum of the field exiting a nonlinear
cavity can be directly obtained from the fluctuation spectrum of normally
ordered products of creation and annihilation operators of the cavity mode. In
this article we show that the output field squeezing spectrum can be derived
also by combining the fluctuation spectra of any pair of s-ordered products of
creation and annihilation operators. The interesting result is that the
spectrum obtained in this way from the linearized Langevin equations is exact,
and this occurs in spite of the fact that no s-ordered quasiprobability
distribution verifies a true Fokker-Planck equation, i.e., the Langevin
equations used for deriving the squeezing spectrum are not exact. The
(linearized) intracavity squeezing obtained from any s-ordered distribution is
also exact. These results are exemplified in the problem of dispersive optical
bistability.Comment: 15 pages, no figures, to be published in Journal of Modern Optic
Quantification of optical pulsed-plane-wave-shaping by chiral sculptured thin films
The durations and average speeds of ultrashort optical pulses transmitted
through chiral sculptured thin films (STFs) were calculated using a
finite-difference time-domain algorithm. Chiral STFs are a class of
nanoengineered materials whose microstructure comprises parallel helicoidal
nanowires grown normal to a substrate. The nanowires are 10-300 nm in
diameter and m in length. Durations of transmitted pulses tend to
increase with decreasing (free-space) wavelength of the carrier plane wave,
while average speeds tend to increase with increasing wavelength. An increase
in nonlinearity, as manifested by an intensity-dependent refractive index in
the frequency domain, tends to increase durations of transmitted pulses and
decrease average speeds. The circular Bragg phenomenon exhibited by a chiral
STFs manifests itself in the frequency domain as high reflectivity for normally
incident carrier plane waves whose circular polarization state is matched to
the structural handedness of the film and whose wavelength falls in a range
known as the Bragg regime; films of the opposite structural handedness reflect
such plane waves little. This effect tends to distort the shapes of transmitted
pulses with respect to the incident pulses, and such shaping can cause sharp
changes in some measures of average speed with respect to carrier wavelength. A
local maximum in the variation of one measure of the pulse duration with
respect to wavelength is noted and attributed to the circular Bragg phenomenon.
Several of these effects are explained via frequency-domain arguments. The
presented results serve as a foundation for future theoretical and experimental
studies of optical pulse propagation through causal, nonlinear, nonhomogeneous,
and anisotropic materials.Comment: To appear in Journal of Modern Optic
Multiple uncontrolled conditions and blood pressure medication intensification: an observational study
Abstract Background Multiple uncontrolled medical conditions may act as competing demands for clinical decision making. We hypothesized that multiple uncontrolled cardiovascular risk factors would decrease blood pressure (BP) medication intensification among uncontrolled hypertensive patients. Methods We observed 946 encounters at two VA primary care clinics from May through August 2006. After each encounter, clinicians recorded BP medication intensification (BP medication was added or titrated). Demographic, clinical, and laboratory information were collected from the medical record. We examined BP medication intensification by presence and control of diabetes and/or hyperlipidemia. 'Uncontrolled' was defined as hemoglobin A1c ≥ for diabetes, BP ≥ 140/90 mmHg (≥ 130/80 mmHg if diabetes present) for hypertension, and low density lipoprotein cholesterol (LDL-c) ≥ 130 mg/dl (≥ 100 mg/dl if diabetes present) for hyperlipidemia. Hierarchical regression models accounted for patient clustering and adjusted medication intensification for age, systolic BP, and number of medications. Results Among 387 patients with uncontrolled hypertension, 51.4% had diabetes (25.3% were uncontrolled) and 73.4% had hyperlipidemia (22.7% were uncontrolled). The BP medication intensification rate was 34.9% overall, but higher in individuals with uncontrolled diabetes and uncontrolled hyperlipidemia: 52.8% overall and 70.6% if systolic BP ≥ 10 mmHg above goal. Intensification rates were lowest if diabetes or hyperlipidemia were controlled, lower than if diabetes or hyperlipidemia were not present. Multivariable adjustment yielded similar results. Conclusions The presence of uncontrolled diabetes and hyperlipidemia was associated with more guideline-concordant hypertension care, particularly if BP was far from goal. Efforts to understand and improve BP medication intensification in patients with controlled diabetes and/or hyperlipidemia are warranted.http://deepblue.lib.umich.edu/bitstream/2027.42/78266/1/1748-5908-5-55.xmlhttp://deepblue.lib.umich.edu/bitstream/2027.42/78266/2/1748-5908-5-55.pdfPeer Reviewe
All Optical Implementation of Multi-Spin Entanglement in a Semiconductor Quantum Well
We use ultrafast optical pulses and coherent techniques to create spin
entangled states of non-interacting electrons bound to donors (at least three)
and at least two Mn2+ ions in a CdTe quantum well. Our method, relying on the
exchange interaction between localized excitons and paramagnetic impurities,
can in principle be applied to entangle a large number of spins.Comment: 17 pages, 3 figure
Combining frequency and time domain approaches to systems with multiple spike train input and output
A frequency domain approach and a time domain approach have been combined in an investigation of the behaviour of the primary and secondary endings of an isolated muscle spindle in response to the activity of two static fusimotor axons when the parent muscle is held at a fixed length and when it is subjected to random length changes. The frequency domain analysis has an associated error process which provides a measure of how well the input processes can be used to predict the output processes and is also used to
specify how the interactions between the recorded processes
contribute to this error. Without assuming stationarity of the input, the time domain approach uses a sequence of probability models of increasing complexity in which the number of input processes to the model is progressively increased. This feature of the time domain approach was used to identify a preferred direction of interaction between the processes underlying the generation of the activity of the primary and secondary endings. In the presence of fusimotor activity and dynamic length changes imposed on the muscle, it was shown that the activity of the primary and secondary endings carried different information about the effects of the inputs imposed on the muscle spindle. The results presented in this work emphasise that the analysis of the behaviour of complex
systems benefits from a combination of frequency and time
domain methods
Observation of second-harmonic generation induced by pure spin currents
Extensive efforts are currently being devoted to developing a new electronic
technology, called spintronics, where the spin of electrons is explored to
carry information. [1,2] Several techniques have been developed to generate
pure spin currents in many materials and structures. [3-10] However, there is
still no method available that can be used to directly detect pure spin
currents, which carry no net charge current and no net magnetization.
Currently, studies of pure spin currents rely on measuring the induced spin
accumulation with optical techniques [5, 11-13] or spin-valve configurations.
[14-17] However, the spin accumulation does not directly reflect the spatial
distribution or temporal dynamics of the pure spin current, and therefore
cannot monitor the pure spin current in a real-time and real-space fashion.
This imposes severe constraints on research in this field. Here we demonstrate
a second-order nonlinear optical effect of the pure spin current. We show that
such a nonlinear optical effect, which has never been explored before, can be
used for the non-invasive, non-destructive, and real-time imaging of pure spin
currents. Since this detection scheme does not rely on optical resonances, it
can be generally applied in a wide range of materials with different electronic
bandstructures. Furthermore, the control of nonlinear optical properties of
materials with pure spin currents may have potential applications in photonics
integrated with spintronics.Comment: 19 pages, 3 figures, supplementary discussion adde
Able-Bodied Wild Chimpanzees Imitate a Motor Procedure Used by a Disabled Individual to Overcome Handicap
Chimpanzee culture has generated intense recent interest, fueled by the technical complexity of chimpanzee tool-using traditions; yet it is seriously doubted whether chimpanzees are able to learn motor procedures by imitation under natural conditions. Here we take advantage of an unusual chimpanzee population as a ‘natural experiment’ to identify evidence for imitative learning of this kind in wild chimpanzees. The Sonso chimpanzee community has suffered from high levels of snare injury and now has several manually disabled members. Adult male Tinka, with near-total paralysis of both hands, compensates inability to scratch his back manually by employing a distinctive technique of holding a growing liana taut while making side-to-side body movements against it. We found that seven able-bodied young chimpanzees also used this ‘liana-scratch’ technique, although they had no need to. The distribution of the liana-scratch technique was statistically associated with individuals' range overlap with Tinka and the extent of time they spent in parties with him, confirming that the technique is acquired by social learning. The motivation for able-bodied chimpanzees copying his variant is unknown, but the fact that they do is evidence that the imitative learning of motor procedures from others is a natural trait of wild chimpanzees
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