263 research outputs found
Non-Markovian Decay and Lasing Condition in an Optical Microcavity Coupled to a Structured Reservoir
The decay dynamics of the classical electromagnetic field in a leaky optical
resonator supporting a single mode coupled to a structured continuum of modes
(reservoir) is theoretically investigated, and the issue of threshold condition
for lasing in presence of an inverted medium is comprehensively addressed.
Specific analytical results are given for a single-mode microcavity resonantly
coupled to a coupled resonator optical waveguide (CROW), which supports a band
of continuous modes acting as decay channels. For weak coupling, the usual
exponential Weisskopf-Wigner (Markovian) decay of the field in the bare
resonator is found, and the threshold for lasing increases linearly with the
coupling strength. As the coupling between the microcavity and the structured
reservoir increases, the field decay in the passive cavity shows non
exponential features, and correspondingly the threshold for lasing ceases to
increase, reaching a maximum and then starting to decrease as the coupling
strength is further increased. A singular behavior for the "laser phase
transition", which is a clear signature of strong non-Markovian dynamics, is
found at critical values of the coupling between the microcavity and the
reservoir.Comment: to appear in Phys. Rev. A (December 2006 issue
Green-function method in the theory of ultraslow electromagnetic waves in an ideal gas with Bose-Einstein condensates
We propose a microscopic approach describing the interaction of an ideal gas
of hydrogenlike atoms with a weak electromagnetic field. This approach is based
on the Green-function formalism and an approximate formulation of the method of
second quantization for quantum many-particle systems in the presence of bound
states of particles. The dependencies of the propagation velocity and damping
rate of electromagnetic pulses on the microscopic characteristics of the system
are studied for a gas of hydrogenlike atoms. For a Bose-Einstein condensate of
alkali-metal atoms we find the conditions when the electromagnetic waves of
both the optical and microwave regions are slowed. In the framework of the
proposed approach, the influence of an external homogeneous and static magnetic
field on the slowing phenomenon is studied.Comment: 15 pages, 6 figure
Statistical Properties of the Reflectance and Transmittance of an Amplifying Random Media
Statistical properties of the transmittance () and reflectance () of an
amplifying layer with one-dimensional disorder are investigated analytically.
Whereas the transmittance at typical realizations decreases exponentially with
the layer thickness just as it does in absorbing media, the average
and \ are shown to
be infinite even for finite due to the contribution of low-probable
resonant realizations corresponding to the non-Gaussian tail of the
distribution of . This tail differs drastically from that in the case of
absorption. The physical meaning of typical and resonant realizations is
discussed.Comment: 5 pages (RevTeX
An insight into polarization states of solid-state organic lasers
The polarization states of lasers are crucial issues both for practical
applications and fundamental research. In general, they depend in a combined
manner on the properties of the gain material and on the structure of the
electromagnetic modes. In this paper, we address this issue in the case of
solid-state organic lasers, a technology which enables to vary independently
gain and mode properties. Different kinds of resonators are investigated:
in-plane micro-resonators with Fabry-Perot, square, pentagon, stadium, disk,
and kite shapes, and external vertical resonators. The degree of polarization P
is measured in each case. It is shown that although TE modes prevail generally
(P>0), kite-shaped micro-laser generates negative values for P, i.e. a flip of
the dominant polarization which becomes mostly TM polarized. We at last
investigated two degrees of freedom that are available to tailor the
polarization of organic lasers, in addition to the pump polarization and the
resonator geometry: upon using resonant energy transfer (RET) or upon pumping
the laser dye to an higher excited state. We then demonstrate that
significantly lower P factors can be obtained.Comment: 12 pages, 12 figure
Smart monitoring system based on adaptive current control for superconducting cable test
A smart monitoring system for superconducting cable test is proposed with an adaptive currentcontrol of asuperconductingtransformer secondary. The design, based on Fuzzy Gain Scheduling, allows thecontrollerparameters to adapt continuously, and finely, to the working variations arising fromtransformer nonlineardynamics. The control system is integrated in a fully digital control loop, with all therelated benefits, i.e., high noise rejection, ease of implementation/modification, and soon. In particular, an accurate model of the system,controlled by aFuzzy Gain Scheduler of the superconducting transformer, was achieved by an experimental campaignthrough the working domain at several current ramp rates. The model performance wascharacterized by simulation, under all the main operating conditions, in order to guidethe controllerdesign. Finally, the proposed monitoring system was experimentally validated at EuropeanOrganization for Nuclear Research (CERN) in comparison to the state-of-the-artcontrol system[P. Arpaia, L. Bottura, G. Montenero, and S. LeNaour, “Performance improvement of a measurement station forsuperconductingcable test,” Rev. Sci. Instrum.83, 095111 (2012)] of theFacility for the Research on Superconducting Cables, achieving a significant performanceimprovement: a reduction in the system overshoot by 50%, with a related attenuationof the corresponding dynamic residual error (both absolute and RMS) up to 52%
Inhibiting the urokinase-type plasminogen activator receptor system recovers STZ-induced diabetic nephropathy.
The urokinase‐type plasminogen activator (uPA) receptor (uPAR) participates to the mechanisms causing renal damage in response to hyperglycaemia. The main function of uPAR in podocytes (as well as soluble uPAR ‐(s)uPAR‐ from circulation) is to regulate podocyte function through αvβ3 integrin/Rac‐1. We addressed the question of whether blocking the uPAR pathway with the small peptide UPARANT, which inhibits uPAR binding to the formyl peptide receptors (FPRs) can improve kidney lesions in a rat model of streptozotocin (STZ)‐induced diabetes. The concentration of systemically administered UPARANT was measured in the plasma, in kidney and liver extracts and UPARANT effects on dysregulated uPAR pathway, αvβ3 integrin/Rac‐1 activity, renal fibrosis and kidney morphology were determined. UPARANT was found to revert STZ‐induced up‐regulation of uPA levels and activity, while uPAR on podocytes and (s)uPAR were unaffected. In glomeruli, UPARANT inhibited FPR2 expression suggesting that the drug may act downstream uPAR, and recovered the increased activity of the αvβ3 integrin/Rac‐1 pathway indicating a major role of uPAR in regulating podocyte function. At the functional level, UPARANT was shown to ameliorate: (a) the standard renal parameters, (b) the vascular permeability, (c) the renal inflammation, (d) the renal fibrosis including dysregulated plasminogen‐plasmin system, extracellular matrix accumulation and glomerular fibrotic areas and (e) morphological alterations of the glomerulus including diseased filtration barrier. These results provide the first demonstration that blocking the uPAR pathway can improve diabetic kidney lesion in the STZ model, thus suggesting the uPA/uPAR system as a promising target for the development of novel uPAR‐targeting approaches
Characterization of Two Novel Missense Mutations in the AQP2 Gene Causing Nephrogenic Diabetes Insipidus
Here, we report the aquaporin 2 (AQP2) mutational analysis of a patient with nephrogenic diabetes insipidus heterozygote due to two novel missense mutations. Direct sequencing of DNA in the male patient revealed that he was compound heterozygote for two mutations in the AQP2 gene: a thymine-to-adenine transversion at position 450 (c.450T>A) in exon 2 and a guanine-to-thymine at nucleotide position 643 (c.643G>T) in exon 4. The double heterozygous 450T>A and 643G>T transversion causes the amino acid substitution D150E and G215C. Direct sequencing of exons 2 and 4 of the AQP2 gene from each of the parents revealed that the c.450T>A mutation was inherited from the father while the c.643G>T mutation was inherited from the mother. Analysis of AQP2 excretion demonstrated that no AQP2 was detectable in the urine of the proband, whereas normal AQP2 levels were measured in both parents. When expressed in renal cells, both proteins were retarded in the endoplasmic reticulum and no redistribution was observed after forskolin stimulation. Of note, homology modeling revealed that the two mutations involve two highly conserved residues providing important clues about the role of the wt residues in AQP2 stability and function
Multiphoton transitions in Josephson-junction qubits (Review Article)
Two basic physical models, a two-level system and a harmonic oscillator, are
realized on the mesoscopic scale as coupled qubit and resonator. The realistic
system includes moreover the electronics for controlling the distance between
the qubit energy levels and their populations and to read out the resonator's
state, as well as the unavoidable dissipative environment. Such rich system is
interesting both for the study of fundamental quantum phenomena on the
mesoscopic scale and as a promising system for future electronic devices. We
present recent results for the driven superconducting qubit-resonator system,
where the resonator can be realized as an LC circuit or a nanomechanical
resonator. Most of the results can be described by the semiclassical theory,
where a qubit is treated as a quantum two-level system coupled to the classical
driving field and the classical resonator. Application of this theory allows to
describe many phenomena for the single and two coupled superconducting qubits,
among which are the following: the equilibrium-state and weak-driving
spectroscopy, Sisyphus damping and amplification, Landau-Zener-St\"uckelberg
interferometry, the multiphoton transitions of both direct and ladder- type
character, and creation of the inverse population for lasing.Comment: 20 pages, 15 figure
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