16,324 research outputs found
Chromospheric Dynamics and Line Formation
The solar chromosphere is very dynamic, due to the presence of large
amplitude hydrodynamic waves. Their propagation is affected by NLTE radiative
transport in strong spectral lines, which can in turn be used to diagnose the
dynamics of the chromosphere. We give a basic introduction into the equations
of NLTE radiation hydrodynamics and describe how they are solved in current
numerical simulations. The comparison with observation shows that
one-dimensional codes can describe strong brightenings quite well, but the
overall chromospheric dynamics appears to be governed by three-dimensional
shock propagation.Comment: Lecture notes and review, held at Kodaikanal Winter School on Solar
Physics, Dec 2006. This version contains corrected page numbers for some of
the reference
Defect Grating Simulations: Perturbations with AFM-like Tips
A defect grating in a silicon on insulator waveguide is simulated. We consider spectral changes in the optical transmission when a thin silicon nitride or silicon tip is scanned across the defect. The tip perturbs the resonance field, moving its peak wavelength andpossibly changing its shape and quality factor. For the nitride tip, the influence is mostly a spectral shift; for silicon, the change of the resonance shape is pronounced. In particular for the nitride tip we observe a close correspondence between the wavelength shift as a function of tip position, and the local intensity in the unperturbed structure
Fermi level alignment in single molecule junctions and its dependence on interface structure
The alignment of the Fermi level of a metal electrode within the gap of the
highest occupied and lowest unoccupied orbital of a molecule is a key quantity
in molecular electronics. Depending on the type of molecule and the interface
structure of the junction, it can vary the electron transparency of a
gold/molecule/gold junction by at least one order of magnitude. In this article
we will discuss how Fermi level alignment is related to surface structure and
bonding configuration on the basis of density functional theory calculations
for bipyridine and biphenyl dithiolate between gold leads. We will also relate
our findings to quantum-chemical concepts such as electronegativity.Comment: 5 pages, 2 figures, presented at the ICN+T 2006 conferenc
Multimode circular integrated optical microresonators: Coupled mode theory modeling
A frequency domain model of multimode circular microresonators for filter
applications in integrated optics is investigated. Analytical basis modes of 2D
bent waveguides or curved interfaces are combined with modes of straight
channels in a spatial coupled mode theory framework. Free of fitting
parameters, the model allows to predict quite efficiently the spectral response
of the microresonators. It turns out to be sufficient to take only a few
dominant cavity modes into account. Comparisons of these simulations with
computationally more expensive rigorous numerical calculations show a
satisfactory agreement
Modeling of grating assisted standing wave microresonators for filter applications in integrated optics
A wide, multimode segment of a dielectric optical waveguide, enclosed by Bragg reflectors and evanescently coupled to adjacent port waveguides, can constitute the cavity in an integrated optical microresonator. It turns out that the device can be described adequately in terms of an approximate coupled mode theory model which involves only a few guided modes as basis fields. By reasoning along the coupled mode model, we motivate a simple design strategy for the resonator device. Rigorous two dimensional mode expansion simulations are applied to verify the predictions of the approximate model. The results exemplify the specific spectral response of the standing wave resonators. As refinements we discuss the single resonance of a device with nonsymmetrically detuned Bragg reflectors, and the cascading of two Fabry-Perot cavities, where the coupling across an intermediate shorter grating region establishes a power transfer characteristic that is suitable for an add-drop filter
Comparison of coupled mode theory and FDTD simulations of coupling between bent and straight optical waveguides
Analysis of integrated optical cylindrical microresonators involves the coupling between a straight waveguide and a bent waveguide. Our (2D) variant of coupled mode theory is based on analytically represented mode profiles. With the bend modes expressed in Cartesian coordinates, coupled mode equations can be derived in a classical way and solved by numerical integration. Proper manipulation of the propagation matrix leads to stable results even in parameter domains of compact and/or radiative structures, which seemed unsuitable for a perturbational approach due to oscillations of the matrix elements along the propagation. Comparisons with FDTD calculations show convincing agreement
Global and local cutoff frequencies for transverse waves propagating along solar magnetic flux tubes
The propagation of linear transverse waves along a thin isothermal magnetic
flux tube is affected by a global cutoff frequency that separates propagating
and non-propagating waves. In this paper, wave propagation along a thin but
non-isothermal flux tube is considered and a local cutoff frequency is derived.
The effects of different temperature profiles on this local cutoff frequency
are studied by considering different power-law temperature distributions as
well as the semi-empirical VAL C model of the solar atmosphere. The results
show that the conditions for wave propagation strongly depend on the
temperature gradients. Moreover, the local cutoff frequency calculated for the
VAL C model gives constraints on the range of wave frequencies that are
propagating in different parts of the solar atmosphere. These theoretically
predicted constraints are compared to observational data and are used to
discuss the role played by transverse tube waves in the atmospheric heating and
dynamics, and in the excitation of solar atmospheric oscillations.Comment: To be publishd in ApJ Vol. 763. 10 pages, 3 Postscript figure
Pion-mass dependence of three-nucleon observables
We use an effective field theory (EFT) which contains only short-range
interactions to study the dependence of a variety of three-nucleon observables
on the pion mass. The pion-mass dependence of input quantities in our
``pionless'' EFT is obtained from a recent chiral EFT calculation. To the order
we work at, these quantities are the 1S0 scattering length and effective range,
the deuteron binding energy, the 3S1 effective range, and the binding energy of
one three-nucleon bound state. The chiral EFT input we use has the inverse 3S1
and 1S0 scattering lengths vanishing at mpi_c=197.8577 MeV. At this
``critical'' pion mass, the triton has infinitely many excited states with an
accumulation point at the three-nucleon threshold. We compute the binding
energies of these states up to next-to-next-to-leading order in the pionless
EFT and study the convergence pattern of the EFT in the vicinity of the
critical pion mass. Furthermore, we use the pionless EFT to predict how doublet
and quartet nd scattering lengths depend on mpi in the region between the
physical pion mass and mpi=mpi_c.Comment: 24 pages, 9 figure
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