192 research outputs found
The Design Optimization and Experimental Investigation of the 4.4 μm Raman Laser Basedon Hydrogen-filled Revolver Silica Fiber
Optical properties of hollow-core revolver fibers are numerically investigated depending on various parameters: the hollow-core diameter, the capillary wall thickness, the values of the minimum gap between the capillaries, the number of capillaries in the cladding and the type of glass (silica and chalcogenide). Preliminary, similar calculations are made for simple models of hollow-core fibers. Based on the obtained results, the optimal design of the revolver fiber for Raman laser frequency conversion (1.56 μm → 4.4 μm in 1H2) was determined. As a result, efficient ns-pulsed 4.42 μm Raman laser based on 1H2-filled revolver silica fiber is realized. Quantum efficiency as high as 36 % is achieved and output average power as high as 250 mW is demonstrated
Review of scientific topics for Millimetron space observatory
This paper describes outstanding issues in astrophysics and cosmology that
can be solved by astronomical observations in a broad spectral range from far
infrared to millimeter wavelengths. The discussed problems related to the
formation of stars and planets, galaxies and the interstellar medium, studies
of black holes and the development of the cosmological model can be addressed
by the planned space observatory Millimetron (the "Spectr-M" project) equipped
with a cooled 10-m mirror. Millimetron can operate both as a single-dish
telescope and as a part of a space-ground interferometer with very long
baseline.Comment: The translation of the original article in Physics Uspekhi
http://ufn.ru/ru/articles/2014/12/c
Structure and Stability of an Amorphous Metal
Using molecular dynamics simulations, with a realistic many-body
embedded-atom potential, and a novel method to characterize local order, we
study the structure of pure nickel during the rapid quench of the liquid and in
the resulting glass. In contrast with previous simulations with pair
potentials, we find more crystalline order and fewer icosahedra for slower
quenching rates, resulting in a glass less stable against crystallization. It
is shown that there is not a specific amorphous structure, only the arrest of
the transition from liquid to crystal, resulting in small crystalline clusters
immersed in an amorphous matrix with the same structure of the liquid.Comment: 4 pages, 4 ps figs., to appear in Phys. Rev. Let
Coulomb gap in a model with finite charge transfer energy
The Coulomb gap in a donor-acceptor model with finite charge transfer energy
describing the electronic system on the dielectric side of the
metal-insulator transition is investigated by means of computer simulations on
two- and three-dimensional finite samples with a random distribution of equal
amounts of donor and acceptor sites. Rigorous relations reflecting the symmetry
of the model presented with respect to the exchange of donors and acceptors are
derived. In the immediate neighborhood of the Fermi energy the the
density of one-electron excitations is determined solely by
finite size effects and further away from is described by
an asymmetric power law with a non-universal exponent, depending on the
parameter .Comment: 10 pages, 6 figures, submitted to Phys. Rev.
Simulations of M87 and Sgr A* imaging with the Millimetron Space Observatory on near-Earth orbits
High resolution imaging of supermassive black holes shadows is a direct way
to verify the theory of general relativity at extreme gravity conditions. Very
Long Baseline Interferometry (VLBI) observations at millimeter/sub-millimeter
wavelengths can provide such angular resolution for supermassive black holes,
located in Sgr A* and M87. Recent VLBI observations of M87 with the Event
Horizon Telescope (EHT) has shown such capabilities. The maximum obtainable
spatial resolution of EHT is limited by Earth diameter and atmospheric phase
variations. In order to improve the image resolution longer baselines are
required. Radioastron space mission has successfully demonstrated the
capabilities of Space-Earth VLBI with baselines much larger than Earth
diameter. Millimetron is a next space mission of the Russian Space Agency that
will operate at millimeter wavelengths. Nominal orbit of the observatory will
be located around Lagrangian L2 point of the Sun-Earth system. In order to
optimize the VLBI mode, we consider a possible second stage of the mission that
could use near-Earth high elliptical orbit (HEO). In this contribution a set of
near-Earth orbits is used for the synthetic space-ground VLBI observations of
Sgr A* and M87 in joint Millimetron and EHT configuration. General-relativistic
magnetohydrodynamic models (GRMHD) for black hole environment of Sgr A* and M87
are used for static and dynamic imaging simulations at 230 GHz. A comparison
preformed between ground and space-ground baselines demonstrates that joint
observations with Millimetron and EHT significantly improve the image
resolution and allow the EHT+Millimetron to obtain snapshot images of Sgr A*
probing dynamics at fast timescales.Comment: 12 pages, 6 figure
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