1,869 research outputs found
Integrable models of galactic discs with double nuclei
We introduce a new class of 2-D mass models, whose potentials are of
St\"ackel form in elliptic coordinates. Our model galaxies have two separate
strong cusps that form double nuclei. The potential and surface density
distributions are locally axisymmetric near the nuclei and become {\it highly}
non-axisymmetric outside the nucleus. The surface density diverges toward the
cuspy nuclei with the law . Our model is sustained by
four general types of regular orbits: {\it butterfly}, {\it nucleuphilic
banana}, {\it horseshoe} and {\it aligned loop} orbits. Horseshoes and
nucleuphilic bananas support the existence of cuspy regions. Butterflies and
aligned loops control the non-axisymmetric shape of outer regions. Without any
need for central black holes, our distributed mass models resemble the nuclei
of M31 and NGC4486B. It is also shown that the self-gravity of the stellar disc
can prevent the double nucleus to collapse.Comment: 8 pages, accepted for publication in MNRA
Self-imaging silicon Raman amplifier
We propose a new type of waveguide optical amplifier. The device consists of
collinearly propagating pump and amplified Stokes beams with periodic imaging
of the Stokes beam due to the Talbot effect. The application of this device as
an Image preamplifier for Mid Wave Infrared (MWIR) remote sensing is discussed
and its performance is described. Silicon is the preferred material for this
application in MWIR due to its excellent transmission properties, high thermal
conductivity, high damage threshold and the mature fabrication technology. In
these devices, the Raman amplification process also includes four-wave-mixing
between various spatial modes of pump and Stokes signals. This phenomenon is
unique to nonlinear interactions in multimode waveguides and places a limit on
the maximum achievable gain, beyond which the image begins to distort. Another
source of image distortion is the preferential amplification of Stokes modes
that have the highest overlap with the pump. These effects introduce a tradeoff
between the gain and image quality. We show that a possible solution to this
trade-off is to restrict the pump into a single higher order waveguide mode.Comment: 11 pages, 5 figures and 5 sections. Submitted to Optics Expres
Real-time Spectroscopy with Sub-GHz Resolution using Amplified Dispersive Fourier Transformation
Dispersive Fourier transformation is a powerful technique in which spectral
information is mapped into the time domain using chromatic dispersion. It
replaces a spectrometer with an electronic digitizer, and enables real-time
spectroscopy. The fundamental problem in this technique is the trade-off
between the detection sensitivity and spectral resolution, a limitation set by
the digitizer's bandwidth. This predicament is caused by the power loss
associated with optical dispersion. We overcome this limitation using Raman
amplified spectrum-to-time transformation. An extraordinary loss-less -11.76
ns/nm dispersive device is used to demonstrate single-shot gas absorption
spectroscopy with 950 MHz resolution--a record in real-time spectroscopy.Comment: The following article has been accepted by Applied Physics Letter
Hydrodynamic acoustic plasmon resonances in semiconductor nanowires and their dimers
The hydrodynamic Drude model known from metal plasmonics also applies to
semiconductor structures of sizes in between single-particle quantum
confinement and bulk. But contrary to metals, for semiconductors two or more
types of plasma may have to be taken into account in order to properly describe
their plasmonic properties. In this combined analytical and computational
study, we explore predictions of the recently proposed two-fluid hydrodynamic
Drude model for the optical properties of plasmonic semiconductor nanowires, in
particular for thermally excited InSb nanowires. We focus on the low-frequency
acoustic surface and bulk plasmon resonances that are unique fingerprints for
this model and are yet to be observed. We identify these resonances in spectra
for single nanowires based on analytical calculations, and they are in complete
agreement with our numerical implementation of the model. For dimers of
nanowires we predict substantial increase of the extinction cross section and
field enhancement of the acoustic localized surface plasmon resonance, which
makes its observation in dimers more likely.Comment: I would like to inform that Dr.Abbas Zarifi is the corresponding
author of this pape
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