7,441 research outputs found
Efficient analysis and design of low-loss whispering-gallery-mode coupled resonator optical waveguide bends
Waveguides composed of electromagnetically-coupled optical microcavities
(coupled resonator optical waveguides or CROWs) can be used for light guiding,
slowing and storage. In this paper, we present a two-dimensional analysis of
finite-size straight and curved CROW sections based on a rigorous Muller
boundary integral equations method. We study mechanisms of the coupling of
whispering gallery (WG) modes and guiding light around bends in CROWs composed
of both identical and size-mismatched microdisk resonators. Our accurate
analysis reveals differences in WG modes coupling in the vicinity of bends in
CROWs composed of optically-large and wavelength-scale microcavities. We
propose and discuss possible ways to design low-loss CROW bends and to reduce
bend losses. These include selecting specific bend angles depending on the
azimuthal order of the WG mode and tuning the radius of the microdisk
positioned at the CROW bend.Comment: 8 pages with 10 figures (to appear in IEEE/OSA J. Lightwave
Technology, 2007
Molecules as magnetic probes of starspots
Stellar dynamo processes can be explored by measuring the magnetic field.
This is usually obtained using the atomic and molecular Zeeman effect in
spectral lines. While the atomic Zeeman effect can only access warmer regions,
the use of molecular lines is of advantage for studying cool objects. The
molecules MgH, TiO, CaH, and FeH are suited to probe stellar magnetic fields,
each one for a different range of spectral types, by considering the signal
that is obtained from modeling various spectral types. We have analyzed the
usefulness of different molecules (MgH, TiO, CaH, and FeH) as diagnostic tools
for studying stellar magnetism on active G-K-M dwarfs. We investigate the
temperature range in which the selected molecules can serve as indicators for
magnetic fields on highly active cool stars and present synthetic Stokes
profiles for the modeled spectral type. We modeled a star with a spot size of
10% of the stellar disk and a spot comprising either only longitudinal or only
transverse magnetic fields and estimated the strengths of the polarization
Stokes V and Q signals for the molecules MgH, TiO, CaH, and FeH. We combined
various photosphere and spot models according to realistic scenarios. In G
dwarfs, the molecules MgH and FeH show overall the strongest Stokes V and Q
signals from the starspot, whereas FeH has a stronger Stokes V signal in all G
dwarfs, with a spot temperature of 3800K. In K dwarfs, CaH signals are
generally stronger, and the TiO signature is most prominent in M dwarfs.
Modeling synthetic polarization signals from starspots for a range of G-K-M
dwarfs leads to differences in the prominence of various molecular signatures
in different wavelength regions, which helps to efficiently select targets and
exposure times for observations.Comment: 9 pages, 5 figures, 1 tabl
Exceeding the solar cell Shockley-Queisser limit via thermal up-conversion of low-energy photons
Maximum efficiency of ideal single-junction photovoltaic (PV) cells is
limited to 33% (for one sun illumination) by intrinsic losses such as band edge
thermalization, radiative recombination, and inability to absorb below-bandgap
photons. This intrinsic thermodynamic limit, named after Shockley and Queisser
(S-Q), can be exceeded by utilizing low-energy photons either via their
electronic up-conversion or via thermophotovoltaic (TPV) conversion process.
However, electronic up-conversion systems have extremely low efficiencies, and
practical temperature considerations limit the operation of TPV converters to
the narrow-gap PV cells. Here we develop a conceptual design of a hybrid TPV
platform, which exploits thermal up-conversion of low-energy photons and is
compatible with conventional silicon PV cells by using spectral and directional
selectivity of the up-converter. The hybrid platform offers
sunlight-to-electricity conversion efficiency exceeding that imposed by the S-Q
limit on the corresponding PV cells across a broad range of bandgap energies,
under low optical concentration (1-300 suns), operating temperatures in the
range 900-1700K, and in simple flat panel designs. We demonstrate maximum
conversion efficiency of 73% under illumination by non-concentrated sunlight. A
detailed analysis of non-ideal hybrid platforms that allows for up to 15% of
absorption/re-emission losses yields limiting efficiency value of 45% for Si PV
cells.Comment: 28 pages, 9 figure
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