314 research outputs found
Modulational instability in a layered Kerr medium: Theory and Experiment
We present the first experimental investigation of modulational instability
in a layered Kerr medium. The particularly interesting and appealing feature of
our configuration, consisting of alternating glass-air layers, is the
piecewise-constant nature of the material properties, which allows a
theoretical linear stability analysis leading to a Kronig-Penney equation whose
forbidden bands correspond to the modulationally unstable regimes. We find very
good {\it quantitative} agreement between theoretical, numerical, and
experimental diagnostics of the modulational instability. Because of the
periodicity in the evolution variable arising from the layered medium, there
are multiple instability regions rather than just one as in the uniform medium.Comment: 4 pages, 4 figures, contains experimental + computational +
theoretical results, to appear in Physical Review Letter
“Si Loko” sebagai Maskot PT. Kereta Api Indonesia
Upaya meningkatkan promosi dan komunikasi pemasaran, PT. Kereta Api Indonesia
menetapkan Si Loko sebagai maskot PT. KAI dan juga duta yang memberikan berbagai
informasi yang ingin diketahui oleh masyarakat pengguna kereta api. Si Loko adalah tokoh
imajiner yang diciptakan untuk menjadi maskot PT. KAI yang digunakan untuk menyapa
pelanggan dan stakeholder kereta api secara langsung. Sebagai duta PT. KAI, Si Loko juga
merupakan wujud peningkatan pelayanan kepada pelanggan berkaitan dengan kebutuhan
informasi produk maupun kebijakan yang dikeluarkan Perusahaan yang juga dapat digunakan
sebagai media promosi dan komunikasi pemasaran. Penelitian ini bermaksud mengungkap
maksud penunjukan, fungsi, dan penetapan Si Loko sebagai maskot PT Kereta Api Indonesia.
Metode yang digunakan dalam penelitian ini adalah metode deskriptif kualitatif.Hasil penelitian
menunjukan bahwa penetapan Si Loko Sebagai Maskot PT. Kereta Api Indonesia (Persero),
dilakukan karena Perusahaan ingin menunjukkan sebuah Perubahan dimana Perusahaan tidak
hanya mengembangkan sarana transportasinya saja, melainkan juga dapat memberikan harapan
baru bagi pengembangan perkeretaapian di Indonesia yang semakin dekat dan peduli dengan
masyarakat. Selain itu, Si Loko juga menjadi sarana mediator komunikasi antara Perusahaan
dengan masyarakat
Holographic capture of femtosecond pulse propagation
We have implemented a holographic system to study the propagation of femtosecond laser pulses with high temporal (150 fs) and spatial resolutions (4 μm). The phase information in the holograms allows us to reconstruct both positive and negative index changes due to the Kerr nonlinearity (positive) and plasma formation (negative), and to reconstruct three-dimensional structure. Dramatic differences were observed in the interaction of focused femtosecond pulses with air, water, and carbon disulfide. The air becomes ionized in the focal region, while in water long plasma filaments appear before the light reaches a tight focus. In contrast, in carbon disulfide the optical beam breaks up into multiple filaments but no plasma is measured. We explain these different propagation regimes in terms of the different nonlinear material properties. © 2006 American Institute of Physics
Most precise single redshift bound to the variability of the fine-structure constant
Verification of theoretical predictions of an oscillating behavior of the
fine-structure constant, alpha, with cosmic time requires high precision
measurements at individual redshifts, while in earlier studies the mean Delta
alpha/alpha values averaged over wide redshift intervals were usually reported.
This requirement can be met via the Single Ion Differential alpha Measurement
(SIDAM) procedure. We apply SIDAM to the FeII lines associated with the damped
Ly-alpha system observed at z=1.15 in the spectrum of HE0515-4414. The weighted
mean calculated on base of carefully selected 34 FeII pairs is <Delta
alpha/alpha>=(-0.07+/-0.84)10^{-6}. The precision of this estimate represents
the absolute improvement with respect to what has been done in the measurements
of Delta alpha/alpha.Comment: 2 pages, to appear in the Proceed. of IAU Symp.232 "The Scientific
Requirements for Extremely Large Telescopes", eds. P. Whitelock, B.
Leibundgut, and M. Dennefel
Nonlinearity management in optics: Experiment, theory, and simulation
We conduct an experimental investigation of nonlinearity management in optics using femtosecond pulses and layered Kerr media consisting of glass and air. By examining the propagation properties over several diffraction lengths, we show that wave collapse can be prevented. We corroborate these experimental results with numerical simulations of the (2+1)-dimensional focusing cubic nonlinear Schrödinger equation with piecewise constant coefficients and a theoretical analysis of this setting using a moment method. © 2006 The American Physical Society
Nitrogen Abundances in Damped Ly alpha Galaxies
Nitrogen abundances have been derived in Damped Ly alpha (DLA) galaxies at
Zabs = 2.309, 2.827 and 3.025 toward the QSOs 0100+1300, 1425+6039 and
0347-3819 respectively. The behaviour of nitrogen relative to iron-peak and
alpha-elements has been investigated by considering all the extant NI
determinations for a total of 9 DLA galaxies. We have estimated the fraction of
iron locked into dust grains to convert the observed [N/Fe] ratios into overall
(dust plus gas) relative abundances, [N/Fe]corr. The ratios [N/alpha] have been
mostly determined by using sulphur as a tracer of alpha-elements which is
unaffected by dust. The [N/Fe] and [N/alpha] ratios show high dispersions, of
one order of magnitude or more, which have no equivalent in other
element-to-element ratios in DLAs. The lowest values of the [N/Fe]corr and
[N/alpha] ratios are at variance with the values measured in Galactic halo
stars of similar metallicity suggesting that part of the DLA galaxies do not
follow the chemical evolution of the Milky Way. The DLA nitrogen abundances and
their dispersion show some similarities with those observed in dwarf galaxies.
The behaviour of nitrogen abundance ratios can be ascribed, in general to the
delayed release of nitrogen in the course of evolution. However it is difficult
to conciliate this interpretation with the lowest [N/alpha] values measured,
since an expected enhancement of alpha-elements respect to the iron-peak
elements is not observed simultaneously in these DLA galaxies. In two cases,
relatively high [N/alpha] values are observed which require also a more complex
chemical evolution to be explained.Comment: 29 pages including 5 tables and figure captions,LaTeX, 8 figures, ApJ
accepte
Ultrafast Electron Diffraction: Visualizing Dynamic States of Matter
Since the discovery of electron-wave duality, electron scattering instrumentation has developed into a powerful array of techniques for revealing the atomic structure of matter. Beyond detecting local lattice variations in equilibrium structures with the highest possible spatial resolution, recent research efforts have been directed towards the long sought-after dream of visualizing the dynamic evolution of matter in real-time. The atomic behavior at ultrafast timescales carries critical information on phase transition and chemical reaction dynamics, the coupling of electronic and nuclear degrees of freedom in materials and molecules, the correlation between structure, function and previously hidden metastable or nonequilibrium states of matter. Ultrafast electron pulses play an essential role in this scientific endeavor, and their generation has been facilitated by rapid technical advances in both ultrafast laser and particle accelerator technologies. This review presents a summary of the remarkable developments in this field over the last few decades. The physics and technology of ultrafast electron beams is presented with an emphasis on the figures of merit most relevant for ultrafast electron diffraction (UED) experiments. We discuss recent developments in the generation, manipulation and characterization of ultrashort electron beams aimed at improving the combined spatio-temporal resolution of these measurements. The fundamentals of electron scattering from atomic matter and the theoretical frameworks for retrieving dynamic structural information from solid-state and gas-phase samples is described. Essential experimental techniques and several landmark works that have applied these approaches are also highlighted to demonstrate the widening applicability of these methods. Ultrafast electron probes with ever improving capabilities, combined with other complementary photon-based or spectroscopic approaches, hold tremendous potential for revolutionizing our ability to observe and understand energy and matter at atomic scales
Holographic recording of laser-induced plasma
We report on a holographic probing technique that allows for measurement of free-electron distribution with fine spatial detail. Plasma is generated by focusing a femtosecond pulse in air. We also demonstrate the capability of the holographic technique of capturing the time evolution of the plasma-generation process. © 2004 Optical Society of America
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