41,427 research outputs found
Theory of plasmon-enhanced high-harmonic generation in the vicinity of metal nanostructures in noble gases
We present a semiclassical model for plasmon-enhanced high-harmonic
generation (HHG) in the vicinity of metal nanostructures. We show that both the
inhomogeneity of the enhanced local fields and electron absorption by the metal
surface play an important role in the HHG process and lead to the generation of
even harmonics and to a significantly increased cutoff. For the examples of
silver-coated nanocones and bowtie antennas we predict that the required
intensity reduces by up to three orders of magnitudes and the HHG cutoff
increases by more than a factor of two. The study of the enhanced high-harmonic
generation is connected with a finite-element simulation of the electric field
enhancement due to the excitation of the plasmonic modes.Comment: 4 figure
Analysis of short pulse laser altimetry data obtained over horizontal path
Recent pulsed measurements of atmospheric delay obtained by ranging to the more realistic targets including a simulated ocean target and an extended plate target are discussed. These measurements are used to estimate the expected timing accuracy of a correlation receiver system. The experimental work was conducted using a pulsed two color laser altimeter
Stochastics theory of log-periodic patterns
We introduce an analytical model based on birth-death clustering processes to
help understanding the empirical log-periodic corrections to power-law scaling
and the finite-time singularity as reported in several domains including
rupture, earthquakes, world population and financial systems. In our
stochastics theory log-periodicities are a consequence of transient clusters
induced by an entropy-like term that may reflect the amount of cooperative
information carried by the state of a large system of different species. The
clustering completion rates for the system are assumed to be given by a simple
linear death process. The singularity at t_{o} is derived in terms of
birth-death clustering coefficients.Comment: LaTeX, 1 ps figure - To appear J. Phys. A: Math & Ge
Camera for QUasars in EArly uNiverse (CQUEAN)
We describe the overall characteristics and the performance of an optical CCD
camera system, Camera for QUasars in EArly uNiverse (CQUEAN), which is being
used at the 2.1 m Otto Struve Telescope of the McDonald Observatory since 2010
August. CQUEAN was developed for follow-up imaging observations of red sources
such as high redshift quasar candidates (z >= 5), Gamma Ray Bursts, brown
dwarfs, and young stellar objects. For efficient observations of the red
objects, CQUEAN has a science camera with a deep depletion CCD chip which
boasts a higher quantum efficiency at 0.7 - 1.1 um than conventional CCD chips.
The camera was developed in a short time scale (~ one year), and has been
working reliably. By employing an auto-guiding system and a focal reducer to
enhance the field of view on the classical Cassegrain focus, we achieve a
stable guiding in 20 minute exposures, an imaging quality with FWHM >= 0.6"
over the whole field (4.8' * 4.8'), and a limiting magnitude of z = 23.4 AB mag
at 5-sigma with one hour total integration time.Comment: Accepted for publication in PASP. 26 pages including 5 tables and 24
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Plasma Processing of Large Curved Surfaces for SRF Cavity Modification
Plasma based surface modification of niobium is a promising alternative to
wet etching of superconducting radio frequency (SRF) cavities. The development
of the technology based on Cl2/Ar plasma etching has to address several crucial
parameters which influence the etching rate and surface roughness, and
eventually, determine cavity performance. This includes dependence of the
process on the frequency of the RF generator, gas pressure, power level, the
driven (inner) electrode configuration, and the chlorine concentration in the
gas mixture during plasma processing. To demonstrate surface layer removal in
the asymmetric non-planar geometry, we are using a simple cylindrical cavity
with 8 ports symmetrically distributed over the cylinder. The ports are used
for diagnosing the plasma parameters and as holders for the samples to be
etched. The etching rate is highly correlated with the shape of the inner
electrode, radio-frequency (RF) circuit elements, chlorine concentration in the
Cl2/Ar gas mixtures, residence time of reactive species and temperature of the
cavity. Using cylindrical electrodes with variable radius, large-surface
ring-shaped samples and d.c. bias implementation in the external circuit we
have demonstrated substantial average etching rates and outlined the
possibility to optimize plasma properties with respect to maximum surface
processing effect
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