37,787 research outputs found
Dual regimes of ion migration in high repetition rate femtosecond laser inscribed waveguides
Ion migration in high repetition rate femtosecond laser inscribed waveguides
is currently being reported in different optical glasses. For the first time we
discuss and experimentally demonstrate the presence of two regimes of ion
migration found in laser written waveguides. Regime-I, corresponds to the
initial waveguide formation mainly via light element migration (in our case
atomic weight < 31u), whereas regime-II majorly corresponds to the movement of
heavy elements. This behavior brings attention to a problem which has never
been analyzed before and that affects laser written active waveguides in which
active ions migrate changing their local spectroscopic properties. The
migration of active ions may in fact detune the pre-designed optimal values of
active photonic devices. This paper experimentally evidences this problem and
provides solutions to avert it.Comment: 4 pages, 5 figure
The Cosmic Near Infrared Background: Remnant Light from Early Stars
The redshifted ultraviolet light from early stars at z ~ 10 contributes to
the cosmic near infrared background. We present detailed calculations of its
spectrum with various assumptions about metallicity and mass spectrum of early
stars. We show that if the near infrared background has a stellar origin,
metal-free stars are not the only explanation of the excess near infrared
background; stars with metals (e.g. Z=1/50 Z_sun) can produce the same amount
of background intensity as the metal-free stars. We quantitatively show that
the predicted average intensity at 1-2 microns is essentially determined by the
efficiency of nuclear burning in stars, which is not very sensitive to
metallicity. We predict \nu I_\nu / \dot{\rho}_* ~ 4-8 nW m^-2 sr^-1, where
\dot{\rho_*} is the mean star formation rate at z=7-15 (in units of M_sun yr^-1
Mpc^-3) for stars more massive than 5 M_sun. On the other hand, since we have
very little knowledge about the form of mass spectrum of early stars,
uncertainty in the average intensity due to the mass spectrum could be large.
An accurate determination of the near infrared background allows us to probe
formation history of early stars, which is difficult to constrain by other
means. While the star formation rate at z=7-15 inferred from the current data
is significantly higher than the local rate at z<5, it does not rule out the
stellar origin of the cosmic near infrared background. In addition, we show
that a reasonable initial mass function, coupled with this star formation rate,
does not over-produce metals in the universe in most cases, and may produce as
little as less than 1 % of the metals observed in the universe today.Comment: 37 pages, 7 figures, (v2) Changes to abstract to emphasize that the
excess near infrared background can solely be explained by stars with
significant metals. (Metal-free stars are not necessarily needed.) (v3)
Expanded discussion on the metallicity constraint. Accepted for publication
in Ap
New nonlinear coherent states and some of their nonclassical properties
We construct a displacement operator type nonlinear coherent state and
examine some of its properties. In particular it is shown that this nonlinear
coherent state exhibits nonclassical properties like squeezing and
sub-Poissonian behaviour.Comment: 3 eps figures. to appear in J.Opt
Performance Study of the Robot Operating System 2 with QoS and Cyber Security Settings
2020 IEEE International Systems Conference (SysCon)Throughout the Department of Defense, there are ongoing efforts to increase cybersecurity and improve data transfer in unmanned robotic systems (UxS). This paper explores the performance of the Robot Operating System (ROS) 2, which is built with the Data Distribution Service (DDS) standard as a middleware. Based on how quality of service (QoS) parameters are defined in the robotic middleware interface, it is possible to implement strict delivery requirements to different nodes on a dynamic nodal network with multiple unmanned systems con- nected. Through this research, different scenarios with varying QoS settings were implemented and compared to baseline values to help illustrate the impact of latency and throughput on data flow. DDS security settings were also enabled to help understand the cost of overhead and performance when secured data is compared to plaintext baseline values. Our experiments were performed using a basic ROS 2 network consisting of two nodes (one publisher and one subscriber). Our experiments showed a measurable latency and throughput change between different QoS profiles and security settings. We analyze the trends and tradeoffs associated with varying QoS and security settings. This paper provides performance data points that can be used to help future researchers and developers make informative choices when using ROS 2 for UxS.This work was funded and sponsored by the Office of Naval Research via the Consortium for Robotics and Unmanned Systems Education and Research (CRUSER) at NPS
Phase properties of a new nonlinear coherent state
We study phase properties of a displacement operator type nonlinear coherent
state. In particular we evaluate the Pegg-Barnett phase distribution and
compare it with phase distributions associated with the Husimi Q function and
the Wigner function. We also study number- phase squeezing of this state.Comment: 8 eps figures. to appear in J.Opt
Existence and Stability of Steady Fronts in Bistable CML
We prove the existence and we study the stability of the kink-like fixed
points in a simple Coupled Map Lattice for which the local dynamics has two
stable fixed points. The condition for the existence allows us to define a
critical value of the coupling parameter where a (multi) generalized
saddle-node bifurcation occurs and destroys these solutions. An extension of
the results to other CML's in the same class is also displayed. Finally, we
emphasize the property of spatial chaos for small coupling.Comment: 18 pages, uuencoded PostScript file, J. Stat. Phys. (In press
Strings at future singularities
We discuss the behaviour of strings propagating in spacetimes which allow
future singularities of either a sudden future or a Big-Rip type. We show that
in general the invariant string size remains finite at sudden future
singularities while it grows to infinity at a Big-Rip. This claim is based on
the discussion of both the tensile and null strings. In conclusion, strings may
survive a sudden future singularity, but not a Big-Rip where they are
infinitely stretched.Comment: REVTEX 4.0, 4 pages, no figures, references adde
Binding branched and linear DNA structures: from isolated clusters to fully bonded gels
The proper design of DNA sequences allows for the formation of well defined
supramolecular units with controlled interactions via a consecution of
self-assembling processes. Here, we benefit from the controlled DNA
self-assembly to experimentally realize particles with well defined valence,
namely tetravalent nanostars (A) and bivalent chains (B). We specifically focus
on the case in which A particles can only bind to B particles, via
appropriately designed sticky-end sequences. Hence AA and BB bonds are not
allowed. Such a binary mixture system reproduces with DNA-based particles the
physics of poly-functional condensation, with an exquisite control over the
bonding process, tuned by the ratio, r, between B and A units and by the
temperature, T. We report dynamic light scattering experiments in a window of
Ts ranging from 10{\deg}C to 55{\deg}C and an interval of r around the
percolation transition to quantify the decay of the density correlation for the
different cases. At low T, when all possible bonds are formed, the system
behaves as a fully bonded network, as a percolating gel and as a cluster fluid
depending on the selected r.Comment: 15 pages, 11 figure
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