9,416 research outputs found
Power and wavelength polarization bistability with very wide hysteresis cycles in a 1550nm-VCSEL subject to orthogonal optical injection
We have measured optical power and wavelength polarization bistability in a 1550nm-Vertical Cavity Surface-Emitting Laser (VCSEL) subject to orthogonally-polarized optical injection into the orthogonal polarization of the fundamental transverse mode. Optical bistability with very wide hysteresis cycles, up to four times wider than previously reported results has been measured for both the optical power and wavelength domain. We also report the experimental observation of three different shapes of polarization bistability, anticlockwise, clockwise and X-Shape bistability, all of them with wide hysteresis cycles. This rich variety of behaviour at the important wavelength of 1550 nm offers promise for the use of VCSELs for all-optical signal processing and optical switching/routing applications. ©2009 Optical Society of America
Optical neuron using polarisation switching in a 1550nm-VCSEL
We report a new approach to mimic basic functionalities of a neuron using a 1550 nm Vertical Cavity Surface Emitting Laser (VCSEL) which is based on the polarisation switching (PS) that can be induced in these devices when subject to polarised optical injection. Positive and negative all-optical threshold operations are demonstrated experimentally using external optical injection into the two orthogonal polarizations of the fundamental transverse mode. The polarisation of the light emitted by the device is used to determine the state of the VCSEL-Neuron, active (orthogonal) or inactive (parallel). This approach forms a new way to reproduce optically the response of a neuron to an excitatory and an inhibitory stimulus. © 2010 Optical Society of America
Arrest of Domain Coarsening via Antiperiodic Regimes in Delay Systems
Motionless domains walls representing heteroclinic temporal or spatial orbits
typically exist only for very specific parameters. This report introduces a
novel mechanism for stabilizing temporal domain walls away from the Maxwell
point opening up new possibilities to encode information in dynamical systems.
It is based on anti-periodic regimes in a delayed system close to a bistable
situation, leading to a cancellation of the average drift velocity. The results
are demonstrated in a normal form model and experimentally in a laser with
optical injection and delayed feedback.Comment: 6 pages, 5 figures, resubmitted manuscrip
Symmetries in Fluctuations Far from Equilibrium
Fluctuations arise universally in Nature as a reflection of the discrete
microscopic world at the macroscopic level. Despite their apparent noisy
origin, fluctuations encode fundamental aspects of the physics of the system at
hand, crucial to understand irreversibility and nonequilibrium behavior. In
order to sustain a given fluctuation, a system traverses a precise optimal path
in phase space. Here we show that by demanding invariance of optimal paths
under symmetry transformations, new and general fluctuation relations valid
arbitrarily far from equilibrium are unveiled. This opens an unexplored route
toward a deeper understanding of nonequilibrium physics by bringing symmetry
principles to the realm of fluctuations. We illustrate this concept studying
symmetries of the current distribution out of equilibrium. In particular we
derive an isometric fluctuation relation which links in a strikingly simple
manner the probabilities of any pair of isometric current fluctuations. This
relation, which results from the time-reversibility of the dynamics, includes
as a particular instance the Gallavotti-Cohen fluctuation theorem in this
context but adds a completely new perspective on the high level of symmetry
imposed by time-reversibility on the statistics of nonequilibrium fluctuations.
The new symmetry implies remarkable hierarchies of equations for the current
cumulants and the nonlinear response coefficients, going far beyond Onsager's
reciprocity relations and Green-Kubo formulae. We confirm the validity of the
new symmetry relation in extensive numerical simulations, and suggest that the
idea of symmetry in fluctuations as invariance of optimal paths has
far-reaching consequences in diverse fields.Comment: 8 pages, 4 figure
Geometric analysis of Lorentzian distance function on spacelike hypersurfaces
Some analysis on the Lorentzian distance in a spacetime with controlled
sectional (or Ricci) curvatures is done. In particular, we focus on the study
of the restriction of such distance to a spacelike hypersurface satisfying the
Omori-Yau maximum principle. As a consequence, and under appropriate hypotheses
on the (sectional or Ricci) curvatures of the ambient spacetime, we obtain
sharp estimates for the mean curvature of those hypersurfaces. Moreover, we
also give a suficient condition for its hyperbolicity.Comment: Final version (January 2009). To appear in the Transactions of the
American Mathematical Societ
Metastability and Avalanches in a Nonequilibrium Ferromagnetic System
We present preliminary results on the metastable behavior of a nonequilibrium
ferromagnetic system. The metastable state mean lifetime is a non-monotonous
function of temperature; it shows a maximum at certain non-zero temperature
which depends on the strengh of the nonequilibrium perturbation. This is in
contrast with the equilibrium case in which lifetime increases monotonously as
the temperature is decreasesed. We also report on avalanches during the decay
from the metastable state. Assuming both free boundaries and nonequilibrium
impurities, the avalanches exhibit power-law size and lifetime distributions.
Such scale free behavior is very sensible. The chances are that our
observations may be observable in real (i.e. impure) ferromagnetic
nanoparticles.Comment: 6 pages, 4 figures, to be published in 2002 Granada Seminar
Proceeding
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