5,985 research outputs found
An optical clock with neutral atoms confined in a shallow trap
We study the trap depth requirement for the realization of an optical clock
using atoms confined in a lattice. We show that site-to-site tunnelling leads
to a residual sensitivity to the atom dynamics hence requiring large depths (50
to for Sr) to avoid any frequency shift or line broadening of the
atomic transition at the level. Such large depths and the
corresponding laser power may, however, lead to difficulties (e.g. higher order
light shifts, two-photon ionization, technical difficulties) and therefore one
would like to operate the clock in much shallower traps. To circumvent this
problem we propose the use of an accelerated lattice. Acceleration lifts the
degeneracy between adjacents potential wells which strongly inhibits
tunnelling. We show that using the Earth's gravity, much shallower traps (down
to for Sr) can be used for the same accuracy goal
Spin-Glass Model Governs Laser Multiple Filamentation
We show that multiple filamentation patterns in high-power laser beams, can
be described by means of two statistical physics concepts, namely
self-similarity of the patterns over two nested scales, and nearest-neighbor
interactions of classical rotators. The resulting lattice spin model perfectly
reproduces the evolution of intense laser pulses as simulated by the Non-Linear
Schr\"odinger Equation, shedding a new light on multiple filamentation. As a
side benefit, this approach drastically reduces the computing time by two
orders of magnitude as compared to the standard simulation methods of laser
filamentation.Comment: 8 pages, 4 figure
Laser filamentation as a new phase transition universality class
We show that the onset of laser multiple filamentation can be described as a
critical phenomenon that we characterize both experimentally and numerically by
measuring a set of seven critical exponents. This phase transition deviates
from any existing universality class, and offers a unique perspective of
conducting two-dimensional experiments of statistical physics at a human scale.Comment: 8 pages, 9 figure
Transition from plasma- to Kerr-driven laser filamentation
While filaments are generally interpreted as a dynamic balance between Kerr
focusing and plasma defocusing, the role of the higher-order Kerr effect (HOKE)
is actively debated as a potentially dominant defocusing contribution to
filament stabilization. In a pump-probe experiment supported by numerical
simulations, we demonstrate the transition between two distinct filamentation
regimes at 800\,nm. For long pulses (1.2 ps), the plasma substantially
contributes to filamentation, while this contribution vanishes for short pulses
(70 fs). These results confirm the occurrence, in adequate conditions, of
filamentation driven by the HOKE rather than by plasma.Comment: 6 pages, 4 figures. Accepted for publication in Physical Review
Letter
Reversibility of laser filamentation
We investigate the reversibility of laser filamentation, a self-sustained,
non-linear propagation regime including dissipation and time-retarded effects.
We show that even losses related to ionization marginally affect the
possibility of reverse propagating ultrashort pulses back to the initial
conditions, although they make it prone to finite-distance blow-up susceptible
to prevent backward propagation.Comment: 12 pages, 3 figure
Direct Numerical Simulation of structural vacillation in the transition to geostrophic turbulence
The onset of small-scale fluctuations around a steady convection pattern in a
rotating baroclinic annulus filled with air is investigated using Direct
Numerical Simulation. In previous laboratory experiments of baroclinic waves,
such fluctuations have been associated with a flow regime termed Structural
Vacillation which is regarded as the first step in the transition to
fully-developed geostrophic turbulence.Comment: 6 page
DNS of bifurcations in an air-filled rotating baroclinic annulus
Three-dimensional Direct Numerical Simulation (DNS) on the nonlinear dynamics
and a route to chaos in a rotating fluid subjected to lateral heating is
presented here and discussed in the context of laboratory experiments in the
baroclinic annulus. Following two previous preliminary studies by Maubert and
Randriamampianina, the fluid used is air rather than a liquid as used in all
other previous work. This study investigated a bifurcation sequence from the
axisymmetric flow to a number of complex flows. The transition sequence, on
increase of the rotation rate, from the axisymmetric solution via a steady,
fully-developed baroclinic wave to chaotic flow followed a variant of the
classical quasi-periodic bifurcation route, starting with a subcritical Hopf
and associated saddle-node bifurcation. This was followed by a sequence of two
supercritical Hopf-type bifurcations, first to an amplitude vacillation, then
to a three-frequency quasi-periodic modulated amplitude vacillation (MAV), and
finally to a chaotic MAV\@. In the context of the baroclinic annulus this
sequence is unusual as the vacillation is usually found on decrease of the
rotation rate from the steady wave flow. Further transitions of a steady wave
with a higher wave number pointed to the possibility that a barotropic
instability of the side wall boundary layers and the subsequent breakdown of
these barotropic vortices may play a role in the transition to structural
vacillation and, ultimately, geostrophic turbulence.Comment: 31 page
Free space laser telecommunication through fog
Atmospheric clearness is a key issue for free space optical communications
(FSO). We present the first active method to achieve FSO through clouds and
fog, using ultrashort high intensity laser filaments. The laser filaments
opto-mechanically expel the droplets out of the beam and create a cleared
channel for transmitting high bit rate telecom data at 1.55 microns. The low
energy required for the process allows considering applications to
Earth-satellite FSO and secure ground based optical communication, with
classical or quantum protocols.Comment: 4 pages + 2 pages supplementary text and movie
Non-linear Synthesis of Complex Laser Waveforms at Remote Distances
Strong deformation of ultrashort laser pulse shapes is unavoidable when
delivering high intensities at remote distances due to non-linear effects
taking place while propagating. Relying on the reversibility of laser
filamentation, we propose to explicitly design laser pulse shapes so that
propagation serves as a non-linear field synthesizer at a remote target
location. Such an approach allows, for instance, coherent control of molecules
at a remote distance, in the context of standoff detection of pathogens or
explosives.Comment: 17 pages, 6 figure
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