121 research outputs found
Light mediated emergence of surface patterns in azopolymers at low temperatures
Polymer thin films doped with azobenzene molecules do have the ability to
organize themselves in spontaneous surface relief gratings (SRG) under
irradiation with a single polarized beam. To shed some light in this still
unexplained phenomenon, we use a new method that permits us to access
experimentally the very first steps of the pattern formation process.
Decreasing the temperature, we slow down the formation and organization of
patterns, due to the large increase of the viscosity and relaxation time of the
azopolymer. As a result decreasing the temperature allows us to access and
study much shorter time scales,in the physical mechanisms underlying the
pattern formation, than previously reported. We find that the patterns organize
themselves in sub-structures which size increase with the temperature,
following the diffusion coefficient evolution of the material. That result
suggests that the pattern formation and organization is mainly governed by
diffusive processes, in agreement with some theories of the SRG formation.
Decreasing further the temperature we observe the emergence of small voids
located at the junction of the sub-structures.Comment: 6 figures, 13 pages
Birefringence properties and surface relief grating formation on methlacrylate polymers with photochromic side chains
International audienc
Surface relief grating formation on nano-objects
We exploit the photoinduced migration effect in azopolymer thin films to induce surface relief patterning of nano-objects. Manipulation and precise control of the molecular order is achieved at the nanoscale. Interaction between a laser beam from an argon laser and the azopolymer nano-objects induces structures on the surface. The self-patterning process is observed to depend on the laser beam polarization
Subwavelength atom localization via amplitude and phase control of the absorption spectrum
We propose a scheme for subwavelength localization of an atom conditioned
upon the absorption of a weak probe field at a particular frequency.
Manipulating atom-field interaction on a certain transition by applying drive
fields on nearby coupled transitions leads to interesting effects in the
absorption spectrum of the weak probe field. We exploit this fact and employ a
four-level system with three driving fields and a weak probe field, where one
of the drive fields is a standing-wave field of a cavity. We show that the
position of an atom along this standing wave is determined when probe field
absorption is measured. We find that absorption of the weak probe field at a
certain frequency leads to subwavelength localization of the atom in either of
the two half-wavelength regions of the cavity field by appropriate choice of
the system parameters. We term this result as sub-half-wavelength localization
to contrast it with the usual atom localization result of four peaks spread
over one wavelength of the standing wave. We observe two localization peaks in
either of the two half-wavelength regions along the cavity axis.Comment: Accepted for publication to Physical Review
An Incentive Mechanism for Cooperative Data Replication in MANETs - a Game Theoretical Approach
Wireless ad hoc networks have seen a great deal of attention in the past years, especially in cases where no infrastructure is available. The main goal in these networks is to provide good data accessibility for participants. Because of the wireless nodes' continuous movement, network partitioning occurs very often. In order to subside the negative effects of this partitioning and improve data accessibility and reliability, data is replicated in nodes other than the original owner of data. This duplication costs in terms of nodes' storage space and energy. Hence, autonomous nodes may behave selfishly in this cooperative process and do not replicate data. This kind of phenomenon is referred to as a strategic situation and is best modeled and analyzed using the game theory concept. In order to address this problem we propose a game theory data replication scheme by using the repeated game concept and prove that it is in the nodes' best interest to cooperate fully in the replication process if our mechanism is used
Theoretical study of dark resonances in micro-metric thin cells
We investigate theoretically dark resonance spectroscopy for a dilute atomic
vapor confined in a thin (micro-metric) cell. We identify the physical
parameters characterizing the spectra and study their influence. We focus on a
Hanle-type situation, with an optical irradiation under normal incidence and
resonant with the atomic transition. The dark resonance spectrum is predicted
to combine broad wings with a sharp maximum at line-center, that can be singled
out when detecting a derivative of the dark resonance spectrum. This narrow
signal derivative, shown to broaden only sub-linearly with the cell length, is
a signature of the contribution of atoms slow enough to fly between the cell
windows in a time as long as the characteristic ground state optical pumping
time. We suggest that this dark resonance spectroscopy in micro-metric thin
cells could be a suitable tool for probing the effective velocity distribution
in the thin cell arising from the atomic desorption processes, and notably to
identify the limiting factors affecting desorption under a grazing incidence.Comment: 12 pages, 11 figures theoretical articl
Group velocity control in the ultraviolet domain via interacting dark-state resonances
The propagation of a weak probe field in a laser-driven four-level atomic
system is investigated. We choose mercury as our model system, where the probe
transition is in the ultraviolet region. A high-resolution peak appears in the
optical spectra due to the presence of interacting dark resonances. We show
that this narrow peak leads to superluminal light propagation with strong
absorption, and thus by itself is only of limited interest. But if in addition
a weak incoherent pump field is applied to the probe transition, then the peak
structure can be changed such that both sub- and superluminal light propagation
or a negative group velocity can be achieved without absorption, controlled by
the incoherent pumping strength
Mouvement photomoléculaire sur des nano-objets en azopolymère: Vers de nouvelles applications en photonique
Date du colloque : 08/2010</p
Photomolecular motions on azopolymer nano-objects: Toward new applications for photonics
International audienc
Transformation of high concentrations of chlorophenols by the white-rot basidiomycete Trametes versicolor immobilized on nylon mesh
Free-cell cultures of Trametes versicolor were compared with cultures
immobilized on nylon mesh in a 2-litre bioreactor for transformation of
pentachlorophenol (PCP) and 2,4-dichlorophenol (2,4-DCP), added at
intervals to the liquid culture medium over a period of 816 hrs.
Increasing amounts of PCP from 200 ppm to 2000 ppm added batchwise to
cultures permitted acclimatization of the fungus to these toxic
pollutants. A total addition of 2000 ppm of 2,4-DCP and 3400 ppm PCP
were removed from the immobilized cultures with 85% of 2,4-DCP and 70%
of PCP transformed by enzymes (laccase and Mn-peroxidase), 5% 2,4-DCP
and 28% PCP adsorbed by the biomass and 10% 2,4-DCP and 2% PCP retained
in the medium at the termination of the fermentation after 1020 hrs. In
contrast free-cell cultures in the same medium with the same addition
regime of PCP and 2,4-DCP, transformed 20% 2,4-DCP and 12% PCP by
enzyme action, adsorbed 58% 2,4-DCP and 80% PCP by the biomass, and
retained 22% 2,4-DCP and 8% PCP in the medium. The use of nylon mesh as
an immobilization matrix for removal of PCP and 2,4-DCP facilitates
more efficient removal of chlorophenols and can be adapted to scale-up
for application of large volumes of chlorophenol-containing aqueous
effluents
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