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

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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&nbsp;: 08/2010</p

Photomolecular motions on azopolymer nano-objects: Toward new applications for photonics

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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