Image transfer through a chaotic channel by intensity correlations

The three-wave mixing processes in a second-order nonlinear medium can be used for imaging protocols, in which an object field is injected into the nonlinear medium together with a reference field and an image field is generated. When the reference field is chaotic, the image field is also chaotic and does not carry any information about the object. We show that a clear image of the object be extracted from the chaotic image field by measuring the spatial intensity correlations between this field and one Fourier component of the reference. We experimentally verify this imaging protocol in the case of frequency downconversion.Comment: 17 pages, 7 figure

The plasma-wall transition layers in the presence of collisions with a magnetic field parallel to the wall

International audienceThe plasma-wall transition is studied by mean of a particle-in-cell (PIC) simulations in the configuration of a parallel to the wall magnetic field (B), with collisions between charged particles vs. neutral atoms taken into account. The investigated system consists in a plasma bounded by two absorbing walls separated by 200 electron Debye lengths (λ d). The strength of the magnetic field is chosen such as the ratio λ d /r l , with r l the electron Larmor radius, is smaller or larger than the unity. Collisions are modelled with a simple operator that reorients randomly ion or electron velocity, keeping constant the total kinetic energy of both the neutral atom (target) and the incident charged particle. The PIC simulations show that the plasma-wall transition consists in a quasi-neutral region (pre-sheath), from the center of the plasma towards the walls, where the electric potential or electric field profiles are well described by an ambipolar diffusion model, and in a second region at the vicinity of the walls, called the sheath, where the quasi-neutrality breaks down. In this peculiar geometry of B and for a certain range of the mean-free-path, the sheath is found to be composed by two charged layers, a first, positive, close to the walls, and a second one, negative, towards the plasma and before the neutral pre-sheath. Depending on the amplitude of B, the spatial variation of the electric potential can be non-monotonic and presents a maximum within the sheath region. More generally, the sheath extent as well as the potential drop within the sheath and the pre-sheath are studied with respect to B, the mean-free-path and the ion and electron temperature

Plasma sheath properties in a magnetic field parallel to the wall

International audienceParticle in cell simulations were carried out with a plasma bounded by two absorbing walls and a magnetic field applied parallel to them. Both the sheath extent and the potential drop in it were derived from simulations for different plasma parameters such as the electron and ion temperature T i , particle density and ion mass. Both of them exhibit a power law dependent on the Larmor to plasma ion pulsation ratio Ω i. For increasing values of the magnetic field, the potential drop within the sheath decreases from a few T i /e down to zero, where e stands for the electron charge. The space charge extent increases with Ω i and saturates to 2.15 ion Larmor radius. A simple model of sheath formation in such a magnetic field configuration is presented. Assuming strongly magnetized electrons, and neglecting collisions and ionizations, a new typical length is evidenced, which depends on the ratio Ω i. The charge separation sheath width is theoretically found to increase from a combination of the electron gyroradius and the ion Debye length for low Ω i ratios up to several ion gyroradii for strongly magnetized ions. Both the calculated sheath extent and plasma potential show a fair agreement with the numerical simulations

Collective action for innovation and small farmer market access: The Papa Andina experience

"The Andean highlands are home to some of the poorest rural households in South America. Native potato varieties and local knowledge for their cultivation and use are unique resources possessed by farmers in these areas. As the forces of globalization and market integration penetrate the Andes, they present both challenges and opportunities for farmers there. This paper reports on how the Papa Andina Regional Initiative is promoting the use of collective action to reduce poverty in the Andes, by developing market niches and adding value to potatoes, particularly the native potatoes grown by poor farmers. Since 1998, Papa Andina has worked with partners in Bolivia, Ecuador and Peru to stimulate pro-poor innovation within market chains for potato-based products. Market chain actors (including small-scale potato producers, traders, and processors), researchers, and other service providers have engaged in innovation processes via two principal tools for facilitating collective action: the Participatory Market Chain Approach (PMCA) and Stakeholder Platforms. The PMCA fosters commercial, technological, and institutional innovation through a structured process that builds interest, trust, and collaboration among participants. Stakeholder Platforms provide a space for potato producers, other market chain actors, and service providers to come together to identify their common interests, share knowledge, and develop joint activities. The PMCA and Stakeholder Platforms have empowered Andean potato farmers by expanding their knowledge of markets, market agents, and business opportunities. Social networks built up among producers, market agents, and service providers have stimulated commercial innovation, which in turn has stimulated technical and institutional innovation. These innovations have allowed small farmers to market their potatoes on more favorable terms and other market chain actors to increase their incomes. This paper describes experiences with collective action in Bolivia, Ecuador, and Peru, via the PMCA and Stakeholder Platforms. Based on these experiences, a number of lessons are formulated for using collective action to stimulate innovation, market access, and poverty reduction in other settings." authors' abstractCollective action, Small farmers, Potato, Participatory methods, Innovation, stakeholders, Markets,

Conductivity in organic semiconductors hybridized with the vacuum field

Organic semiconductors have generated considerable interest for their potential for creating inexpensive and flexible devices easily processed on a large scale [1-11]. However technological applications are currently limited by the low mobility of the charge carriers associated with the disorder in these materials [5-8]. Much effort over the past decades has therefore been focused on optimizing the organisation of the material or the devices to improve carrier mobility. Here we take a radically different path to solving this problem, namely by injecting carriers into states that are hybridized to the vacuum electromagnetic field. These are coherent states that can extend over as many as 10^5 molecules and should thereby favour conductivity in such materials. To test this idea, organic semiconductors were strongly coupled to the vacuum electromagnetic field on plasmonic structures to form polaritonic states with large Rabi splittings ca. 0.7 eV. Conductivity experiments show that indeed the current does increase by an order of magnitude at resonance in the coupled state, reflecting mostly a change in field-effect mobility as revealed when the structure is gated in a transistor configuration. A theoretical quantum model is presented that confirms the delocalization of the wave-functions of the hybridized states and the consequences on the conductivity. While this is a proof-of-principle study, in practice conductivity mediated by light-matter hybridized states is easy to implement and we therefore expect that it will be used to improve organic devices. More broadly our findings illustrate the potential of engineering the vacuum electromagnetic environment to modify and to improve properties of materials.Comment: 16 pages, 13 figure

Indirect excitation of Er3+ ions in silicon nitride films prepared by reactive evaporation

International audienceEr-doped silicon nitride films were obtained by reactive evaporation of silicon under a flow of nitrogen ions and were annealed at temperatures up to 1300°C. Samples were studied by infrared absorption and Raman spectrometries and by transmission electron microscopy. The 1.54 m Er-related photoluminescence ͑PL͒ was studied in relation with the structure with pump excitation at 488 and 325 nm. Steady-state PL, PL excitation spectroscopy, and time-resolved PL were performed. The results demonstrate that Er 3+ ions are indirectly excited both via silicon nanocrystals and via localized states in the silicon nitride matrix. Er-doped silicon-based materials have attracted much attention in the scientific community because of their potential use for optoelectronics. 1 Indeed, Er 3+ ions can emit sharp luminescence at 1.54 m, which is the commonly used wavelength for optical communications. The Er sensitization has been widely studied in Si rich SiO 2 layers. In silica containing silicon nanocrystals ͑Si-nc͒, the Er-related photolu-minescence is strongly improved due to a strong energy transfer from Si-nc to Er 3+ ions. 2-4 The Er 3+ ions can then be indirectly excited by Si-nc which have an absorption cross section several orders of magnitude higher than that of direct Er excitation. While SiN x is a particularly interesting host matrix for electrically pumped light-emitting devices, the Er excitation mechanism in silicon nitride films is still not clear. Similarly to the SiO x based samples, the sensitization of Er 3+ ions by Si nanoparticules has been reported in SiN x samples prepared by plasma enhanced chemical vapour deposition ͑PECVD͒ 5 or by magnetron sputtering. 6 However, some works have also demonstrated that indirect excitation of Er 3+ ions could occur via electronic states localized in the SiN x band tail states. 7,8 In this letter, we study the Er-related PL at 1.54 m in Er-doped silicon nitride thin films prepared by an ion-beam-assisted evaporation technique. The evolutions of the structure and of the PL properties with the annealing treatments are studied. It is demonstrated that the Er excitation is indirect and that Si-nc is able to improve the PL intensity. It is also shown that another indirect excitation path presumably exists in the amorphous SiN x matrix. Silicon was evaporated from an electron beam gun with a deposition rate equal to 0.1 nm/s. The 200 nm thick films were deposited on silicon substrates maintained at 100°C. The nitrogen ions were provided by an electron cyclotron resonance microwave plasma source. The nitrogen flow was regulated by maintaining the total pressure in the evaporation chamber at 2 ϫ 10 −5 Torr. The Er doping was performed from an effusion cell. Rutherford backscattering spectrom-etry was used to analyze the chemical content of the film. The Si, N, O, and Er atomic concentrations are equal to 47%, 48%, 5%, and 0.3%, respectively. The oxygen content is due to the low density of the layer and to exposure to the air. This concentration corresponds to a 12 at. % Si excess compared to the Si 3 N 4 equilibrium stoichiometry. The Fourier transform infrared ͑FTIR͒ experiments were carried out with a spectrometer with a resolution of 2 cm −1. Raman measurements were carried out with a mutichannel spectrometer equipped with a 1800 grooves mm −1 grating. The samples were excited by the 514 nm line from an argon laser. Transmission electron microscopy was performed with a 200 keV microscope. For the steady-state PL experiments, the samples were excited by a 30 mW He-Cd laser using the 325 nm line or by a 60 mW laser diode emitting at 488 nm. For the PL excitation ͑PLE͒ experiments, the samples were excited by an optical parametric oscillator laser. The PL signal was measured by a photomultiplier tube cooled at 190 K. For the time-resolved PL experiments, the samples were pumped by the 355 nm line of a frequency-tripled YAG:Nd laser. The laser pulse frequency, energy, and duration were typically equal to 10 Hz, 50 J, and 20 ns, respectively. The time response of the detection system was better than 1 s. Figure 1͑a͒ shows the FTIR spectra of the films for as-deposited sample and samples annealed at 1000 and 1100°C. The spectrum shows a very intense band at around 850 cm −1 , characteristic of the asymmetric stretching vibration of the SiN bonds. 9 The spectra are not significantly modified for annealing temperatures lower than 1000°C since only a 6 cm −1 shift occurs to higher wavenumbers. For higher annealing temperature, the peaks shift again a few cm −1 and a shoulder appears at high wavenumbers, demonstrating a modification of the SiN bonds, which could be correlated to the precipitation of Si-nc. 1

Experimental measurements of the RF sheath thickness with a cylindrical Langmuir probe

International audienc

The plasma-wall transition with collisions and an oblique magnetic field: reversal of potential drops at grazing incidences

International audienceThe plasma-wall transition is studied by using 1d3V particle-in-cell (PIC) simulations in the case of a one dimensional plasma bounded by two absorbing walls separated by 200 Debye lengths (λ d). A constant and oblique magnetic field is applied to the system, with an amplitude such that r < λ d < R, where r and R are the electron and ion Larmor radius respectively. Collisions with neutrals are taken into account and modelled by an energy conservative operator, which randomly reorients ion and electron velocities. The plasma-wall transition (PWT) is shown to depend on both the angle of incidence of the magnetic field with respect to the wall θ, and on the ion mean-free-path to Larmor radius ratio, λ ci /R. In the very low collisionality regime (λ ci R) and for a large angle of incidence, the PWT consists in the classical tri-layer structure (Debye sheath / Chodura sheath / Pre-sheath) from the wall towards the center of the plasma. The drops of potential within the different regions are well consistent with already published models. However, when sin θ ≤ R/λ ci or with the ordering λ ci < R , collisions can not be neglected, leading to the disappearance of the Chodura sheath. In these case, a collisional model yields analytic expressions for the potential drop in the quasi-neutral region, and explains, in qualitative and quantitative agreement with the simulation results, its reversal below a critical angle derived in the paper, a regime possibly met in the SOL of tokamaks. It is further shown that the potential drop in the Debye sheath slightly varies with the collision-ality for λ ci R. However, it tends to decrease with λ ci in the high collisionality regime, until the Debye sheath finally vanishes

CIP-OFIAGRO: estudio de perdidas pos-cosecha en la cadena de la papa en Ecuador.

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