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      Dynamics of the population grating formation in erbium-doped fibers

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          Abstract

          Experimental results on formation dynamics of population gratings in erbium-doped fiber (EDF) investigated via two-wave mixing (TWM) are reported. It is shown that at all three utilized essentially different recording wavelengths 1492, 1526 and 1568nm the transient TWM response in EDF with low erbium concentration (≈ 640 ppm) has the characteristic rate significantly larger than that of the population of the Er3+ meta-stable level evaluated from the fluorescence dynamics. Additionally in EDF with high erbium concentration (≈ 5600 ppm) the TWM response is faster (even at low recording power) and the relative TWM amplitude is lower than that in EDF with low concentration. General increase of the TWM response rate is explained by contribution of a transient decrease of the fiber average absorption level, while above-mentioned differences observed in heavily doped EDF - by spatial migration of the excited states. The spectral dependence of the later effect is attributed to the difference in the absorption/emission cross-section ratio of the Er3+ ions at the utilized wavelengths.

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          Photo-induced charge transfer across the interface between organic molecular crystals and polymers

          Photo-induced charge transfer of positive and negative charges across the interface between an ordered organic semiconductor and a polymeric insulator is observed in the field-effect experiments. Immobilization of the transferred charge in the polymer results in a shift of the field-effect threshold of polaronic conduction along the interface in the semiconductor, which allows for direct measurements of the charge transfer rate. The transfer occurs when the photon energy exceeds the absorption edge of the semiconductor. The direction of the transverse electric field at the interface determines the sign of the transferred charge; the transfer rate is controlled by the field magnitude and light intensity.
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            Caustic activation of rain showers

            We show quantitatively how the collision rate of droplets of visible moisture in turbulent air increases very abruptly as the intensity of the turbulence passes a threshold, due to the formation of fold caustics in their velocity field. The formation of caustics is an activated process, in which a measure of the intensity of the turbulence, termed the Stokes number St, is analogous to temperature in a chemical reaction: the rate of collision contains a factor exp(-C/St). Our results are relevant to the long-standing problem of explaining the rapid onset of rainfall from convecting clouds. Our theory does not involve spatial clustering of particles.
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              The ferroelectric-paraelectric phase transition in BiFeO3: crystal structure of the orthorhombic beta-phase

              A detailed investigation using variable temperature powder neutron diffraction demonstrates that BiFeO3 undergoes a phase transition from the ferroelectric alpha phase (rhombohedral, R3c) to a paraelectric beta phase (orthorhombic, Pbnm) between 820 oC and 830 oC. Co-existence of both phases over a finite temperature interval, together with abrupt changes in key structural parameters, confirm that the transition is firstorder. The beta phase corresponds to the GdFeO3-type perovskite. The metastability of BiFeO3 relative to Bi2Fe4O9 above 810 oC precludes observation of a reported cubic gamma phase above 925 oC under the present experimental conditions.
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                Author and article information

                Journal
                rmf
                Revista mexicana de física
                Rev. mex. fis.
                Sociedad Mexicana de Física (México, DF, Mexico )
                0035-001X
                December 2015
                : 61
                : 6
                : 458-465
                Affiliations
                [01] Ensenada Baja California orgnameCentro de Investigación Científica y de Educación Superior de Ensenada México
                Article
                S0035-001X2015000600009 S0035-001X(15)06100600009
                91547dec-d927-4fdf-b3df-9604fa8d3977

                This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

                History
                : 28 July 2015
                : 18 September 2015
                Page count
                Figures: 0, Tables: 0, Equations: 0, References: 25, Pages: 8
                Product

                SciELO Mexico

                Categories
                Research

                erbium doped fibers,two-wave mixing,Dynamic population gratings

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