84 research outputs found

    On exponential cosmological type solutions in the model with Gauss-Bonnet term and variation of gravitational constant

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    A D-dimensional gravitational model with Gauss-Bonnet term is considered. When ansatz with diagonal cosmological type metrics is adopted, we find solutions with exponential dependence of scale factors (with respect to "synchronous-like" variable) which describe an exponential expansion of "our" 3-dimensional factor-space and obey the observational constraints on the temporal variation of effective gravitational constant G. Among them there are two exact solutions in dimensions D = 22, 28 with constant G and also an infinite series of solutions in dimensions D \ge 2690 with the variation of G obeying the observational data.Comment: 21 pages, 12 figures, LaTex; eq. (2.1) is modified, several sentences are added, a typo in eq. (3.13) is eliminate

    Exact (1+3+6)(1 + 3 + 6)-dimensional cosmological-type solutions in gravitational model with Yang-Mills field, Gauss-Bonnet term and Λ\Lambda-term

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    We consider 1010-dimensional gravitational model with SO(6)SO(6) Yang-Mills field, Gauss-Bonnet term and Λ\Lambda-term. We study so-called cosmological type solutions defined on product manifold M=R×R3×KM = R \times R^3 \times K, where KK is 6d6d Calabi-Yau manifold. By putting the gauge field 1-form to be coinciding with 1-form spin connection on KK, we obtain exact cosmological solutions with exponential dependence of scale factors (upon tt-variable), governed by two non-coinciding Hubble-like parameters: H>0H >0, hh, obeying H+2h0 H + 2 h \neq 0. We also present static analogs of these cosmological solutions (for H0H \neq 0, hHh \neq H and H+2h0 H + 2 h \neq 0). The islands of stability for both classes of solutions are outlined.Comment: 17 pages, 3 figures, LaTex, Revised version: 3 paragraphs are added into Introduction, new references are included and few references (self-citations) are omitte

    Modulation control and spectral shaping of optical fiber supercontinuum generation in the picosecond regime

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    Numerical simulations are used to study how fiber supercontinuum generation seeded by picosecond pulses can be actively controlled through the use of input pulse modulation. By carrying out multiple simulations in the presence of noise, we show how tailored supercontinuum Spectra with increased bandwidth and improved stability can be generated using an input envelope modulation of appropriate frequency and depth. The results are discussed in terms of the non-linear propagation dynamics and pump depletion.Comment: Aspects of this work were presented in Paper ThJ2 at OECC/ACOFT 2008, Sydney Australia 7-10 July (2008). Journal paper submitted for publication 30 July 200

    Optical pump–terahertz probe study of HR GaAs:Cr and SI GaAs:El2 structures with long charge carrier lifetimes

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    The time dynamics of nonequilibrium charge carrier relaxation processes in SI GaAs:EL2 (semi-insulating gallium arsenide compensated with EL2 centers) and HR GaAs:Cr (high-resistive gallium arsenide compensated with chromium) were studied by the optical pump–terahertz probe technique. Charge carrier lifetimes and contributions from various recombination mechanisms were determined at different injection levels using the model, which takes into account the influence of surface and volume Shockley–Read–Hall (SRH) recombination, interband radiative transitions and interband and trap-assisted Auger recombination. It was found that, in most cases for HR GaAs:Cr and SI GaAs:EL2, Auger recombination mechanisms make the largest contribution to the recombination rate of nonequilibrium charge carriers at injection levels above ~(0.5–3)·1018 cm−3, typical of pump–probe experiments. At a lower photogenerated charge carrier concentration, the SRH recombination prevails. The derived charge carrier lifetimes, due to the SRH recombination, are approximately 1.5 and 25 ns in HR GaAs:Cr and SI GaAs:EL2, respectively. These values are closer to but still lower than the values determined by photoluminescence decay or charge collection efficiency measurements at low injection levels. The obtained results indicate the importance of a proper experimental data analysis when applying terahertz time-resolved spectroscopy to the determination of charge carrier lifetimes in semiconductor crystals intended for the fabrication of devices working at lower injection levels than those at measurements by the optical pump–terahertz probe technique. It was found that the charge carrier lifetime in HR GaAs:Cr is lower than that in SI GaAs:EL2 at injection levels > 1016 cm−3.В ст. ошибочно: Irina A. Kolesnikov

    Stochasticity, periodicity and localized light structures in partially mode-locked fibre lasers

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    Physical systems with co-existence and interplay of processes featuring distinct spatio-temporal scales are found in various research areas ranging from studies of brain activity to astrophysics. The complexity of such systems makes their theoretical and experimental analysis technically and conceptually challenging. Here, we discovered that while radiation of partially mode-locked fibre lasers is stochastic and intermittent on a short time scale, it exhibits non-trivial periodicity and long-scale correlations over slow evolution from one round-trip to another. A new technique for evolution mapping of intensity autocorrelation function has enabled us to reveal a variety of localized spatio-temporal structures and to experimentally study their symbiotic co-existence with stochastic radiation. Real-time characterization of dynamical spatio-temporal regimes of laser operation is set to bring new insights into rich underlying nonlinear physics of practical active- and passive-cavity photonic systems