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End date: 2023 × Start date: 2020 × DDC: 600 × Has files: Yes × WGL Institute: WIAS ×

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Now showing 1 - 6 of 6
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    Longtime behavior for a generalized Cahn-Hilliard system with fractional operators
    (Messina : Accademia Peloritana dei Pericolanti, 2020) Colli, Pierluigi; Gilardi, Gianni; Sprekels, Jürgen
    In this contribution, we deal with the longtime behavior of the solutions to the fractional variant of the Cahn-Hilliard system, with possibly singular potentials, that we have recently investigated in the paper Well-posedness and regularity for a generalized fractional Cahn-Hilliard system. More precisely, we study the ω-limit of the phase parameter y and characterize it completely. Our characterization depends on the first eigenvalues λ1≥0 of one of the operators involved: if λ1>0, then the chemical potential μ vanishes at infinity and every element yω of the ω-limit is a stationary solution to the phase equation; if instead λ1=0, then every element yω of the ω-limit satisfies a problem containing a real function μ∞ related to the chemical potential μ. Such a function μ∞ is nonunique and time dependent, in general, as we show by an example. However, we give sufficient conditions for μ∞ to be uniquely determined and constant.
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    Topology- and Geometry-Controlled Functionalization of Nanostructured Metamaterials
    (Basel : MDPI, 2023) Fomin, Vladimir M.; Marquardt, Oliver
    [no abstract available]
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    Semiconductor laser linewidth theory revisited
    (Basel : MDPI, 2021) Wenzel, Hans; Kantner, Markus; Radziunas, Mindaugas; Bandelow, Uwe
    More and more applications require semiconductor lasers distinguished not only by large modulation bandwidths or high output powers, but also by small spectral linewidths. The theoretical understanding of the root causes limiting the linewidth is therefore of great practical relevance. In this paper, we derive a general expression for the calculation of the spectral linewidth step by step in a self-contained manner. We build on the linewidth theory developed in the 1980s and 1990s but look from a modern perspective, in the sense that we choose as our starting points the time-dependent coupled-wave equations for the forward and backward propagating fields and an expansion of the fields in terms of the stationary longitudinal modes of the open cavity. As a result, we obtain rather general expressions for the longitudinal excess factor of spontaneous emission (K-factor) and the effective α-factor including the effects of nonlinear gain (gain compression) and refractive index (Kerr effect), gain dispersion, and longitudinal spatial hole burning in multi-section cavity structures. The effect of linewidth narrowing due to feedback from an external cavity often described by the so-called chirp reduction factor is also automatically included. We propose a new analytical formula for the dependence of the spontaneous emission on the carrier density avoiding the use of the population inversion factor. The presented theoretical framework is applied to a numerical study of a two-section distributed Bragg reflector laser.
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    Impact of energy dissipation on interface shapes and on rates for dewetting from liquid substrates
    ([London] : Macmillan Publishers Limited, part of Springer Nature, 2018) Peschka, Dirk; Bommer, Stefan; Jachalski, Sebastian; Seemann, Ralf; Wagner, Barbara
    We revisit the fundamental problem of liquid-liquid dewetting and perform a detailed comparison of theoretical predictions based on thin-film models with experimental measurements obtained by atomic force microscopy. Specifically, we consider the dewetting of a liquid polystyrene layer from a liquid polymethyl methacrylate layer, where the thicknesses and the viscosities of both layers are similar. Using experimentally determined system parameters like viscosity and surface tension, an excellent agreement of experimentally and theoretically obtained rim profile shapes are obtained including the liquid-liquid interface and even dewetting rates. Our new energetic approach additionally allows to assess the physical importance of different contributions to the energy-dissipation mechanism, for which we analyze the local flow fields and the local dissipation rates. Using this approach, we explain why dewetting rates for liquid-liquid systems follow no universal power law, despite the fact that experimental velocities are almost constant. This is in contrast to dewetting scenarios on solid substrates and in contrast to previous results for liquid-liquid substrates using heuristic approaches.
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    Topology- and Geometry-Controlled Functionalization of Nanostructured Metamaterials
    (Basel : MDPI, 2023) Fomin, Vladimir M.; Marquardt, Oliver
    [No abstract available]
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    Grain boundary assisted photocurrent collection in thin film solar cells
    (Les Ulis : EDP Sciences, 2015) Harndt, Susanna; Kaufmann, Christian A.; Lux-Steiner, Martha C.; Klenk, Reiner; Nürnberg, Reiner
    The influence of absorber grain boundaries on the photocurrent transport in chalcopyrite based thin film solar cells has been calculated using a two dimensional numerical model. Considering extreme cases, the variation in red response is more expressed than in one dimensional models. These findings may offer an explanation for the strong influence of buffer layer preparation on the spectral response of cells with small grained absorbers.