DIFFRACTION GRATINGS FORMED BY BENT-CORE LIQUID CRYSTALS IN THE TWIST – BEND NEMATIC PHASEMuhammad Ali
, 2021, doctoral dissertation
Abstract: In this thesis, we study the structure and optical transmission properties of the twist-bend nematic liquid crystalline phase, made of bent dimers, confined in thin planar cells. Confinement leads to the formation of a periodic modulated structure, the formation of which is explained as follows. The twist-bend nematic phase is characterized by a heliconical modulation of the molecular long axes. Due to a short pitch of modulation (approximately 10 nm), the twist-bend nematic phase behaves as a pseudo-layered medium. At temperatures below the nematic – twist-bend nematic phase transition, the heliconical pitch and thus the thickness of the pseudo-layers reduces, which leads to a two-dimensional undulation of pseudo-layers in the direction perpendicular to the cell surfaces and along the surfaces. The undulated structure is responsible for a stripe texture observed under a polarizing microscope and acts as a diffraction grating.
We constructed theoretical models to predict the pseudo-layer structure of a confined twist-bend nematic phase and to describe the properties of light diffracted on such cells. The free energy of the two-dimensional pseudo-layer structure of the twist-bend nematic phase is expressed in terms of the nematic director field, by which we describe the direction of the heliconical axis, and a complex smectic order parameter, the gradient of which gives the direction of the layer normal. At first, we assume that pseudo-layers are perpendicular to the surfaces (bookshelf geometry) and find a stable structure by assuming an ansatz for the pseudo-layer displacement from the bookshelf geometry and then minimizing the free energy at a very strong and very weak surface anchoring. In this way a threshold condition for the onset of the modulated structure is obtained, as well as the amplitude and period of modulation. Next, we assume that, at the onset of the twist-bend nematic phase, pseudo-layers are formed at some angle (pre-tilt) with respect to the surface. We find that in both cases, the bookshelf and pre-tilted one, the calculated period of modulation far from the phase transition is always approximately twice the cell thickness, which agrees with experimental observations.
The properties of light diffracted by the spontaneously formed grating were studied both experimentally and theoretically. We measured the intensity and polarization properties of the first two orders of the diffracted light and the temperature dependence of the polarization of the second order diffraction peaks. To predict the observed properties of the diffracted light and to simplify the description of such gratings, we consider different preliminary models of a one-dimensional spatial variation of the optic axis, the direction of which is given by two angles. A transfer matrix method is used and a good agreement between the experimental and theoretical results is obtained. In a more comprehensive approach, we determine the spatial variation of the optic axis direction from the modeled structure. The electric field in the diffracted light is obtained by using the transfer matrix method and beam propagation method. In the case of a pre-tilt of the pseudo-layers and very strong surface anchoring both methods give good qualitative agreement with experimental results, only in the case of the temperature dependence of the second order diffraction peaks, a more complex beam propagation method is superior to the transfer matrix method.
The thesis is divided into three parts. In the first part, we focus on the physical properties of the twist-bend nematic phase and its structure in thin planar cells. In the second part, a continuum model is proposed and finally, the properties of diffracted light are discussed and theoretically predicted by using the beam propagation method and transfer matrix method.
Keywords: Bent-dimer liquid crystals, twist-bend nematic phase, undulation of pseudo-layers, polarization, diffraction grating, beam propagation method, transfer matrix method.
Published in DKUM: 21.10.2021; Views: 1058; Downloads: 81
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An overview of selected pseudo-stereophonic techniques : B.Sc. ThesisMichele Perrone
, 2020, undergraduate thesis
Abstract: Stereophony is a method of sound recording and sound reproduction that uses two separate audio channels, two for each process. In contrast, monophony uses only one audio channel. Stereophonic sound is thus affine, to some degree, to human binaural hearing because it can reproduce some of the ambience and localization cues that were present during the recording session. For this reason, stereophonic music recordings are generally preferred over their monophonic counterparts. To make the sound of monophonic recordings more natural, many pseudo-stereophonic techniques have been developed; these techniques process monophonic sound into stereophonic sound. In this thesis, we review in detail five such techniques, of which we implemented and tested two. Our results show that pseudo-stereophony, when applied appropriately, can create more natural nuances in monophonic recordings and can reduce listening fatigue.
Keywords: Pseudo-stereophony, audio processing, music
Published in DKUM: 14.12.2020; Views: 985; Downloads: 62
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Rekonstrukcija funkcionalnih mrež iz dinamike pankreasnih celic betaJakob Murko
, 2019, master's thesis
Abstract: V magistrskem delu obravnavamo funkcionalno povezanost celic beta. Te celice se nahajajo v Langerhansovih otočkih v trebušni slinavki in so ključne pri uravnavanju koncentracije glukoze v krvi, saj ob povečani ravni glukoze v organizmu izločajo hormon inzulin. V prvi fazi dela dinamiko električno vzdraženih celic beta simuliramo z modelom, v drugi fazi pa se osredotočimo na analizo eksperimentalno izmerjene dinamike znotrajceličnega kalcija. Za opis dinamike posameznih celic uporabimo fenomenološki model, t.i. iterativno mapo Rulkova. Posamezne Rulkove oscilatorje med seboj povežemo v mrežo, s čimer simuliramo funkcijo presledkovnih stikov, s katerimi so povezane celice beta v realnem tkivu. Da bi naš večcelični model čimbolj približali realnemu obnašanju, vpeljemo tudi naključno in prostorsko definirano heterogenost celic. Na podlagi podatkov aktivnosti celic iz simulacij določimo funkcionalno povezanost le-teh. Predstavimo dve metodi določitve funkcionalne mreže, to je korelacijsko in metodo pseudo-inverza kovariance. Analiza simuliranih podatkov pokaže, da sta obe metodi zanesljivi, zato jih preizkusimo tudi na eksperimentalno pridobljenih podatkih, in sicer na in situ meritvah kalcijeve aktivnosti v svežih tkivnih rezinah mišje trebušne slinavke. Ugotovimo, da sta si funkcionalna mreža na podlagi simulacijskih in eksperimentalnih podatkov podobni, kar nam kaže na natančno obnašanje modela. Z analizo obeh funkcionalnih mrež ustvarjenih na podlagi eksperimentalnih podatkov lahko vsaj delno sklepamo na strukturo povezav presledkovnih stikov v opazovani plasti Langerhansovega otočka. Pomembna je tudi ugotovitev, da v funkcionalni mreži celic beta opazimo celice z nadpovprečnim številom povezav, tudi daljnosežnih, kar je v skladu z ugotovitvami preteklih raziskav.
Keywords: celice beta, Langerhansov otoček, funkcionalna mreža, Rulkov model, celična heterogenost, korelacijski koeficient, pseudo-inverz kovariance.
Published in DKUM: 05.11.2019; Views: 1392; Downloads: 91
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An alternative sorting procedure for interactive group decision support based on the pseudo-criterion conceptAndrej Bregar
, József Györkös
, Ivan Rozman
, 2003, original scientific article
Abstract: An original interactive procedure is proposed, which aims at overcoming some of the major weaknesses of existent pseudocriterion based methods for group decision analysis. It refers to absolute judgements of feasible alternatives and is focused on complementary activities of opinion elicitation and robustness analysis. As a foundation, four interdependent principles are introduced - problem localization, interactivity on the basis of aggregation/disaggregation approach, automatic derivation of criteria weights according to selective effects of veto tresholds, and group consensus seeking. The principles are grounded and realized by appropriate methodological solutions.
Keywords: decision analysis, pseudo-criterion concept, decision models, group consensus, alternative sorting, group work, fuzzy relation
Published in DKUM: 01.06.2012; Views: 1678; Downloads: 90
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