| 1. |
- Lagerwall, J. P. F., et al.
(author)
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Chiral smectic C subphases induced by mixing a bistereogenic antiferroelectric liquid crystal with a non-chiral liquid crystal
- 2005
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In: Ferroelectrics. - : Informa UK Limited. - 0015-0193 .- 1563-5112. ; 315, s. 221-230
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Journal article (peer-reviewed)abstract
- By mixing a bistereogenic antiferroelectric liquid crystal (AFLC) compound, exhibiting only the SmQ* and SmC a * mesophases, with the achiral N-SmC liquid crystal HOAB we could induce all three AFLC SmC-type subphases, SmC α *, SmC β * and SmC γ *. This seems to be in contradiction with two recent postulations regarding the subphase stability, one of which suggests that the subphases appear as a result of strong chiral interactions, the other that these phases require high smectic order, something one would generally not expect in mixtures. We have studied the helical pitch, optical tilt angle, spontaneous polarization and the x-ray diffraction due to the smectic layering, as a function of mixing ratio in order to better understand the relation between phase sequence and mixture composition. The smectic layer spacing shows a strongly non-linear behavior, suggesting that the basic structure of the pure AFLC substance is retained up to a HOAB content of about 75%.
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| 2. |
- Bouligand, Yves, et al.
(author)
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Blue-phase drops on a glass interface and their decoration at the cholesteric transition
- 2008
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In: Comptes Rendus Chimie. - : Elsevier BV. - 1631-0748. ; 11:3, s. 212-220
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Journal article (peer-reviewed)abstract
- Free-surface drops of blue phase attached to a glass slide present topologically concentric lines, which we first believed to be steps at the air interface, as this was often observed in smectic drops or in other liquid crystals. Actually these contours lie at the glass interface, or in its close vicinity, but do not really form steps. While the existence of steps at the air interface cannot be excluded, we did not observe them in this study. Reproducible decorations were observed at the transition from the blue phase to the cholesteric liquid crystal, and showed a geometry close to that found in certain biological structures, which may be considered as stabilized analogues of blue phases.
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| 3. |
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| 4. |
- Caillaud, B., et al.
(author)
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Safe and ergonomic welding masks by fast liquid crystals
- 2008
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In: Molecular Crystals and Liquid Crystals. - : Informa UK Limited. - 1563-5287 .- 1542-1406. ; 494, s. 195-204
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Journal article (peer-reviewed)abstract
- Recent developments in welding technology, in particular the high intensity pulsed welding arc, have sharpened the requirements on speed and adjustable attenuation for safe and ergonomic operations. With this advent the conventional welding helmets based on nematic liquid crystals are facing difficulties because the nematics are not responding fast enough to stroboscopically follow the changing intensity of the arc during pulsed welding. We propose here a new generation of welding shutters based on smectic (ferroelectric or antiferroelectric) liquid crystals which are fast enough to be synchronized to follow pulsed arc regimes and hence permitting a better visibility of the object under work. The two varieties of smectic materials used are polymer-stabilized in order to make the device shock-proof, to prevent the formation of specific defects such as stripes and chevrons, and to reduce the back-relaxation time in the antiferroelectric case.
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| 6. |
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| 7. |
- Lagerwall, Sven, 1934
(author)
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Linear and Non-Linear Polar Effects in Liquid Crystals
- 2011
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In: Molecular Crystals and Liquid Crystals. - : Informa UK Limited. - 1563-5287 .- 1542-1406. ; 543, s. 3-47
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Journal article (peer-reviewed)abstract
- In cholesterics and chiral smectics there are linear electro-optic modes of considerable interest. As they are fast-switching and have a natural gray scale we will discuss their future potential, in general for active matrix addressing. In spite of differences in physical origin they are all dielectric effects (even in the smectic C* phase) and are generically related by symmetry. This leads to the common feature that there is only one time constant involved (not one oONo and another oOFFo as for effects related to dielectric anisotropy). Furthermore this time constant is independent on the applied electric field. We will also see that cholesterics and chiral smectics have more in common - but seldom pointed out - than just being chiral. In surface-stabilized tilted chiral smectics there are instead strongly non-linear effects one of which may achieve a more or less perfect memory (bistability). If, in addition, the liquid crystal is polymer-stabilized, the cell can be made monostable with a continuous gray scale. As the effects are very fast they were originally successfully developed for passive matrix addressing. Industrial interest in these modes (ferroelectric and antiferroelectric) declined, nevertheless, in the late 1990s. This was not because other types of liquid crystals had been discovered but rather because a cost-effective thin-film-transistor (TFT) technology had been developed. With a TFT matrix every liquid crystal pixel is addressed individually and needs neither to have memory nor to switch at higher speed than can be achieved with nematics. As, however, active-matrix addressing is now commonplace there is naturally a renewed interest in these modes, especially for the future transfer from displays with color filters to displays in which the full color is generated by modulating the red, green and blue LEDs in the time domain.
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| 8. |
- Lagerwall, Sven, 1934, et al.
(author)
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Liquid Crystals: Cholesteric, Guest-Host, and Polymer-Dispersed
- 2015
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In: Encyclopedia of Optical and Photonic Engineering Second Edition. - 9781351247177 ; , s. 1443-1450
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Book chapter (other academic/artistic)abstract
- If we abandon the mirror symmetry present in the ordinary nematics, a new world opens up for applications, for photonics as well as for displays. The basic new phenomenon that now appears is scattering. In practice, we enter this domain by using cholesterics, which is the chiral form of nematics. The nematogenic molecules might themselves be chiral, but we could also, in principle, obtain the cholesteric by adding chiral dopants to the material. If we add dichroic dyes or polymerizable materials to the liquid crystal, we can create highly interesting devices based on either absorption or scattering or both. Cholesteric and nematic devices based on scattering and dichroic effects will be discussed in this entry.
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| 9. |
- Lagerwall, Sven, 1934, et al.
(author)
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Liquid Crystals: Electro-Optical Modes and Applications
- 2015
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In: Encyclopedia of Optical and Photonic Engineering Second Edition. - 9781351247177 ; , s. 1451-1464
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Book chapter (other academic/artistic)abstract
- Ferroelectric liquid crystals are particular because they have vector properties: There is a local polarization P everywhere in the volume. This leads to an electrooptic effect that depends on the sign of the applied field E.For a polarization to appear, the structure has to be chiral, and the common representative for this kind ofmaterial is smectic C*, the chiral form of smectic C. Antiferroelectric liquid crystals also have a local polarization, but this polarization is already cancelled on the molecular level as the P vectors of adjacent smecticlayers are antiparallel to each other. While the smectic C* phase is not ferroelectric per se, it may, togetherwith surfaces, form a ferroelectric structure. This led to the first manufactured bistable liquid-crystal displays,demonstrating several attractive electrooptic properties like in-plane switching, broad viewing angle, andhigh speed. Several nematic devices have been elaborated in the last decade that feature one or several of theseproperties. Their principles will be explained after the presentation of the ferroelectric and antiferroelectricdevices. Finally, some nondisplay applications will be discussed.
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| 10. |
- Lagerwall, Sven, 1934, et al.
(author)
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Liquid Crystals: Non-Linear Optical Effects
- 2015
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In: Encyclopedia of Optical and Photonic Engineering Second EditionP. - 9781351247177 ; , s. 1465-1472
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Book chapter (other academic/artistic)abstract
- Today, inorganic crystals are the completely dominating materials for nonlinear optics (NLO) applications. The interest in organic materials for NLO has been constantly growing due to several factors. First of all, the polarization response to an externally applied field is of molecular electronic origin and, therefore, practically lossless and extremely rapid. Ultrafast, nonresonant nonlinearities in combination with relatively low value of dielectric permittivity can be found in organic single crystals, but these are expensive to grow. Polymer materials are cheap, but normally have very low polar order. Here, liquid crystals and liquid crystalline polymers have a particular interest due to their inherent order. While nematics show some peculiar effects which can be described as macroscopic, the polar smectics have a nonlinear response on the microscopic, electronic level which is fairly similar to what is found in crystals. We first briefly introduce the basic concepts and then discuss macroscopic and electronic effects in turn.
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