2014 On a étudié les caractéristiques de seuil d'une instabilité dans la forme des domaines flexoélectriques apparaissant dans une fine couche de cristal liquide soumise à un champ électrique continu ; les échantillons sont des phases nématiques obtenues par mélanges de composés esters et azoxybenzènes. On a déterminé les variations de la tension seuil et de la périodicité des domaines en fonction de l'anisotropie diélectrique. On a montré que la distorsion maximum d'une couche homogène correspond à la partie centrale de cette couche. En comparant des données expérimentales avec la théorie de Bobylev et Pikin pour les domaines flexoélectriques à deux dimensions, on a calculé la différence de coefficients flexoélectriques e* = e11 2014 e33 pour les cristaux liquides étudiés.Abstract. 2014 The threshold characteristics of an instability in the form of longitudinal flexoelectric domains, arising in thin layers of liquid crystals exposed to a d.c. field were investigated for two nematic mixtures based on azoxy-and ester compounds. The experimental dependences of the threshold voltage and the domain period as functions of dielectric anisotropy were obtained. It was shown that maximum distortion of a homogeneously oriented layer corresponds to the central region of the layer. The difference of the flexo-electric moduli e* = e11 -e33 was calculated for the mixtures by comparing the experimental data with the theory for two-dimensional flexo-electric deformation developed by Bobylev and Pikin. LE JOURNAL DE PHYSIQUE TOME 39, AVRIL 1978, Classification Physics Abstracts 61.30

A wide aperture liquid crystal polarized light source (LC-PLS) is presented which converts unpolarized-into circular- and/or linearly polarized light with an efficiency better than 80%. Moreover, new operating modes for electro-optical field-effects on which liquid crystal displays (LCDs) are based are shown to increase the transmission of twisted nematic (TN)-LCDs by a factor of two. Besides, a 25% shift of the TN-LCD transmission-voltage characteristics towards lower voltages is achieved. One of the new, reflective LCD-operating modes is shown to avoid light absorption by thin film transistor structures in actively addressed LCD-projection systems. Within the bandpass region of the cholesteric liquid crystal filters the combination of LC-PLS with appropriate LCD-operating mode is shown to increase the transmission of LCD-projection systems three to five times compared with conventional LCD-projectors.

The temperature dependence of the surface polarization has been measured for both the planar and homeotropic orientation of a nematic liquid crystal at a solid substrate. A conventional liquid crystal 5CB, pure and doped with an azo-dye, was used in cells with controlled asymmetry for light absorption. The measurements have been made by a pyroelectric technique using short pulses of a YAG laser to create a temperature increment. The latter, in turn, was measured independently by a novel time-resolved "optical thermometer" technique monitoring temperature-dependent birefringence by a He-Ne laser beam. In accordance with the symmetry of the order parameter, the surface polarization has different sign for the two orientations, its magnitude ranges from −4 to +2 pC/m. The same technique has been used for the measurement of the flexoelectric polarization in hybrid cells. The sum of the flexoelectric coefficients is e 1 + e3 = −13 pC/m at 25 • C.

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