2014 Des transitions de l'orientation moléculaire induites par un champ électrique ou magnétique ont été mises en évidence dans différents composes smectiques A. Deux régimes de distorsion ont été observés : 2014 près de la transition smectique A ~ nématique, une distorsion périodique, de vecteur d'onde perpendiculaire au plan (n, n0) ; 2014 à plus basse température, deux réseaux périodiques de défauts, perpendiculaires au plan (n, n0) permettent le pliage des couches. Les dépendances du champ seuil en fonction de l'épaisseur de l'échantillon et de la température, et la variation de l'angle de basculement des molécules en fonction du champ appliqué sont discutées pour le second régime de distorsion. Abstract. 2014 Observations of electric and magnetic field induced transitions of the molecular orientation in smectics A are presented. Two regimes of distortion can be distinguished : 2014 close to the nematic/smectic A transition, a periodic distortion appears with a wave vector normal to the plane (n, n0). 2014 for lower temperatures, two periodic arrays of defects, normal to the plane (n, n0), allow for the folding of the smectic layers. The dependence of threshold field versus sample thickness and temperature, and the variation of the tilt angle of the molecules versus applied field are discussed for the second regime of distortion.
R6sumB.-Des observations d'instabilites 6lectrohydrodynamiques dans des materiaux pour lesquels le paramktre de Helfrich 5 2 depend trks fortement de la temperature sont prksentees et discutees. Quand 5 2 tend vers un par valeurs supkrieures la tension seuil du regime de conduction diverge, ainsi que le champ seuil du rkgime dielectrique. Les deux types d'instabilite disparaissent pour 5 2 = 1. Pour 5 2 < 1 il existe encore des instabilites qui sont interpretdes comme provenant de l'amplification de fluctuations de torsion du directeur. Cette amplification peut se produire si le coefficient de friction a s est positif, et si le rapport des conductivitks electriques parallele et perpendiculaire aux molecules all/q est plus petit que un.
Laboratoires d'Electronique et de Physique App1iqui.e 3 , avenue Descartes 94450 Limeil Br6vannes (France) ABSTMCTThe scanning limitations of rms responding liquid crystal devices due to the lack of sharpness of the transmittance characteristic can be overcome in the case of the twisted nematic configuration. Indeed, it is possible to obtain a linear output polarization from an electrically excited cell, and then to decrease the transmittance level of the non selected dots without altering the optical efficiency for the selected dots by rotating the output polarizer perpendicularlyto the transmitted vibration. A 64 x 64 element matrix display using a biphenylcompound has been constructed and operated in the transmissive mode. The contrast ratio obtained with a duty factor equal to 1/128 corresponding to a 128 line display is 20:l. The address and the information pulses respectively applied to the lines and the columns are V1 = 12.4 V and v2 = f 1.1 v.
The scanning limitations of a liquid-crystal matrix display due to the lack of sharpness of the transmission characteristic can be extended in the case of the twisted nematic configuration. Indeed, the output polarization from an electrically excited cell, generally elliptic, can be made linear when the linear input polarization is rotated from the molecular alignment at the entrance of the cell. Thus it is possible to decrease the transmission of the nonselected dots while keeping a suitable transmission for the selected dots. A theoretical treatment of the output polarization versus the input polarization and the π/2-twisted nematic parameter u=2dΔn/λ shows the existence of linear output polarizations. Experiments made on a 64×64-dot matrix display agree with the predictions. With a duty cycle equal to 1/128, the contrast ratio is 20 : 1, while the response time is 100 ms at 20 °C; the address and the data pulses, applied to the rows and to the columns, respectively, are V1=12.4 V and V2=±1.1 V.
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