J. Phys. II France
Volume 1, Numéro 5, May 1991
Page(s) 559 - 570
DOI: 10.1051/jp2:1991189
J. Phys. II France 1 (1991) 559-570

Second harmonic generation in a liquid-crystalline elastomer

Harald Hirschmann, Dolors Velasco, Helmut Reinecke and Heino Finkelmann

Institut für Makromolekulare Chemie der Universität Freiburg, Germany

(Received 29 October 1990, accepted in final form 1 February 1991)

A liquid-crystalline elastomer with NLO active nitrogroups bonded covalently to a siloxane network has been investigated. By means of polarizing microscopy, X-ray diffraction and UV-VIS spectroscopy it was found that the elastomer exhibits a S $_{\rm A}$ phase and is macroscopically ordered. The elastomer is transparent for wavelenghts above 450 nm and the anisotropy of refractive index at room temperature is about 0.1. Applying an external electric poling field breaks centrosymmetry and second harmonic generation similar to crystals can be observed. The poling process is single exponential with an activation energy of 190 kJ/mol and is identified as a $\delta$-process. In contrast to low molar mass liquid crystals no director reorientation is observed by the laser field strength. From Maker fringe experiments in the liquid crystalline phase at room temperature the second-order susceptibilities are: $\chi_{zzz} = 0.92$ pm/V and $\chi_{zxx} = 0.06$ pm/V for a poling field of 19 V/ ${\rm\mu}$m.

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