Nonlinear optical and piezoelectric behavior of liquid-crystalline elastomers

Liquid-crystalline elastomers have been investigated with respect to their nonlinear optical and piezoelectric behavior. A liquid-crystalline elastomer with NLO active nitrogroups bonded covalently to a siloxane network was chosen for the NLO experiments. The elastomer exhibits a SA phase and is macroscopically ordered. The elastomer is transparent for wavelengths 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 d3 3 equals 0.92 pm/V and d3 1 equals 0.06 pm/V for a poling field of 19 V/micrometers . Cholesteric elastomers are noncentrosymmetric due to their helicoidal structure. Above a threshold deformation, where the elastomer becomes uniformly aligned with respect to the helicoidal axis, the helicoidal pitch strongly changes with deformation and a piezoelectric voltage of up to 40 mV is observed.