Photomechanically induced ferroelectricity in smectic liquid crystals

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Abstract
We report new photomechanical effects in the ferroelectric liquid crystal SCE13 doped with a photoisomerizing guest azo dye. Low concentrations of dye (∼5 per cent wt:wt) are shown to cause an isothermal, reversible disruption of smectic phases when the system is illuminated with low power density (∼ 1 mW cm−2) UV light. In the case of a sample initially in the S*c phase, this results in a fall in the magnitude of spontaneous electrical polarization (P s) and changes in electro-optic switching characteristics. If the sample is illuminated in the SA phase, the electroclinic switching decreases. In contrast to this, when systems containing higher concentrations of dye (≥ 10 per cent wt: wt) are UV illuminated in the SA phase, a reversible, isothermal transition to a biphasic S*c/isotropic state occurs. In this case, the P s is seen to rise from zero in the SA phase to a finite value(∼2 nC cm−2) in the biphasic mixture and hysteresis occurs in the electro-optic switching. When these higher dye concentration mixtures are held initially in the S*c phase and UV illuminated, a more complicated variation of P s occurs with the sample again undergoing a transition to a biphasic S*c/isotropic state. Possible mechanisms for the transition are discussed.