The intrinsic photorefractive effect in the smectic C* phase of a chiral azobenzene

The ferroelectric properties in the liquid-crystalline smectic C* phase of the chiral mesogenic azobenzene (S)-4-(4-decyloxy-phenylazo)benzoic acid 4-(1-methylhexyloxy)-3-nitro-phenyl ester (‘W470’) were studied and found to be strongly dependent on the intensity of incident light. The light-induced trans to cis isomerization of the azobenzene chromophore which is an intrinsic constituent of this SmC* mesogen leads to a pronounced photoferroelectric effect since the ferroelectric SmC* ordering is considerably disturbed by the presence of bent-shaped cis-isomer molecules: At increasing light intensity a strong decrease of both SmC* order parameters, spontaneous electric polarization Ps and director tilt angle θ, is observed. It is further seen that even the nature of the ferroelectric transition is affected by the photoisomerization which leads to a crossover from the nearly tricritrical transition observed in the absence of light to a typical second-order transition. This result is interpreted theoretically taking into account a coupling between the director tilt and the biaxial order parameter which is strongly affected by the light-induced generation of the bent-shaped cis-isomers. The dynamics of the intrinsic photoferroelectric effect in W470 was studied by time-resolved measurements of spontaneous polarisation and director tilt