INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTER J. Phys.: Condens. Matter 18 (2006) 9415–9425 doi:10.1088/0953-8984/18/41/008 Investigations of the opto-dielectric effects in the vicinity of the smectic-A–smectic-C * A transition Geetha G Nair 1 , Gurumurthy Hegde 1 , S Krishna Prasad 1 and Y S Negi 2 1 Centre for Liquid Crystal Research, Jalahalli, Bangalore 560 013, India 2 Centre for Materials for Electronics Technology, Panchwati, Off Pashan Road, Pune 411 008, India Received 4 May 2006, in final form 23 August 2006 Published 29 September 2006 Online at stacks.iop.org/JPhysCM/18/9415 Abstract Dielectric investigations of the photoinduced effects in the vicinity of the smectic A (Sm-A)–antiferroelectric smectic C (Sm-C ∗ A ) transition of a guest– host system consisting of photoactive azobenzene-based guest molecules and non-photoactive host molecules have been carried out. The frequency- dependent dielectric data is analysed in terms of a molecular mode and a high- frequency mode. We observe that the dielectric parameters associated with the two modes show primary as well as secondary photoferroelectric effects, in agreement with the effects observed in our previous study (Nair et al 2006 Phys. Rev. E 73 011712, referred to as [1] in the present article) for polarization and response time. Here the primary photoelectric effect is associated with changes in the polar ordering and/or transverse molecular dipole moment at a constant reduced temperature and the secondary effect is due to the radiation- induced reduction in the transition temperature. The temporal variation of the relaxation parameters shows that the photoinduced isothermal transition as well the thermal back relaxation occurs on fast timescales. 1. Introduction Since the discovery of antiferroelectricity [2] in liquid crystals, a number of materials exhibiting the antiferroelectric Sm-C ∗ or Sm-C ∗ A phase have been reported. In the Sm-C ∗ A phase the molecules in the neighbouring layers are tilted from the smectic layer normal in almost opposite directions. Generally, the Sm-C ∗ phase intervenes between the Sm-A and Sm-C ∗ A phases and systems with a direct transition from the Sm-A to the Sm-C ∗ A phase are rare [3]. A particular aspect of the Sm-C ∗ A phase that has been well debated is its dielectric property, especially the relaxation modes it exhibits. The phenomenon of reversible shape transformation of chromophoric molecules, such as azobenzenes, driven by photo-induced isomerization has been extensively studied [4]. The principle behind the phenomenon is outlined in the following. Upon UV irradiation (around 0953-8984/06/419415+11$30.00 © 2006 IOP Publishing Ltd Printed in the UK 9415