Thermally stable nonlinear optical activity in a smectic-A liquid crystal

Abstract

THE key requirement for a material to exhibit nonlinear optical (NLO) activity is the presence of non-centrosymmetric (polar) order, an attribute that is usually restricted to certain crystalline classes and ferroelectric liquid crystals^1,2. NLO activity can also be obtained in some amorphous organic materials by applying an intense electric field (corona discharge) above the glass transition temperature, T_g, and subsequently quenching the field-induced polar orientation order^3,4. Such materials are attractive for NLO device applications, as they promise lower costs and easier processibility than their crystalline organic and inorganic counterparts⁵. But field-induced polar order is not stable, and the eventual return to equilibrium (apolar) order results in a deterioration of NLO activity, particularly at temperatures near T_g (refs 6, 7). Here we show that this thermally activated decay of polar order can be circumvented by using a liquid crystal in which both mesogens (molecules that induce a liquid-crystal phase) and NLO-active chromophores are appended to macromolecular siloxane rings. We find that a shear-aligned melt of these composite macromolecules gives rise to a material with a monodomain lamellar superstructure that retains bistable, field-induced polar order above T_g. We attribute the thermal stability of these materials to an energetically favoured polar packing arrangement of the constituent macro-molecules.