THERMALLY STABLE NONLINEAR-OPTICAL ACTIVITY IN A SMECTIC-A LIQUID-CRYSTAL

THE key requirement for a material to exhibit nonlinear optical (NLO) activity is the presence of non-centrosymmetric (polar) order, an attr ibute that is usually restricted to certain crystalline classes and fe rroelectric liquid crystals(1,2). NLO activity can also be obtained in some amorphous organic materials by applying an intense electric fiel d (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 pr omise lower costs and easier processibility than their crystalline org anic and inorganic counterparts(5). But field-induced polar order is n ot stable, and the eventual return to equilibrium (apolar) order resul ts in a deterioration of NLO activity, particularly at temperatures ne ar T-g (refs 6, 7). Here we show that this thermally activated decay o f polar order can be circumvented by using a liquid crystal in which b oth mesogens (molecules that induce a liquid-crystal phase) and NLO-ac tive chromophores are appended to macro-molecular siloxane rings. We f ind that a shear-aligned melt of these composite macromolecules gives rise to a material with a monodomain lamellar superstructure that reta ins bistable, field-induced polar order above T-g. We attribute the th ermal stability of these materials to an energetically favoured polar packing arrangement of the constituent macromolecules.