The use of a multi-functional, chromophore containing, polymerisable compound for producing an optical element having non-linear optical properties (an NLO element) wherein the multi-functional chromophore is a compound capable of being polymerised into a cross-linked network and of being poled under the influence of an electric field whilst being polymerised into the cross-linked network, the compound having the generalised formula (D) (C) (A) (F) D, C, A and F representing covalently linked moieties where C represents at least one conjugated system of π-bonds,
A and D represent at least one respectively π-electron acceptor and π -electron donor groups linked at opposite ends of the conjugated system, and are preferably conjugated with the conjugated system of π-bonds (C) and where more than one moiety A, C or D is present they may be the same or different,
F represents at least two polymerisable functional groups, which may be the same or different, attached to the π-conjugated system C or the groups A and D via substituent groups R₁ and R₂, where R₁ and R₂ are selected from alkyl, cycloalkyl, aryl heteroalkyl, alkenyl, cycloalkenyl, and alkynyl including groups which have been substituted or contain heteroatom replacement, the groups F being linked to the compound so that after the compound has been polymerised to give a cross-linked network the interpolymerised functional groups prevent significant realignment of the chromophore groups when the optical element is subject to temperature changes during any further fabrication and in use.