Electric-Field Poling of Polymers

Amorphous polymers with strong second-order optical nonlinearities [1, 2] that are potentially useful for electro-optical or all-optical device applications [4] contain molecular chromophore dipoles [3] as guest molecules, as side groups, or as main-chain segments. In order to break the inherent centro-symmetry of the initially isotropic dipole orientation in these materials and to render them nonlinear optically active (and also piezo- and pyro-electric), preferential dipole alignment by means of poling is required [5]. Poled nonlinear optical (NLO) polymers are molecular dipole electrets [6, 7] so that the full range of techniques for the preparation and investigation of polar electrets may be utilized on them. In particular, the poling of polar polymers such as polyethylene terephthalate (PETP, Mylar) or polymethyl methacrylate (PMMA, Plexiglass) is not at all different from the poling of NLO polymers. Strongly pyro- and piezoelectric polymer electrets such as polyvinylidene fluoride (PVDF) and its copolymers or the odd polyamides (PA or Nylon 5, 7, 9, and 11), on the other hand, are ferroelectric and do therefore not require heating during poling and subsequent cooling under the electric field; however, their ferroelectric polarization switches only at field strengths higher than the respective coercive field.