ELSEVIER Synthetic Metals 81 (1996) 117-120 _ Udl'fiITlUl Tflll Nonlinear optical properties of chiral polymers Martti Kauranen, Thierry Verbiest, Jeffery J. Maid, Andr6 Persoons Laboratory of Chemical and Biological Dynamics and the Center for Research on Molecular Electronics and Photonics, University of Leuven, Celestijnenlaan 200 D, B-3001 Heverlee, Belgium Abstract The efficiency of second-harmonic generation from chiral surfaces is different for fundamental beams that are left- and right-hand circularly polarized or that have two appropriately chosen linear polarizations. These effects can occur within the electric-dipole approximation. However, for the case of thin films of chiral polymers, we provide evidence of magnetic-dipole contributions to this form of nonlinear optical activity. Keywords: Chiralpolymers;Optical properties 1. Introduction Chiral materials have low symmetry and possess no mirror planes. For example, chiral molecules occur in two different forms (enantiomers) that are mirror images of each other [ 1 ]. Nonlinear optical investigations of chiral molecules axe important for two complementary reasons. First, nonlinear techniques can provide new probes of chirality for applica- tions in biomedical and pharmaceutical sciences [2]. Sec- ondly, chiral molecules are interesting as nonlinear optical materials [ 3 ]. In the electric-dipole approximation, second- order nonlinear processes are allowed only in noncentrosym- metric materials [4]. Chiral molecules and macroscopic assemblies of a single enantiomer of a chiral molecule are necessarily noncentrosymmetric [5,6]. On the other hand, magnetic-dipole transitions of chiral molecules are relatively strong. Symmetry properties of magnetic-dipole transitions [ 7 ] are different from those of electric-dipole transitions and allow second-order nonlinearities also in centrosymmetric systems [8]. The linear response of chiral molecules is different to left- and right-hand circularly polarized light [ 1 ]. In isotropic solutions, such optical activity arises from the magnetic con- tributions to the linear optical properties of chiral media. In nonlinear optics, circular-difference effects have been observed in second-harmonic generation from various types of chiral surfaces [9-13]. This form of nonlinear optical activity has been explained within the electric-dipole approx- imation [ 14,15 ] and by including the magnetic contributions to the surface nonlinearity [ 15]. Recently, it has also been shown that appropriately chosen linear input polarizations can be used as probes of surface chirality [ 16]. 0379-6779/96/$15.00 © 1996 ElsevierScience S.A. All rights reserved PHS0379-6779(96)03762-9 In this paper, we summarize our theory of second-har- monic generation from chiral surfaces that includes the elec- tric and magnetic contributions to the nonlinearity [ 15]. We also present results on second-harmonic generation from thin films of chiral polymers. Our experimental technique allows us to assess the relative importance of the electric and mag- netic contributions to the nonlinearity [ 17 ]. We also compare linear and circular input polarizations as probes of surface chirality. 2. Theory We consider the geometry of Fig. 1 in which a laser beam at frequency oJ is applied on a chiral surface. Second-har- monic fields at frequency 2~o are generated in the reflected and transmitted directions [ 18 ]. We assume that the surface is isotropic in the plane of the surface and that the media on both sides of the surface have no nonlinear optical response. We include the contributions of both electric and magnetic dipole transitions [ 15 ] to the surface nonlinearity. The mag- z l ~IcP ~¢ reflection / / chiral | k'~/ surface 2~ transmission Fig. 1. Geometry of second-harmonic generation froma chiral surface.The surface layer is in the x-y plane. The incident beam at frequency o9gives rise to second-harmonicgeneration in the reflected and transmitted direc- tions. The angleof incidence is 0.