Journal of Photochemistry and Photobiology A: Chemistry 145 (2001) 113–115 Magnetic-dipole nonlinearities in chiral materials Thierry Verbiest, Sonja Sioncke, André Persoons ∗ KU Leuven, Laboratory of Chemical and Biological Dynamics, University of Leuven, Celestijnenlaan 200 D, B-3001 Leuven, Belgium Received 17 April 2001; received in revised form 4 July 2001; accepted 9 July 2001 Abstract We propose a new measurement procedure to study the role of magnetic-dipole contributions to the second-order nonlinearity in chiral materials. The procedure is based on the measurement of the ellipticity of the second-harmonic light generated in the chiral material. A better insight in the magnetic-dipole nonlinearities of materials could eventually lead to the observation of new phenomena in the field of photochemistry and photophysics. © 2001 Published by Elsevier Science B.V. Keywords: Chirality; Second-harmonic generation 1. Introduction Chiral molecules show extremely interesting second-order nonlinear optical (NLO) properties. For example, the NLO properties of chiral molecules and polymers can be significantly enhanced by optimizing magnetic-dipole contributions to the nonlinearity, in addition to the usual electric-dipole contributions [1]. Furthermore, the symme- try properties of magnetic-dipole interactions are different from those of electric-dipole interactions. As a consequence, they can support electric-dipole forbidden second-order NLO processes in highly symmetric media [2]. Hence, the importance of magnetic interactions in chiral media could therefore provide a new approach to the development of second-order NLO materials. In general, a careful study of magnetic-dipole nonlinearities in chiral materials could lead to a variety of new optical phenomena and applica- tions, such as selective photodestruction of enantiomers through a NLO interaction, or probing the enantiomeric excess in surfaces and membranes. For this reason, measure- ment techniques that specifically probe magnetic contribu- tions to the nonlinearity are needed. Recently, evidence of magnetic-dipole contributions to the nonlinearity of chiral thin films has been obtained from second-harmonic genera- tion (SHG) experiments: a detailed study of the dependence of the s- and p-polarized SHG signals on the polarization of the fundamental beam revealed strong (i.e. comparable ∗ Corresponding author. Tel.: +32-1632-7197; fax: +32-1632-7982. E-mail address: andre.persoons@fys.kuleuven.ac.be (A. Persoons). to the electric-dipole contributions) magnetic-dipole contri- butions [3,4]. In this paper, we propose a new measurement procedure based on the detection of elliptically polarized second-harmonic light that can be used to study the role of magnetic contributions in chiral materials. 2. Theory and discussion All optical phenomena are governed by the Maxwell equations [5]: ∇× E =- 1 c ∂ B ∂t (1) ∇× H = 1 c ∂ D ∂t + 4π c J (2) ∇· D = 4π ρ (3) ∇· B = 0 (4) where J and ρ are the current and charge densities, E the electric field, D the electric displacement, H the magnetic field and B the magnetic induction. E and D, and H and B are connected through the constitutive relations D = E + 4π P - 4π ∇· Q (5) B = H + 4π M (6) where P, Q and M are the electric polarization, the elec- tric quadrupolarization and the magnetization, respectively. 1010-6030/01/$ – see front matter © 2001 Published by Elsevier Science B.V. PII:S1010-6030(01)00566-4