Res. Chem. Intermed., Vol. 34, No. 2–3, pp. 217–228 (2008)  Koninklijke Brill NV, Leiden, 2008. Also available online - www.brill.nl/rci State of the art of the sphere method, a unique characterization technique for non-linear crystals PATRICIA SEGONDS 1,∗ , BENOIT BOULANGER 1 , BERTRAND MENAERT 2 and JULIEN ZACCARO 2 1 Institut Néel, Centre National de la Recherche Scientifique & Université Joseph Fonrier, BP 166, 38402 Grenoble Cedex 9, France 2 Laboratoire de Cristallographie, Centre National de la Recherche Scientifique, 25 Avenue des Martyrs, BP 166, 38042 Grenoble Cedex 09, France Received 12 November 2006; accepted 20 January 2007 Abstract—We report the sphere method as a unique characterization technique for the complete study of non-linear optical properties for frequency conversion in new materials belonging to the uniaxial or biaxial optical class. It relies on the use of a single crystal with millimetre dimensions cut as a sphere, combined with a tuneable laser source. With the sphere method we perform direct measurements of phase-matching angles and associated conversion efficiencies for second harmonic, sum- and difference-frequency generation. Furthermore, we follow the orientation of the dielectric frame as a function of the wavelength for monoclinic and triclinic crystals. It also allows the determination of the magnitude of the principal refractive indices in biaxial crystals based on the study of the double refraction affect at the exit of a sphere. By combining the analysis of all these data simultaneously, we determine Sellmeier equations reliable over the whole transparency domain and we are able to get the non-zero elements of the second-order susceptibility tensor of uniaxial or biaxial crystals. Finally, the sphere method is completely self-sufficient for the study of biaxial crystals. Keywords: Metrology; non-linear optical properties; frequency conversion; birefringent crystals. INTRODUCTION When dealing with non-linear crystals, the knowledge of the dispersion equations of the magnitudes of the principal indices, in the whole transparency range of the studied material, is of prime importance. Indeed, the phase-matching directions for sum- or difference-frequency conversion depend on the ratios between the principal refractive indices [1]. Crystals belonging to the uniaxial and biaxial optical classes are concerned. For example, biaxial crystals have three principal ∗ To whom correspondence should be addressed. E-mail: psegonds@ujf-grenoble.fr