We demonstrate that scanning second-harmonic microscopy with a mode-locked laser can be used as a nondestructive technique to image ferroelectric domain structures with micron resolution in both lateral and axial directions. This method is expected to have significant impact particularly on the further development of nonlinear optical bulk and waveguide devices with periodically poled ferroelectric crystals. PACS: 07.60.Pb; 42.30; 77.80.Dj Ferroelectrics such as LiNbO3, KNbO3, or BaTiO3 play an important role in nonlinear optics and electro-optics because of their large second-order nonlinearities. A ferroelectric crystal may be composed of domains with different polar orientations.LiNbO3, for example, whose crystal symmetry is 3m, can show two different domain orientations. They differ in the signs of all non-vanishing components of the nonlinear optical tensor χ(2). Hence, for nonlinear optical experiments and applications, it is crucial to control the domain structure of the crystals. Devices for holographic data storage and optical parallel processing [1, 2] or for nonlinear frequency conversion with birefringent phase-matching usually require crystals with a single domain, whereas in recent years a lot of interest has been attracted by crystals fabricated with a periodic domain structure, mainly for quasi-phase-matched frequency conversion [3, 4]. The technique of quasi-phasematching (QPM) opens a number of very attractive possibilities in nonlinear optics: virtually any nonlinear interaction of waves within the transparency region of the crystal can be noncritically phase-matched at room temperature, and in addition QPM devices can be significantly less sensitive to the photorefractive effect [5], which is particularly important for the generation of visible light. The crucial prerequisite ∗ To whom correspondence should be addressed ∗∗ Present address:Institute of Quantum Electronics, Swiss Federal Institute of Technology, ETH-Hönggerberg, HPT E 17, CH-8093 Zürich, Switzerland, Phone: +41-1 /633-6825, Fax: +41-1 /633-1059, E-mail: paschott@iqe.phys.ethz.ch for using QPM is the ability to fabricate crystals with periodic domain structures of good quality, typically with periods in the range from3μm to 30μm. Various techniques have been used for this purpose, most successfully the technique of electric-field poling [6–8]. In any case, the further development of such methods requires techniques for the characterization of the obtained domain structures. First we briefly review the currently available techniques and later describe our new technique which is non-destructive, reproducible, quick and versatile, and provides images of domain structures with micron resolution in both lateral and axial directions. 1 Alternative techniques for the observation of ferroelectric domains Polarization microscopy is well suitable for the observation of the anisotropic grains in polycrystalline matter. The linear optical technique, however, does not allow us to distinguish between the antiparallel domains in ferroelectrics. Domain boundaries can cause visible structures due to stress birefringence or due to the electro-optic effect originating from charges on the boundaries. The unambiguous identification of domain structures, however, seems not to be possible with this method. Selective etching can be used to transform the domain structure on the surface of a crystal into a topographic structure [9, 10] that can be observed with a usual microscope. This method, however, is destructive, provides information about the domain structure on a polished surface but not from the interior of the specimen, and it works only on certain crystal faces (for example, on the Y and Z face, but not on the X face of aLiNbO3 crystal). Another method applied to crystal surfaces is the deposition of electrically charged powder particles [11]. This technique is non-destructive but suffers from a poor accuracy of the obtained images. Other nondestructive techniques such as atomic force microscopy [12– 14] and scanning secondary-electron microscopy [15, 16] have recently been applied to ferroelectric crystal surfaces. However, they also do not provide information on the interior domain structure. X-ray topography studies can reveal strain,