DOI: 10.1007/s00340-006-2156-7 Appl. Phys. B 83, 273–278 (2006) Lasers and Optics Applied Physics B k.a. kuznetsov 1 h.c. guo 2 gs.kh. kitaeva 1, ✉ a.a. ezhov 1 d.a. muzychenko 1 a.n. penin 1 s.h. tang 2 Characterization of periodically poled LiTaO 3 crystals by means of spontaneous parametric down-conversion 1 Department of Physics, M.V. Lomonosov Moscow State University, 119992 Moscow, Russia 2 Department of Physics, National University of Singapore, Singapore 117542 Received: 20 November 2005/ Revised version: 31 January 2006 Published online: 29 March 2006 • © Springer-Verlag 2006 ABSTRACT We describe the observation of quasi-phase- matched spontaneous parametric down-conversion in eee- geometry in periodically poled LiTaO 3 crystals. For scattered light, the two-dimensional angular-frequency intensity distri- bution was studied. Several detuning curves were recorded, corresponding to high orders m =−2, −3, −4, −5 of quasi- phase matching. The measured periods of domain gratings agree with the data obtained by atomic-force microscopy for the etched crystal surfaces. The presence of both odd and even orders indicates that the lengths of positive and negative do- mains are unequal. To determine the mean duty cycle for regular domain gratings we propose comparing of the intensities of spontaneous parametric down-conversion in different orders of quasi-phase-matching. PACS 42.65.Lm; 42.70.Mp; 81.70.Fy; 78.67.Pt; 42.65.Yj 1 Introduction Periodically poled (PP) ferroelectric crystals are widely used as frequency converters for electromagnetic ra- diation. Characterization of the quality of PP crystals is an important task directly connected with the problem of their optimal fabrication. For manufacturing PP crystal elements, as-grown and post-growth techniques are used [1]. For as-grown PP crys- tals, fabricated directly under the Czochralsky growth proced- ure, the important working parameter to be tested is the degree of periodicity. The grown samples differ strongly in this pa- rameter and achieving high-quality periodicity is one of the main problems in fabrication and engineering of such crys- tals. On the contrary, for PP crystals, which are manufactured by post-growth techniques, periods of non-linear gratings are usually constant, and only the stability of the required duty- cycle is in question. The necessity to control this parameter is a specific feature of post-growth PP crystals since the duty cycle can be different for the z -surface and for the bulk of the crystal. There are a lot of topographic and linear-optical methods to diagnose these structures: chemical etching and subsequ- ent topographic measurements by atomic-force microscopy ✉ Fax: +7-495-939-11-04, E-mail: kit@qopt.phys.msu.su (AFM) [2], electrostatic force microscopy (EFM) [3], op- tical microscopy in polarizing modes [4], X-ray diffraction imaging [5], etc. But all these methods have some limitations and inconvenience when characterizing the nonlinear-optical properties of the samples. For example, it is often difficult to interpret the topography obtained after chemical etching, and to determine the size of domains according to this topography. Another problem is that the microscopic methods character- ize small parts of the crystal surfaces only, whereas one needs to obtain the properties of the whole sample. Linear optical methods can characterize the working volume as a whole, but peculiar properties of the non-linear coefficient are not meas- ured directly. Non-linear optical methods, such as second harmonic gen- eration (SHG) [6–8], and terahertz technique [9], are free of these limitations. In a number of recent experimental and theoretical works, the distribution of the nonlinear suscep- tibility was diagnosed via (SPDC) [10–13]. This method is based on the analysis of the angular or frequency lineshapes of the scattered radiation. Until now this method was ap- plied to as-grown periodically poled crystals of lithium nio- bate (PPLN), doped PPLN and PP barium sodium niobate (Ba 2 NaNb 5 O 15 ), which were manufactured under Czochral- ski growth procedure. In this work we study periodically poled lithium tantalate crystals (PPLT), prepared by the post-growth electric poling technique. Single-domain lithium tantalate crystals are usu- ally unsuitable for frequency conversion in the IR-range due to their small birefringence. Making periodical domain grat- ings permits use of the excellent non-linear optical properties of this crystal in the quasi-phase matching regime. The SPDC method has been never applied before for testing PPLT, as well as for testing any other PP crystal elements manufactured by a post-growth technique. For the first time we study here: 1. peculiarities of quasi-phase matched SPDC spectra in lithium tantalate 2. diagnostic abilities of the SPDC-based characterization method applied to domain gratings of this type 2 Experimental 2.1 Samples Two samples were cut from an as-grown boule. The samples had the following dimensions: the length along the crystallographic X -axis was approximately 10 mm, the