Optical characterization of periodically-poled KTiOPO4 W. H. Peeters and M. P. van Exter Huygens Laboratory, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands [email protected] Abstract: We demonstrate how the Maker fringes that are observable in spontaneous parametric down-conversion (SPDC) give a direct visualization of the poling quality of a periodically-poled crystal. Identical Maker fringes are observed in the optical spectrum of collinear SPDC and the temperature dependence of second harmonic generation. We analyze these Maker fringes via a unified treatment of the tuning curve in crystals with small and slowly-varying deformations of the poling structure. Our theoretical model, based on a Fourier analysis of the poling deformations, distinguishes between duty-cycle variations and variations of the poling phase. The analysis indicates that the poling phase is approximately fixed, while the duty-cycle typically varies between 36% and 64%. © 2008 Optical Society of America OCIS codes: (190.4410) Nonlinear optics, parametric processes; (130.2260) Ferroelectrics; (190.4400) Nonlinear optics, materials; (270.0270) Quantum Optics References and links 1. M. M. Fejer, G. A. Magel, D. H. Jundt, and R. L. Byer, “Quasi-phase-matched second harmonic generation: tuning and tolerances,” IEEE J. Quantum Electron. 28, 2631 (1992). 2. M. Hou´e and P. D. Townsend, “An introduction to methods of periodic poling for second-harmonic generation,” J. Phys. D: Appl. Phys. 28, 1747–1763 (1995). 3. C. Canalias, V.Pasiskevicius, and F. Laurell, “Periodic Poling of KTiOPO4: From Micrometer to Sub-Micrometer Domain Gratings,” Ferroelectrics 340, 27–47 (2006). 4. F. Laurell, M. G. Roelofs, W. Bindloss, H. Hsiung, A. Suna, and J. D. Bierlein, “Detection of ferroelectric domain reversal in KTiOPO4 waveguides,” J. Appl. 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Watanabe, “First-order quasi-phase matched LiNbO3 waveguide peri- odically poled by applying an external field for efficient blue second-harmonic generation,” Appl. Phys. Lett. 62, 435 (1993). 24. G. Rosenman, K. Garb, A. Skliar, M. Oron, D. Eger, and M. Katz, “Domain broadening in quasi-phase-matched nonlinear optical devices,” Appl. Phys. Lett. 73, 865 (1998). 25. G. Rosenman, A. Skliar, D. Enger, M. Oron, and M. Katz, “Low temperature periodic electrical poling of flux- grown KTiOPO4 and isomorphic crystals,” Appl. Phys. Lett. 73, 3650 (1998). 1. Introduction Phase matching is required for an efficient operation of second-order nonlinear optical processes such as second harmonic generation (SHG) and spontaneous parametric down con- version (SPDC). In uniform crystals, phase matching is achieved by utilizing the modal dis- persion resulting from the crystal’s birefringence. Another method called quasi phase matching (QPM) relies on the fabrication of a ferroelectric domain structure with periodically-inverting polarization. QPM allows for a larger choice of optical frequencies and allows access to more, and potentially larger, elements of the nonlinear χ (2) tensor. An extensive discussion on the tuning and tolerances of QPM can be found in an article of Fejer et al. [1]. Different poling techniques are discussed in Refs. [2, 3]. The quality of the fabricated poling structure is generally characterized via imaging of the domain boundaries at the crystal surface. Important techniques comprise surface-charge se- lective etching [4], electrostatic force microscopy [5], secondary-electron microscopy [6], and piezoresponse-assisted atomic force microscopy [7]. Another way to characterize the poling quality is by studying the phase-matching conditions of a second-order nonlinear process like SHG or SPDC. Common techniques in this category use the wavelength dependence of the con- version efficiency of SHG [8, 9]. A three-dimensional characterization of the poling duty-cycle has been demonstrated using ultrashort pulses and SGH [10]. A third technique characterizes the crystal structure by mapping the angle-frequency distribution of radiation that is generated via SPDC [11]. In this paper we present high quality measurements and a detailed analysis of the ring-shaped angular SPDC pattern, generated in a periodically-poled KTiOPO 4 crystal (PPKTP). Our analy- sis is very different from earlier results presented by Kitaeva et al. [11], as these authors analyze the large scale angle-frequency distribution of the SPDC pattern, whereas we concentrate on the fine structure close to phase matching. We also demonstrate that the same fringe pattern is present in the optical spectrum in collinear SPDC and the temperature dependence of SHG. We denote the observed patterns as Maker fringes, as the typical sinc-type intensity depen- dence was first discussed by Maker et al.[12]. We perform a meticulous comparison between #93533 - $15.00 USD Received 6 Mar 2008; revised 30 Apr 2008; accepted 4 May 2008; published 6 May 2008 (C) 2008 OSA 12 May 2008 / Vol. 16, No. 10 / OPTICS EXPRESS 7345 the measurements and our theoretical predictions of the fringe shape, hereby using up to date knowledge of KTP’s material properties. Finally, we analyze the observed fringe patterns in terms of small and slowly-varying deformations of the periodic poling structure. The paper is organized as follows. Section 2 gives a theoretical description of the tempera- ture, angle, and wavelength dependence of the phase-matching condition in periodically-poled crystals. Section 3 describes the three experimental setups that are used for (A) the measure- ment of the angular SPDC pattern, (B) the spectrum of SPDC light, and (C) the temperature dependent conversion efficiency in SHG. Sections 4.1, 4.2, and 4.3 discuss the Maker fringes that are experimentally obtained via the three above-mentioned methods. In Sec. 5, we analyze the observed deviations from the ideal sinc-shaped Maker fringes in terms of small and slowly- varying deformations of the poling structure. We hereby use a theoretical treatment, given in Sec. 5.1, that is based on a Fourier analysis of small and slowly-varying deformations of the poling structure. 2. Phase matching in a periodically-poled crystal We consider the nonlinear processes of second harmonic generation (SHG) and spontaneous parametric down conversion (SPDC) in a periodically-poled crystal.