DOI: 10.1007/s00339-006-3750-6 Appl. Phys. A 86, 165–170 (2007) Invited paper Materials Science & Processing Applied Physics A j. burghoff 1, ✉ h. hartung 1 s. nolte 1 a. t ¨ unnermann 1,2 Structural properties of femtosecond laser-induced modifications in LiNbO 3 1 Institut für Angewandte Physik, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, 07743 Jena, Germany 2 Fraunhofer-Institut für Optik und Feinmechanik, Albert-Einstein-Strasse 7, 07745 Jena, Germany Received: 5 Oktober 2006/Accepted: 10 Oktober 2006 Published online: 17 November 2006 • © Springer-Verlag 2006 ABSTRACT Structural modifications induced by femtosecond laser pulses in LiNbO 3 were studied. The influence of the pro- cessing and focusing parameters was investigated. Two different types of modifications could be identified. High laser fluences cause a refractive index decrease, material damage and stresses in the surrounding crystalline lattice. At low laser fluences, an extraordinary index increase was observed that allows for opti- cal waveguiding. This kind of modification is thermally unstable and correlates to a weak distortion of the lattice. The elec- trooptic coefficient measured in a waveguide was found to be substantially reduced. The mechanisms underlying the struc- tural modifications are discussed. PACS 61.80.Ba; 77.84.Dy; 42.82.Et; 42.65.Re 1 Introduction Lithium niobate (LiNbO 3 ) is one of the most im- portant materials in waveguide fabrication for optical infor- mation and communication technology because of the wide range of transparency and large electrooptic coefficients. Con- ventionally the waveguide fabrication is based on ion diffu- sion or proton exchange. Recently, it has been demonstrated that LiNbO 3 can be structured by femtosecond laser pulses. In this field, a variety of phenomena has been observed that includes filamentation and voids [1, 2]. The fabrication of op- tical waveguides, first demonstrated by Gui et al., has received particular attention because of its potential for nonlinear in- tegrated optical applications [3]. Specifically, Thomson et al. identified two distinct waveguide types that appeared with different processing parameters [4]. One type of modifica- tion was observed to decay over time. Similar results were obtained by our group [5]. Furthermore, we demonstrated ef- ficient second harmonic generation in a fs laser-written wave- guide [6]. For the ultrashort pulse laser structuring, a strong dependence on pulse duration was recently observed [7]. The origin of the reported phenomena is still under discus- sion. The influence of stress in the crystalline lattice, move- ment of ions and the photorefractive effect was proposed but ✉ Fax: +493641657680, E-mail: burghoff@iap.uni-jena.de no detailed investigation exists to date [4, 8, 9]. In this article, we discuss the influence of the laser parameters and present results of the thermal, nonlinear and structural properties of the modifications. We thus draw the first comprehensive pic- ture of fs laser-induced changes in LiNbO 3 to the best of our knowledge. 2 Experimental For the laser structuring, a Spectra-Physics Spitfire laser system was used which operated at a repetition rate of 1 kHz. The pulses had a temporal width of 40 fs at a wave- length of 800 nm. A half-wave plate and a polarizer were used to allow for a continuous variation of the output power. The laser was focused into the samples with a 40×, NA = 0.65 microscope objective and was incident on the x - or z -face for x - or z -cut LiNbO 3 , respectively. Before the objective, the laser beam radius was 2.4 mm at 1/e 2 intensity. The sam- ple was moved with an air-bearing xyz positioning stage to fabricate straight lines of modifications. The writing laser po- larization was perpendicular to the lines. For all structures shown in this article, a translation velocity of 100 μ ms −1 was used. The LiNbO 3 samples were either x - or z -cut crystals from Crystal Technology. After laser processing, the end facets were polished and inspected with a transmission microscope. Optical mode characterization was performed by fiber-coup- ling of laser light at 633 nm wavelength into the structures and imaging the end facet. For measuring the refractive index changes, thin slices of the samples were polished to a thick- ness of 40 μ m and mapped with a shearing interferometric microscope at a wavelength of 550 nm. 3 Results Using the experimental setup described above, structural modifications were inscribed in an x - and a z -cut LiNbO 3 sample. The typical results are summarized in Fig. 1 where the refractive index distributions of the cross sections of two waveguides in different crystal cuts are shown. For z -cut LiNbO 3 (Fig. 1a and b), laser pulses with a duration of 420 fs and an energy of 1 μ J were used. The laser was incident from the bottom and focused at a depth of 130 μ m. In 1a and b, the extraordinary and ordinary index changes, ∆n e and ∆n o , are shown. Figure 1c and d show index profiles for ∆n e and ∆n o