Abstract-Periodic inversion of ferroelectric domains is realized in a lithium niobate crystal by focused femtosecond near-infrared laser beam. One and two-dimensional domain patterns are fabricated. Quasiphase matched frequency doubling of 815nm light is demonstrated in a channel waveguide with an inscribed periodic domain pattern with conversion efficiency as high as 17.45%.Lithium niobate (LiNbO 3 ) waveguides play a significant role in integrating many optical devices due to excellent electro-optic, acousto-optic, and nonlinear properties of this important material. In nonlinear interactions such as frequency conversion the phases of interacting waves can be synchronized in LiNbO 3 waveguide through the process of quasi-phase matching (QPM) [1], which requires a periodic ferroelectric domain inversion to change the sign of the second-order nonlinear susceptibility at every coherence length. A common method used for ferroelectric domain engineering is electric field poling [2], where a spatially modulated electric field is applied along the polar axis of the crystal. This method involves a photolithographic process to fabricate periodic electrodes and application of sequence of high voltage pulses to induce ferroelectric domain flipping. Apart from its complexity, this technique suffers from the restrictions imposed by the crystallographic orientation of the crystal. Consequently, while the Z-cut crystals are easily poled, other orientations require sophisticated design of electrodes and involve etching of the crystal [3].There have been considerable efforts in developing alternative methods of ferroelectric domain reversal to overcome the drawbacks of traditional electric field poling. In particular, the optical poling uses intense laser * E-mail: yan.sheng@anu.edu.au irradiation to either assist or directly induce domain inversion [4]. This approach becomes particularly interesting because of its unique advantages. For example, it overcomes the restriction that the electric field must be applied along the polar axis of the crystal. In addition, the light field can be manipulated more accurately with a resolution up to the diffraction limit. Therefore, it enables one to fabricate fine ferroelectric domains with better defined details than those produced by electric-field poling alone. The UV radiations are usually used in direct laser poling as their strong absorption by ferroelectric can produce a very high local temperature gradient for domain inversion. However, such a strong absorption of UV light restricts inverted domains into a shallow surface layer (few hundred nanometers). This severely limits the application of such an optically created domain pattern. So far no report has been made on the performance of these structures as a QPM frequency convertor.In this work we demonstrate that the inversion of ferroelectric domains can be efficiently realized by using a tightly focused femtosecond infrared laser inside the LiNbO3 crystal without application of any electric field. As the LiNbO3 is transparent in...