Quasi-phase matching via femtosecond laser-induced domain inversion in lithium niobate waveguides.

We demonstrate an all-optical fabrication method of quasi-phase matching structures in lithium niobate (LiNbO₃) waveguides using a tightly focused femtosecond near-infrared laser beam (wavelength of 800 nm). In contrast to other all-optical schemes that utilize a periodic lowering of the nonlinear coefficient χ⁽²⁾ by material modification, here the illumination of femtosecond pulses directly reverses the sign of χ⁽²⁾ through the process of ferroelectric domain inversion. The resulting quasi-phase matching structures, therefore, lead to more efficient nonlinear interactions. As an experimental demonstration, we fabricate a structure with the period of 2.74 μm to frequency double 815 nm light. A maximum conversion efficiency of 17.45% is obtained for a 10 mm long waveguide.