Monolithic fabrication of quasi phase-matched waveguides by femtosecond laser structuring the χ(2) nonlinearity

Abstract

We demonstrate second harmonic generation in quasi phase-matched waveguide structures fabricated by direct laser writing. Circular waveguides are inscribed in z-cut lithium niobate that provide well confined guiding of the fundamental and second harmonic wave. In contrast to classic schemes that employ periodically poled crystals, quasi phase-matching is realized by a laser-induced modulation of the nonlinearity inside the waveguide core. The proposed design allows monolithic integration of buried frequency conversion devices with tailored nonlinear response and excellent compatibility to on-chip optical elements. Second harmonic generation of 1064 nm radiation is demonstrated for different grating periods and associated matching temperatures. A maximum conversion efficiency of 5.72% is obtained for a 6 mm long, laser-induced quasi phase-matching grating.