Second harmonic generation by quasi-phase matching in a lithium niobate thin film

Lithium niobate thin film becomes a promising material platform for integrated nonlinear optics. Quasi-phase matching is an important way to realize optical frequency conversion. Due to the strong dispersion of the high-refractive-index-contrast lithium niobate thin film waveguides, a small poling period (several micrometers) is needed, which becomes a challenge for the poling process. In this paper, a periodic reversed domain with a period of 3.8 ??m was realized by poling with an external electric field in the x-cut lithium niobate thin film. The reversed domains were characterized by piezoresponse force microscopy and the confocal Raman spectroscopy. The second harmonic generation was realized in the ridge waveguides, and the normalized conversion efficiency was 684%W???1cm???2. The waveguide loss, the irregularity of the reversed domain, and the inhomogeneities effective index along the waveguide were analyzed to be the main sources of the conversion efficiency decrease. The simulations showed that the phase matching was very sensitive to waveguide geometry. This work provided useful information for studying second harmonic generation in the lithium niobate on insulator.