Spatial optical solitons in nonlinearly coupled lithium niobate waveguides

We numerically investigate a novel type of quadratic spatial solitons in the system of surface and buried planar waveguides obtained via direct- and reverse-proton exchange in z-cut lithium niobate, respectively, and coupled by the second-order susceptibility. Through frequency doubling, transverse-electric and transverse-magnetic modes lock into a 1 + 1D solitary wave encompassing orthogonal field components at the two harmonic wavelengths. The phase matching scheme leads to a wavelength-comb filter with adjustable linewidths