  Lithium niobate (LN) thin film has recently emerged as an important platform
for nonlinear optical investigations for its large $\chi^{(2)}$ nonlinear
coefficients and ability of light localization. In this paper, we report the
first fabrication of LN on insulator (LNOI) ridge waveguides with generalized
quasi-periodic poled superlattices using the electric field polarization
technique and microfabrication techniques. Benefiting from the abundant
reciprocal vectors, we observed efficient second-harmonic and cascaded
third-harmonic signals in the same device, with the normalized conversion
efficiency 1735% W$^{-1}$cm$^{-2}$ and 0.41% W$^{-2}$cm$^{-4}$, respectively.
This work opens a new direction of nonlinear integrated photonics based on LN
thin film.
