  The lithium niobate integrated photonic platform has recently shown great
promise in nonlinear optics on a chip scale. Here, we report second-harmonic
generation in a high-Q lithium niobate microring resonator through modal phase
matching, with a conversion efficiency of 1,500% W$^{-1}$. Our device also
allows us to observe difference-frequency generation in the telecom band. Our
work demonstrates the great potential of the lithium niobate integrated
platform for nonlinear wavelength conversion with high efficiencies.
