  High quality lithium niobate thin film microresonators provide an ideal
platform for on-chip nonlinear optics applications. However, the phase matching
condition for efficient parametric process sets a high requirement for
fabrication control. Here we demonstrate a photonic molecular structure
composed of two strongly coupled lithium niobate microdisks of different
diameters fabricated using femtosecond laser micromachining and focused ion
beam milling. With a continuous wave excitation, rich nonlinear optical
processes including cascaded four-wave mixing and stimulated Raman scattering
were observed around the second harmonic wavelength. Our results indicate that
the coupled microdisks form a superior system for nonlinear optical process.
