  Nonlinear optical processes in whispering gallery mode (WGM) microresonators
have attracted much attention. Owing to the strong confinement of light in a
small volume, a WGM microresonator can dramatically boost the strength of light
field, thereby promoting the nonlinear interaction between the light and the
resonator material. However, realization of efficient nonlinear parametric
process in microresonators is a challenging issue. The major difficulty is to
simultaneously ensure the phase matching condition and a coherent multiple
resonance condition for all the waves participating in the nonlinear conversion
process. Here, we demonstrate highly efficient second harmonic generation (SHG)
in an on-chip lithium niobate microresonator fabricated by femtosecond laser
direct writing. We overcome the difficulty of double resonance for the phase
matched pump and second harmonic waves by selectively exciting high order modes
in the fabricated thin-disk microresonator. Our technique opens opportunities
for integrated classical and quantum photonic applications.
