  We demonstrated broadband highly efficient optical nonlinear processes in
on-chip integrated lithium niobate (LN) microdisk resonators. The Q factors of
the micro-resonators fabricated by femtosecond laser writing and
chemo-mechanical polishing are reliably above 10^8, approaching the intrinsic
material absorption limit of LN. Broadband nonlinear processes, including
optical parametric oscillation (OPO), second harmonic generation (SHG), third
harmonic generation, and fourth harmonic generation, were observed with
ultrahigh efficiencies in the same LN microdisk without introducing domain
inversion, thanks to the natural quasi phase-matching and the dense spectral
modes of the X-cut LN microdisk with millimeter diameter. The threshold of OPO
and the absolute conversion efficiency of SHG are 19.6 microwatt and 66%, both
surpass the state-of-the-art values among on-chip LN micro-resonators
demonstrated so far. The broadband and highly efficient nonlinear frequency
conversions achieved with the ultrahigh-Q LN microdisk resonators promise
high-density integration of nonlinear photonic devices such as frequency
convertors and entangled photon sources.
