High-quality lithium niobate photonic crystal nanocavities

Lithium niobate (LN) exhibits unique material characteristics that have found many important applications. Scaling LN devices down to a nanoscopic scale can dramatically enhance light–matter interaction that would enable nonlinear and quantum photonic functionalities beyond the reach of conventional means. However, developing LN-based nanophotonic devices turns out to be nontrivial. Although significant efforts have been devoted to this in recent years, the LN photonic crystal structures developed to date exhibit fairly low quality (Q). Here we demonstrate LN photonic crystal nanobeam resonators with optical Q as high as 105, more than two orders of magnitude higher than other LN photonic crystal nanocavities reported to date. The high optical Q, together with tight mode confinement, leads to an extremely strong nonlinear photorefractive effect, with a resonance tuning rate of ∼0.64  GHz/aJ, or equivalently ∼84  MHz/photon, three orders of magnitude greater than other LN resonators. In particular, we observed an intriguing quenching of photorefraction that has never been reported before. The devices also exhibit strong optomechanical coupling with a gigahertz nanomechanical mode with a significant f·Q product of 1.47×1012  Hz. The demonstration of high-Q LN photonic crystal nanoresonators paves a crucial step toward LN nanophotonics that could integrate the outstanding material properties with versatile nanoscale device engineering for diverse and intriguing functionalities.