Hanxiao Liang

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 in recent years, LN photonic crystal structures developed to date exhibit fairly low quality. Here we demonstrate LN photonic crystal nanobeam resonators with optical Q as high as 10^5, more than two orders of magnitude higher than other LN nanocavities reported to date. The high optical quality together with tight mode confinement leads to 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 intriguing quenching of photorefraction that has never been reported before. The devices also exhibit strong optomechanical coupling with gigahertz nanomechanical mode with a significant f*Q product of 1.47*10^12 Hz. The demonstration of high-Q LN photonic crystal nanoresonators paves a crucial step towards LN nanophotonics that could integrate the outstanding material properties with versatile nanoscale device engineering for diverse intriguing functionalities.

Nonlinear optical oscillation dynamics in high-Q lithium niobate microresonators.

Recent advance of lithium niobate microphotonic devices enables the exploration of intriguing nonlinear optical effects. We show complex nonlinear oscillation dynamics in high-Q lithium niobate microresonators that results from unique competition between the thermo-optic nonlinearity and the photorefractive effect, distinctive to other device systems and mechanisms ever reported. The observed phenomena are well described by our theory. This exploration helps understand the nonlinear optical behavior of high-Q lithium niobate microphotonic devices which would be crucial for future application of on-chip nonlinear lithium niobate photonics.

On-chip second-harmonic generation and broadband parametric down-conversion in a lithium niobate microresonator

Nonlinear wavelength conversion is essential for many classical and quantum pho-tonic applications. The underlying second-order nonlinear optical processes, however, generally exhibit limited spectral bandwidths that impact their application potential. Here we use a high-Q X-cut lithium niobate microdisk resonator to demonstrate both second-harmonic generation and spontaneous parametric down-conversion on chip. In particular, our lithium niobate microresonator, with its wide-range cyclic phase matching and rich optical mode structures, is able to achieve ultra-broadband spontaneous parametric down-conversion, with a bandwidth over 400 nm, inferred from recorded spectra of the down-converted photons. The produced biphoton pairs exhibit strong temporal correlation, with a coincidence-to-accidental ratio measured to be 43.1. Our device is promising for integrated quantum photonics where optical frequency could be used as a degree of freedom for signal processing.

Ultra-broadband dispersion engineering of nanophotonic waveguides

We propose double-cladded and multi-cladded waveguide structures that enable flexible engineering of group-velocity dispersion over multi-octave spanning spectral range, allowing to produce intriguing dispersion characteristics that oscillate between normal and anomalous dispersion in a sinusoidal-like fashion with controllable magnitude, which will be very useful for nonlinear photonic applications. The proposed waveguides are easy to fabricate in practice, which we expect to have great potential for broad applications in nonlinear photonics, quantum optics, and optical communication.

Multicolor cavity soliton

We show a new class of complex solitary wave that exists in a nonlinear optical cavity with appropriate dispersion characteristics. The cavity soliton consists of multiple soliton-like spectro-temporal components that exhibit distinctive colors but coincide in time and share a common phase, formed together via strong inter-soliton four-wave mixing and Cherenkov radiation. The multicolor cavity soliton shows intriguing spectral locking characteristics and remarkable capability of spectrum management to tailor soliton frequencies, which would be very useful for versatile generation and manipulation of multi-octave spanning phase-locked Kerr frequency combs, with great potential for applications in frequency metrology, optical frequency synthesis, and spectroscopy.

Nonlinear frequency conversion in one dimensional lithium niobate photonic crystal nanocavities

We demonstrate flexible nonlinear frequency up-conversion in high-Q lithium niobate photonic crystal nanobeam resonators. The high optical Q together with strong optical mode confinement allows us to observe clear second harmonic generation and sum frequency generation with an optical power around only tens of microWatts. These demonstrations show that high-Q lithium niobate photonic crystal nanoresonators are of great promise for nonlinear photonic applications.We demonstrate flexible nonlinear frequency up-conversion in high-Q lithium niobate photonic crystal nanobeam resonators. The high optical Q together with strong optical mode confinement allows us to observe clear second harmonic generation and sum frequency generation with an optical power around only tens of microWatts. These demonstrations show that high-Q lithium niobate photonic crystal nanoresonators are of great promise for nonlinear photonic applications.