With the longtime development of electronic information technology, we have come to the age of information. However the bottleneck of the electronic technology has limited the further development of the information technology. The photonic circuit has been placed great expectations, and on-chip photonic devices have attracted great interests these years.

Lithium niobate (LN) exhibits a significant optical nonlinearity which has been applied for many important nonlinear and quantum photonic applications. In general, nonlinear optical processes rely critically on the optical intensity, which can be dramatically increased by miniaturizing the device structure, leading to enhanced nonlinear conversion efficiency. This great potential has excited significant interest in recent years to explore nonlinear optics in on-chip LN photonic devices.

The device employed is a high Q one-dimensional photonic crystal nanocavity, which is fabricated on an X-cut LN-on-insulator wafer, with a lattice constant of 545 nm. The suspended nanobeam has a thickness of 250 nm, with a 2-lm gap from the silicon substrate. The device structure was patterned using electron beam lithography and etched by an argon-ion milling process. The buried silica layer between the LN nanobeam and the silicon substrate was finally undercut by diluted hydrofluoric acid.

A photonic crystal nanocavity exhibits superior capability of confining light in subwavelength dimension; thus it is of great promise for nonlinear photonic application.  Surprisingly, at a drop power of 80 lW, the spot is so bright that it can be seen even by naked eyes.

This work was done by Prof. Yuping Chen, Prof. Xianfeng Chen, and Prof. Qiang Lin from University of Rochester in collaboration. The result is published in Appl. Phys. Lett. 113, 021104 (2018)（https://doi.org/10.1063/1.5039948）.  The demonstration of flexible nonlinear frequency conversion in these devices shows great promise of nonlinear photonic applications using high-Q LN photonic crystal nanoresonators.

The Laboratory of Advanced Photonic Materials and Physics in Shanghai Jiao Tong University has keep in close collaboration with Qiang Lin’s group in the University of Rochester for several years. With various exchanges and visits, and joint PhD programs, we have successfully cooperated in several researches, including Fast response of photorefraction in lithium niobate microresonators  （https://doi.org/10.1364/OL.42.003267）, On-chip second-harmonic generation and broadband parametric down-conversion in a lithium niobate microresonator （https://doi.org/10.1364/OE.25.024531）, Self-referenced temperature sensing with a lithium niobate microdisk resonator （https://doi.org/10.1364/OL.42.001281）, and High-quality lithium niobate photonic crystal nanocavities（https://doi.org/10.1364/OPTICA.4.001251）.