Research progress of photonics devices on lithium-niobate-on-insulator thin films

Lithium niobate (LiNbO3, LN) crystals have excellent electro-optical and nonlinear optical properties, and they have been regarded as one of the most promising materials for constructing the multifunctional photonic integrated systems. Due to the excellent optical properties of LN crystal, the emerging LN thin film technology has received great attention in the research of integrated photonics in recent years. With the help of advanced micro-nano fabrication technologies, many high-performance lithium niobate integrated photonic devices have been realized. Integrated photonic platform can incorporate high-density, multi-functional optical components, micro-nano photonics structures, and optical materials on a monolithic substrate, which can flexibly implement a variety of photonic functions. At the same time, it also provides a low-cost, small-size, and scalable solution for miniaturizing and integrating the free-space optical systems. Photonic chips based on LN have been widely used in fast electro-optic modulation, nonlinear optical frequency conversion and frequency comb generation. In particular, periodically poled lithium niobate (PPLN) based on quasi-phase matching has gradually become a mature integrated photonic platform and has been widely used in the field of nonlinear optics. As wafer bonding technology is matured, the lithium-niobate-on-insulator (LNOI) thin films made by the “smart-cut” process have been commercialized. The thickness of the LN film on a Si or SiO2 substrate can reach several hundred nanometers, and good uniformity in film thickness at a larger size (3 inches) can be ensured. With the development of micro-nano fabrication technologies, the quality and functions of photonic devices on LNOI chips have been significantly improved in recent years, and research on integrated photonic devices based on LNOI has also been developed rapidly in recent years. In this article we briefly review the development of LNOI technology, introducing the applications of several advanced micro-nano fabrication techniques and summarizing their applications in the micro-/nano-fabrication of on-chip photonic devices based on LNOI wafers. In addition, in this article we also summarize the latest advances in the functionality of LNOI on-chip photonic devices and give a short prospective on their future applications in integrated photonics.