Single-crystal lithium niobate (LiNbO3, LN), with its large diagonal second-order susceptibilities (χ(2)zzz as high as 41.7 pm/V), wide transmission window (400 nm to 5 µm, with an OH – absorption peak at 2.87 µm), relatively high ordinary and extraordinary refractive indices (no = 2.21, ne = 2.14 at 1550 nm, respectively), as well as large piezoelectricity, is an excellent nonlinear optical material that has already found many applications in communication science and technology. Conventional LN components, including electro-optical modulators or periodically-poled (PPLN) wavelength conversion devices, are realized in weakly-perturbed LN waveguides with very low index contrast (Δn < 0.1) and large device dimensions. An integrated LN nanophotonic platform, which combines the unique material properties of LN with the superior light confinement in wavelength scale optical waveguides and cavities, could overcome these limitations and enable efficient, low-cost and highly-integrated nonlinear optical systems.
Based on commercially available LN-on-insulator (LNOI) substrates, we have developed simple and robust fabrication techniques to realize high quality integrated LN nanophotonic devices. As an example, we have demonstrated LN microdisk resonators with quality factors over 100,000 [Opt Express 22, 25(2014)]. Other types of nanophotonic elements including micro-ring resonatos, nanowaveguides and Mach-Zehnder interferometers are also readily achievable with the same fabrication method. As a proof-of-concept, on-chip second second-harmonic generation (SHG) was shown in the aforementioned microdisk resonators, with an internal conversion efficiency of 0.109 W-1. The goal of this project is to build compact, highly-efficient and mass-producible on-chip electro-optical modulators and wavelength conversion systems, which could find broad applications ranging from telecommunications to quantum information processing.
LN microdisk resonators and fabrication procedures
Transmission spectrum of LN microdisk resonators
Waveguide-coupled LN race-track resonators
