Cascading of wave-mixing processes plays an important role in optics through which one can achieve complex manipulation or highly enhanced conversion of photons beyond a direct process. The concept has been implemented in a vast range of applications in both classical and quantum optics. Among these, the cascading of the electro-optic (EO) effect and nonlinear wave mixing is of significance for enhancement or control of the optical process with a simple method of applying voltages. Previous proposals and demonstrations suffer from inefficiency in both sub-processes in bulk structures. The emerging technology of lithium niobate on insulator (LNOI) successfully solved this problem. It combines versatile properties of lithium niobate and a high refractive index contrast of it with respect to silica or air for integrated photonic applications. The efficiency of EO coupling and nonlinear wave mixing can be significantly enhanced.

Fig. (a) Schematic of the cascading EO coupling and SHG process in the PPLNOI ridge waveguide. (b) Calculated fundamental mode profiles of the FW (1590 nm) and SH (795 nm) in the PPLNOI ridge waveguide.

We use the ridge periodically polarized lithium niobate on insulator (PPLNOI) waveguide to realize the cascade of lateral EO coupling and second harmonic generation (SHG) at the same time. The fundamental wave (FW) needs to satisfy the quasi-phase matching (QPM) condition of the two processes, where SHG is a type-0 QPM. The polarization control of FW is realized by changing the voltage, which affects the intensity of SHG. The integration of the cascading process is demonstrated at a low-voltage fast-speed drive for EO coupling and a relatively low input power for SHG. The demonstration shows a new type of on-chip functionality on the LNOI platform, which implies one step further towards on-chip dynamic control of nonlinear wave mixing.

This research was published in “ingting Ding, Yuanlin Zheng, and Xianfeng Chen, Integration of cascaded electro-optic and nonlinear processes on a lithium niobate on insulator chip, Optics Letters, 44(6), 1524-1527 (2019).”