Nonlinear optics is the study of the interaction between light and matters. It originated from the first observation of second harmonic generation by Franken and coworkers in 1961. Since then, interest in this area has been continuously growing. Now nonlinear optics has been extended to numerous applications such as diversified lasers, communications, sensing and different quantum technologies, in both industry and the research of fundamental science.
Recently, with many breakthroughs in the demonstrations of nonlinear devices at chip-scale, the area of nonlinear photonics has evolved and went through rapid progress. For example, frequency comb generation, the Nobel-winning technology, has been enabled by micro-resonators on multi photonic platforms. Other advanced nonlinear processes, such as second harmonic generation, optical parametric oscillation, as well as many opto-mechanics processes, have also been realized in different on-chip devices.
However, even though those demonstrations open up many new possibilities for nonlinear applications by using photonics approach, currently the nonlinear photonic technologies still have some major problems at both individual device and system level. The efficiency of nonlinear process on chip is still not high enough to enable strong frequency conversion at the pump power level of integrated lasers, which is usually around a few milli-watts. Another challenge is that current nonlinear devices are not integration compatible with other active photonic components, which prevents the realization of fully integrated nonlinear systems on chip.
To address those problems, we proposed and developed the heterogeneous integration technology for nonlinear photonics. Heterogeneous integration has been widely used in silicon photonics to transfer III-V or other material layers on Si waveguides by wafer- or die- bonding. Introducing it into nonlinear photonics enables the access to high quality single crystalline films, which can have much higher nonlinearities compared to the materials used for previous nonlinear devices. Furthermore, it also offers the possibilities to integrate nonlinear and active photonic components together, which paves the way to the revolutionary nonlinear photonic integrated circuits (PICs) in the future.
This thesis presents two nonlinear platforms we developed based on heterogeneous integration: the (Al)GaAs on insulator platform as well as the lithium niobate (LiNbO3) on insulator platform. Multiple efficient nonlinear processes on those platforms, including second harmonic generation by waveguides and micro-resonators, as well as frequency comb generation, have been realized. We also demonstrated CMOS compatible platforms with nonlinear functionalities by heterogeneously integrating silicon nitride with either (Al)GaAs or LiNbO3. The design considerations, fabrication processes, and the performance of each nonlinear device are discussed in details. Furthermore, we discuss a system level demonstration, an optical frequency synthesizer based on efficient nonlinear devices, and the possibility to fully integrate those nonlinear systems in PICs based on the heterogeneous integration technology.
