Piezoelectric transduction of Silicon Nitride photonic system

  

Integrated photonics has provided an elegant way to bring the table-top bulky optical systems from the research lab to our daily life, thanks to its compact size, robustness, and low power consumption. Over the past decade, Silicon Nitride (Si3N4) photonics has become a leading material platform, benefiting from its record-low loss, large Kerr nonlinearity, and compatibility with the foundry process. However, the lack of electro-optical effect makes it challenging to actively tune the Si3N4 photonic circuits for advanced applications, such as LiDAR, spectroscopy, and atomic clocks. During my PhD research, I have developed a new platform of piezoelectric control of Si3N4 photonics through stress-optical effect. By integrating an
Aluminum Nitride (AlN) piezoelectric actuator, I demonstrated the tuning of Si3N4 optical microring resonator at sub-microsecond speed with nano-Watt power consumption. Microwave frequency (GHz) acousto-optic modulation (AOM) is realized by exciting high-overtone bulk acoustic wave resonant modes (HBAR), which are tightly confined in an acoustic Fabry-Pérot cavity. Maximum of 9.2 GHz modulation is achieved which falls into the microwave X-band. 

The applications of the Piezo-on-Photonic platform are extensively explored in the quasi-DC and high frequency regimes. By working as a stress-optical tuner at low frequency, it allows me to actively tune a Kerr frequency comb into different states, and stabilize it over several hours, which can serve as the light source for the next-generation chip-based LiDAR engine. On the other hand, the GHz frequency AOM has helped me demonstrate a magnetic-free integrated optical isolator, a device that transmits light in only one direction. Three AlN HBAR actuators are integrated closely on the same Si3N4 microring resonator, which generate an effective rotating acoustic wave and break the transmission reciprocity of the light. A maximum of 10 dB isolation is achieved under 300 mW total radiofrequency power, with minimum insertion loss of 0.1 dB. Finally, the application of the same technique in quantum microwave to optical converter is theoretically analyzed, showing potential for building future quantum networks. The initial experimental attempt and outlook for future improvements are investigated. 

In conclusion, this thesis investigated a novel Piezo-on-Photonic platform for flexible and efficient control of the Si3N4 photonic system, and its applications in a wide variety of advanced devices are demonstrated, with the potential of being key building blocks for future optical systems on-chip.