EPFL Doctorate Award 2021 – Junqiu Liu

For groundbreaking experiments in the field of chip-scale frequency combs and the extraordinary record of scientific accomplishments.

Microresonator-based, dissipative Kerr soliton frequency combs (“soliton microcomb”) constitute chip-scale optical frequency combs of broad bandwidth and repetition rate in the terahertz to microwave domain. To build microcombs using photonic integrated circuit (PIC), silicon nitride (Si₃N₄) has emerged as the most mature CMOS-compatible platform due to its low optical loss. My PhD thesis concerns the development and application of ultralow-loss Si₃N₄ PIC for nonlinear photonics such as microcomb generation. Through careful PIC design, systematic optimization of Si₃N₄ wafer-scale fabrication process, and comprehensive characterization of the final chip devices, a novel Si₃N₄ fabrication technology featuring ultralow optical loss and wafer-level yield has been developed. Integrated Si₃N₄ microresonators of quality factors exceeding 30 million (corresponding to a linear optical loss of 1 dB/m) have been fabricated, enabling soliton microcombs with unprecedentedly low power budgets and repetition rates down to 10 GHz. In addition, monolithic integration of Si₃N₄ with aluminium nitride piezoelectric modulators, and hybrid integration with semiconductor lasers, have been also demonstrated, as well as applications such as microwave synthesis, coherent LiDAR and optical communication. The Si₃N₄ photonics technology developed in this PhD thesis can be readily merged with silicon photonics foundry process and proliferate new applications for frequency metrology, spectroscopy and photonic computing.