The first optical frequency combs (OFC) were realized 20 years ago and have since found wide-ranging applications in timing, spectroscopy, and metrology. Traditionally, OFCs are constructed based on mode-locked lasers that generate periodic ultrashort pulses in the time domain. These ultrashort pulses appear in the frequency domain as thousands of optical frequencies that are equally spaced. Therefore, OFCs are “frequency rulers” for precise frequency and time measurement, and are widely used in optical atomic clocks, quantum communications, ultra-stable lasers and microwaves, astronomical observations, and physical constant measurements.
Dr. Junqiu Liu, currently a postdoctoral scientist at École polytechnique fédérale de Lausanne (EPFL), Switzerland, has been working on OFC and related applications. In 2007, his Ph.D. supervisor Prof. Tobias J. Kippenberg discovered that OFCs can be generated in ultralow-loss, highly nonlinear optical microresonators. The OFCs generated in this scheme is referred to as “microcombs”today. Importantly, by operating the microcombs in dispersion-optimized microresonators, dissipative Kerr solitons (DKS) – which are stable circulating ultrashort pulses with maintained shape – are generated. These DKS microcombs are intrinsically stable and coherent OFCs on-chip and allow massive and low-cost production of chip-scale OFCs for a vast of applications.
Dr. Liu’s research work has addressed multiple central topics in the development of chip-based microcombs using CMOS nanofabrication technology. He developed and optimized the “photonic Damascene process”, a novel but mature CMOS nanofabrication technology for Si3N4 photonic integrated circuit (PIC) that enables ultralow optical loss of only 0.5 dB/m of optical length, the world's lowest value in any nonlinear integrated platform. Using this ultralow-loss Si3N4 PIC, he has demonstrated the first integrated soliton microcombs of 10 and 20 GHz repetition rates, which could have a disruptive impact for coherent optical communications and low-noise microwave synthesis. In addition, with his collaborators, he has achieved the first demonstration of hybrid integration of DFB laser chips or diodes with Si3N4 chips which enables the most compact and smallest microcomb modules operated at CMOS frequencies, and monolithic integration of acoustic-optic modulators on Si3N4 using MEMS-based piezoelectric aluminum nitride.
Dr. Liu has published more than 30 publications in high-profile journals, including 4 Nature, 4 Nature Photonics / Physics, and 9 Nature Communications. He has received several awards for his achievements in Si3N4 integrated photonics. Today, the Si3N4 chips fabricated by Dr. Liu have been used in many research institutes and industrial partners. Meanwhile, the Si3N4 nanofabrication technology developed during his Ph.D. thesis has been transferred to several startups that are currently commercializing ultralow-loss Si3N4 photonic circuits and related laser products.