Octave-spanning microcomb generation in 4H-silicon-carbide-on-insulator photonics platform

Optical frequency combs have changed science and technology as we know it. They are the enabler of applications including frequency generation and synthesis, image and sensing, light detection and ranging, parallel optical communication and computation, and quantum information processing. Given the importance, half of the 2005 Nobel Physics Prize was awarded to the two inventors of the optical comb technology: John L. Hall and Theodor W. Hansch.

 

In the past decade, miniaturizing the optical comb technology using chip-scale device platforms attracted a lot of research efforts, which can significantly reduce its size, weight, and power consumption, all of which are critically important for its broader adoption in practical applications. Thus far, several photonics materials such as silicon, silicon nitride, aluminum nitride and lithium niobate have been extensively studied and achieved varying successes.

 

Recently, silicon carbide, a wide-bandgap material that has been traditionally used as an abrasive or for electronic devices that operate at high temperatures or high voltages, emerged as a promising candidate due to its unique properties, including possessing strong second- and third-order nonlinear coefficients and hosting various color centers that can be utilized as single photon sources or quantum memories. As such, various polytypes of silicon carbide, including 3C, 4H and 6H, are being actively investigated by the research community. In addition, low-loss silicon-carbide-on-insulator photonics platforms have been successfully developed.

 

However, microcomb generation based on the silicon carbide platform has only achieved limited performance. In particular, the reported comb bandwidth was relatively narrow, with the largest wavelength span covering from 1300 nm to 1700 nm so far. For many metrology and timekeeping related applications, it is necessary to obtain a comb bandwidth on the order of one octave or more.

 

To expand the microcomb bandwidth, the research group led by Prof. Qing Li from Carnegie Mellon University reported octave-spanning microcomb generation in the silicon-carbide-on-insulator platform for the first time. The research results are published in Photonics Research, Volume 10, No. 4, 2022 (Lutong Cai, Jingwei Li, Ruixuan Wang, Qing Li. Octave-spanning microcomb generation in 4H-silicon-carbide-on-insulator photonics platform[J]. Photonics Research, 2022, 10(4): 04000870).

 

The significantly increased comb bandwidth was achieved by engineering the dispersion of a compact silicon carbide microring resonator on the one hand and optimizing the nanofabrication to obtain intrinsic quality factors above 1 million on the other hand. With these efforts, the researchers realized a microcomb with wavelength coverage from 1100 nm to 2400 nm with approximately 120 mW on-chip power, as shown in Fig. 1. The dispersion engineering was achieved by simply varying the ring waveguide width, and a good agreement between the simulation and experimental results was achieved.

 

Fig.1 Octave-spanning microcomb generation in the 4H-silicon-cabide-on-insulator platform

 

"Our results have paved the way for the chip-scale implementation of the f-2f self-referencing in the silicon-carbide-on-insulator platform," said by Dr. Li, "and more importantly, lent strong support to the competitiveness of silicon carbide for a variety of nonlinear applications when compared to more mature nonlinear materials such as silicon nitride and aluminum nitride."

 

The goal is to develop a series of efficient device technologies, including frequency generation, modulation, and conversion, and combine them with the quantum technologies implemented in the silicon carbide platform in a seamless fashion. This will eventually lead to powerful information processors that can handle classical and quantum information simultaneously, all based on silicon carbide.