Silicon oxynitride microresonators for Kerr frequency combs

On-chip Kerr frequency combs have attracted significant attention because of their compact footprint and numerous applications. While many integrated material systems are being investigated for generating the on-chip Kerr frequency combs, so far only silica devices have achieved quality factors above 100 million, which is important for decreasing the threshold and power consumption of the system. However, as an intrinsic property of silica, the hydroxyl groups present on the surface of the devices will attract water molecules in the air, which decreases the quality factor of the devices. To maintain the performance of the frequency combs, methods like putting the devices in nitrogen purged boxes or building covers for the system are proposed, which would largely increase the complexity of the system. Here we studied another material system, silicon oxynitride microtoroids, whose quality factors can achieve and stay constant at more than 100 million because of the lack of the hydroxyl groups on the surface. Kerr frequency combs are generated from the SiOxNy microtoroids with normal dispersion with avoided mode crossing. Thresholds as low as 280 mu W are achieved as a result of the high quality factor. The comb spectrum remains the same for the same pump power over the nine day period after fabrication, which indicates that the performance of the frequency combs remains constant despite the silicon oxynitride devices being stored in ambient atmosphere without any special treatment the whole time.