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 μW 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.
Recent advances in optical materials have enabled the development of a wide range of integrated photonic devices from high speed modulators to frequency combs. With low optical loss over a wide wavelength range and environmental stability in ambient environments for several weeks, silicon oxynitride (SiOxNy) shows potential in many of these applications. However, unlike many classic optical materials, the thermo-optic response (dn/dT) in both the visible and near-IR is poorly characterized, limiting researcher’s ability to accurately model device performance. Here, we leverage the intrinsic thermal response of resonant cavities to measure the dn/dT of SiOxNy with a 12.7:1 and 4:1 oxygen to nitrogen ratio based on EDX measurements. The thermo-optic coefficient is measured in the visible and near-IR and compared with SiO2. The refractive indices of the silicon oxynitride films were also measured using spectroscopic ellipsometry. Based on an analysis of the O:N ratio and a comparison with both SiO2 and Si3N4, an expression for the dependence of the dn/dT on the stoichiometric ratio is developed.
High quality whispering gallery mode resonators can greatly enhance the optical field by trapping the light through total internal reflection, which makes these resonators a promising platform for many areas of research, including optical sensing, frequency combs, Raman lasing and cavity QED. Among these resonators, silica microtoroidal resonators are widely used because of their ability to be integrated and to achieve ultrahigh quality factors (above 100 million). However, quality factors of traditional silica toroids gradually decrease over time because there is an intrinsic layer of hydroxyl groups on the silica surface. This layer of hydroxyl groups attracts water molecules in the atmosphere and results in high optical losses. This property of silica degrades the behavior and limits the applications of the integrated silica toroids. In this work, we address this limitation by fabricating integrated microtoroids from silicon oxynitride. The surface of silicon oxynitride has a mixture of hydroxyl groups and fluorine groups. This mixture prevents the formation of a layer of water molecules that causes the optical losses. Our experiments demonstrate that the quality factors of the silicon oxynitride toroids exceed 100 million, and these values are maintained for over two weeks without controlling the storage conditions. As a comparison, quality factors of traditional silica toroids fabricated and stored under same conditions decayed by approximately an order of magnitude over the same duration.
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