Kerr nonlinearity based frequency combs and solitons have been generated from
on-chip optical microresonators with high quality factors and global or local
anomalous dispersion. However, fabrication of such resonators usually requires
materials and/or processes that are not standard in semiconductor manufacturing
facilities. Moreover, in certain frequency regimes such as visible and
ultra-violet, the large normal material dispersion makes it extremely difficult
to achieve anomalous dispersion. Here we present a concentric racetrack-shaped
resonator that achieves anomalous dispersion in a 300 nm thick silicon nitride
film, suitable for semiconductor manufacturing but previously thought to result
only in waveguides with high normal dispersion, a high intrinsic Q of 1.5
million, and a novel mode-selective coupling scheme that allows coherent combs
to be generated. We also provide evidence suggestive of soliton-like pulse
formation in the generated comb. Our method can achieve anomalous dispersion
over moderately broad bandwidth for resonators at almost any wavelength while
still maintaining material and process compatibility with high-volume
semiconductor manufacturing
