Optical whispering gallery mode (WGM) resonators combining small size with high quality factor are very attractive for a wide range of applications in optics, photonics and radiophotonics. Silicon is a major semiconductor material for modern technology, so samples of high purity and homogeneity are easily available. Besides that, it is a very promising material for Kerr frequency comb generation  in near and mid-infrared frequency range due to the high value of Kerr nonlinearity and significant reduction of two-photon absorption in mid-IR. However, application of silicon microresonators was limited due to the relatively low demonstrated quality factors at some promising spectral ranges. to date, integrated silicon WGM resonators demonstrated a quality factor about 107 that is approximately two orders smaller than conventional quality factor of fluoride crystalline microresonators and much worse if we consider the record value 1011 in CaF2.
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