Whispering gallery mode (WGM) resonators are particularly interesting platforms for the emerging optical Kerr frequency comb generations, as they are able to feature quality (Q) factors above one billion and thus allow comb generations with very low threshold pump power in the sub-mw regime. The flexibility of choosing various optical materials also facilitates the extension of the spectral ranges and applications of such combs. We have thereby investigated the material dispersion of different optical materials that can support 109 Q factors.
We find that fluoride crystals have flatter anomalous dispersion profiles, among which, barium fluoride has a very attractive profile for potential broadband Kerr comb generations in the mid-infrared (mid-IR) range. By using an analytical approximation of WGM resonant frequencies in toroidal disk resonators, we also report a systematic study of the cavity dispersion effect on the total group velocity dispersion (GVD). We show that the major radius and the radial transverse mode index play an important role in tailoring the GVD values. The latter one can lead to very different GVD values in a single resonator, which is experimentally evidenced by Kerr comb generations in a magnesium fluoride disk.