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We report the generation of Kerr frequency combs in magnesium fluoride (MgF 2 ) microdisk resonators with normal dispersion. Two MgF 2 microdisks with Q factors in the order of 108 and radii of 180 μm and 85 μm were fabricated and characterized. Despite being in the normal dispersion regime at a wavelength of 1550 nm, the microcavities exhibited the generation of Kerr frequency combs. Remarkably, one of the microcavities, when pumped with a 1550 nm laser, produced a comb with a spectral range spanning over 250 nm. This unexpected phenomenon highlights the unique nonlinear properties of MgF 2 microresonators and opens up new perspectives for compact Kerr comb generators based on ultra-high Q crystalline whisperinggallery mode resonators. Additionally, the transparency of MgF 2 in the ultraviolet (UV) wavelength range suggests the potential for extending Kerr frequency combs into the UV spectrum, further enhancing the versatility of MgF 2 microcavities in nonlinear photonics applications.
Lin et al. (Wed,) studied this question.