Widespan Kerr Comb Generation Using Sub-Millimeter-Size Magnesium Fluoride Whispering-Gallery Resonators in the Normal Dispersion Regime

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
$10^{8}$
and radii of
$180\,\mu$
m and
$85\,\mu$
m 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 whispering-gallery 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.