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

IEEE J. ST Quantum Electron. 30, 2100208 (2024)https://ireap.umd.edu/10.1109/JSTQE.2024.33799372024Journal ArticleNonlinear and Quantum Photonics

We report the generation of Kerr frequency combs in magnesium fluoride (MgF₂) microdisk resonators with normal dispersion. Two MgF₂ 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 MgF₂ 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 MgF₂ in the ultraviolet (UV) wavelength range suggests the potential for extending Kerr frequency combs into the UV spectrum, further enhancing the versatility of MgF₂ microcavities in nonlinear photonics applications.