Precisely Dispersion Tailored Crystalline Microresonator with a Q Exceeding 10⁸ Fabricated by Computer-Controlled Machining

Ultrahigh-Q crystalline microresonators have various potential applications including as microresonator frequency combs [1] and low-noise radio frequency oscillators [2]. Since crystalline materials are transparent in the visible to mid-infrared wavelength region, magnesium fluoride (MgF ₂ ) and calcium fluoride (CaF ₂ ) microresonators have been expected to achieve broad bandwidth Kerr comb generation. The demand for precise geometrical dispersion control to compensate for material dispersion has recently been increasing because material dispersion in the visible or mid-infrared region is not feasible for obtaining a Kerr comb [3,4]. However, the dispersion engineering of crystalline microresonators remains a major challenge since hand polishing is generally needed after diamond turning to obtain a high Q [5]. This restricts the possibility of tailoring the resonator dispersion because it degrades the controllability of the cross-sectional shape.