Fabrication of three-dimensional microdisk resonators in calcium   fluoride by femtosecond laser micromachining
  We report on fabrication of on-chip calcium fluoride (CaF2) microdisk resonators using water-assisted femtosecond laser micromachining. Focused ion beam (FIB) milling is used to create ultra-smooth sidewalls. The quality (Q)-factors of the fabricated microresonators are measured to be 4.2x10^4 at wavelengths near 1550 nm. The Q factor is mainly limited by the scattering from the bottom surface of the disk whose roughness remains high due to the femtosecond laser micromachining process. This technique facilitates formation of on-chip microresonators on various kinds of bulk crystalline materials, which can benefit a wide range of applications such as nonlinear optics, quantum optics, and chip-level integration of photonic devices. 
Jintian Lin, Yingxin Xu, Jialei Tang, Nengwen Wang, Jiangxin Song, Fei   He, Wei Fang, and Ya Cheng
physics.optics