Barium fluoride whispering-gallery-mode disk-resonator with one billion quality-factor

Area of Science:

• Optics and Photonics
• Materials Science

Background:

• Whispering-gallery-mode resonators are crucial for various photonic applications.
• Achieving ultra-high quality (Q) factors in resonators is essential for enhancing performance.
• Barium fluoride (BaF₂) is a promising material, but achieving high Q-factors in soft crystals has been challenging.

Purpose of the Study:

• To demonstrate a monolithic optical whispering-gallery-mode resonator fabricated with barium fluoride (BaF₂).
• To achieve an ultra-high quality (Q) factor exceeding 10⁹ at 1550 nm.
• To explore the feasibility of using these resonators for high-energy particle scintillation detection.

Main Methods:

• Fabrication of a monolithic BaF₂ whispering-gallery-mode resonator.
• Measurement of the Q factor using linewidth and cavity-ring-down methods.
• Characterization of surface roughness using vertical scanning optical profilometry.

Main Results:

• An ultra-high Q factor above 10⁹ was achieved for the BaF₂ resonator at 1550 nm.
• A root mean square surface roughness of 2 nm was achieved on the mm-size disk resonator.
• Demonstrated that a billion Q-factor is achievable in soft crystals (Mohs hardness of 3) via precision polishing.

Conclusions:

• Monolithic BaF₂ whispering-gallery-mode resonators can achieve ultra-high Q factors, surpassing previous limitations for soft optical crystals.
• The achieved Q factor and surface quality open avenues for advanced nonlinear optics, microwave photonics, and novel high-energy particle scintillation detection.
• Complex thermo-optical dynamics were observed, indicating the potential for further investigation and application tuning.